K Wolski Directeur De Recherche Mathematiques

Appliquees O Roustant

Maître-Assistant Mecanique

Ingenierie S Drapier

Professeur Informatique O Boissier

Genie Des

Maître Procedes F Gruy

Nabil Absi

Génie Industriel Mr

Augusto Cmp

Vincent

Stéphane Pr2

Tria Assia

Valdivieso François

DR Jean Paul

Wolski Krzystof

Xie Xiaolan

Yugma Gallian

Jan Vanfleteren

Stéphanie Escoubas

Spécialités doctorales Responsables Spécialités doctorales Responsables SCIENCES ET GENIE DES MATERIAUX

d'avoir accepté de participer à mon jury de thèse. Je voudrais remercier mon directeur de thèse Prof. Sylvain Blayac pour ses conseils et son enthousiasme tout au long de cette thèse. Sylvain, merci particulièrement pour ton aide précieuse dans la structuration de mon travail, et pour ton coaching et ta confiance lors de cette période cruciale qu'est la fin de thèse. Je voudrais remercier mon encadrant industriel, David Coulon, sans qui cette thèse n'aurait pas été possible. Merci de m'avoir enseigné avec enthousiasme les différents aspects du processus industriel, et de m'avoir incluse dans chacune de ses étapes. Je remercie chaleureusement mes encadrants académiques, Roger Delattre et Marc Ramuz, pour leur suivi et leurs précieux conseils tout au long de ma thèse. Roger, merci pour ton aide « sur le terrain », et de m'avoir enseigné les tenants et les aboutissants de la fabrication des circuits souples. Marc, merci pour tes nombreux conseils et pour ton soutien dans l'élaboration de mes idées. J'ai beaucoup apprécié nos discussions, qui m'ont guidée autant dans ma recherche que d'un point de vue personnel. De manière générale, je souhaiterais remercier les différents services du CMP, notamment l'équipe salle blanche, les équipes informatique et infrastructure, ainsi que le service administratif, pour leur aide et contribution à un bon environnement de travail. Mes quatre années au CMP ont été très enrichissantes pour moi autant d'un point de vue professionnel que personnel, et le berceau de nouvelles amitiés. Merci à la team Panaxium pour leur aide sur le parylene, et pour l'ambiance en salle blanche. Merci à BEL de m'avoir acceptée comme l'une des leurs autour d'un (ou plus) café(s). Je souhaite remercier mes collègues thésards de l'open-space d'avoir permis une bonne ambiance de travail, et pour les discussions enrichissantes autour de délicieux gâteaux. Un grand merci à Simon d'avoir accepté Integration into textile .......

Je voudrais dédier ce travail de thèse à ma grand-mère, qui aurait écouté, fidèlement et avec attention, chaque mot de ma défense de thèse.

Acknowledgment

First of all, I would like to thank my thesis committee. I am very grateful to Professor Jan Vanfleteren, from the University of Ghent, and Professor Bernhard Wolfrum, from the Technical University of Munich, for the time they have dedicated to carefully review my thesis.

I am also very grateful to Stéphanie Escoubas, professor at the University of Aix-Marseille, as well as Thierry Legou, research engineer at the University of Aix-Marseille, and Christian Rivero, research engineer at STMicroelectronics, for their participation to my jury committee and for the interest they showed for my work.

I would like to thank my supervisor Prof. Sylvain Blayac for his numerous advices and his enthusiasm throughout this thesis. Thank you for helping me structuring my work, and for your coaching and trust at the crucial period that is the end of a thesis. I would like to thank my industrial supervisor, David Coulon, without whom this thesis would not have been possible. Thank you for enthusiastically teaching me the different aspects of the industrial process, and for including me in every steps. I warmly thank my academic supervisors, Roger Delattre and Marc Ramuz, for their follow-up and advices throughout my thesis. Roger, thank you for your precious help "in the field", and for teaching me the ins and outs of flexible circuits fabrication. Marc, thank you for your numerous advices and for your support in the development of my ideas. I really enjoyed our discussions, which guided me both in my research and from a personal point of view.

In general, I would like to thank the different departments in CMP, including the clean room team, the IT and infrastructure teams, and the administrative department, for their help and contribution to a good working environment.

These four years at CMP have been very rewarding for me both professionally and personally, and the cradle of new friendships. I would like to thank the Panaxium gangstas for their help with parylene, and for the good atmosphere inside the clean room. I would like to thank BEL for accepting me as one of their members for one (or more) coffee(s). I would like to thank my fellow open-space PhD students for providing a good working atmosphere, and for the many enriching discussions around delicious cakes. A big thank you to Simon for graciously accepting the role of cobaye many times. I would like to thank Michelle and Anaïs for their help and kindness. Health Organization estimated that 31% of all deaths worldwide were victims of cardiovascular diseases (17.9 million people), with 85% due to heart attack and strokes. [1] A study conducted in the Cardiology Department of Aix-en-Provence's hospital showed that a continuous monitoring over months of a patient's electrocardiogram (ECG) could lead to up-front detection of cardiovascular diseases and prevent cardiovascular incidents. [2] Nowadays, the only commercialized solution for extended monitoring is a Holter device. The system is rigid, bulky and uncomfortable for the patient, who only wears it a few days. Therefore, improvement of heart attack prevention goes through the development of a connected garment that can be worn continuously without disturbing the patient. In this context, this thesis aims at the fabrication of imperceptible wearable electronics integrated in clothes for long-term and continuous medical monitoring.

List of Figures and Tables

Wearable devices have gained increasing interest in the past two decades. Fields of application include human-machine interfaces [3][4][5] , monitoring of physiological signals [5][6][7][8] , display applications [9] and environmental analysis [10,11] . Despite comfort and practical advantages, truly imperceptible devices struggle to evolve from laboratory prototype to commercialized device.

Current imperceptible electronics in literature are dedicated to sensing parts, as the one depicted in Figure 1-1. They integrate the notion of "ultra-thin" layers, with thickness below 10 microns and a notion of imperceptibility through softness and conformability. Nonetheless, laboratory prototypes are connected to rigid boards for data processing and transmission as shown in Figure 1-1c andd, which can ruin the imperceptibility of the system and prevent from real-life application. On the opposite side, currently commercialized wearable systems are bulky and rigid, as the one depicted in Figure 1-2. In those systems, electronics are confined in rigid removable package, strategically integrated on flat parts on the body, and that the user must unplug for washing in case of garment. A) Neural electrode arrays on simulated brain models to illustrate flexibility. Reproduced from [12] . B) Imperceptible electronic foil of large-area active-matrix sensor laminated on the back of a hand. Reproduced from [13]. C) Wearable salivary uric acid mouthguard biosensor with integrated wireless electronics. Reproduced from [14]. D) Illustrations of graphene electronic tattoos (GET) electrooculography (EOG) system worn by a subject. A schematic of the GET EOG sensors connected to an OpenBCI board, which performs DAQ and wirelessly transmits EOG data to the laptop. The laptop performs data classification and wirelessly sends commands to the quadcopter. Photo of visually imperceptible EOG sensor mounted on the skin.

Photo of back view of the GET EOG sensor system worn on a subject. Reproduced from [3] The wearable electronics we aim at should meet four main features:

 The first criterion is technical: electronics should be performant in data acquiring, analysis, and transmission for fast-responsive smart devices. To date, only inorganic rigid components meet the required electrical and computational performances.

 Imperceptibility of the device is essential, in terms of weight, flexibility, and thickness, as targeted applications impose continuous wearing over months.

 The system should be reliable through everyday stresses such as wearing and washing. We aim at permanent integration in textile for a more convenient and transparent use. Due to the integration of rigid components on flexible substrates, specific mechanical issues raise at soft/rigid interfaces and must be addressed.

 In the context of this thesis, a last criterion is the development of a close-to-standards process, as we are aiming at near-future commercialization.

To build robust and imperceptible systems, we separately address the weak points of the different elements of the system (i.e. the substrate, tracks and components' joints). Analytical model of stacks' mechanics helps us analyze the failure modes and optimizing existing materials; while their robustness is characterized using specifically developed mechanical setups. Wish for thinner, lighter and softer devices drives this work, alongside with better mechanical durability. Finally, we are able to determine the most robust assembly of materials with improved imperceptibility. Chapter 2 gives a broad overview of the four main approaches that were explored, to date, to fabricate wearable smart devices. Each approach is evaluated in terms of: feeling advantages (degree of imperceptibility), mechanical robustness, density of component's integration, and processability. The methods vary from transistors merged inside the core of the fiber to flexible circuit board (FCB) integrated on clothes. Different solutions are also described to address the mechanical transition between rigid components and flexible substrates. All those methods were motivated either by high imperceptibility, or by electronic performances needed for smart and real-time applications.

Chapter 3 addresses mechanical failures, under bending and washing, of conventional laminated layers of FCB industry. We first investigate their mechanical limits as flexible substrates using a dedicated bending setup. To avoid mechanical failure, researchers have investigated encapsulation and thickening structures, but final thickness of a few millimeters can spoil the device imperceptibility. We use a theoretical model of strain distribution under bending to understand how layers' thickness and Young modulus influence the overall mechanical reliability under bending. Finally, we improve the reliability of aluminum/Polyethylene Terephthalate (PET) films with tuned encapsulation and final overall thickness below one millimeter.

In chapter 4, we focus on component integration methods to build mechanically robust systems.

The integration of rigid inorganic components is compulsory but competes with the need for highly compliant substrates. Indeed, heterogeneous integration generates weak points due to mismatch in mechanical properties. In this chapter, ultra-thin substrates and soft contacting glues are compared to conventional materials of flexible circuit industry such as polyimidecopper stacks and tin alloy. We develop bending setups related to real-life solicitation modes in order to characterize the mechanical durability of combinations of substrate and integration method and determine the most robust system.

In the last chapter, we present a new process for fabrication of flexible and ultra-thin circuits that reach microelectronics standards. The novelty of the process lies in the use of copper leaves with an average thickness of 450 nm. This thickness between evaporated metal layers and laminated films of FCB provides advantages from both sides: the layer is thin enough to ensure high compliance and thick enough for good electrical conductivity. The process includes maskfree laser patterning that reaches fine-pitch integration, conventional soldering methods, and a three-step via and assembly process for multilayered circuits. Final devices differ from flexible circuits usually found in literature based on PI/copper laminates of a few dozen of microns.

They integrate a supplementary notion of imperceptibility thanks to their thinness and lightness.

We demonstrate the functionality of the process through an electrocardiogram monitoring device.

Introduction

Integration of complex computational systems into our everyday outfits is an important milestone of wearable technologies. Embedded applications that require distributed sensing, computing and actuating, urge the need for textile-integrated devices. Among them: medical embedded devices such as electrocardiogram make precise measurements from pinpointed locations on the body; energy harvesting devices may collect energy from different positions on the garment into supercapacitors and batteries located in non-exposed regions. We envision smart textiles as complete electronic systems with various functionalities distributed all over the fabric such as: environmental sensing, energy generation, energy storage or even display.

Textile-integration of electronic components such as sensors [1][2][3] , solar cells, [4][5][6][7] batteries and supercapacitors [8][9][10][11] have been extensively covered in literature and will not be addressed in this review. [12][13][14][15] However, all these integrated functionalities require robust electronic circuit to be properly monitored and synchronized. Smart systems perform data acquiring and in situ analysis, deliver consistent output and/or transmit the measured data to the outside world.

Therefore, real-time applications require time-efficiency, reliability and high computational performances. Although this aspect is essential for the functioning of complex integrated systems, it is often neglected in literature due to its technical aspect compared to the more appealing sensing applications mainly reported. It results in only partially integrated devices:

research demonstrators are plugged on computers, and commercialized products feature bulk rigid electronic boxes that hinder the natural feeling of the garment. In this paper, we focus on the textile-integration of the electronic circuitry required to build complete and fully integrated smart wearable systems.

To guarantee correct functioning of real-time devices, computational performances of classic rigid silicon-based components seem compulsory. For the past eighty years, technology progresses in the field of electronics increased the computational abilities and functionalities of electronic devices, while at the same time shrinking their dimensions. This integration has to comprise two important characteristics any wearable systems should fulfill: imperceptibility for the user, as real-wearability and comfort-in-use are compulsory for mass market success; and high reliability under use, therefore resistance to repetitive stresses clothes are prone to during wearing, washing, and everyday activities. Integration of electronic circuits in textile can be reached at three levels:

 At garment level, where clothing and electronics are separately processed and merged at a later stage.

 At fabric level, circuits are partially fixed on the surface of the fabric, with conductive fibers interconnections sewn into the fabric.

 At fiber level, basic electronic elements, such as transistors, are integrated inside the core of the fiber, which will further constitute the fabrics.

The higher the degree of integration, the more standard manufacturing process of textiles and electronics must be revisited and adapted. Development of integration technologies is crucial for wearable systems to meet the required comfort and reliability. Four different approaches to smart textile fabrication are identified: textile circuits, functionalized fibers, flexible circuits and hybrid stretchable circuits. In the two first, fabric serves as a substrate for the electronic circuit to maintain essential textile properties. In basic textile circuits, standard electronic components directly attached to fiber tracks insure elementary electronic functionalities while remaining a non-invasive solution. Then, functionalized fibers replace the rigid electronic parts, to make the electronics even more integrated, merging inside the core of the fiber. These must permission. [131] Copyright 2015, Nanomaterials. (b) Shrinking of electronic components over time. Transistors in current technology are 22nm in size. Reproduced from [132] Scientific endeavor

Wearable technologies are thoroughly studied by scientific community these last years.

This interest can be appraised through the number of articles published per year, here based on Web of Science database. Various subjects in connection with wearables were investigated and are presented in Figure 2-2. From a general point of view, "wearable" is a major subject in scientific literature since 2003 and published articles on this topic currently knows an exponential growth. Secondary topic concerning "smart wearable" is less noticeable in publications but reached a strong growth during the last five years. More focused and late topics, such as "smart clothing" and "E-textile", currently experience a steady growth.

Meanwhile, the economic impact of smart wearables is expected to rise with the development of these technologies.

Smart wearable market

In 2015, the smart wearable market was expected to reach around $14 billion in 2017 and double it in 2020. [16,17] In 2016, this market already accounted for $28.7 billion per year industry [18] , far exceeding expectations. As indicated by the sales of wearable devices between 2015 and 2017, the current market is dominated by smart watches and wristbands. [18,19] However, despite the rapid growth of wearable technology in fitness trackers and smart watches categories, other categories do not constitute a significant part of the market. In particular, smart clothing only represents 1.3% of the global wearable market part this year. The weak growth performance and consumer acceptance in this category is due to several technical factors with sensor size and accuracy, power storage, mechanical reliability to washing/ironing. In addition, there are some cultural challenges to overcome in data privacy, device cost, and fashion.

Markets trend

One may compare smart wearable growth to smartphones since it is the most complex and

transportable device yet available. In 2015, the smartphone market reached $399 billion which is way above the current expectations for the smart wearable market but still way below the clothing market with $1.2 trillion in garment sales. [20] This trend is envisioned to continue for 2019 with $520 billion smartphone sales and $2.2 trillion in garment sales. One would assume, due to the durability and continual growth of both markets, that merger of wearable technology with clothing would be an obvious area for market expansion. However, growth in this area is predicted to be moderate with smart clothing accounting for 9% of the global smart wearable market in 2021. [19] In addition, the e-textile market is expected to reach over $3 billion in 2026, from around $70 million in 2015 [21] , with the appearance of first mass market production around 2020 (Figure 2-3). [22] Social opportunities

Current smart wearables achieved in the literature are already able to measure continuously and accurately several biometric parameters such as electrocardiogram, hearth rate, glucose, respiration, movements, etc. By analyzing the generated data, medical experts are able to establish an early stage diagnostic of health disorders. Patients real time monitoring opens the path to a more reactive and efficient way to treat diseases.

Smart wearable technologies will have an important impact on our social daily life, especially concerning healthcare interactions. [23] In this case, a full and permanent physiological monitoring of patients can prevent serious health issues such as cardiac arrests and seizures.

Thus, a breakthrough may be reached in healthcare by shifting from treatment to prevention. [24] It will induce some social changes by decreasing the economic burden of healthcare services and people interactions to these services. Smart wearables will offer countless opportunities to improve human condition. Through haptic interfaces directly integrated into clothes to provide sensory replacement or enhancement in order to treat patients suffering of sensory impairment.

In addition, smart textile could allow better working conditions in harsh environments by providing real time feedback to users and supervisors. Present evolution of e-textile [21] (left) market and provisional growth [22] (right). Adapted from IDTechEx reports "E-Textiles" 2016-2026 and 2017-2027. A breakthrough is expected within five years with mass market solution of e-textile technology.

In the future, smart wearables may have a great influence on our daily life by doing more than monitoring us. For instance, garments evaluating our stress or fatigue levels will be able to warn us when from performing sensitive tasks (driving, operating heavy machinery, surgery, etc.) in case of lack of concentration. Not allowing a connected car/heavy machinery engine to start, or not granting access to the operating room, could protect the user from potentially dangerous situations. In order to achieve these performances and applications, the integration of electronic circuits allowing precise monitoring and on-site data analysis of the different sensor responses is a key for the success of smart wearable.

Textile circuits

At the early stage development of smart-textile, technologies involved attaching commercial off-the-shelf components directly onto fabric to create textile circuits. Standard silicon-based components ensure the computing functions, and conductive threads embedded in textile replace the copper tracks of conventional printed circuit boards (PCB). The final device results in discrete rigid components distributed on the garment surface, with an interconnection network at the yarn level. This approach allows natural textile's deformations such as "crumpling", which one can define as folding along non-parallel axes. Although the presence of small rigid components decrease the conformability of textiles compared to organic/inorganic functionalized fibers, silicon-based components ensure much higher computational performances, which make them compulsory for any applications of data recording and processing.

Fabric-level interconnects

In order to interconnect the different parts such as sensors, batteries, processing circuits;

conductive tracks have to be embedded into/onto the fabric, as they would lie on a classic motherboard. Electrically conductive fabrics need to be soft and comfortable to wear, rather than hard and rigid. There are mainly four ways to create tracks in/on textile: weaving, knitting, embroidery of conductive threads, and patterning (or coating) of conductive inks. In the first two processes, interconnection network is created during manufacturing process, whereas embroidery and patterning processes occur at later stage on finished fabrics. These four integration methods imply various strains in the conductive material as well as various electrical performances that should be considered.

Conductive materials

Main desirable characteristics for conductive materials in textiles are good electrical conductivity and mechanical properties close to textiles. Firstly, low electrical resistivity and low impedance of the thread is necessary for the high frequencies used in real-time processing.

Secondly, textile-like mechanics give a higher degree of imperceptibility to the electronic and the ability to be processed through commercial textile manufacture. Materials will be presented first in a fiber shape, then as conductive inks for textile coating.

As a quick reminder, a fiber is basically one or several materials embedded into the shape of a single filament (monofilament). The assembly of fibers from same or different materials creates yarns. A thread here means fiber or yarn regardless of the type. Figure 2-4a shows some possible structures for conductive threads.

Several materials have been investigated to integrate conductive tracks in textile. Using standard electrical metallic wires into fabric is an early approach for textile circuit prototypes.

