domain: spi spi

text:
Thibault Vogel

Jean-Christophe C Walrick
email: jean-christophe.walrick@estaca.fr

Pascal Casari
email: pascal.casari@univ-nantes.fr

Composite laminate with embedded electric cables submitted to low energy impact

Keywords: multifunctional structures, low energy impacts, damage law, cohesive zone, dielectric strength 1

Cette étude traite de la résistance mécanique des structures composites fonctionnelles intégrant des systèmes électriques, soumise à l'impact basse énergie. Les matériaux, définis pour des applications aéronautiques sont des stratifiés fibres de verre longues intégrant des couches de câbles électriques. Une question importante porte sur l'endommagement induit par l'impact à basse énergie sur ces structures fonctionnelles et l'influence des couches électriques. L'impact induits des endommagements non forcément visibles en surface qui peuvent induire une dégradation dans l'isolation des conducteurs. Des campagnes d'essais expérimentales sont réalisées à l'aide d'une tour de chute en flexion 3 points dynamique et en impact ponctuel hémisphérique sur éprouvettes clampée s à basse énergie (entre 10 et 25 Joules). Des modèles par éléments finis intégrant des mécanismes d'endommagement progressifs couplés à des critères en rupture donne une prédiction des effets mécaniques observés.

Introduction

In the last decade, research and development related to multifunctional structures have been increasing due to the growing interest of many industries for these structures and their benefits (cost, mass and volume reduction) [START_REF] Gibson | A review of recent research on mechanics of multifunctional composite materials and structures[END_REF]. Among those integration of wires into composite structures for electrical function embedded (structural batteries, integrated super capacitors,..). is a key technological challenge for aeronautical and aerospace industry. While enabling the future of electric and hybrid flight, these structures also have the potential to some weight and volume optimisation as well as to reduce lead time for system installations.

This study focuses on a particular type of multifunctional composite laminated structure composed of 2 laminate embedding layers of flat cable (laminate + cable + laminate) as showed in. Fig. 1. Fig. 1. Representative multifunctional sandwich system with 2 flat cable layers It is well known that impact loadings can be very harmful to polymeric composite structures as they are likely to produce internal damage such as delamination and fibre/matrix cracking, leading to substantial mechanical properties degradation, including strength and stiffness [START_REF] Kim | Impact and delamination failure of woven-fabric composites[END_REF].

To our knowledge very few studies have been carried out to investigate on the impact response of multifunctional structures, and when they do they mainly focus on mechanical properties degradation [START_REF] Vaidya | Impact response of three-dimensional multifunctional sandwich composite[END_REF][START_REF] Pinto | Smart Multifunctional Composite Materials for Improvement of Structural and nonstructural properties[END_REF][START_REF] Walrick | Influence sur la tenue mécanique de l'insertion d'une fonction de protection électromagnétique au sein de composites à renforts de fibres de verre = Strength mechanical influence of electromagnetic shield function embedded in laminated glass composite[END_REF]. This study investigate on the mechanical degradation of such structures, but also the influence of a low energy impact on their electrical functionality by insulation deterioration. These considerations are expected to help in determining electrical degradation and acceptance criteria.

The test specimens are composed of 2 layers of flexible flat cable embedded in a host composite structure made of 4 harness-satin equilibrated woven fabric glass-epoxy composite. The flat cables are approximately 0,45mm thick while each composite lay-up is approximately 1,5mm thick, which results in an overall thickness of 5,4mm. The flat cables are inserted in the composite lay-up and the assembly is co-cured in an autoclave for 3 hours at 160°C. The outer stacking sequences of composite plies are [0/45/0/-45/45/0] s (top and bottom) and the inner stacking sequence is [-45/0/45/0/-45/0] s . The composite plies, flat electrical cables were manually laid up in this configuration while performing intermediate compacting and suitable surface treatment of the electrical cables (such as dry-cleaning) to ensure good interface properties between the composite and the cables.

In parallel, test specimens without embedded cables (called blank test specimen) are also manufactured to act as references.

Three points bending test

The 3 points bend tests are investigated in a first stage for a better understanding of the influence of the integration, through the structure, of flat electrical cables on the damage and critical strength. This test, in short bending configuration involves inter-laminar shear strains expected critical at the interfaces of the electrical cable layers.

Quasi-static short 3 points bending tests

Experimental results are compared on test specimens with and without embedded cables. The short 3 points bend tests (S3PB) are carried out according to normative reference ISO 14130. This loading is used for determination of apparent interlaminar shear strength of fibre-plastic composites by the short beam method. Hence, the width of the test specimens is chosen as 5 times the thickness and the length as 10 times the thickness. Besides, the electrical wires were either oriented in the transverse, or longitudinal direction regarding the specimen length to investigate on their influence on the bending stiffness of the structure (Figure . 3 ) The first observation that can be drawn is the highly non-linear behaviour of the multifunctional test specimens (transverse and longitudinal) as compared to the blank test specimens (sample ref. FDV3). Indeed, the hysteresis loops are much bigger for multifunctional specimens. Hysteresis loops indicates the presence of energy-dissipative mechanisms, which increase with increasing load. These energy-dissipative mechanisms are mainly of two natures: firstly probable friction between the test specimen and the test machine supports. Secondly, damage mechanisms within the composite structure (friction between the warp and weft fibres, and fibre/matrix debonding [START_REF] Asaad | Damage accumulation in hybrid woven fabric composites[END_REF]) and for multifunctional specimen, the inter-laminar shear deformation in the embedded cables and the associated damage which could be visually seen during test. These inter-laminar shear strains is revealed through Digital Image Correlations (DIC). In figure 5 where a qualitative measure of the inter-laminar strain field is presented for a displacement of the crosshead of d = 0.5mm. The two electrical cables involves a highly discontinuous load transfer between the three composite layups and a highly non-uniform distribution of the shear stresses through the thickness of the test specimen.

Interlaminar shear strains  xy observed through DIC on multifunctional S3PB test Tensile-shear failure of composite plies in bottom plies and embedded cables