The electric wires are sewn in a piping, [25,26] woven into knitted structures, [27] or embroidered at the textile surface. [28] Their reliable and high electrical properties are advantageous, and the process does not require complex materials. However, the bulkiness and low flexibility of the wires mismatch with the textile natural properties, and decrease the wearability and comfort of use. This implies an elaborate engineering step for the design integration in order to avoid hindrance during wearing, and therefore limitations in design. User oriented studies were published providing design guidelines for wearability. [29,30] Metallic fibers naturally present low resistivity ranging from 10 -8 to 10 diameter copper fiber around polyester yarn. [31] Brittle metal monofilament can also be blended in multifilament yarns (Figure 2-5b). Another technique inspired by eighteenth century decorative textiles features non-conductive base yarns wrapped in metallic foil or thread. Post et al. reported an average resistance of 10 Ω m -1 for a silk yarn wrapped in a thin copper foil (Figure 2-6a). [25] Another process to reach mechanical properties of standard textile fibers is the metallization of threads (Figure 2-5c). Statex Company (Bremen, Germany) fabricates metallized polyamide fibers and fabrics in a product range named Shieldex, which are claimed to be suitable for textile process such as twisting, knitting, embroidery or weaving. Covering metals are silver, copper, tin or nickel. Shieldex silver-coated polyamide typically shows electrical resistance of 500 Ω m -1 , which limits their use in high power applications. Nonetheless, Linz, Kallmayer and colleagues optimized the embroidery of tracks from metal coated yarns and reduced the overall resistance by multiple sewing. [32] They have demonstrated a functional fully integrated electrocardiogram (ECG) shirt using this technology. [33] physically/chemically coated with a thin metal layer. Reproduced with permission. [START_REF] Locher | Technologies for System-on-Textile Integration[END_REF] Copyright 2006, ETH. (d) Optical microscopy images of the knitted single PU/PEDOT:PSS fibers, and (e) single PEDOT:PSS fibers co-knitted with a commercial spandex yarn. The blue loop ( ) and the black loop ( ) represent PU/PEDOT:PSS fiber and Spandex yarn, respectively. Reproduced with permission. [14] Copyright 2015, American Chemical Society. (f)

FESEM images of single gold/graphene yarn. Cross-section image of single gold/graphene microfiber. High resolution image of edge of microfiber. The yellow arrow indicates the gold nanoparticles. Reproduced with permission. [46] Copyright 2017, Royal Society of Chemistry.

(g) Top: photograph of the process to twist graphene fibers to yarns. Bottom and right: SEM images of the twisted yarn at different magnitudes. Reproduced with permission. [47] Copyright 2012, John Wiley and Sons. metal foil. Reproduced from [25] . (b) Woven fabric with metal fibers. Reproduced with permission. [31] Copyright 2003, IEEE. (c) Ag screen printed P-FCB: Micrograph of the surface 100 X magnified. Reproduced with permission. [57] Copyright 2010, IEEE. (d) Woven textile substrate (Petex);  warp direction,  weft direction. (e) Electrical components interconnected via the wire grid in the fabric (polyester threads omitted). (f) Cross section through a textile via. Reproduced with permission . [50] Copyright 2007, IEEE.

Slade et al. examined in depth the washing of commercialized metallized conductive fibers and fabrics. [34] The washing study showed that 100 % metal fibers and foil-based yarns have the best performance under washing conditions, but should be protected from corrosion of commercial detergents. Fabrics made from individually metallized fibers show significant increase in resistance after 50 washing cycles, with a metal coating lifetime limited by mechanical abuse to which the fibers are subjected. Metal-coated fabrics on the other hand perform poorly, as they lose their metal coating within 10 washing cycles. Tao et al. studied the influence of thermoplastic polyurethane (TPU) film protection for washing of different conductive fibers. [35] Depending on the fiber, electrical resistance value could be maintain over five time more washing cycles compared to fibers without TPU protection.

Polymer materials were also investigated for integration into fabrics as their Young modulus in the range of GPa show good compatibility with mechanical properties of fabrics. [1,[36][37][38][39][40][41] They are being shaped into conductive free-standing threads, or conductive paste for coating of fabrics. Nanofibers composed of polyaniline or polypyrrole were synthesized (60-100 nm in diameter) with high conductivity (120-130 S cm -1 ). In particular, poly-(3,4ethylenedioxythiophene) (PEDOT) is the most successful conducting polymer due to its high conductivity and solution processability and is being explored as electrodes for flexible and wearable capacitors or photodiodes. [14,36] Seyedin et al. achieved a scaled-up fiber wet-spinning production of polyurethane/PEDOT:polystyrene sulfonate fibers, with average electrical conductivity of 9.4 S cm -1 , and highly stretchable with a Young's modulus of around 23.5

MPa. [14] The fibers show mechanical strength appropriate for knitting various structures, including mixing with Spandex yarns as shown in Figure 2 presenting an electrical conductivity of approximately 14 S cm -1 when experiencing repeated bending stress as well as strain during sewing. The conductive textile shows good electrical stability over 5 hand washing cycles in standard domestic washing machine. [36] Other polymers like polypyrrole and polyaniline synthesized by in-situ solution or vapor phase polymerization show conductivity in the range of 10 -6 to 10 -3 S cm -1 . [42][43][44][45] The incorporation of conductive fillers (carbon black, graphite, metal powder or flakes) into polymeric fibers provides a good balance between electrical and mechanical properties.

Carbon-coated thread exhibits resistances of ~140 Ω cm -1 , while being robust enough to be integrated with an embroidery machine. [3] Yun et al. fabricated gold/graphene yarns shown in Figure 2-5f using completely solution-based process, with a conductivity of 2.86 10 2 S cm -1 .

The gold/graphene yarns show good reliability under 1000 bending test to 10° bending angle and 10 washing tests with commercial synthetic detergents, and can be knitted or woven into textiles. [46] Xu et al. developed ultra-strong graphene fibers using giant graphene oxide sheets by wet-drawing and ion-cross-linked, reaching a record tensile strength of 0.5 GPa, and excellent electrical conductivity of 4x10 4 S m -1 . [47] Individual fibers show good flexibility and a Young modulus around 10 GPa, which allow them to be twisted into yarns with an approximate diameter of 45 µm as shown in Figure 2-5g.

Metal fibers meet the best performances in terms of electrical conductivity, but have very poor mechanical strength, cannot be used in standard fabric processes, and show poor feeling for the user. Metal coated threads have better mechanical properties but poor reliability under use and wash, as often the metallic coating is rapidly damaged. On the other hand, polymer based conductive fibers presents lower conductivity (four order of magnitude lower for polypyrole: 120-130 S cm -1 ), but their superior mechanical properties make them attractive for integration in textiles. The addition of conductive fillers to polymer fibers merges good electrical and mechanical properties together. They require dedicated fabrication processes that make them less easy to get, but laboratory scale-up fabrication already showed promising results. as small as 160 µm were measured. [48] The structure of woven textiles is not inherently stretchable, but the strain experienced by the fibers when the textile is deformed depends on the weaving pattern. As an example, Cherenack et al. reported that an Atlas pattern (weft fiber successively goes under one and over seven warp fibers) allows tensile strain up to 11.1% to individual yarns, whereas plain pattern allows only 2.2% of maximal strain. [49] Knitting textiles are the intermeshing of threads forming loops with small bending radii, depending on the desired density of the final fabric. Such structure requires high deformability and reliability of the conductive thread. On the other hand, knitted textile have inherent stretchability that allows high comfort during wearing, and are consequently greatly favored for application that requires tight fitting of the body or for underwear fabrication. For both knitted and woven textiles, the mechanical properties and reliability of the conductive threads must be studied before integration.

To integrate conductive thread during weaving process, conductive wires replace a ratio of nonconductive threads as shown in Figure 2-4b. The conductive threads can be integrated in a regular grid pattern (Figure 2-6b and Figure 2-6d), [31,50] distributed in only one direction (weft or warp) (Figure 2-6a), [25] or replace non-conductive fibers at localized positions. [31,[START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF] In order to create the tracks design, electrical contact must be made at crossing points between conductive fibers. This can be done during the weaving process, with complex weaving techniques connecting up and bottom layers of one fabric. [START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF] Another way to create electrical connection between two strands is to use laser processing. Locher, Tröster and colleagues developed a smart textile named PETEX, in which isolated copper fibers intertwine with polyester fibers in a grid pattern (see Figure 2-6d-f). [50] They use laser processing to create electrical contact between two fibers. First, laser ablation shortens the wires depending on the design; then some conductive adhesive creates electrical contact at wire crossing point, and the contact is protected with a drop of liquid epoxy (see Figure 2-6f). A drawback of the weaving process is that only perpendicular wiring structure is possible, which seriously reduces the freedom of routing of the circuit compared to standard microelectronic processes. The process for making electrical contact between two fibers is not conventional and current methods allow making only one contact at a time, which is very time-consuming.

Standard knitted fabrics can be made entirely from a single thread, which makes this process a great candidate to create stretchable conductive electrodes. Integrating circuit tracks in knitting process can be done with the same manufacturing process used to create color pattern in knitted fabrics. Nonetheless, maximum conductivity is reached along the thread direction, which can be only horizontal. Another way to integrate conductive fiber into knitted fabrics is to weave a conductive thread through the finished knitted structure, as depicted in Figure 2-4b.

Embroidery of conductive threads on finished textiles: As a low-cost and accessible method to integrate conductive threads in textile, embroidery is among the first methods used to create interconnections in textile. It allows a great freedom for designing 2D tracks, as it is processed on finished garment and can be removed or modified at any time. Researchers developed an "ebroidery" process, which is a combination of commercial embroidery process and computeraided design (CAD). [25,32,33] It allows rapid prototyping and precise control on circuit's pattern.

As drawback, the sewing machine processing implies various strains that need to be considered, as conductive threads do not have necessarily the same mechanical properties as natural fibers.

Failure modes implied in embroidery were studied, [START_REF] Kok | [END_REF] and specially designed threads were developed through collaborative works. [32,53] In partnership with Luxion Technologies Inc. Ltd., Acti et al. created a 7-stranded copper yarn that can be used in standard embroidery machines.

The number of filaments is optimized to offer best compromise between flexibility and high tensile strength required for embroidery process. [53] Specialized embroidery machine can be found in commerce today, for example the company ZSK (Krefeld, Germany) builds embroidery machine with head that allows inserting special threads, ribbons or chains. If the embroidery process shows good advantage in terms of freedom in design, the embroidery pattern is not intrinsically stretchable if the conductive thread is not stretchable. This raises two issues: the textile loses its natural stretchable property along the tracks, and the conductive thread may be under mechanical tension when the garment is worn. For integration into knitted structure for example, substrate t-shirt can be pre-stretched during the embroidery process, or conductors can be patterned in a zigzag layout. [33] Each sample was printed in 1 pass (acetone: black circle, MIBK: blue triangle, BCA: green square) except BCA five passes sample (yellow diamond). Reproduced with permission. [62] Copyright 2017, John Wiley and Sons.

Patterning of conductive materials: Another way to add circuit tracks in a textile is to pattern conductive materials at the surface of standard fabrics. Mechanical aspects are important in patterning technologies as the strain endured by the textile substrate is fully transmitted to the patterned material. Therefore, mismatch of mechanical properties between the two parts must be reduced to avoid cracks or delamination in conductive tracks.

In an attempt to popularize the fabrication of electronic textiles, Bueschley et al. cut and attached commercially available metalized fabrics on garments to create electronic textiles. The conductive fabrics are laser or, hand cut and attached to fabrics with heat activated adhesive and ironing process. [54] They can build multilayers circuits by layering conductive and nonconductive fabrics, and the Sn/Cu plated fabric can be soldered with flux. With a nonconductive protecting layer, such tracks show electrical reliability under five washing machine at 20 °C and one high temperature drying process. The facility of process and low-cost materials make this technique very interesting for hobby and scholar activities. Nevertheless, the structure density of the fabric dictates the achievable resolution, which in this case was a minimum width of 3 mm to avoid break under wash. [START_REF] Buechley | Proc. SIGCHI Conf. Hum. Factors Comput. Syst[END_REF] Stencil and screen printing on textiles benefit from well-established and cost-effective fabrication processes, as they are already developed for decorative purposes. A great advantage of ink patterning is that it can be used to create landing pads on textile for the electronic components, in a cleaner way than by laser cutting of conductive threads. Printed surfaces also have a soft finish that make them appealing from a user comfort perspective point. However, meeting good electrical properties of printed materials on fabrics is more challenging. First, the structural roughness and porosity of the fabric's surface complicate the printing of a uniform and conductive layer. Viscous composite inks were developed to be printed either directly on fabrics, [START_REF] Merilampi | [END_REF][57][58] or with an interface layer that smoothens the structural roughness of fabrics' surfaces. [59,60] Sheet resistance of 43 mΩ  -1 was achieved by Merilampi and colleagues on woven textiles. [START_REF] Merilampi | [END_REF] A second issue is the mechanical performances that the patterns must meet.

Most works are performed under low mechanical stress on hardly stretchable woven structures, [START_REF] Merilampi | [END_REF]61] and mechanical tests of the patterned wires report low flexibility (10 mm bending radius achievable) [61] or are not studied at all. Some works show good electrical performances, but low-porosity fabrics and controlled stretching of the textile are requisite conditions to avoid destruction of the dried ink. [57] Matsuhisa et al. have stencil printed an elastic conductor silver ink on a commercial stretchable sport textile. Although the patterns are stretchable up to 218% on smooth elastomer surfaces, they are only flexible when printed on textile. [58] Severe strains endured by smart textiles under wear and wash urge the need for stretchable patterns. In most recent work, Jin, Matsuhisa and colleagues developed conductive stretchable patterns on knitted textiles that could withstand up to 450% of strain. [62] The use of a slow evaporating solvent allows deep permeation into the fiber bundles, in such a way that the ink does not form a uniform surface layer anymore (see Figure 234567). The dried ink benefits from both inherent stretchability of ink material and knitted structure, resulting in mechanically and electrically robust wiring. The sheet resistance initially of 0.06 Ω  -1 increases less than 10 times after 100 cycles at 10% strain. A four-channel electromyogram (EMG) monitoring garment demonstrates their technology. Protected with a 50 µm of thermoplastic polyurethane (TPU), the device remains conductive after 40 washing cycles at 40 °C and shows 100 times increased resistance. To protect the conductive patterns, encapsulation layers can be applied through printing process, and waterproofing of devices can be achieved by printing hydrophobic elastomers. [61,62] Table 2-1 summarizes the performances under mechanical strain and washing of some materials developed for the integration of electronic tracks into textile.

Cilia-assisted transfer printing: In most recent work, Yoon et al. elaborated an indirect method that enables conformal wrapping of electronic tracks around irregular shape with an improved adhesion (Figure 2345678). [63] Adhesive elements incorporating artificial cilia were implemented in the periphery of electronic devices. As claimed by the authors, the promoted adhesion of the cilia to the nearby threads enriches the overall tension of the entire cilia in such a synchronized manner and provides more stability in complex deformation. This improvement of adhesion was combined with the fabrication of ultrathin indium gallium zinc oxide (IGZO)-based 7-stage ring oscillator with peripheral cilia. At VDD=10 V, output voltage oscillates with a frequency of 33 kHz and a propagation delay of 2.2 ms. This advanced work demonstrates the ability to perform integration of microscale electronic tracks and devices directly at the surface of conventional textile. The device could withstand dipping in a detergent solution but failed under mechanical strain applied in a washing machine.

Whatever the final application and the electronic circuit, interconnections between circuit's elements are mandatory. Patterning of conductive inks and embroidery of conductive threads allow great freedom in design and are attractive solutions. Screen-printing on textile results in clean and smooth surfaces on woven textiles, but needs optimization to be applied on stretchable knitted structures. Embroidery machines using CAD assistance are commercialized today, which is an asset from industrialization perspective. However, depending on the chosen technology, the heterogeneous integration of discrete electronic devices on conductive tracks embedded into/onto textile may raise some technological limitations. waveform at VDD=10V and oscillation frequency. Reproduced with permission. [63] Copyright 2016, Nature Publishing Group. Ref.

Metal fibers and foils 10 -8 Ω m Weaving, wrapping around conventional yarns Same as bulk material Narrow woven fabrics show no change in conductivity after 50 regular hot washing cycles in household washing machine [34] Metal coated yarns <100 Ω m -1

(silver-plated polyamide, Statex Shieldex 234/34-2 ply HCB)

Embroidery, weaving, knitting with nonconductive threads

Mechanical properties close to standard yarns

Significative increase in R after 10 regular hot washing cycles in household washing machine [32,34,35,137] Metal coated fabrics Range from 0.09 Ω  -1 to 5.0 Ω  -1

Laser cutting of tracks and thermo adhesive attachment to conventional textiles Same a conventional fabrics Conductivity reduced by 98% after ten washing cycles [34,54] Graphene fibers 4x10 4 S m -1 NC Young Modulus = 10 GPa Tensile strength 0.5 GPa Stable conductivity over 1000 bending cycles [47] Silver ink Rs = 60 mΩ  -1 1) Printing (5 passes)

2) Drying at 90 °C oven for 2 h 3) Heat pressuring at 160 °C for 30 s at 30 kPa Rs < 0.6 Ω  -1 after 100 cycles at 10% strain With TPU encapsulation, device remains conductive after 40 washing cycles at 40 °C, hang dried between cycles [62] Rs = 80 mΩ  -1 1) Printing of a dual interface layer 2) Single printing of Silver ink 3) Printing of dual waterproofing layer

Withstand 10 mm of bending radius

Waterproof: no change in electrical resistance after 24 h immersion in water [61] Polyelectrolytebridged metal/cotton hierarchical structures 1 S cm -1 modified cotton yarns with polyelectrolyte brushes as adhesion layer

Stable conductivity within 30 bending/stretching cycles 5 hand-washing and squeeze-drying cycles [138] PEDOT:PSS 14 S cm -1 dyeing of silk Repeated bending stress No change in conductivity within 5 cycles @ 30 °C, 50 min, spinning at 900 rpm in standard household washing machine + common detergent [36] technology, where a flexible plastic substrate features copper pads and redistributes the contacts in a radial way (Figure 2-9b). [25] The bare silicon die is first fixed in the middle of the chip carrier. The pads of the chips are then electrically connected to the PTCC copper pads by goldwire bonding, which is a standard process for semiconductor device fabrication. The PTCC pads are easily accessible thanks to the pads' redistribution and they can be punched through with conductive threads and connected to the rest of the circuit. The space required to integrate a chip using PTCC technology is function of the number of connected pads and thread spacing. screen-printed chip carrier shown in Figure 2-9d. [57] The chip is glued to the fabric and then wire bonded to the printed pads. A limiting factor of this method is the width of the printed pads, which cannot be smaller than 1 mm for reliability of the printing process. Connecting components with high pin density should consider this limitation. The authors also investigated gold sputtering on fabrics to create chip pads and tracks. Despite showing an interesting pitch resolution of 200 µm, it remains an expensive process, which is reserved for textile with smooth surface. Indeed, surface roughness of fabric must be considered due to the fact that the typical thickness of sputtered layers remains below 1 µm.

To protect the bonding of isolated components, liquid epoxy was widely used in different encapsulation technics. [25,32,50,[START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF][START_REF] Lee | [END_REF] Glob top encapsulation is well suited as it is applied to very small surfaces and easily processed. [START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF][START_REF] Lee | [END_REF] This technic is a standard encapsulation process in microfabrication, and consists of a drop of resin epoxy deposited over a semiconductor chip and its wire bonds to provide mechanical support and environmental barrier. It can be dispensed through a syringe to be cured afterwards. Molding of liquid epoxy was also investigated by Yongsang Kim et al., showing good mechanical performances under strain. [57] Tensile tests were performed on their P-FCB package along x-direction, y-direction and diagonal direction above 19.5 % of elongation, and fabrics are torn before the bonding or package is broken. In their work on transponder system embedded in medical graded suit, Kallmayer et al. developed a two steps encapsulation process where the silicon chip is embedded in a fabric pocket. [START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF] First a conventional underlying material is dispensed inside the pocket, then encapsulation material is screen-printed on both sides of the fabric pocket, with a final total thickness of around 1 mm.

First reliability tests showed that the structure survived the conventional washing procedures without failure. permission. [START_REF] Coyle | Wearable Implant. Body Sens. Netw. 2009 BSN 2009 Sixth Int. Workshop On[END_REF] Copyright 2009, IEEE. (b) Prototype "Plastic Threaded Chip Carrier" package couched onto fabric substrate. Reproduced from [25] . (c) (i) Preparing components for initial design. In the top picture, individual Litz-wires have been soldered to the pins of a SMD component (SOT-6). In the bottom picture, the wires and the component have been isolated and protected using epoxy resin. (ii) Preparing components for the weave. Manual removal of lacquer from the Litz-wire at the desired distance and length, to facilitate attachment to the warp-system. (iii) Testing the attached woven components with a magnet and power supply during the weaving. The LED can be seen through the top-layer, while the hall-effect sensor is not visible. The copper wire used in the pins is barely visible between the weft yarns.

Reproduced with permission. [68] Copyright 2015, ACM. (d) Bonding and packaging on the fabric. Left: system on a chip (SoC) integrated on planar-fashionable circuit board (P-FCB).

Right: molded package. Reproduced with permission. [57] Copyright 2010, IEEE.

Interposer technology

In order to align the component's pads with the corresponding conductive fibers, one must adapt their respective pitches. Interposer technologies were developed to properly interface components and textiles electrical points. [25,50,[START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF][START_REF] Locher | Technologies for System-on-Textile Integration[END_REF] The principle of interposer technology is depicted in Figure 2-10a-c: a small printed circuit board features landing pads for the component on one side, and pads matching with conductive fibers pattern on the other side.

PETEX textile developed by Locher et al. is a woven fabric featuring a wire grid with a constant pitch of 570 µm. This pitch matches with the pitch of the components they want to assemble, but for pin count higher than two, the grid pattern causes shortcuts between aligned pads.

Interposer allows redistributing the pads in non-aligned pattern to avoid these shortcuts. The In all cases, the presence of an interposer requires complex thinking of the circuit design and adds substantial thickness to the device. Locher et al. calculated the minimum area of an interposer as a function of the pitch between conductive threads, and the number of interposer pads that needed to be connected. [50] The authors claimed that even with an interposer, the application range is limited by the wire-ability of electronic components with high pin count.

Integrating components at the fabric level has several mechanical and feeling advantages: for simple electronic design featuring few components, like textile transponder systems, basic gaming/hobbies interfaces, or educational purposes, textile circuits technologies are totally suitable. [25,[START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF]54] The resulting smart garment features local rigid parts and do not hinder the natural loose and soft characteristic of textile at the wearer's scale. The comfort of the user is preserved. For more complex electronics, these technologies show several drawbacks. Loose mechanical properties of textiles imply much bigger tolerances of dimensions in fabrics compared to standard electronics. Interconnections with small rigid components are inclined to misalignment issues, which are multiplied by the number of components, by their pin counts, and therefore by the complexity of the electronics. Each component package must be addressed case by case, and adapting the packaging to interface with standard textile process is often manually operated. Moreover, the contact is rarely done without the presence of an interposer, which increases the area occupied per component. In case of woven fibers, the fiber pitch and the use of interposer limit the pin count. Adding material by dipping, coating or screen printing allow greater flexibility in the design, but the number of pins is limited by the resolution of the additive technology which is often at the millimeter-scale. These space limitations may run counter to the high density of components required for data processing, depending on the circuit's complexity. In the meanwhile, electronic functions directly embedded onto the fiber are researched in order to bring high comfort electronic textile with the objective of increasing the integration of the electronics inside the fabric. Cross-section through interposer-to-fabric connect, 200 x magnified. Reproduced with permission. [50] Copyright 2007, IEEE. (e) Final demonstrator of transponder chip embedded and encapsulated in textile. Reproduced with permission. [START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF] Copyright 2003, IEEE

Functionalized fibers electronic components for logic circuits

Instead of attaching electronics components to the fibers, the textile fibers themselves can be functionalized with special coatings to fabricate logic circuits. This part presents the direct integration of electronic components -mainly transistors for switching purpose -into the fabric fiber. It is the most challenging and advanced degree of integration where electronics and textiles make only one, referred as monolithically integration. This could potentially reduce the number of fabrication steps of e-textile as well as improve the comfort of use. However so far, it is performed to the detriment of the computational capabilities.

Most recent efforts involve integrating electronic function into the yarn's structure to keep textile mechanical properties. [15,[START_REF] Stoppa | [END_REF] Contrary to flexible display intended for few centimeters bending radii, smart fibers can withstand bending radii smaller than 1 millimeter and large tensile strains.

The realization of analogue and digital micro-electronics directly into textile currently attracts important attention but is still marginal compare to rigid / flexible substrates assembly onto the fabric. Transistor is the key electrical component in textile electronics (and in conventional electronics) in the perspective of gaining computational abilities into fabrics. Among the variety of available transistors, mainly two kinds are studied for monolithically integrated e-textile:

namely organic field-effect transistors (OFET) and wire electrochemical transistors (WECT), both depicted in Figure 2-11. Contrary to silicon-based devices, they use organic materials as active transistor channel, which offer superior mechanical properties as well as the ability to be printed directly onto the textile.

Electrochemical transistor into textile

Textile embedded advanced electronic functionalities can be thought as a system in a 1D configuration. This is especially true for the elementary switching element as is the transistors. [73] Recently, the group of D. Wang (College of Materials Science et Engineering, Wuhan Textile University) presented a electrochemical transistor (ECT) directly integrated into a textile fiber. [74,75] Direct polymerization of polypyrrole/ poly(vinyl alcohol-co-ethylene) was achieved into a nylon fiber. Then by crossing two fibers, a functional ECT in which one fiber acts as the channel with source/drain and the other as the gate. On/off ratio of 10 4 , applied voltage below 2V and a trans-conductance around 3mS stand as good electrical performance. switching from the ON to the OFF state occurs when ions are depleted from the polymer semiconductor channel by electrodiffusion through the solid electrolyte under the action of the gate voltage. The WECT structure is symmetrical; the source-drain fibre and gate fibre can be interchanged, which adds flexibility to circuit design. Reproduced with permission. [START_REF] Rossi | [END_REF] Copyright 2007, Nature Publishing Group.

Prior to this, pioneer work on specific woven logic circuit based on electrochemical transistor was released in 2007 by Hamedi and co-worker. [76] Many adaptions of this ECT onto textile fiber sprung from this initial work. [73,77,[77][78][79] The ECT is a transistor in which the drain current is controlled by the injection of ions from an electrolyte into a semiconductor channel. [80] The injection of ions in the channel is controlled through the application of a voltage to the gate electrode. ECTs are different from electrolyte-gated field-effect transistors. In the latter type of device, ions do not penetrate into the channel, but rather accumulate near its surface (or near the surface of a dielectric layer, when such a layer is deposited on the channel). [81] This induces accumulation of electronic charge inside the channel, near the surface. In contrast, in ECTs, ions are injected into the channel and change the electronic charge density throughout its entire volume. As a result of this bulk coupling between ionic and electronic charge, ECTs show a very high transconductance. [82] In the Hamedi et al. paper, [76] the PEDOT:PSS active channel material is drop casted onto various textile fiber such as polyamide, Kevlar and polyester. A key feature is the use of a solid polymer electrolyte containing 33 wt% PSS with a molecular weight of 70 000, 12 wt% glycol, and 8% sorbitol, 0.1 molar NaClO4, and water. As seen in Figure 2-12 andFigure 2-13, by gridding the PEDOT:PSS coated textile fiber with the combination of appropriate contacts (semi-conducting; ohmic or insulating) between crossing fibers, they have fabricated elementary switching circuit onto the textile. The authors speculate that it would be theoretically possible to construct ∼100 000 transistors cm -2 using 10 μm fibers and a two-dimensional mesh. Transistor packing densities would increase by taking advantage of the third dimension of the textile. Reproduced with permission. [76] Copyright 2007, Nature Publishing Group.

Despite its interesting level of integration density, the electrochemical transistors suffer from their slow switching speed. Reliability is also an issue, which still requires extensive study.

However, these ECTs may be relevant as simple switch functionalities for sensors or implants for example. Implantable of multiple bio or chemical sensors could benefit of the presence of simple switchers in order to properly and sequentially record the data in a "slow" mode scheme. polyamide monofilaments with 100 μm diameter. Reproduced with permission. [76] Copyright 2007, Nature Publishing Group.

Organic field effect transistors into textile

Fabrication of wire electrochemical transistor (WECT) onto textile for logical switch purpose has been demonstrated although such a device shows poor performance notably in terms of switching speed. As a matter of fact, ions mobility remains slow compared to electrons.

Moreover, electrochemical transistors require ionic liquid or gel, which limits their integration ability into textile. Thin film transistor based on field effect can potentially solve these issues.

In 2005, Lee and co-worker described a field effect transistor integrated on fiber. [83] These transistors based on pentacene fiber exhibit an on/off ratio around 1000, mobilities of 0.5 cm 2 V -1 s -1 measured at 20 VDD and could operate stably under a wide range of flexion stress. As seen on Figure 2-14, devices are defined and positioned solely by a weaving pattern. Aluminium and stainless steel wires ranging from 125 to 500 µm in diameter were used as gate lines. As gate dielectric, SiO2 or a cross-linked organic polymer based on poly-4-vinylphenol (PVP) was used. On top of the gate dielectric, 90 nm of pentacene channel material was evaporated.

Finally, 100 nm Gold was evaporated as the source and the drain connector (Figure 2-14a-b).

Transistors performances are similar to conventional inverted top-contacted pentacene thin film transistors (TFT); however, the entire transistor is formed on a metallic gate wire without conventional lithography, ensuring easy e-textile integration. As explained by the authors, because of the back-gated structure, the fiber transistors would be most amenable for use in circuits such as a gate array of switching grid. Drive voltage, parasitic capacitances, drive current, and relative gate leakage must be considered in evaluating the suitability of fiber transistors for switching circuits. Clearly, stability and reproducibility issues were highlighted.

Pursuing this initial work on field effect transistors (FET) fabricated onto textile yarns, other groups have presented modified architectures. Polyimide was tested as insulator and PEDOT:PSS as gold contact replacement. [84] With pentacene as channel material of the organic FET, mobility up to 0.06 cm 2 V -1 s -1 and on/off ratio of 3 x 10 3 was demonstrated with an extended ability of the FET to bend thanks to the integration of softer materials. Recently Kim and coworker [85] presented the fabrication of metal/insulator/semiconductor (MIS) coaxial microfibers based on organic semiconductor:insulating polymer blend (Figure 2 Strips of pentacene are visible between the gold pads. Reproduced with permission. [83] Copyright 2005, IEEE. (c) Schematic diagram and top-view OM image of fibriform OFET based on MIS coaxial microfiber. Reproduced with permission. [85] Copyright 2016, John Wiley and Sons Reproduced with permission. [134] Copyright 2017, John Wiley and Sons.

To summarize, the development of the so-called one-dimensional fiber based electronic circuits is at the moment principally limited to simple functionality, such as few transistors and simple switching. The realization of viable textile fibers circuits endowed with active electronic functions is still at its infant stage. Nevertheless, properties of polymer based transistor:

printable, low cost, mechanical properties; fit perfectly with textile application. Among organic transistors, ECTs have the advantages of low voltage. FET based devices offer better switching speed; more compatible for the fabrication of active matrix. The end goal of a concomitant fabrication of electronics devices and fabrics is driving this research. This field is overlapping with e-skin where active sensors / actuators are integrated on ultra-soft substrates for robotic and prosthetic applications. However for important data processing, silicon based devices and chips remain required. As a matter of fact, switching speeds and transistors density developed in functionalized fibers do not meet the computational performances required for real-time processing applications. A full-fledged fiber based microcontroller has yet to be demonstrated.

Therefore, current advanced applications seek for smart textile has to use more standard siliconbased ICs that feature higher computational performances. In this latter case the circuit needs to be moved from the textile substrate to a dedicated substrate that can sustain higher electronic components density.

Flexible Printed Circuits

Complex electronic systems that feature data acquiring, processing, and transmission, require high density of components and good computational performances. Therefore, circuits used for ECG shirt for example cannot be integrated using the methods investigated in part 2 and 3. In this part, we report the integration of flexible circuits that are manufactured separately from the textile. Components are integrated on a flexible substrate using processes close to standard microelectronic fabrication. This convenience eliminates the density and computational performance limitations met previously. Electronic parts and textile are merged at a later stage.

Technologies regarding the electrical circuit substrate and physical connection to textile are explored in order to keep natural textile mechanical performance as well as good electrical reliability of the device.

Fabrication

Flexible circuit boards are commonly processed using conventional microelectronic fabrication techniques. The most widely used flexible substrates are polyimide (PI), polyethylene terephthalate (PET) and polyethylene naphthate (PEN) films. [86] As the two last films deform and melt for temperature over 130 -150 °C, standard components assembly process via solder reflow must be adapted using a low-temperature bonding process. Bismuth alloys or conductive paste are currently used. [START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF] PI films are more suitable to common microelectronic fabrication processes due to their high thermal stability (glass transition temperature over 360 °C) and stable mechanical properties over broad temperature range. [START_REF] Mark | Polymer Data Handbook[END_REF] In one hand, the non-crumpling area is increased because the circuit is assembled on a unique piece of flexible substrate. On the other hand, the overall area occupied by the circuit is diminished because the circuit fabrication benefits of the achievable high resolution of standard microelectronic process. ECG modules were demonstrated on 27 mm x 27 mm PI substrate [33] , and with integrated electrodes on a 50

x 50 mm PI substrate. [START_REF] Khan | [END_REF] Between the weaving of conductive fibers and the integration of flexible circuits, Kindeldei et al. developed an electronic fiber, or E-fiber, using standard microfabrication techniques on polyimide (PI) stripes (Figure 2345678910111213141516). [START_REF] Kinkeldei | Sens. 2009 IEEE[END_REF][START_REF] Kinkeldei | [END_REF] The width of the stripes range from 500 µm to 2 mm.

Thanks to encapsulation optimization and to the thinness of the resulting electronic stripes, they can be integrated as any textile threads into fabric using commercial weaving process. [48,49,[START_REF] Kinkeldei | Sens. 2009 IEEE[END_REF][START_REF] Zysset | Wearable Comput. ISWC 2010 Int. Symp. On[END_REF][START_REF] Bonderover | [END_REF] Nonetheless, when it comes to bigger components like microprocessors, at least twice the width of the fiber is required, which reduces the possible circuitry performances. Reproduced with permission. [START_REF] Kinkeldei | Sens. 2009 IEEE[END_REF] Copyright 2009, IEEE. c) integrated "nose strips" featuring 4 gas sensors each. Reproduced with permission. [START_REF] Kinkeldei | [END_REF] Copyright 2012, Elsevier.

Electronic module integration onto textile and electrical connection to tracks

Different interconnections between flexible modules and conductive tracks integrated in textile were investigated and tested under wearing and washing conditions. Early state of research investigated the soldering of electrical wires to module's pads. The rigid wires and solder withstand only minimal strains and washing tests lead to broken contact. [28] Riveting was also investigated as it is part of common textile process, but showed poor reliability to wash and use. [25,[START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF] Standard snap fasteners are a convenient alternative that allows repetitive manual connecting and removing of the electronics, for example for washing or changing of the supporting garment. [93] This technology is used in today's commercialized sporting equipment: consumer brands like Polar propose "Self-quantifying" devices, featuring heart rate monitors and performance trackers that need to be removed for cleaning of the textile. Only the textile electrodes are washable with the t-shirt.

In most research cases, conductive embroidery through metallized pads of the flexible substrate creates the electrical contact. [33,35] The flexible circuit is placed on the garment, then cycling. [32] The researchers from the Fraunhofer Institute for Reliability and Microintegration optimized the contact pad preparation, the number of embroidery through the pad and the encapsulation to make the electrical contact resistant to washing machine. Figure 2-17b shows a cross-view of the electrical contact. They showed that their interconnections could be reliable up to five washing cycles. Reproduced with permission. [32] Copyright 2005, IEEE. c) Wiring design of the embroidery circuit for the test vehicle. d) and e) Double sided encapsulation of flexible substrates with embroidered interconnections by transfer molding. Reproduced with permission. [93] Copyright 2008, Trans Tech Publications Ltd.

Interconnecting the flexible substrate via embroidery also promotes its attachment to the textile.

If the circuit is attached at several points, the strains occurring in the garment under wearing and washing may lead to mechanical and electrical failure of the electronics. One way to circumvent this issue is to prevent locally the stretching of fabric by framing the module with non-conductive embroidery. In further encapsulation process, using a footprint slightly larger than the module allows the encapsulation compound to adhere to the surface of the textile all around the module. [33] This encapsulation fixes the module to the garment, but also prevents surrounding textile from stretching and damaging the electronics. Such "stiffening" structures may be inevitable when integrating non-stretchable modules onto stretchable structures such as knitted fabrics, to increase the lifetime of the electronics.

Module encapsulation

Electronics embedded in textiles face a harsh environment. During wearing and washing, textiles are being stretched and rubbed against, exposed to various media (water, sweat and chemicals) and at high temperature during washing or specific applications. The encapsulation of electronics and interconnections is mandatory to insure correct lifetime of the module.

Modules can be encapsulated on one side or from both sides. Generally, the flexible module and the embroidered electrical interconnections to fabric are embedded in the encapsulation, as they are the most fragile part of the modules. Transfer molding is commonly used in electronic industry. An epoxy resin filled with SiO2 particles (70-90 wt%) is applied on the substrate with temperature up to 165-185 °C and high pressure through a mold. Thanks to the filler particles, the coefficient of thermal expansion (CTE) of the molding compound adapts to the one of electronic components, which reduces the thermal stress occurring during temperature changes. However, this technique presents some constraints regarding textile applications. First, the particle-filled resin is no longer flexible after curing. In this case, the use of a flexible module is justified by its interesting thinness and ability to be punched through by embroidery process. [93] Second, the temperature and pressure conditions of process can diminish the textile mechanical properties around the module. In several works, Kallmayer and colleagues used a commercially available molding compound, the LOCTITE Hysol GR 9800, with a curing temperature of 165 °C. [32,33,[START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF] The heat degrades the characteristics of the spandex included in the fabric, and locally lowers the elasticity of the garment. The authors claim this change to be acceptable as applied to a small area (27 x 27 mm 2 module in ECG t-shirt demonstrator). [33] In another study, a bigger issue is the damages induced in the unprotected part of the embroidered conductive threads. [93] SEM images show that the silver coating is strongly damaged. Authors suggest it may be due to the high pressure and temperature applied during the molding process, and that it can be overcome with a change in the assembly process. Nonetheless, the transfer molding process shows excellent reliability of the encapsulated parts, which are the electronic flexible module and its components, as well as the embroidered interconnection. All the parts supported 20 washing cycles according to ISO 6330:2000 test condition 6A, which is a 40 °C washing cycle, and through temperature cycling from -40°C to 58°C (JESD22A104 test condition N). Apart from a protective aspect, the molding of the embroidered interconnections improves the electrical contact by pressuring the conductive threads against the metallized pads. [32] Hot melt encapsulation is a low cost alternative molding process that uses thermoplastic materials with melting temperature ranging from 130 to 200 °C. This process does not require high pressure. Because of their high viscosity no filler particles are added to the thermoplastic materials, and therefore there is no adjustment between the CTEs of components and encapsulating compound. Linz, Kallmayer and colleagues compared their results on transfer molding, previously described, with the hot melt encapsulated package. [93] They found that the hot melt package protects the electronics during washing cycles, but would partially delaminate on the package edges and lead to unacceptable increase in contact resistance between threads and metallized pads. Due to high CTEs mismatch between electronics and thermoplastic, the package shows bad protection under thermo-cycling, and all copper tracks are broken around SMD components.

The use of flexible substrates to integrate electronics into textiles has the advantage to rely on standard microfabrication processes. The processes are well-known and industrialized, which makes the fabrication faster and with a clearly higher density of components compared to systems with textile-substrate. The flexibility of the plastic substrate grants the textile natural drapability, which makes the module suitable for integration onto woven fabrics, for example in a jacket. On the other hand, for near-body applications such as ECG, stretchable knitted textiles are compulsory for the comfort of the user. In this case, integration method must be further investigated: as the flexible circuit cannot be stretched, the integration area must be prevented from stretching, and the natural wearing of the garment is distorted. The density of components is interesting because the non-stretchable area is reduced in size, but the resulting circuit can show some stiffness. Substrate optimization will improve the comfort, for example with hybrid substrate allowing some part to stretch as the textile would do.

Stretchable hybrid matrix

Flexibility alone is not enough for circuits to fit closely the complex 3D curvatures implied in clothes. Stretchable modules are highly desirable for electronic textiles, as they would even less hinder the natural feeling of the garment. Standard silicon-based components and bonding materials are rigid and cannot withstand strain. Therefore, they cannot be easily implemented on stretchable substrates. In the past twenty years, the approach of hybrid stretchable matrix was developed. [START_REF] Axisa | Polytronic 2007 6th Int. Conf. On[END_REF][START_REF] Kim | [END_REF][96][97][98][START_REF] Lacour | Proc. IEEE[END_REF] Non-stretchable but flexible or rigid islands carry the less compliant parts of the device, and are electrically connected to each other via stretchable interconnections. This approach allows large area electronics systems, e.g. light-emitting diode (LED) screen to be integrated in t-shirt with optimized wearability and comfort that flexible device cannot perform. Reproduced with permission. [135] Copyright 2011, Cambridge University Press. (right) configuration. Reproduced with permission. [97] Copyright 2010, The American Association for the Advancement of Science.

Fabrication of flex/stretch modules

The principle of flexible/stretchable hybrid matrix and some related works are shown in Figure 2-18. Stretchable lines have been widely investigated during the past twenty years, but two approaches can be described regarding their integration in stretchable matrix.

Stretchable properties may be achieved using special patterning of non-stretchable materials such as silicon or metallic films. Arc shapes [97] (Figure 2-18g) and wavy structural configurations were developed (Figure 2-18f), [START_REF] Kim | [END_REF][START_REF] Lacour | Proc. IEEE[END_REF][START_REF] Lacour | [END_REF] as well as in-plane special patterns that can bend out-of-plane under strains. [START_REF] Axisa | Polytronic 2007 6th Int. Conf. On[END_REF]101,102] Stretchable interconnections and flexible islands may be fabricated from the same flexible material, which enables patterning both parts during the same process step. Ghent University researchers intensively investigated the fabrication of stretchable lines using flexible substrates patterned in horseshoe-shape design. [START_REF] Axisa | Polytronic 2007 6th Int. Conf. On[END_REF]103,104] The fabrication processes are based on standard methodologies for PCB productions (photolithography, copper etching, reflow oven, and lead free soldering), with a pre-step where the flexible substrate is laminated on a temporary carrier (wax, temporary adhesive or tape).

The conductive layer can be laser-designed copper, [START_REF] Axisa | Electron. Syst.-Integr. Technol. Conf. 2008 ESTC 2008 2nd[END_REF] etched copper, [START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF] lithographed metals [START_REF] Verplancke | [END_REF] or electroplated metals, [START_REF] Axisa | Polytronic 2007 6th Int. Conf. On[END_REF] which are materials currently used in standard microfabrication. The conductive layer can be free-standing [START_REF] Axisa | Polytronic 2007 6th Int. Conf. On[END_REF][START_REF] Axisa | Electron. Syst.-Integr. Technol. Conf. 2008 ESTC 2008 2nd[END_REF][START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF] or supported by a flexible substrate layer such as PI. [START_REF] Verplancke | [END_REF][START_REF] Bossuyt | MRS Proc[END_REF] After patterning of the conductive tracks, the carrier is replaced by an elastomer that generally also encapsulates the device at the final step of the process.

Thermoplastic polyurethane [START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF] (TPU) and polydimethylsiloxane [START_REF] Vervust | Stretchable and Washable Electronics for Embedding in Textiles[END_REF] (PDMS) are currently used. Figure 2-18b shows an encapsulated 7 x 8 LED matrix demonstrator from Ghent University. This similarity with standard microelectronic fabrication is motivated by the will to bring electronic textiles to industrial level in the near future.

Other researches use inherently stretchable materials for interconnecting tracks. Such technologies involve more intricate fabrication process because of the different materials and interconnections between them. Matsuhisa et al. investigated non-stretchable PI islands carrying organic thin-film transistor (OTFT) interconnected with printable elastic Ag-based conductor (Figure 2-18c-e). [58] The authors do not report stretchable properties of the ink when printed on textile, but patterned lines can stretch up to 215 % strain with a conductivity of 182 S cm -1 on PDMS substrate. As the conductive ink is inherently and partially stretchable, interconnections can be patterned in a straight-line design. The matrix under strain is shown in composites. [START_REF] Larmagnac | [END_REF] Their technology uses cleanroom-free process to produce soft all-elastomeric printed circuit boards at large scale. Double-sided soft PCBs are created by bonding top and bottom layers together through punched holes and then filled with Ag-PDMS composite. As clamping from hard electronics to soft electronics is useful for powering, data transmitting, or program loading into a microcontroller, the authors were able to clamp commercialized rigid Zero Insertion Force (ZIF) connectors to an 8 leads stretchable ribbon cable. Commercial electrical components are bonded to the elastomeric tracks using Silver epoxy and no mechanical failure is observed after hundreds of 20% stretching cycles. Authors do not report any bending tests or cycling.

Reliability of stretchable interconnects

Meander-shaped stretchable lines have been intensively studied and optimized to increase their maximum tolerable strain. [104,[START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF][111][112][113] Finite Elements Method (FEM) shows the influence of the meander geometry and the substrate's Young modulus on the magnitude of the stresses inside the lines. [103] Among parameters of design, FEM shows that the width of the metallic line drastically increases the stress induced in metal under deformation. As reducing the line's width also impacts its electrical resistance, a trade-off must be made. Axisa et al. also developed a multi-track design that increases the line's stretchability without sacrificing its electrical performance. [START_REF] Axisa | Polytronic 2007 6th Int. Conf. On[END_REF] To bypass the risk of break of the line, Ghent researchers developed a selfhealing structure by adding a deposition of conductive polymer on top of the metallic meander. [START_REF] Axisa | Polytronic 2007 6th Int. Conf. On[END_REF] In case of physical failure of the metallic track, the polymer insures electrical contact by joining over the break, thus avoiding general failure of the device. Good adhesion between the metal and plastic substrate was reported to be a key to achieve strain-resistant metal lines. [114][115][116][117] In their work, Cherenack et al. fabricate straight interconnect lines using 500-nmthick Cu thin-film on PI substrate, rupturing after one-time tensile strains up to around 45 %. [49] Integration into textile

Stretchable electronics modules are well suited for textile integration as they can deform with the garment. Aiming at smart textile industrialization, screen-printing was investigated to attach stretchable modules to fabric, as it is a well-known process in textile industry. Reliability to washing cycles and the comfort of the user remain the two main targets of this integration method. Polyurethanes are widely used in the textile industries, as glue to join textiles, or as semi permeable ultra-thin membrane to waterproof textiles. Löher et al. have used polyurethane (PU) to glue stretchable electronic matrix to textile, to create prototypes of LEDs dresses in the frame of "STELLA" (Stretchable Electronics for Large Area applications) project. Resulting device is shown in Figure 2-19a. The authors reported that the method showed robustness within many wear cycles. [START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF] Vervust et al. used PDMS to bond modules to fabric: after a thin layer (below 1 mm) of PDMS is screen-printed at the surface of the fabric, the module is positioned with slight pressure and the PDMS is cured (Figure 2-19b). [28] One must control the penetration of PDMS through the fabric, as a sufficient bonding needs a thin layer of PDMS on top of fabric and too deep penetration negatively affects the feel and wearability of the textile.

Viscosity of PDMS, curing method, and type of substrate show strong influences on the penetration rate. To further study the impact of adhesion layer on wearability, Vervust and Using stretchable hybrid matrix has the advantage to rely on microelectronic standard process while allowing a great compliance of the circuits toward the textile. Splitting the circuit into several smaller circuits, interconnected via stretchable interconnections, reduce the area of the non-stretchable parts and allow a more natural feeling of the textile. On the other hand, the final device spreads in a larger surface, which can be of importance depending on the available surface on the garment. Pure stretchable electronics show very promising results, but is still at the research laboratory phase and advanced autonomous applications with high integration density of components has still to be demonstrated. Hence, hybrid flexible / stretchable is an interesting compromise for a near future development of advanced electronics fully embedded into textile. Instead of choosing (changing) the substrate, optimum mechanical properties could be achieved by "stack engineering" in order to minimize mechanical strains.

integration into flexible substrates. [START_REF] Balde | Foldable Flex and Thinned Silicon Multichip Packaging Technology[END_REF][START_REF] Rempp | Solid-State Circuits Conf. 2008 ISSCC 2008 Dig. Tech. Pap. IEEE Int[END_REF][START_REF] Marinov | [END_REF][121] However, their strains at failure are substantially lower than those of plastic or elastomeric substrates. In multilayer layouts under bending, the neutral mechanical plane defines the position through thickness that experiences no strain.

Shifting the neutral plane to the most sensitive layers allows higher deformation of the device without mechanical or electrical failure. This approach was widely investigated in literature for fabrication and encapsulation of metallic layers [49,122] and silicon-based thin-film transistors (TFT). [START_REF] Kim | [END_REF]123,124] In their work, Cherenack et al. report that encapsulation by a 50 µm thick PET layer allows the TFT to bend down to 0.25 cm of bending radius before failure, compared to 1 cm when not encapsulated. Lines encapsulated with PI withstand bending radius down to 100 µm without a significant change in electrical resistance, while they fail at radius around 120 µm without encapsulation. [49] Kim et al. developed high-performance, stretchable and foldable IC matrix. [START_REF] Kim | [END_REF] The authors were able to integrate silicon complementary logic gates, ring oscillators, and differential amplifiers into a stretchable 3D wavy structure. The shifting of the neutral plane to the devices layer enables the integration of brittle silicon-based components into stretchable structures, as it facilitates the nondestructive bending required for the wavy patterns (Figure 2-18f).

Geometrical gradient of the layout

The patterning of the substrate can locally influence the system's Young modulus. FEM studies describe the influence of gradually decreasing the track's width on the stress applied at the borders of flexible islands. [START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF][START_REF] Bossuyt | MRS Proc[END_REF]125] Figure 2-20e shows a gradual transition in width of copper tracks and supporting PI substrate. This gradient reduces the local plastic strain in the copper and increases its mechanical reliability. Another way to create a gradient of softness is to add reinforcing structures around the most fragile parts of the design. Löher et al. added stiffening copper structure around component positions to improve their reliability under strain (d). [START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF] These structures are patterned at the same time as the rest of the copper tracks and do not need additional step for fabrication. These structures can protect the component interconnections up to 500 % of elongation.

Encapsulation influence on circuit's mechanical behavior

Devices can be locally encapsulated to minimize their deformation, but interconnects should be protected and stay flexible to maintain essential textile properties such as drapability. One approach to bypass the mechanical issue at the interface between stiff and compliant parts is to smooth the mechanical transition by introducing a local gradient of stretchability

Molded encapsulation for controlled 3D profile

Encapsulation, apart from bringing chemical and waterproofness to electronic parts, has an important role in the mechanical behavior of the interface. Researchers have controlled the encapsulation thickness profile to bring more rigidity to weak parts of the system (Figure 2-20ac). [28,[START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF][START_REF] Bossuyt | MRS Proc[END_REF] The whole system is encapsulated in silicone rubber by over-molding process (Figure 2-20a), which allows good control on the thickness profile. The profile of encapsulation, shown in Figure 2-20b, has thicker parts (4mm) on flexible islands and thinner parts (1mm) in the regions where serpentine lines are defined. A thickness gradient along the interface brings a stretchability gradient and reduces the fragility of the region. [28,[START_REF] Bossuyt | MRS Proc[END_REF] Different materials with different Young moduli can be used respectively for stretchable interconnections and for fragile parts such as components. Löher et al. have reported the use of a polyurethane glob top encapsulation of the IC with a cast over molding process, and a TPU film for the rest of the circuit. Once again, the polyurethane encapsulation profile is controlled to smooth out the transition between the thick, rather stiff polyurethane encapsulation and the thinner part containing copper interconnections. [START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF] 2.8.

Composite encapsulation

Recent researches involve composite encapsulations with a spatial gradient of mechanical properties. [58,126] Matsuhisa et al. realized a horizontal gradient of Young modulus using two PDMS mixture with different ratio of curing agent. [58] A stiffer PDMS with a relatively high concentration of curing agent (12.5 wt%) is first patterned around flexible polyimide islands. A second layer of softer PDMS (5 wt% curing agent) cover the matrix before the curing of the first PDMS layer. A gradient of stiffness is formed during the curing process, as the curing agent diffuses between the two PDMS preparations. [127] A schematic view of the encapsulation is shown in Figure 2-21a. Libanori et al. created a polymeric encapsulation with a vertical gradient of Young modulus by layering gradually reinforced PU sheets. [102] The PU soft matrix are hardened with inorganic materials, and their Young moduli can be tuned from 4 MPa to 7

GPa. [128] The resulting 2-mm thick composite shows an out-of-plane gradient in elastic modulus spanning over five orders of magnitude, and a gold layer patterned on the composite could be stretched up to 10 times at 80 % strain without a change in resistance. Comparison between with graded and non-graded encapsulation of a stiff component under strain is shown in Figure 2-21b-d. permission. [START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF] Copyright 2009, IEEE. (e) Non-optimized (left) and optimized (right) design at a transition area between rigid/flexible and stretchable zones. Gradual transitions in width of the copper and corresponding supporting polyimide reduce the local plastic strain in the copper. Sharp edges need to be omitted. 8 Copper track width is ∼ 100 μ m. Reproduced with permission. [136] Copyright 2011, Cambridge University Press.

Another approach using photo-controllable elastomer modulus allows smooth gradient of stretchability. [129,130] By mixing a photoinhibitor solution to standard PDMS, Cotton et al.

prepared an elastomeric substrate with photo-patternable (PP) mechanical compliance. PP-PDMS is exposed to UV through a positive mask to define "stiff" regions with a Young modulus value of 2.88 MPa. The elastic modulus of unexposed PP-PDMS is 1.35 MPa. PI islands embedded at the bottom of the elastomer membrane strengthen the elastomeric membrane and emphasize the mechanical gradient at the same time. Pentacene thin-film transistors patterned on top of the elastomeric membrane could sustain applied strain up to 13% without electrical degradation and mechanical fracture.

Up to now, few processes allow stretchability gradient efficient enough to improve the reliability of mechanical interfaces. Mechanical gradients were successfully implemented but require several additional steps of the process, which need to be taken into account when thinking about the industrialization phase of the product. Stretchability gradient can be divided into in plane (horizontal) and out-of-plane (vertical) gradients. In out-of-plane cases, the overall thickness of the device needs to be controlled, as a too much thick layer would once again impede the natural feeling of the garment. Stack engineering modulates the neutral plane position inside the stack by playing on its composition which is promising as it does not necessarily increase the overall thickness; albeit improving the robustness. However, mechanical optimization has been demonstrated at the component scale and still need to be obtained at the circuit level in order to reach a fully washable electronics integrated onto textile.

Conclusion

Depending on the required computational performances of the application, on the desired integration level, and on the feeling requested by the user, one of the previously described approaches should be chosen on a case-by-case basis. Especially for clothing, natural textile properties such as softness, drapability, stretchability and conformability are essential. The fabrication of electronic devices, on the other hand, favors the electrical and computational performances over mechanical compliance. Multiple techniques are currently investigated to fabricate electronic textile devices merging essential aspects of both disciplines, by integrating electronics at different scales. When the textile serves as a substrate for the electronics, important natural properties of the textiles are preserved. Standard electronic components are separately attached to conductive threads embedded in textile to create electronics textile circuits. The natural feeling and draping of the garment is preserved, but complex textile circuits are hard to achieve because each electronic package has to be specifically adapted and attached to fabric structures. Functional fibers do not have this interfacing issue as the component directly merges inside the conductive tracks. They represent the highest degree of integration as electronic functions are embedded in the core of the fiber, and their process and mechanical properties perfectly match with textile application. Nonetheless, current achievable computing performances do not meet real-time smart technology requirements, as switching speeds and transistors density of integration in textile are too low. To address these issues and for a foreseeable future development of advanced smart textile, standard silicon-based components and microfabrication processes are favored. Stretchable and flexible electronics modules, which are fully fabricated apart from the textiles and merged with the cloth in a later stage, show interesting mechanical properties while providing high computational performances. These technologies use mostly well-established standard microelectronic fabrication processes, which allow currently highest achievable resolution and components density. A remaining issue is the mechanical reliability of the hybrid structures. As they feature several different mechanical properties, each mechanical transition represents a weak point for the device under strain. In order to improve the lifetime of the device, many fabrication processes are developed to create stretchability gradient at these fragile mechanical interfaces, using in-plane and out-of-plane gradients, as well as encapsulation and stack optimizations.

Although sensors, power generator and storage may be directly integrated into garment's fibers, a full-fledged smart garment, for healthcare purposes for instance, can only be achieved by combining them with technologies depicted in this review. In fact, data acquisition and processing have to currently be handled by silicon-based IC. However, this raises the question of the overall mechanical reliability of such heterogeneous system that has to be thoroughly investigated. Indeed, the main challenge of smart clothing to be useful is to withstand washing environments where chemical and mechanical aggressions are fatal to embedded electronics.

In this respect, researching washable heterogeneous electronics circuits integrated into clothes

Polymer substrates Young Modulus (Pa) ref

Polyethylene terephthalate (PET) (GoodFellow) 2.8 x [1] Polyimide (PI) (Kapton® HN DuPont) 2.8 x [2] Polydimethylsiloxane (PDMS) (Sylgard 184) 2.0 x [3] Polyurethane (PU) 2.5 x [1] Parylene C (PaC) (Curtiss Wrights) 3.2 x [4] Polyvinylidene Fluoride (PVDF) 2.7 x [5] Silicone rubber (EcoFlex 0030™ SmoothOn) 125 x 10 3 with high computational performances are required to achieve multiple tasks, such as signal acquirement, processing, and transmission. This study particularly is part of the development of smart clothes for presymptomatic detection of cardiovascular diseases. Such wearable systems should fulfill two main characteristics. On one hand, electronics should be imperceptible for the user, and therefore they shouldn't alter the textile's natural wearability and comfort. This implies that the module shows compliancy, with limited overall thickness.

On the other hand, circuits must be robust enough to endure the stress our everyday outfits are subjected to, such as crumpling, especially during domestic washing cycle. The challenge consists in the concurrency between circuit's complexity, imperceptibility, and mechanical robustness.

Researchers have explored different approaches to build textile electronics. Functionalized fibers [7][8][9][10][11][12][13][14] meet the most advanced degree of integration, with transistors merged directly inside the core of the fibers. Textile mechanical properties are greatly preserved, but at the cost of computational capabilities, which limits current fiber-based electronics to basic logic circuits.

In order to increase computational performances on textile substrates, rigid silicon-based integrated circuits are directly bonded to the surface of fabrics. [15][16][17][18] However, due to natural loose properties of textile substrates, misalignment and spatial resolution issues occur when integrating microelectronic components. On the contrary, flexible circuit boards (FCB) have the advantage to rely on conventional industrial process of microelectronics, and can be implemented in textiles. [19][20][21][22][23][24][25] Resulting flexible devices achieve spatial resolution, component density and computational performances similar to those of conventional printed circuit boards.

FCB are composed of a flexible substrate (most often a polymer film), and a functional layer containing tracks design (metallic layer). Rigid, conventional silicon-based integrated circuits (microcontrollers, Bluetooth communicating devices, etc.) are integrated on the stack to create a flexible electronic device. In this work, we focus on the flexible stack itself, without any integrated component.

Laminated films of aluminum on polyethylene-terephthalate (PET), and copper on polyimide (PI), are widely available and standard substrates for FCB. However, their mechanical properties are hardly sufficient for textile-integrated electronics. Indeed, applications in FCB industry so far (bus ticket, antitheft stickers, ribbon cable, etc.) imply limited stress and/or short lifetime. Textile electronics on the contrary are subjected to high and dynamic mechanical stress during daily domestic washing. Folding or creasing of polymer substrates are reported after conventional cleaning process. [26] Cracks and delamination of the metallic tracks appears after washing and can lead to failure of the device.

Encapsulation of flexible electronics integrated in clothes is mandatory for two reasons. First, encapsulation protects electronics from moisture, chemicals and water, to allow washing of the device. Secondly, it can improve the mechanical reliability of the device, for example by preventing high deformations. Several encapsulation methods were explored to enhance mechanical performances of FCB under washing. Stiffening structures such as transfer molding or glob top encapsulation using epoxy resin [15,17,18,20,27] offers good reliability through 20 washing cycles at 40°C. [28] Unfortunately, resulting devices are no longer flexible. Soft silicone molding encapsulations, such as PDMS, were investigated to improve mechanical behavior under washing, with a final thickness of a few millimeters. [19,29] Whereas such encapsulations offer good chemical and mechanical protection, they diminish the compliancy of the module and can hinder the natural feeling of the garment. [30] A tradeoff must be made between stack stiffness preventing high strain, and compliance required for user's comfort.

In this work, we study how conventional substrates of the FCB industry can be adapted for robust washable electronics, while preserving compliancy and thinness of the final substrate. In a first part, we focus on the polymeric substrates. Despite their high flexibility, one washing cycle is enough to induce permanent damages in the films. We study the films' elastic limitations under bending using to a customized setup allowing measurements of radius down to 100 µm. In regards of our results, we select a 13 µm-thick PET layer for the rest of the study.

In a second part, we focus on the electrical behavior of a stack composed of PET substrate, aluminum conductive tracks, and PDMS encapsulation. We demonstrate the impact of the thickness of the different layers on the mechanical reliability of the stack under washing. We show that our experimental results are consistent with an analytical model of stacks under bending.

Investigation of polymeric films plasticization Test methodology

We consider two kinds of deformations: non-permanent (elastic) deformations, and permanent (plastic) deformations. It is important for flexible electronics to undergo only elastic deformations under use, as plastic deformations would shorten lifetime of the device and discomfort for the user. Table 3-2 shows different substrates with and without metallization, before and after washing cycle in domestic machine. For easier reading, films and stacks are identified by the material name followed by the thickness in microns: a stack composed of 18 µm-thick copper film on 50 µm-thick PI substrate is referred as PI50/Cu18. Several small creases in PET50 and PI50 films, and coiling of PI50/Cu18 sample, can be visually observed after wash, and show that plasticization occurred during washing. Whereas those materials show good flexibility and can be fully bent without fracture, one washing cycle is enough to induce permanent deformations in films. PET and PI films show multiple small creases, and metallized sheets have larger permanent folds.

To our knowledge, no standard test or modeling of strain implied in domestic cleaning machine is currently available. To evaluate and compare materials' mechanical performances, we assume that most part of damages induced in flexible films is due to clothes' crumpling during cleaning process, which translates to the films as flexion movements. Therefore, elastic limits of materials under bending condition their mechanical reliability through washing cycles. The high flexibility of our materials presages radius thresholds in the millimeter range. In order to address this measurement, we built a setup using a uniaxial moving stage with side-view imaging of the sample under bending, as shown in Figure 3 (e) Theoretical plot of maximum strain in films under bending shows that thinner layer endures lower strain at same bending radius.

This setup allows minimum measurements of 100 µm, whereas more conventional rolling procedure is limited to few millimeters in radius. [31][32][33] A first LabView program synchronizes the moving stage with the camera pictures (see Appendix 1 We measured the bending radius threshold for plasticization of the substrates as explained in Figure 3-1b. We define the plasticization ratio  as𝛼 = 𝑟 𝑟 ⁄ , with 𝑟 and 𝑟 the natural, or stress-free, radii of curvature of the film, respectively before and after bending.

Samples are bent down to different radii of curvature, and the ratio  decreases when permanent deformations occur in the film.

Results

For PI50 and PET50 film, these thresholds are around 2.3 mm and 1.1 mm, respectively.

Therefore, plastic deformations of PET50 foil in Table 3-1 suggests that radii of 1.1 mm or less apply to the flexible module under washing. Thinner layer of PET13 shows the best performance as it can be bent down to 0.4 mm without plastic deformations. Laminated polymer/metal stacks were also tested after the metal was completely removed by laserengraving, to investigate the impact of the laser process on the polymer material. These films showed similar results than polymer substrates, which suggests that laser ablation does not alter the polymer's mechanical properties upon folding. In comparison, mechanical characterization of plain polymer/metal laminated films revealed very low elasticity, with plasticization over 10 mm of curvature, as shown in Figure S 3-2 in supplementary material. To understand the differences in elastic domains, we used a model of strain implied in films under bending depicted in Figure 3-1e. For a layer bending downward, top surface is in tension whereas bottom surface is in compression. The neutral plane ℎ is the surface inside the film where the strain is null. Maximum tensile strain arises at the top surface of the film and depends on bending radius 𝑅 and thickness of the film:

𝜀 = 𝑡 -ℎ 𝑅 (1)
with 𝑅 the radius of curvature of the neutral plane. In a uniform monolayer, the neutral axis lies in the middle of the film, and therefore:

𝜀 = 𝑡 2 * 𝑅 (2)
Equation ( 2) implies that 𝜀 at constant radius increases with the thickness of the film. This result is in correlation with plots in Figure 3-1d, where PET13 can elastically withstand smaller radius than PET50. On the other hand, PI50 and PET50 are subjected to the same surface strain.

The difference between their plasticization thresholds is consistent with their mechanical properties, as elastic limit is around 4% of strain for PET [34] and 0.8% for PI. [35] Those thresholds are reported in Figure 3-1e.

Because of its higher elastic limit, PET film can be bent to smaller radius while remaining in its elastic domain. Analytical model and experimental results show that reducing the substrate's thickness widens the elastic domain under bending. Moreover, decreasing the thickness lowers the bending stiffness, [36] which allows more conformable electronics. This conclusion is consistent with our ambition of thin and imperceptible electronics. In the following study, we focus on PET-based substrates because of their lower plasticization threshold.

Study of the washability of PET/Al substrates

Methods

Mechanical stresses during washing induce cracks and opening in the conductive tracks, causing failure of the device. In this part, we study the impact of the stack's composition on electrical reliability of the functional layer under washing. Tested circuits were matrix composed of 800 µm-wide lines, wide enough for future integration of surface mounted components like light-emitting diodes, and to apply probes for electrical measurements. The lines have 0°, 45° and 90° orientations, which are standard geometrical features in microelectronics. Using a four-point probing, we measured the change in electrical resistance of the tracks as an indicator of the interconnections' degradation. Our samples were integrated into shirts in two different ways for washing tests, to approach the behavior of wearable devices embedded in textile. First, they were put free-standing inside a closed pocket, which allows circuit moving independently from the surface of fabric. In a second approach, samples were stitched to the fabrics on both shorter sides, which allows partial transmission of stretching movement from the fabric to the stack. In both cases, the films are not attached to the fabric on their whole surfaces. Moreover, impact of PDMS encapsulation on the samples' mechanical behavior was tested, as integrated electronic circuits will further be encapsulated for chemical resistance. PET12/Al9 samples, despite their completely shriveled aspect (see inset pictures in Figure 3-3),

Results

show similar electrical results to encapsulated PET12/Al9 samples.

Discussion

We investigate the evolution the neutral plane position using a model of the stack under bending. As a matter of fact, in case of multilayer structures, neutral plane ℎ shifts through the stack depending on its composition. Da Silva proposes an analytical modeling of the elastic deformation of multilayer structures due to applied external uniform bending. [37] If we consider a uniaxial geometry (film bending in only one direction), the neutral plane position ℎ through the stack is given by: With 𝑌 and 𝑡 respectively Young modulus and thickness of layer 𝑖, and ℎ the coordinate of the middle of layer 𝑖. Maximum tensile strain is then calculated using Equation (1) as a function of radius of curvature.

Based on this analytical model, we plot on Figure 3-3a the neutral plane position in the aluminum layer as a function of PDMS thickness, for PET/Al/PDMS stacks. We do not include the textile's layer in the model, as the stack is not monolithically integrated onto the fabric. It is important to lower as much as possible the strains inside the metal, as it is the functional layer of the stack. Here we aim at placing the neutral plane in the middle of the aluminum layer, to reduce compressive and tensile strains inside the metal upon folding. In PET50/Al9, the neutral plane lies in the PET substrate, approximatively 5.3 µm away from the middle of the aluminum.

An 800 µm-thick layer of PDMS improves the electrical robustness, as it brings the neutral plane closer to the aluminum layer, at PET-Al interface. According to the model, 2 mm of PDMS coating is needed to ideally reach the middle of the functional layer. This thickness counters to our goal to provide imperceptible flexible electronics integrated in clothes. On the contrary, non-encapsulated PET12/Al9 stack naturally possesses a neutral plane close to the middle of the metallic layer, approximately 0.5 µm below it. This explains why PET12/Al9 samples show good electrical measurements despite the completely shriveled aspect of the

ℎ = ∑ 𝑌 x𝑡 xℎ ∑ 𝑌 x𝑡 (3)
𝜀(𝑦) = 𝑦 -ℎ 𝑅 (4)
samples, indicator of high deformations. An 800 µm-thick PDMS encapsulation shifts the neutral plane to the same distance but above the middle of the aluminum. We can assume that strain distribution in the metallic layer is not highly impacted by the displacement of the neutral plane, which explains the low change in electrical behavior in Figure 3-2b. According to the analytical model, a 600 µm-thick PDMS coating is needed to reach optimum position of the neutral plane, for an overall thickness below 1 mm. This result in thin, compliant, and electrically robust stack, suitable for integration in washable textiles. Based on this model, one may determine the optimal encapsulation depending on stack's composition and desired final thickness. Figure 3-3b compares the different thicknesses required to bring the neutral plane of PET12/Al9 stack in the middle of aluminum layer, considering the encapsulation material. Softer materials will have lower mechanical impact on stack and need thicker layer to shift the neutral plane. Whereas materials like parylene C (Young modulus = 2.8 GPa) and polyvinylidene fluoride (PVDF) (Young modulus = 2.7 GPa) already impact the mechanics of the stack with thickness below 20 µm. These two materials should be more suitable for thicker layer of PET substrate, as they can mechanically balance the stack while keeping a reduced overall thickness.

Conclusion

In this work, we have demonstrated that commercialized flexible substrates could be mechanically improved to bring their properties closer to the requirements of washable electronics. We approached the issue of plastic deformations in polymer substrates under washing, by correlating it to the elastic limit of materials under bending. Due to the high flexibility of our films, we used a customized setup on a moving stage with camera monitoring allowing measurements of bending radius down to 0. Plain metallic layer on top of the polymer substrate highly affects the mechanical behavior of the stack, as the plasticization threshold radius is 10 times higher than for polymer substrates without metal. Study of mechanical robustness for heterogeneous integration of rigid components on highly-compliant substrates

Wearable devices are subjected to high strains consequently to their proximity with the human body. In previous chapter, we investigated the mechanical reliability of commercially available substrates for flexible electronics, considering the conductive track alone. One conclusion of previous chapter is that thinner tracks have better robustness under bending. In the following chapter, we focus on the mechanical robustness of systems integrating rigid components on flexible interconnections.

For a smart device integrated in clothes, global deformations at macroscopic scale induce micro-deformations inside the circuit. The locations and values of those strains is related to the system's mechanical structure. Particularly, stress concentrations occur at rigid-to-flexible interfaces. On one side, the use of highly compliant substrates benefits wearable technologies in terms of comfort, as well as mechanical bending robustness as seen in previous chapter. On the other hand, decreasing rigidity of the substrate implies increasing the mechanical mismatch with rigid components. This chapter characterizes the mechanical robustness of ultra-thin substrates (thickness below 10 µm) integrating rigid components, in comparison with conventional substrates of flexible circuit board industry (thickness superior to 10 µm). We compare integrating methods including rigid conventional solder and flexible conductive pastes. The aim of this chapter is to determine which mechanical system {substrate; conductive paste} ensure the highest durability under bending.

Publication Status: This chapter is a manuscript in preparation.

S. de Mulatier 1 , R. Delattre 1 , S. Blayac 1 and M. Ramuz 1 mechanical interface between rigid components and ultra-thin tracks that generates weak points in the structure [10] . A key technical challenge is the fabrication of a mechanically robust heterogeneous integration. Global deformations of the support transmit to the embedded electronics as local deformations. These local deformations depend on several factors: the integration method, which will influence how strains are relayed from support to circuit; the mechanical stiffness of the circuit, which will allow more or less deformations; and the mechanics of the layers, which integrate materials with different Young moduli and thicknesses. The latest aspect is complex in wearable electronics, as they integrate rigid components on soft substrates. Silicon-based integrated circuits (IC) and other inorganic components are non-flexible but compulsory for fast-responsive systems. On the other side, highly compliant substrates are desirable for imperceptible electronics. Moreover, ultra-thin interconnections have shown better mechanical robustness than thicker films. [9] To the best of our knowledge, there is no study dedicated to the characterization of bending limits and durability of conductive joints between rigid components and ultra-thin tracks. Jeong et al. presented electronic tattoos integrating rigid inorganic components on skin-like substrates, [11] but investigated minimum radius is related to on-skin application and therefore of 8 mm, which is too big compared to sharp bending in clothes. Vasquez Quintero et al.

presented a smart contact lens integrating an electronic layer with total thickness of 10 µm comparable to copper leaf stacks. They report delamination of bump attachment of the chip after 200 cycles of external bending of the lens at 1.7 mm of radius. Most reports present bending tests of a few millimeters on final prototypes, and do not focus especially on the components attachment. Different methods were investigated to smoothen the mechanical mismatch with strain-engineered solutions. Stiffening islands made of polyimide (PI), [12,13] and poly(methyl methacrylate) (PMMA), [14] are deposited below rigid components to stiffen and/or shift the maximum stress area. Other works build a gradient of stretchability to smoothen the transition, like photopatternable silicones with tunable progressive hardness. [12,15] Encapsulation molding in flexible silicone [16] and rigid epoxy resin [17] were also investigated to smoothen transition at soft/rigid interfaces, or prevent bending of sensitive parts. These methods provide great robustness but they result in thickening and/or stiffening the circuit.

Moreover, additional steps such as photolithography are necessary, which can be cost-and time-consuming. Finally, many works investigate the integration and bending robustness of thinned silicon chips. Chip as thin as 20 µm can be bend to a radius of 4 mm. [18,19] Those works are relevant for integration of application-specific integrated circuits (ASIC). [19,20] They are dedicated to mass-production devices, and we are interested in the integration of discreet inorganic components as they grant freedom of design and cost-effective solution for prototyping.

In this paper, we report on the study of mechanical behavior of ultra-thin tracks (thickness 10 µm) integrating rigid surface mounted components. Two bending setups are developed, considering first the bending mechanics of the whole system, and second the flexibility of the joint itself. The study compares mechanical behavior and bending durability of ultra-thin tracks with the ones of current flexible circuit board (FCB) solutions based on PI-copper stacks (thickness 21 µm). We compare robustness of joints made of conventional reflow soldering with silver/silicone conductive pastes. We assess the superior mechanical durability of ultrathin tracks for all three conductive pastes, whereas FCB industry substrate can be improved with conductive soft glue.

Materials and Methods

Materials

Thin and ultra-compliant films present two opposing aspects. Previous work demonstrated that thinner films with tracks alone withstand more severe bending stresses than thick films. [21] On the other hand, a more compliant film induces a larger mechanical gap with the rigid component. Table 4-1 describes the two substrates investigated. The ultra-thin stack is composed of thin copper layer (0.5 µm) on top of parylene C (PaC) substrate (5 µm). PaC is widely investigated as soft substrate for bioelectronics devices as its thickness can be tuned in micron-and submicron-range, creating soft biocompatible substrates with mechanical properties close to bio-tissues. The fabrication of the copper-PaC stack is described elsewhere. [9] We chose for comparison a laminated film of copper on PI as it is widely used in FCB industry. We chose two substrates with different thicknesses and similar Young Moduli (respectively 2.76 GPa and 3.2 GPa, for PI and PaC) to assess the impact of the film's compliance on the system's reliability.

not shown here for easier reading of the graph but it is available in Supporting Information shows almost no mechanical resistance to applied deformation, which makes it a great candidate for imperceptible integration in textile. For monolayer films, the stiffness is calculated as [17] :

𝑆 = 𝐸𝑡 12 (1)
With E the Young Modulus and t the thickness of the layer. With similar Young moduli, the stiffness ratios between PaC5, PI12 and PI18 layers are approximatively the ratio of their thicknesses cubed: PI50 has a stiffness 72 times higher than PI12, and PI12 has a stiffness 14 times higher than PaC5. In comparison, PaC5Cu0.5 presents a great asset regarding the imperceptibility. Conventional soldering paste is studied and compared to flexible conductive glues. Table 4-2

Material

shows the different conductive pastes investigated. The idea of using flexible conductive glues with two different Young Modulus is to assess the impact of flexibility gradient along the rigid component's edges. We chose to investigate these materials and processes, as they do not bring major changes in original soldering process. Only reflow temperature and times are changed for lower temperatures.

the middle of the metal. [21] For PaC5Cu0.5 layers, calculated thickness is 5 µm, and 10.9 µm of PaC for encapsulation of PI12Cu9. This encapsulation relates to real-life conditions, as circuits are always encapsulated for protection from external chemicals, water or sweat. In our case, bringing the NMP in the middle of the metallic layer allows better mechanical robustness at this position reduces maximum strains suffered by the copper track under bending. On PI12Cu9 substrates, PU1000 is the only glue that remains stable over 5000 cycles, as shown in Figure 4 3c. With tin alloy, two over three samples remain stable whereas the last one holds 3000 cycles of bending before increasing its resistance. For ICP4000, only one sample over three remains stable under cycling. The two other samples show variations over 30 Ω, but do not loose electrical contact. Those increases are due to small cracks inside joints that progressively spread but do not entirely break the interconnection or glue. Up and down variations of resistance are attributed to cracks formation during bending and the recovery over the flattening of the substrate. By nature, this recovery is random which generate these fluctuations.

Results show that the substrate has a real influence on the durability of the assembly. As seen on Figure 4 3b, d and f, PaC5Cu0.5 assemblies show better durability than PI12Cu9 integrating the same paste. This superior mechanical robustness is due to the higher compliance of PaC.

As shown in Figure 4 1, PaC5Cu0.5 stack is more soft and compliant than PI12Cu9 stack. This difference influences how the bending strains distribute along the sample. Figure 4 4 shows side view picture of PaC5Cu0.5 and PI12Cu9 substrates with tin alloy Sn42Bi58 under test. For PaC substrates, the rigid tin solder leads the mechanics of deformation. The system preferentially evacuates stresses by deforming the compliant and soft substrate than at the component area, which is stiffer. Therefore, several curvatures occur along the sample, allowing strain relaxation. In the case of PI12Cu9, the layer is too stiff to accept and evacuate the stresses, and the bending preferentially occurs locally, at the interface between two stiff areas. Stress concentration therefore is more important at interfaces in the case of PI12Cu9. Moreover, top surface strains are related to the thickness of the interconnection stacks by [21] :

𝜀 = 𝑡 -ℎ 2 * 𝑅 (2)
with t the total thickness, ℎ the neutral plane position, and R the bending radius. The samples were made so the neutral plane lies in the middle of metallic layer. With a thinner stack, associated top strains are lower in PaC5Cu0.5 samples than PI12Cu9 samples, which also explain higher robustness through equal applied bending.

Failure localizations

Cycling of 5000 bending was also performed at a very small radius of 0.05 mm (full folding), and all the 6 configurations {substrate; conductive paste} showed electrical failure (raw data available in Supporting Information Figure S 4 2). The limit of reliability under bending is between 0.5 and 0.05 mm of radius for configurations like PU1000 on PI12Cu9, and PU1000, ICP4000 and Sn42Bi58 tin alloy on PaC5Cu0.5. Permanent damages occur in the sample at a radius of 0.05 mm. The electrical conductivity is not lost immediately, but the defaults propagate rapidly over multiplied bending with loss of electrical contact after few cycles. The apparition of permanent defaults allowed us to localize where damages occur in the system.

Pictures of the samples after 5000 cycles at 0.05 mm of radius are depicted in Figure 4 5. For both substrates integrating tin alloy, the solder leads the stack and imposes main bending axe at the edge of the solder drop, due to its high stiffness. For the PU1000 glue, two bending modes happen. Main crack and electrical failure happen at the edge of the glue drop, as highlighted with a red arrow in Figure 4 4-7d reveal how the stiff metallic joint actually delaminates from the substrate, as its stiffness prevent from conforming to the sharp edge. One bending is enough for the solder joint to start delaminate from the copper pad, but electrical contact is ensured by the small area that remains attached. Nonetheless, we can assume that after several cycles at this radius, the resistor will fully detach and electrical contact will be completely lost. This result concurs with the localization of bending in PaC5Cu0.5 and PI12Cu9 substrates in Figure 4-5a and B), where the tin alloy drives the mechanics and forces the bending at its edge.

On the other side, conductive glues like PU1000 and ICP4000 particularly benefit from their flexibility in this bending situation, as they show high flexibility associated with small change in electrical conductivity. Under free-bending deformations as in Figure 4-3c, if a substrate as stiff as PI12Cu9 imposes bending inside the PU1000 joint, the conductive glue is able to evacuate some of the deformation by bending and shows better reliability through cycling.

Conclusion

Real success of wearable smart systems is tied to their imperceptibility. The integration of inorganic rigid elements raises mechanical weak points at soft/rigid interfaces. In this work, we study the influence of ultra-thin and compliant substrates on the mechanical behavior and durability of electronics under bending. The choice of a system composed of one conductive paste and one substrate must take into account the external strains of final application. Under free deformations, mechanical structures built on ultra-thin PaC5Cu0.5 substrates show a more robust mechanical behavior than conventional, thicker FCB substrates, with stable resistance over 5000 cycles at 0.5 mm radius of curvature. Component integration using tin alloy Sn42Bi58 on PaC5Cu0.5 presents the advantages of imperceptibility coupled with conventional reflow soldering. Mechanical behavior of thicker PI12Cu9 substrates is improved by mounting component with a soft glue PU1000. For devices in close contact with the human body, or that should withstand high deformations in unpredictable locations, soft conductive glues benefit from flexibility and absorb strains in sensitive zones. Further work includes fatigue tests on the conductive glues under localized bending. The fabrication of robust imperceptible electronics not only benefits to computing parts, but also to the whole device, as soft sensors could be built in one block with the electronics. This perspective eliminates interfaces and additional interconnections between sensing and computing parts of a device.

Introduction

In the past few decades, electronics have evolved from bulk and static to small-size portable devices, and are now trending into imperceptible wearable/implanted systems. Conformable and lightweight electronics are highly desired for imperceptible devices, integrated on clothes or on the skin to perform health monitoring, assistive systems, or human-machine interfaces.

Such systems find a critical purpose in medical applications as they allow local data processing and therefore noise reduction in low-voltage biosignal recording. For example, in case of cardiovascular diseases, continuous assessment of the electrocardiogram (ECG) could not only lead to up-front detection with better chance of curing and avoiding a cardiovascular event, but also to important insights in diagnostics and in the understanding of therapeutic effects. [1] Autonomous and responsive, smart devices contain sensors and/or actuators, an energy source, and a data processing circuit that allows the system to react accordingly to the measurement.

We define the latter as "complex electronics" in terms of density of components and tracks, diversity of tasks, and high computational performances required for fast responsive systems.

Those criteria today are only met in rigid silicon-based components, which we should use in the smallest package available for improved imperceptibility when used in body-implanted device for instance. This urges the need for high-resolution process for patterning of flexible interconnections. The term "leaf" refers to an ultra-thin foil of metal originally intended for gilding and art restoration purposes. It is processed manually from a bulk metal piece that is first thinned and stretched through rollers, then hammered between sheets of acetate until reaching a thickness below a micron with the resistivity of bulk copper. The leaf is so thin it conforms to fingerprints features (Figure 5-1a). Other works investigated the use of this material as nanoporous large area electrodes [18] , or for flexible electrodes as decorative user-interface tattoos [19] . In this work, we focus on the attributes of the copper thin film as a conductive, highly flexible material, as well as its suitability for fabrication of microelectronics. The process flow for copperleaf electronics is shown in Figure 5 We use a commercially available PCB prototyping laser equipment to pattern the circuit. The laser beam hatches the copper's surface and reduces it to powder to reveal the circuit's tracks. [20] The thinness of the copper-leaf enables low-power laser ablation compared to standard copper films on polyimide (PI). Indeed, only 4 watts hatches the copper leaf, where 16 watts are needed to remove the 18-µm-thick copper films on PI, which tends to burn the PI surface. It also affects the patterning resolution, which is a crucial point for mounting integrated circuits (IC). Highly integrated packages reduce the non-flexible areas of the circuit, but require high-resolution of patterning. The narrowest line we are able to pattern is 25 µm, with a minimum space of 25 µm corresponding to the diameter of the laser beam (Figure 5-1e). For high aspect ratio lines (15-mm-long), we found 50 µm to be the reliable minimum width of tracks. This resolution is an order of magnitude smaller than other dry patterning methods studied in literature [16,19] , and allows integration of fine-pitch packages of microelectronics, as shown in Figure 5-1f, and in (over 10 samples), which is not critical for our application as conductive tracks in circuits.

Integration of microelectronic components can be challenging when the conductive layer is not compatible with standard processes like soldering. Researchers investigated silver epoxy [3,9,21] and liquid gallium-indium [6] for bonding on flexible and stretchable surfaces, but they need through-mask patterning steps to achieve sub-millimeter resolution. Soldering on pre-treated polymers, [22] or laser-induced forward transfer (LIFT) [12] reach high resolutions but they require several steps of pre-treatment of the substrates and components. We chose to use copper among metals available in the form of leaves (silver and gold) as it is compatible with off-the-shelf materials like tin-soldering. In consideration of the melting temperature of parylene (290 °C)

and PVA (200 °C), we used a solder paste of Sn42Bi58 alloy with a reflow temperature of 138 °C. Regular drops of paste are dispensed on contact pads using a pressure-controlled syringe.

Passive components are aligned using a pick-and-place machine and left at the surface of solder, where they remain attached by capillary effect for the reflow process, operating in a convection Another important asset related to the copper's thinness is its impact on the mechanical reliability of the stack, as maximum strain under bending is directly correlated to the thickness of the stiffest material (in our case copper). [15,23,24] To assess the mechanical and electrical reliability of the stack, we performed bending tests of copper tracks embedded in parylene using a dedicated setup depicted in Figure 5-2a. [15] We furthermore limited the strain induced in copper by using the ideal configuration where mechanical neutral plane lies in the metallic layer, therefore encapsulation and substrate have the same thickness. [24] First, a single folding test was performed on a 110-µm-width track (Figure 5 We finally demonstrate the functionality of ultra-thin copper-leaf circuits with an ECG monitoring device. The circuit is an open source design, single-lead ECG monitoring device from Sparkfun. [26] The board is two-layered and features an analog front-end integrated circuit for ECG (AD8232, Analog Devices) and 25 passive components (resistors and capacitors) for filtering. The graph shows that the two boards can measure identical signals, with the same quality of reading of the characteristic ECG peaks. Figure 5-3d shows the labeled ECG peaks acquired from the copper-leaf board. The total thickness of the flexible material without any components is 13.5 µm, compared to 1.6 mm for standard PCB boards. Including the copper wires and headers for interconnections, the overall device weights 700 mg, compared to 4.6 g for the original PCB. This demonstration evidences the interests of copper-parylene stack for ultrathin, ultra-light imperceptible electronics, without loss of signal quality. Importantly, no modification of the device was brought to adapt the circuitry to flexible electronics, nor of the components, which is a non-negligible aspect for rapid and efficient prototyping of imperceptible devices. One main limitation in this circuit is the ECG chip, which have a thickness of 750 µm. Further work should include integration of unpackaged dies. Final circuits could be used for local processing of data with a direct attach to epidermal/implantable sensors, instead of today's bulky and uncomfortable solution such as Holter devices.

In this paper, we present a new fabrication method for imperceptible circuits featuring commercialized decorative copper-leaf of a few hundreds of nanometers of thickness. The material shows high robustness with no change in electrical resistance over 5000 folding cycles, independently of the track's width and stack's thickness. The process implies stencil-free, laser patterning of the tracks with a resolution of 50 µm suitable for high-integrated IC with finepitch packages. We have developed a three-step process for vias mounting that simplifies multilayering for flexible microelectronic circuits. Crucial asset of the described method is its compatibility with standard soldering processes, and the possibility to mount vias and components in a unique step. The process allows rapid prototyping of flexible circuits, as it is a direct transfer of layout and use off-the-shelves tools of PCB fabrication. Such circuits could be integrated on wearable systems for imperceptible computational electronics requiring fast processing and response. In biosignals monitoring involving measurement of low voltage signals, the distance between sources and processing circuits could be reduced by integrating our circuits directly at the output of the electrodes. Further work implies study of the mechanical attach of the components and robustness of the via, the influence of the components thickness on the mechanical reliability and the use of unpackaged, flexible components.

Experimental Section

Lamination of copper leaf on glass: A few milliliters of water was deposited on the surface of the glass wafer to form a continuous surface, on which the leaf immediately adheres and spreads thanks to high surface tensions (see Movie Figure S 5-1, Supporting Information). The water was then manually swept from underneath the metal with a soft brush, and the leaf remained attached to the glass with Van der Waals forces. Same process operates during the different steps of fabrication that require moving of the parylene-copper stack. Water was replaced by PVA solution when a stronger attach is needed (2 w% or 5 w% depending on the step).

Patterning of the circuitry: The copper-parylene stack was fixed on a glass wafer with copper facing upwards. A LPKF ProtoLaser S-Series equipment, with a beam wavelength of 1064 nm, hatched the copper without damaging the transparent carriers (parylene, PVA, and glass wafer).

The beam was powered at 4W with a frequency of 75 kHz. The hatching pattern was a grid of line spaced by 10 µm, to eliminate all residues of copper on parylene. With 𝜌 the resistivity of the bulk (1.68 x 10 -8 Ω m), 𝐿 and 𝑤 respectively the length (15 mm) and width of the line, and 𝑡 the thickness of copper, varying from 350 nm to 450 nm. In-between line shows resistance for a thickness of 400 nm. Values occurring out of the theoretical plots highlight that some parts of copper may exceed 500 nm in thickness. The layer detaches within minutes.

Mounting

  gracieusement et maintes fois le rôle de cobaye. Merci à Michelle et Anaïs pour leur aide et leur bonne humeur. Je remercie tout particulièrement Bastien, Marie, Aimie, Anaïs, Magali et Yanid, pour leur précieuse amitié. Merci pour tous les bons moments pendant et en dehors de la thèse, qui ont contribué autant à ma productivité qu'à rendre cette expérience de thèse agréable. Merci pour votre soutien infaillible. Pour finir, je souhaite remercier chaleureusement ma famille et mes amis pour leur soutien durant ces quatre années intenses. Merci à celles et ceux qui sont descendus me voir et m'ont offert une bulle d'air dans ce long parcours. Merci à mon assistante d'une semaine, Aí-Linh, pour sa motivation et sa curiosité pour mon travail. Pensée toute particulière pour ma famille adoptive aixoise : Bruno et Brigitte, merci pour tout, et pour votre présence à divers moments folkloriques de mon expérience à Aix. Merci à mon grand-père et à mes parents d'avoir traversé la France pour venir me soutenir à ma défense de thèse, et pour avoir organisé un pot digne de ce nom. Merci particulièrement à ma petite soeur Hortense, et à ma cousine Camille, pour leur soutien inconditionnel à distance durant ces 4 années. Merci à ma grande soeur Clélia de m'avoir montrée la voie du doctorat, d'avoir partagé son amour pour les sciences et la vulgarisation depuis toute petite, et pour m'avoir fait il y a quelques années, la démonstration d'une soutenance de thèse exemplaire.

Figure 1 - 1 . 2 Figure 1 - 2 . 3 Figure 2 - 1 . 10 Figure 2 - 2 . 12 Figure 2 - 3 . 13 Figure 2 - 4 . 14 Figure 2 - 5 .

 112123211022122313241425 Figure 1-1. Examples of wearables technologies found in literature. ........................................ 2

Figure 2 - 6 . 18 Figure 2 - 7 . 22 Figure 2 - 8 . 26 Figure 2 - 9 . 29 Figure 2 - 10 . 31 Figure 2 - 11 . 33 Figure 2 - 12 . 34 Figure 2 - 13 .Figure 2 -

 26182722282629292103121133212342132 Figure 2-6. Different examples of conductive textiles and methods to create conductive tracks in textiles .................................................................................................................................. 18

Figure 2 -

 2 Figure 2-15. (a) Schematic illustration for fabrication process of 1D fiber-type CMOS circuitry. (b) A noise margin characteristic (at VDD = 5.0 V) and (inset) circuit configuration of the complementary SWCNT inverter. ............................................................................................

Figure 2 -

 2 Figure 2-16. 500 µm wide sensor stripes a) before and b) after being woven into textile band. ..................................................................................................................................................

Figure 2 -

 2 Figure 2-17. a) Flexible electronic module connected with conductive yarn by embroidery. b) Conductive thread held by the gap created by punching through the pad with the sewing needle. ..................................................................................................................................................

Figure 2 -

 2 Figure 2-18. Flexible/stretchable hybrid matrix technologies: ................................................

Figure 2 - 19 .

 219 Figure 2-19. Picture and process for stretchable system integrated in fabrics. ........................

Figure 2 - 20 .

 220 Figure 2-20. Different technics developed to create a gradient of stretchability. ...................

Figure 2 - 21 .

 221 Figure 2-21. Methods for creating gradient of stretchability ...................................................

Figure 3 - 1 .

 31 Figure 3-1. Bending setup for plasticization threshold and results. .........................................

Figure 3 - 2 .

 32 Figure 3-2. Electrical resistance of aluminum tracks after one washing cycle, normalized to the electrical resistance before washing, for (a) PET50/Al9 and (b) PET12/Al9 samples. ...........

Figure 3 - 3 .

 33 Figure 3-3. Neutral plane position in stack determined using mechanical model of stacks under bending. ....................................................................................................................................

Figure S 3 - 1 .

 31 Figure S 3-1. Labview programs for (a) acquirement of side-view image of the sample under bending, and (b) image treatment for measurement of the bending radius. .............................

Figure S 3 - 3 .

 33 Figure S 3-3. Electrical resistance of Aluminum lines .............................................................

Figure 4 - 1 .

 41 Figure 4-1. Setup and results of comparison of bending stiffness between the substrates ......

Figure 4 - 2 .

 42 Figure 4-2. Setup and samples for the two bending methodologies ........................................

Figure 4 - 3 .

 43 Figure 4-3. Durability testing of tracks integrating zero-ohms resistors. .................................

Figure 4 - 4 .

 44 Figure 4-4. Comparison between bending mechanics of PI substrate and PaC substrate ........

Figure 4 - 5 .

 45 Figure 4-5. Localization of cracks and bending marks in the six configurations. ...................

Figure 4 - 6 .

 46 Figure 4-6. Side views of electrical connections made with two different glues .....................

Figure 4 - 7 .

 47 Figure 4-7. Results of bending setup localized inside the joint ...............................................

Figure S 4 - 1 .

 41 Figure S 4-1. Load-displacement curves for PI50Cu18, PI12Cu9 and PaC5Cu0.5. ................

Figure S 4 - 2 .

 42 Figure S 4-2. Four-probe resistance measurements over 5000 bending cycles at 0.05 mm of radius. .......................................................................................................................................

Figure 5 - 1 . 103 Figure 5 - 2 . 106 Figure 5 - 3 . 108 Figure S 5 - 1 . 112 Figure S 5 - 2 . 112 Figure S 5 - 3 . 113 Figure S 5 - 4 . 113 Figure S 5 - 5 .Figure S 5 - 6 .Figure S 5 - 7 .

 51103521065310851112521125311354113555657 Figure 5-1. Copper-leaf process presentation. ....................................................................... 103

Figure S 5 - 9 .

 59 Figure S 5-9. Raw data of the ECG signals from Sparkfun and copper-leaf boards. ............

Figure S 5 - 10 .

 510 Figure S 5-10. Live monitoring of the ECG signal (Capture) ................................................

Figure 1 - 1 .

 11 Figure 1-1. Examples of wearables technologies found in literature.

Figure 1 - 2 .

 12 Figure 1-2. Examples of commercialized wearable electronic devices.

Figure 2 -

 2 1a shows a few milestones of the transfer of computing devices, from an immobile desk position to the human body. Figure2-1b presents the rapid miniaturization of silicon-based electronic elementary functions, which have reached submicronic features. This size reduction can be seen as an improvement toward their integration in textile, since dedicated area to rigid components is consequently reduced. However, a new technological limitation arises. Actually, interfacing conventional macro-scaled conformable textile with microelectronic rigid devices currently requests breakthrough embedding technologies.

  recent works are today limited to basic logic circuitry and quasi-static applications. The third and fourth approaches are flexible and stretchable circuit boards embedded in textile. They have the advantage to rely on more established process technologies with a dedicated rigid/flexible/stretchable substrate separately processed from the garment. Despite a loss in wearing comfort, the superior computational performances, motived by economic and social aspects, are driving this axis of research.Societal and economical aspects of the smart wearable market including electronic textileSince the 1970's, the wearable technology market is continuously growing. The appearance of smart wearable devices, such as fitness trackers and smartwatches, has recently accelerated the market growth. One may first define what Smart wearable implies. "Smart" stands for electronic devices embedded with active electronics such as sensors, data processors, humanmachine feedbacks and/or wireless communication. Therefore, smart devices tend to be autonomous in term of connectivity as well as powering. "Wearable" may indiscriminately refer to a wide range of products as soon as users may comfortably wear it. Smart wearables are divided in several sub-categories including smart watches, smart wristband, earwears, and smart clothing. These devices serve a wide range of applications from sport and fitness activity to healthcare purposes and safety notifications. It also emphasizes the autonomous aspect since data communication and powering are integrated on a moving subject.

Figure 2 - 1 .

 21 Figure 2-1. Evolution of integrated technologies

Figure 2 - 2 .

 22 Figure 2-2. Number of articles published per year for different topics in connection with wearables

Figure 2 - 3 .

 23 Figure 2-3. Past-evolution and future prevision of e-textile market

Figure 2 - 4 .

 24 Figure 2-4. Example of conductive structures

Figure 2 - 5 .

 25 Figure 2-5. Different types of conductive yarns

Figure 2 - 6 .

 26 Figure 2-6. Different examples of conductive textiles and methods to create conductive tracks in textiles

  -5d and Figure 2-5e. Ryan et al. recently developed a conductive silk fabric through dyeing with PEDOT:PSS

2. 4 . 1 . 2 .

 412 Integration of conductive tracks into textile Weaving and knitting of conductive textiles: Woven and knitted fabrics are the most used in everyday clothes. Both structures are depicted in Figure 2-4b. During manufacturing process of woven fabrics, fibers in two perpendicular directions (weft and warp) are intertwined and radii

Figure 2 - 7 .

 27 Figure 2-7. Permeation properties of silver fluoroelastomer composite inks which were fabricated with acetone, MIBK, and BCA solvent

Figure 2 - 8 .

 28 Figure 2-8. (a-c) Photograph and selected SEM images of 2x2 open square pattern arrays of PI (thickness=2 mm)/Cr (70 nm)/PI (2 mm) layered structure with PI (thickness=1.8 mm) peripheral cilia transferred onto a textile

Finest

  practical thread spacing realized by Post et al. is 1.27 mm in their optimized plastic chip carrier. Yongsang Kim et al. introduced planar-fashionable circuit board (P-FCB) with a

Figure 2 - 9 .

 29 Figure 2-9. Optimized packaging of components

  authors use standard PCB materials to create an interposer, and classic soldering process to attach the component to the interposer. Electrical connections between interposer pads and conductive wires in fabric are made with conductive adhesive (Figure 2-10d). Kallmayer et al. developed a flexible interposer technology featuring thinned silicon chips and a 45 µm-thick metallized polyimide tape. Low temperature soldering of the interposer pads onto the integrated metallized fibers forms a stable connection. Further encapsulation between two layers of fabrics and encapsulation filler results in an overall thickness of approximately 1 mm (Figure 2-10e).

Figure 2 - 10 .

 210 Figure 2-10. Interposer technologies:

Figure 2 -

 2 Figure 2-11. Working principles of OFET and electrochemical WECT transistors for 1D textile integration.

Figure 2 - 12 .

 212 Figure 2-12. Pictures of planar and cylindrical electrochemical transistors (ECTs).

  (a) Schematic picture of a wire ECT (WECT). (b) Optical micrograph of a WECT constructed at the junction of two PEDOT/PSS-coated polyamide monofilaments with 10 μm diameter.

Figure 2 -

 2 Figure 2-13. Design and electrical characterization of logic circuits on fibres.

  -14c). This field effect based transistor uses poly(methyl methacrylate) (PMMA) as insulator and a vertically phase-separated 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT) blended with PMMA served as channel. Mobility and on/off ratio in the range of 0.2 cm 2 V -1 s -1 and 10 4 was achieved, respectively. In 2016, Heo and coworker reported highperformance reel-processed complementary metal-oxide-semiconductor (CMOS) integrated circuits based on chemical-doped single walled carbon nanotube (SWCNT) transistors on 1D fiber (Figure2-15). By a selective doping using (4-(1,3-dimethyl-2,3-dihydro-1Hbenzoimidazol-2-yl)phenyl)-dimethylamine (N-DMBI), n-type and p-type SWCNT transistors were realized on a single fiber (optical one) substrate as well as their integrated circuits. The pand n-type SWCNT transistors exhibit field-effect mobility of 4.03 and 2.15 cm 2 V -1 s -1 , respectively. Their SWCNT CMOS inverter circuits demonstrate a gain of 6.76 and relatively good dynamic operation at a supply voltage of 5 V. The outcomes of this work are impressive but the level of integration onto classical textile fiber has to be optimized in order to extend the potential of applications.

Figure 2 -

 2 Figure 2-14. (a) Schematic cross section of device, showing masking layer, and (b) Arrays of 500 µm x 50 µm transistors formed using weave-based masking.

Figure 2 -

 2 Figure 2-15. (a) Schematic illustration for fabrication process of 1D fiber-type CMOS circuitry. (b) A noise margin characteristic (at VDD = 5.0 V) and (inset) circuit configuration of the complementary SWCNT inverter.

Figure 2 -

 2 Figure 2-16. 500 µm wide sensor stripes a) before and b) after being woven into textile band.

  interconnections and electrical tracks are embroidered in a same step. An integrated device from Linz and colleagues' work is shown in Figure 2-17a. The solder resist of the flexible module shows several openings at the contact pads where the conductive yarns is embroidered to create electrical interconnection. The reliability of the interconnection contact was investigated by Linz, Kallmayer and colleagues under accelerated ageing, temperature cycling and washing

Figure 2 -

 2 Figure 2-17. a) Flexible electronic module connected with conductive yarn by embroidery. b) Conductive thread held by the gap created by punching through the pad with the sewing needle.

Figure 2 -

 2 17c and Figure 2-17d show a flexible module before and after encapsulation respectively.

Figure 2 -

 2 Figure 2-18. Flexible/stretchable hybrid matrix technologies:

  (c)Illustrations of an organic thin-film transistor (OTFT) cell on a rigid island with elastic conductor wiring. (d) Pictures of magnified OTFT cells. Scale bar, 5 mm; (e) Active matrix is stretched to about 60%. Scale bars, 20 mm. Reproduced with permission.[58] Copyright 2015, Nature Publishing Group. (f) Stretchable silicon circuit in a wavy geometry, compressed in its center by a glass capillary tube (main) and wavy logic gate built with two transistors (top right inset). (g) Stretchable silicon circuit with a mesh design, wrapped onto a model of a fingertip, shown at low left), moderate (center) and high (right) magnification. The red (left) and blue (center) boxes indicate the regions of magnified views in the center and right, respectively. The image on the right was collected with an automated camera system that combines images at different focal depths to achieve a large depth of field. (h) Array of organic transistors interconnected by elastic conductors on a sheet of PDMS in a stretched (left) and curvilinear

Figure 2 -

 2 Figure 2-18e. Larmagnac et al. created stretchable electronics based on Ag-PDMS

  colleagues measured two parameters: stretchability of the textile with PDMS and extra weight add-on. They showed that a 1 mm thick layer of PDMS could increase by 10 times the textile's weight, and by 5 its Young Modulus. These results lead them to design open PDMS structures instead of plain sheets, as shown in Figure 2-18b. The optimized integration method shows good reliability and no delamination after 50 standard washing cycles at 40°C without tumbledrying.

Figure 2 - 20 .

 220 Figure 2-20. Different technics developed to create a gradient of stretchability.

  would be the next step toward the advent of autonomous smart clothing. Beyond the device fabrication aspect, many tools need to be developed to frame the research and applications in smart textile. No standard testing protocols were developed to characterize mechanical and electrical behavior of smart textiles. Washing is the most demanding test mechanically speaking, and yet no clear scientific understanding of the diverse mechanical strains implied in the washing machine was developed. Electronic textile is at the edge of multiple disciplines, and cannot completely fulfill the standard demands from each field. Nevertheless the standardization of testing protocols dedicated to smart textiles will accelerate the development of this high potential research field.

Figure 3 - 1 .

 31 Figure 3-1. Bending setup for plasticization threshold and results.

  ). A second Labview program allows measurements of the bending radius from the pictures (see Appendix 2). Both programs are shown in the supplementary material in Figure S 3-1. Labview programs for (a) acquirement of side-view image of the sample under bending, and (b) image treatment for measurement of the bending radius.

Figure 3 -

 3 1c and 1d show  ratio for PET and PI films under bending. Each plot has two distinct regions. A first horizontal line translates typical elastic behavior where material returns to its initial state (𝑟 = 𝑟 ) after bending. The second region conveys plastic behavior (𝑟 < 𝑟 with decreasing  while increasing strain (decreasing bending radius). Transition between the two regions indicates the plasticization threshold of the material.

Figure 3 -

 3 Figure 3-2a and b reports the relative electrical resistance of aluminum lines after washing for PET50/Al9 and PET12/Al9 samples. For both stacks, pocket configuration samples show slightly better electrical results than stitched samples. This could be explained by the fact that, in stitched-configuration, bending and stretching movements of the fabric are transferred to the stack during washing, whereas samples embedded in pocket only suffer from bending movements. This highlights that further integration of flexible electronics in textile should minimize physical ties with the fabric, and preferably group interconnections on the same side of the device.

Figure 3 - 2 .

 32 Figure 3-2. Electrical resistance of aluminum tracks after one washing cycle, normalized to the electrical resistance before washing, for (a) PET50/Al9 and (b) PET12/Al9 samples.

Figure 3 - 3 .

 33 Figure 3-3. Neutral plane position in stack determined using mechanical model of stacks under bending.

  1 mm. Comparison of curvature before and after bending reveals the plasticization threshold, which decreases with Young Modulus and thickness of the film. Therefore, 13 µm-thick PET film elastically bends down to 0.4 mm, whereas 50 µm-thick PI film plasticizes at 2.3 mm. Then, we measured the change in electrical resistance of aluminum tracks on polymer substrate, to monitor the damages induced in the metallic layer under washing. For PET50/Al9 samples, an 800 µm-thick PDMS encapsulation reduces the change of electrical resistance, from 5% to 2%, after one washing cycle. An analytical model of stacks under bending shows how the encapsulation layer brings the neutral plane closer to the aluminum layer, lowering the strains applied upon folding. For thinner and more compliant PET12/Al9 stack, the neutral plane is naturally close to the middle of the aluminum layer and the stack does not require a PDMS encapsulation to show good electrical results after washing. Nonetheless a thin layer helps counterbalance the high deformability of the thin film and prevent from high deformations. As a result, the mechanical behavior of conventional films of FCB industry can be improved by controlling of the stack and with final overall thickness below one millimeter. Further materials with balanced thicknesses and Young moduli in regards of the analytical model could be investigated for even thinner devices.MethodsPatterning of electrical tracks: Tested substrates were commercialized laminated films of copper on polyimide (Pyralux® AC from DuPont), PI films (Kapton® HN from Dupont), laminated films of aluminum on PET and PET films (ADDEV Materials). To create the aluminum tracks, unwanted metallic parts were removed by laser-hatching using a LPKF Protolaser S equipment, with a 1064 nm wavelength beam, powered at 9 W with a frequency of 75 kHz. The diameter of the laser beam is 25 µm.

Figure S 3 - 2 .

 32 Figure S 3-2. Measurement of plasticization threshold under bending of metallized films.

Figure S 3 - 3 .

 33 Figure S 3-3. Electrical resistance of Aluminum lines

Figure S 4 - 1 .

 41 Figure S 4-1. Substrates with such thickness are intended for low dynamic bending applications like ribbon-cable, or to condense electronics in static high-density devices. Up to 4 mm displacement, the measured force remains below 0.1 N for both PI12Cu9 and PaC5Cu0.5, whereas PI50Cu18 substrate requires a force of 0.37 N. Both PI12Cu9 and PaC5Cu0.5 substrates shows much higher compliance than thick PI50Cu18. Film PaC5Cu0.5 especially

Figure 4 - 2 .

 42 Figure 4-2. Setup and samples for the two bending methodologies

Figure 4 - 3 .

 43 Figure 4-3. Durability testing of tracks integrating zero-ohms resistors.

Figure 4 - 4 .

 44 Figure 4-4. Comparison between bending mechanics of PI substrate and PaC substrate

Figure 4 - 5 .

 45 Figure 4-5. Localization of cracks and bending marks in the six configurations.

  -5c and d. In addition, on soft PaC5Cu0.5 substrate, the component induces stresses, which translate as bending marks in the copper pad, around the edges of the components, as shown in Figure4-5d (white arrows). This is related to the glue's flexibility that allows it to deform around component's edges. These kind of deformations do not appear in ICP4000 samples on PaC5Cu0.5, despite its lower Young Modulus. To explain this difference, we must consider the geometry of the connection. Side view of the conductive connections are shown in Figure4-6. The paste PU1000 dries faster and deposits as a thin layer with uniform thickness of roughly 120 µm (Figure4-6a). ICP400 remains liquid until curing step and forms a triangular shape along the component's edge with maximum thickness of 300 µm (Figure4-6b). Regarding Equation (1), at the edge of the resistor, the stiffness of ICP4000 is therefore roughly 6 times higher than in PU1000.

Figure 4 - 6 .

 46 Figure 4-6. Side views of electrical connections made with two different glues

Figure 4 - 7 .

 47 Figure 4-7. Results of bending setup localized inside the joint

Figure 5 - 1 .

 51 Figure 5-1. Copper-leaf process presentation.

  -1d. First, the leaf is laminated on a glass wafer, where it remains attached by weak Van der Waals forces (see Movie Figure S 5-1, Supporting Information). A Parylene-C (PaC) film is deposited on top of copper by chemical vapor deposition, to serve as the substrate. We chose parylene as soft carrier for two main reasons: it is transparent to the near-infrared laser beam (wavelength of 1064 nm), and its thickness it tunable depending on the desired final softness. After deposition, the copper-parylene stack easily peels off the wafer due to the weak bonding forces involved. With a weight of 0.72 mg cm -2 (plain copper on 2-µm-thick parylene), our stack is light enough to stand on soap bubbles (Figure5-1b). To proceed with laser patterning, the stack is turned upside down with copper facing upwards. A few drops of polyvinyl alcohol (PVA) solution in water (2 w%) serves as sacrificial adhesive to fix the stack on a glass wafer carrier through the process. The PVA is also transparent to the laser beam and is easily dissolved in water within minutes at the end of the process (see Movie Figure S 5-4, Supporting Information).

Figure 5 -

 5 Figure 5-1g with LEDs as thin as 145 µm (83 µm gap between anode and cathode). By eliminating processes such as photolithography, metal evaporation and mask fabrication, this high-resolution patterning process can greatly reduce the time and cost of flexible microelectronic circuits. The stencil-free and direct laser hatching especially enables rapid prototyping directly from a digital layout of a circuit. We assess the influence of intrinsic irregularities in the thickness of the copper leaves on electrical properties of patterned tracks (Figure S 5-2. Characterization of the copper layer thickness, Supporting Information). The average resistance of 90-µm-wide, 15-mm-long lines is 7 Ω with a standard deviation of 1.4 Ω

oven for 3

 3 min at 170 °C. With this technique, one can mount surface mounted devices (SMD) as small as 0402 (1.0 mm x 0.5 mm x 0.35 mm). Packages with smaller pitch like the Lead-Frame Chip Scale Packages (LFCSP, 500 µm pitch) are mounted with a dedicated vertical aligning machine (Figure 5-1f).

  -2b). The track shows electrical stability under progressive bending, down to complete folding corresponding to 0.1 mm of radius (right inset picture). We repeated the same folding test over 5000 cycles to investigate the endurance of the stack (Figure5-2c). As we can see, stacks with different parylene thicknesses show excellent electrical reliability through the 5000 cycles. The role of encapsulation is demonstrated with a non-encapsulated sample, which shows progressive increase of resistance translating damages of the metallic layer. The same fatigue test on narrower lines (Figure5-2d) assess the robustness of lines as thin as 50 µm width. A small decrease of resistance was observed for 50-µm and 110-µm-wide lines (respectively 4 % after 200 cycles and 2 % after 100 cycles), which could be due to the flattening of copper at bending edge. After stabilization, no electrical change occur superior than 1 %.

Figure 5 - 2 .

 52 Figure 5-2. Bending setup and results for characterization of mechanical robustness under bending of interconnection tracks.

Figure 5 - 3 .

 53 Figure 5-3. ECG monitoring demonstrator

Figure 5 -

 5 3a and Figure 5-3b show original rigid Sparkfun board and its copper-leaf based counterpart on the backside of a hand. The inset picture highlights the thinness of the device fixed on a finger. ECG signals simultaneously acquired from original and transferred circuits are shown in Figure 5-3c. Both boards are plugged on 50 Hz notch filters before we apply a 3-bits digital filter. Raw data are available in Supporting Information (Figure S 5-9).

  of the components: We applied a Sn42Bi58 alloy paste from Shenzhen Winghongtai Tin CO., LTD, with a Metcal DX-250 pressure-controlled dispenser, for a consistent and repeatable fluid dispensing. Drops were deposited with a tip with internal diameter 0.41 mm, at a pressure of 4.5 Pa and pressure-time of 0.07 s. A high accuracy SMD manipulator PRECITEC®.ELITE from CIF allows rapid mounting of 0402 components. We integrated smaller-pitch components like the ECG chip (AD8232 from Analog Devices) with an aligner from Metcal series APR-5000-DZ-TAB. LED dies from Cree (reference C450SA1000) had a gap of 83 µm between anode and cathode. They can be mounted under microscope after applying a drop of flux underneath it, then proceed with reflow. The reflow process operated in a batch reflow oven FT03 from CIF, with a heating program configured at 170 °C for 3 min.Electrical characterization under bending: Bending tests were performed on a customdesigned XY motion table. Guiding ramps curve the sample to promote the sharpest bending in the middle of the sample, and to avoid damaging the sample on the edge of the clamp. A Foculus IEEE 1394 digital camera took high-resolution pictures of the side of the sample. This setup allow measurement of bending radius down to 0.1 mm. At the same time, 4-probes measurements monitors the electrical resistance on a Keithley. The setup was synchronized using a homemade Labview script (see Appendix 1). After collecting the pictures and associated electrical measurements, bending radii were extracted from the pictures with a second program developed on LabView Vision module (see Appendix 2).ECG measurements: A generator (TTi CPX400DP) supplied 3.3 V to power the ECG device.Three medical grade electrodes (Ag/AgCl ECG electrodes from TIGA-MED) were placed based on the Einthoven's triangle: on the chest near the right and left arms, and above the right, lower abdomen. All connections from the soft device were made with thin copper wires of a diameter of 150 µm, soldered on the device using the reflow process depicted for components mounting. The outputs from both rigid and flexible boards were plugged on notch filters to eliminate the 50 Hz supply noise. We simultaneously acquired the signals on a LeCroy WaveRunner 6100A oscilloscope. Data depicted in Figure5-3c and Figure5-3d are processed using a 3-bits digital filter from the LeCroy oscilloscope. Supplementary filter (100 Hz) was added on the signal from the Sparkfun board.

Figure S 5 - 3 .

 53 Figure S 5-3. Infrared camera monitoring of tracks' temperature as a function of applied current.

Figure S 5 - 4 .

 54 Figure S 5-4. Delamination of the parylene -copper stack from the rigid carrier by dilution of PVA in water (Capture)

Figure S 5 - 5 .Figure S 5 - 6 .

 5556 Figure S 5-5. Comparison between tin alloy soldering on evaporated gold and copperleaf

Figure S 5 - 7 .

 57 Figure S 5-7. Stretching test on 100-µm wide line on parylene.

Figure S 5 - 8 .

 58 Figure S 5-8. Copper-parylene device is more than 6 times lighter than its PCB counterpart is.

Table 2 -

 2 1. Integration method, mechanical reliability and electrical reliability of conductive materials

	Conductive material	Electrical property	Integration method	Mechanical property	Reliability under wash or mechanical testing

Table 3 -

 3

1. Young's Moduli of investigated substrates

Table 3 -

 3

2. Standard substrates of FCB industry

Samples dimensions are 20 mm x 50 mm. Damages induced by washing are visually noticeable:

  -1a.

	Substrate's thickness	PET 50µm	PI 50µm	PET 50µm	PI 50µm
	Metal's thickness	-	-	Al 9µm	Cu 18µm
	Sample	PET50	PI50	PET50/Al9	PI50/Cu18
	Before wash
	After wash

Table 4 -

 4 2. Conductive pastes and mechanical properties of tested pastes.

IMT Mines Saint-Etienne, Center of Microelectronics in Provence, Department of Flexible Electronics, F -13541 Gardanne, France

Acknowledgements

We thank the scientists who organized the "Electronic textiles" symposium of the 2017 E-MRS spring meeting. Ecole Nationales Supérieure des Mines de Saint-Etienne is acknowledged for financial support.

Acknowledgment

We would like to thank Thierry LEGOU, from CREx in the Brain and Language Research Institute in Marseille, for his precious help and time on the ECG board. This work was partially done in the ID-FAB prototyping platform funded by the European Union and local authorities.

Discrete integration of electronic components into textile

The simplest case of device integration for electronic circuitry purpose consists in attaching components directly onto a conductive fiber integrated in the textile. In this approach, the integrated circuits (IC) are individually integrated on textile by attaching them directly to integrated tracks. Such circuit shows mechanical differences with a flexible circuit integrated in textile. First, splitting the circuit into its different functional parts avoids large stiff areas in the garment, which can positively impact the imperceptibility of the device. The location of rigid components can be chosen at non-disturbing strategic places by extending the interconnecting wires. Secondly, textile circuits can crumple non-destructively as textile would do, which cannot be achieved with more conventional flexible circuits. Nonetheless, the nature of the board, which is here the textile, causes a noticeable difference in the components integration process. Methods for fabrication of flexible circuit lean on standard PCB fabrication, as the flexible substrate can be temporary stiffened during process. Here, on the contrary, the loose nature of textiles brings several difficulties for the discrete integration of rigid components. Several methods are developed to mechanically and electrically attach surface mounted components to textiles.

Package optimization

Stitching of e-textile-tuned components on textile was an early approach to create demonstrators of wearable devices. The textile-tuned components are common silicon-based components mounted on stitchable printed circuit boards (PCBs) like Arduino Lily's pad boards (Figure 2-9a), [START_REF] Buechley | Proc. SIGCHI Conf. Hum. Factors Comput. Syst[END_REF][START_REF] Sugathan | Cond. Assess. Tech. Electr. Syst. CATCON 2013 IEEE 1st Int. Conf. On[END_REF][START_REF] Coyle | Wearable Implant. Body Sens. Netw. 2009 BSN 2009 Sixth Int. Workshop On[END_REF][START_REF] Shaw | Proc. 12th ACM Int. Conf. Adjun. Pap. Ubiquitous Comput.-Adjun[END_REF][START_REF] Ugulino | [END_REF] or mounted on a special socket button, [25,54] or with extension of the chip's pads to allow stitching or weaving (Figure 2-9c). [25,54,68] Despite the relatively bulky aspect of the resulting garment, this process remains simple and does not require complex equipment, while keeping a great liberty in designing simple circuits. These aspects can explain the popularity of the technique among designers and hobbyists, and the commercial availability of the required materials.

Direct bonding and packaging of silicon chip on fabric were developed as well. The main challenge in this technology is to bond chip-scale pads to fiber-scale landing pads. For example, the pitch between two pads is 400 µm in a wafer level chip scale package (WLCSP) for a microcontroleur chip. [START_REF] Stmicroelectronics | Datasheet STM32F205xx[END_REF] A solution that was investigated is to create a new packaging (a new chip carrier) that would redistribute the pads at a larger scale and in a suitable way for handmade integration in textile. Post et al. imagined a plastic threaded chip carrier (PTCC) (a) Stretchable electronic system laminated to fabrics. Reproduced with permission. [START_REF] Löher | Electron. Packag. Technol. Conf. 2009 EPTC09 11th[END_REF] Copyright 2009, IEEE. (b) Textile bonding by screen printing PDMS. Reproduced with permission. [135] Copyright 2011, Cambridge University Press.

Mechanical transition between heterogeneous materials

Smart textile comprises a large variety of sensors, devices and electronic circuits, with heteroclite mechanical properties. As electronic textiles are subjected to high and repetitive strains under wearing and washing, a mechanical mismatch between two connected parts is prone to failure. This appears at interfaces between stretchable interconnections and flexible islands in hybrid matrix, or when integrating rigid electronic components into soft circuits. Noncompliant materials may break under repetitive deformations whereas soft parts are the subject of scratching or damaging when connected to less-soft parts. Because of a relatively new development, smart textile technologies do not have dedicated standard for reliability testing.

Washing standard for textiles exists, as well as reliability test to temperature and humidity cycles for electronics, but no common test merge both aspects. Fraunhofer IZM and Ghent University use the ISO 6330 Textiles -Domestic washing and drying procedures for textile testing -to test their wearable devices. This normalized test requires standard domestic machine, includes drying process and washing temperatures ranging from 30°C to 92°C. At this time, no test reproduces the mechanical strains applied in washing and drying machine, and test conditions should be related to the application environment. [START_REF] Kallmayer | Electron. Compon. Technol. Conf[END_REF] Shifting of the neutral plane Standard silicon-based microelectronic components possess high electrical performances, but present mechanical disadvantages for integration in soft device as their absence of stretchability and show low flexural rigidity. Ultra-thin silicon chips obtained by wafer thinning are more flexible than conventional silicon components, and were widely investigated for their [58] Copyright 2015, Nature Publishing Group. (b,c) Resistance of the graded architecture against interfacial failure under stretching as opposed to the complete delamination observed for the non-graded sample containing the stiffest patch. The experimental samples are identified by the letter E followed by the elastic modulus of the patches in MPa. Scale bars, 2 mm. (d) Distribution of strains yy for the architectures S-0040 and S-Grad after applying a longitudinal global strain (g) of 25% in the y-direction. Reproduced with permission. [129] Copyright 2012, Nature Publishing Group.

Mechanical characterization of soft substrates for wearable and washable electronic systems

First chapter has highlighted the advantages and drawbacks of state-of-the-art solutions to build wearable electronics. Regarding the integration density needed for data acquiring, processing, and transmission, flexible circuit board (FCB) is a good compromise.

Conventional substrates of FCB industry are laminated films of aluminum on PET, and copper on PI. They are used for static applications such as in anti-theft devices, inside electronic devices with restrained volume such as cameras, or inside electronic cards for near-field communication (NFC) technologies. Their fabrication advantageously relies on circuit board processes, which guarantees the same integration density and makes them great candidates for prototyping in laboratories.

Their use in harsh environment such as washing machine revealed mechanical and electrical failures: metallic tracks under high repeated stresses shows cracks and loss of electrical contact; PET and PI substrates deform plastically which could discomfit the user and/or finally break.

Solutions have been explored, such as stiffening of the stack, or graduated thickness of encapsulation. Nonetheless, they often result in rigid parts inside textile, or final thickness over 1 mm, which can spoil the circuit's imperceptibility.

In this chapter, we investigate stacks robustness and bendability through two different tests. We assess elastic limits of polymer films through radius monitoring, and the electrical stability of metallic tracks under washing. Results are correlated with an analytical modeling of stress repartitions under bending, depending on Young moduli and thicknesses of each films. Table 3-1 groups the Young moduli of investigated materials. Our goal is to improve the robustness of the films while preserving there imperceptibility level.

Publication Status: This paper has been accepted and published in APL Materials (DOI: 10.1063/1.5063671) S. de Mulatier, 1,2 M. Ramuz, 1 D. Coulon, 2 S. Blayac 1 and R. Delattre 1 1 IMT Mines Saint-Etienne, Center of Microelectronics in Provence, Department of Flexible Electronics, F -13541 Gardanne, France 2 @HEALTH, Europarc de Pichaury, bâtiment B10, F-13290 Aix-en-Provence, France E-mail: thierry.djenizian@mines-stetienne.fr Abstract Integration of complex electronics into our everyday outfit is a key challenge for tomorrowwearable technologies. Performing data processing and communications, these circuits are an essential part of sensing and actuating smart systems. The choice of an appropriate electronic substrate is essential for robust and reliable, yet imperceptible systems. In this paper, we study and mechanically improve conventional substrates of flexible circuit board (FCB) industry for highly flexible and washable textile-integrated circuits. First, we study the mechanical limits of polymeric substrates to better apprehend how permanent damages occur during washing cycle.

Measurements of plasticization threshold of our films under bending showed that bending radii below 1 mm occur during washing, and that 13 µm-thick polyethylene terephthalate (PET) film can be elastically deformed down to 0.4 mm of bending radius. Secondly, we focus on the electrical and mechanical behavior of a stack composed of PET substrate, aluminum conductive tracks, and polydimethylsiloxane (PDMS) encapsulation. An analytical model of stacks under bending is correlated to experimental results to demonstrate the impact of the thicknesses of the different layers, and to improve the reliability under washing. Using the same model, we investigate different materials for encapsulation to further optimize the mechanical reliability of FCB while decreasing its overall thickness.

Introduction

Seamless integration of complex electronics into textile is a keystone for wearable technologies to become integral part of our lives. As for any other processing systems, complex circuitry Washing test and electrical measurements: Washing cycles were performed in a domestic Samsung frontload washing machine WD0804W8E. To mimic real-life conditions, "daily" washing program was selected with a washing temperature of 30°C and 800 rpm tumble dry step. A whole cycle was 1 hour and 4 minutes long. No detergent was added to limit the washing effects to mechanical strain. For each cycle, the machine was filled in with same textile loading (2 kg). After wash, the samples were manually removed from textile and air-dried at room temperature before electrical testing.

Electrical tests were performed using a four-probe setup, with a National Instruments PXIe-

Abstract

Imperceptibility is a key asset for wearable electronic devices embedded in textiles. The use of an ultra-thin, highly compliant substrate benefits this imperceptibility. On the other hand, it increases the mechanical gap between soft substrate and rigid components, which generates weak points at interfaces such as mounting joints. Current approaches for flexible wearable circuits are based on fabrication processes and materials of flexible circuit board industry. They do not address precisely the mechanical robustness of the mounting joint under bending. Several methods smoothen the soft/rigid transition by using stretchability gradient or stiffening islands, but they all require several additional process steps, or increase the thickness and stiffness of the device. In this paper, we characterize the bending durability of conductive attach of rigid components on ultra-thin substrates based on conventional mounting process. Two bending setups are developed related to real-life solicitations in wearables. We characterize and compare the mechanical robustness of systems composed of standard polyimide -copper substrate and substrates with different thickness, and conductive pastes with different softness. Results and comparison with FCB materials allow determining the more robust system depending on targeted solicitations. When the system is free to adapt under external bending, ultra-thin substrates integrating rigid components with solder mounting show reliability through 5000 bending at a radius of 0.5 mm. Under stresses localized inside the conductive joint, flexible glues such as ICP4000 and PU1000 show mechanical robustness down to 0.35 mm of radius.

Introduction

Wearable electronics are in close contact with the human body. They are embedded in textiles [1,2] , integrated on the skin [3][4][5] , or directly placed on organs like smart contact lenses [6- 8] . The interest of textiles as support for wearable devices stands out in medical applications that require distributed sensing, computing and energy sourcing. To reach true wearability and imperceptibility for the user, the electronic circuit must meet comfort aspects, such as lightweight, thinness, and high flexibility. To perform smart and fast-responsive systems, they should also integrate inorganic rigid components, as their computational performances remain unequaled by their flexible organic/inorganic counterparts. Ultra-thin circuits based on copper leaves are good candidates for imperceptible computational electronics. [9] The bending stiffness is defined as the mechanical resistance of the film or stack in flexure.

Figure 4-1a depicts the setup for comparison of the bending stiffness. A cantilever with a radius of 10 mm translates downwards imposing a flexure movement to the substrate. Depending on the substrate's stiffness, the cantilever experiences more or less resistance to its displacement.

In application, the stiffness of a wearable device is directly correlated to the mechanical resistance the human body perceives during movements. Therefore the lowest stiffness is needed for wearable electronics.

Methods

To the best of our knowledge, there is no norm to characterize the mechanical durability of electronic textile. Evaluation of materials implies developing a methodology test in regards of expected operating conditions. The choice of circuit's integration in textile greatly influences the transmission of strain from textile to circuitry. When the circuit is fully attached, stretching as well as bending strains must be considered. This kind of integration was investigated, but includes stretchable electronics [1,25] , or flexible electronics molded in rigid encapsulation to avoid stretching of the circuit and of the fixture points on textile [17] . Non-stretchable or nonflexible areas inside clothes can create discomfort for the user and develop weak points inside textile and circuitry. Moreover, to avoid mechanical failure, we want to avoid stretching deformations transmitting to flexible electronics. We consider the circuit anchored on one side that gather all the circuit's outputs. The circuit is hold inside a closed pocket of textile, but only fixed on one side. In this way, the fabric is free to stretch naturally, without disturbance from the circuit, and without applying elongation to the circuit.

In this context, only bending strains apply to the circuit. We characterize two types of flexural deformations. First, we apply a global deformation to the system. In this setup, the system is free to adapt its curvature to release bending stresses, depending on the mechanics of the layers.

The sample is clamped on its two sides, away from the component, with moving stage inducing folding. A camera monitors the side view of the sample in order to measure the bending radius. Copper-leaf based process for imperceptible computational electronics

We have previously highlighted the mechanical advantages of thinner layers under bending solicitations. In literature, two main classes of thin interconnections are developed: ultra-thin evaporated metallic films, with thickness below 1 µm for time and cost issues, and challenging integration of components; and commercialized laminated films with thickness over 20 µm, and therefore limited flexibility. The metallic layer we use in the next chapter is a copperleaf with an intermediate thickness of 450 nm. This form is interesting as it meets our need for thin layer (two order of magnitude thinner than laminated films), granting mechanical robustness to harsh solicitations; while remaining thick enough to grant electrical conductivity of bulk copper and allow standard integration process such as soldering. This chapter describes the process we developed, including laser-patterning with fine-pitch integration, soldering, multilayering and via contacting, for the fabrication of ultra-thin (12 µm for encapsulated double-layered stack) and lightweight (6 times lighter than PCB) flexible electronic circuits.

Publication Status: This review has been accepted in Advanced Electronic Materials (DOI: https://doi.org/10.1002/aelm.201900787) S. de Mulatier, 1,2 D. Coulon, 2 S. Blayac 1 , R. Delattre 1 and M. Ramuz 1 literature, but there is a lack of methods to build the electronic part itself. In this paper, we present a new process for the fabrication of flexible and imperceptible circuits that reach standards of microelectronics associated with significant integration density of components and interconnections, as well as multilayering. The stack entails a commercialized decorative copper leaf with a thickness of 450 nm as conductive layer and a parylene substrate of a few microns. The copper leaf shows both high compliance and conductivity of bulk copper (5.96 x 10 7 S/m), with electrical resistance stable over 5000 cycles of complete folding. The process implies stencil-free laser patterning that reaches fine-pitch integration, conventional soldering methods, and a three-step via fabrication and assembly process for multilayered circuits. We demonstrate the functionality of an ultra-thin, multilayered, lightweight, electrocardiogrammonitoring device. This work allows direct transfer of microelectronic designs, for rapid prototyping from rigid board to imperceptible electronics.

We distinguish two types of flexible interconnections based on thin films and bulk laminated films. Thin films for soft electronics entail evaporated metals, graphene films, [2] printed nanoparticles ink, [3,4] and liquid alloys. [5,6] They reach the highest degree of imperceptibility as their thicknesses in the sub-micron range provide high conformability [2,7,8] and lightweight, even invisibility to the eye. [2,7] Investigations have highlighted the advantage of thin layers for mechanical robustness. [9,10] Nonetheless, methods to integrate components for circuitry fabrication remain non-conventional and challenging.

On the other hand, commercialized stacks of copper on polyimide are widely used in literature, [11][12][13][14] as they show the advantage to be easily adapted to standard microelectronic processes and packages. However, their thickness often exceeds a few tens of microns, which limits their actual flexibility and robustness through bending, and spoils their imperceptibility. [15] Moreover, they need costly and time-consuming photolithography processes to achieve desired resolution. Other approach implies low-cost and chemical-free cutter-plotter techniques for fast prototyping of tattoos electronics, [14,16,17] but with resolution limited to 200 µm.

In this work, we have developed a new process for flexible and ultra-thin circuitry. The novelty of the process lies in the use of a copper leaf, shown in Figure 5-1a, diverted from its original purpose to serve as conductive tracks. The copper thickness of 450 nm is transitional between thin and bulk films, and provides a combination of their advantages: the metallic leaf is highly flexible, and shows the resistivity of the bulk (1.68 x 10 -8 Ω m). Associated to a parylene substrate with tunable thickness, we achieve lightweight and soft electronics that can float on soap bubbles (Figure 5-1b and 5-1c). We assess the electrical stability of tracks down to 50 µm of width through 5000 bending cycles at 0.1 mm of radius. The circuitry process includes laser patterning with a resolution of 50 µm suitable for fine-pitch integration, conventional reflow soldering for mounting of components and vias, and stacking for multilayered circuits.

Application of a functional ECG monitoring device transferred from a double-layered printed circuit board (PCB) without loss in the signal quality is also presented. In microelectronics, the density of components and tracks most often demands splitting the circuitry in several layers, to allow a denser repartition of components, and therefore smaller systems. Flexible via fabrication implies several steps such as pre-conditioning of substrate, metallization/electroplating, and photolithography. [12,25] We have developed a three-step fabrication process for vias using a new approach depicted in Figure 5-1h. After patterning, the top layer is transferred to a silicon wafer for opening of the via holes. Unlike glass and parylene, silicon absorbs the laser beam, generating a local plasma that create openings in above parylenecopper films. This is the first demonstration of this process for the creation of flexible vias. Top and bottom layers are then aligned under microscope and fixed together with PVA solution (5 w%). At this step, the PVA serves several purposes. First, the top layer spreads by capillarity upon contact with the liquid, which is an important effect considering the ultra-thin layer's propensity to collapse during manipulation. PVA also brings enough adhesion to complete the mounting process until final encapsulation. After alignment, solder paste is deposited on vias and reflowed to connect electrically the two layers. This method is possible thanks to the high compliance of each layer. Indeed, similar technique was attempted on laminated films of polyethylene terephthalate (PET) (thickness 50 µm) -aluminum (thickness 9 µm) with conductive silver paste, shown in left picture Figure 5-1h. Stiffness of the films prevents from tight contact between the layers, which results in paste leaking and contact failure. On the contrary, the parylene-copper films have enough conformability to perfectly fit to each other.

Right picture in Figure 5-1h shows the area where tin paste alloyed with copper without overflow. A crucial asset of the process is that both vias and components can be integrated in a single step. Finally, parylene deposition encapsulates the circuit and permanently fix the layers together. The circuit can then be released from the wafer carrier in water to dissolve the sacrificial PVA.

Supporting information

All movies are available online at: https://doi.org/10.1002/aelm.201900787 A first technological bottleneck of wearable electronics lies in the substrate material. Substrates most commonly used in literature and industry show mechanical and electrical failures under washing stresses. Current solutions imply stiffening and/or thickening of the sensitive parts, but often result in ruining the imperceptibility. In chapter 3, we investigated how layers of the electronic stack could be chosen to improve robustness while preserving imperceptibility. We developed a state-of-the-art bending setup able to monitor radius down to 0.1 mm, and used it to determine elastic limits of polymer films under folding. We used a theoretical model of strain distribution to mechanically optimize the mechanical behavior of the stack by shifting the neutral plane inside metal. We improved the reliability through washing of laminated films of aluminum on PET with an 800-µm thick PDMS encapsulation, with final thickness below one millimeter.

In chapter 4, we addressed the mechanical challenge raised by heterogeneous integration of rigid components on soft substrates. Researchers have explored stiffening islands and stretchability gradients, but at the cost of many additional steps that are not adapted to standard processes, or with reduced imperceptibility. We characterized the mechanical behavior of heterogeneous integration upon two different bending tests for various interconnecting pastes and substrates. Standard reflow soldering on ultra-thin substrates provides robust systems under 5000 bending cycles at 0.5 mm of radius. Soft conductive glues benefit heterogeneous integration as they support sharp bending at 0.35 mm of radius.

In the last chapter, we described the development a new process for flexible, ultra-thin circuits with component density comparable to standard printed circuit board processes. Copper leaves with an average thickness of 450 nm provided a unique combination of advantages in terms of flexibility and electrical conductivity. Mechanically, its thinness provides excellent electrical reliability through 5000 folding cycles, as predicted by the model described in chapter 3. Our criteria of close-to-conventional processes was met with laser patterning of fine-pitch resolution, reflow soldering, via fabrication and stacking for multilayered circuits. Therefore, we could reproduce a standard printed circuit board without further modification of design or components. Compared to conventional flexible substrates, the developed electronic stack entails a superior notion of imperceptibility through its thinness and lightness. We demonstrated the functionality of our process with an ECG monitoring circuit.

We have demonstrated the mechanical advantages of ultra-thin layers in flexible electronics, as well as their increased imperceptibility compared to current solutions based on FCB processes.

The use of conventional microelectronic components and processes make our fabrication method accessible. To further reduce the imperceptibility of the electronics, next steps could be the integration of flexible passive components, such as screen-printed inductors, capacitors and resistors. The challenge is different for replacement of integrated circuits, as density reached in semiconductor technology significantly surpasses those of organic technologies. Recent work proposes a transistor density of 347 unit per square centimeter, which is promising but still far from today's integration density in silicon.

Another aspect that was not addressed in this thesis is the communication method, for communication and transmission of data and energy. Imperceptibility and transparency of use would greatly benefit from contactless communication methods. It will reduce mechanical interfaces with rigid electronics as well as repeated plugging/unplugging of sensitive circuits.

Copper leaves have high enough conductivity for the fabrication of near-field communication antenna like the prototype depicted in chapter 5. Nonetheless, extensive study of the antenna response under bending and crumpling, taking into account the thinness of the copper layer, should be performed.

We discussed the possibility of imperceptible electronics as local data processing circuits;

another promising perspective is the building of sensing and computing parts in a unique imperceptible block, thanks to similar mechanical properties. This would present several advantages such as reducing the fabrication time and the overall size of the device; it would also eliminate additional physical interconnections between sensing and computing parts like currently used flexible ribbon cable, which multiply contacting steps in process and mechanical weak points, and allow data amplifying as close as possible to the source, with noise reduction.

Flexible components, contactless communication methods and single-substrate fabrication provide further level of imperceptibility and integration, and pave the way to wearable devices and implantable modules. Wearable technologies represent a milestone for tomorrow's medical monitoring. Clothing is an interesting medium for these technologies that we want imperceptibly integrated in our daily lives. The objective of this thesis is to design electronics that meet the following three criteria: electrical and computational performances for smart reactive systems; imperceptibility for the user; and robustness through the multiple mechanical stresses applied under use. Few standards exist today that characterize the durability of electronics embedded in the human body. Based on hypotheses of integration in textiles, we have therefore established a methodology for the qualification of materials and assemblies. The study of the assemblies highlighted the main mechanical weaknesses, located at the connection between the rigid components and the flexible substrates. A detailed mechanical study of these connections made it possible to compare the assembly techniques and to highlight the best couple of mechanical connection materials/processes according to the mechanical load mode. The technological integration processes developed allowed the fabrication of an electrocardiogram acquisition system that is compact and embedded on the human body. Compared to a conventional Holter system, it has the advantages of being ultra-thin, ultra-light, flexible and can be integrated into textile, for continuous and imperceptible cardiological monitoring.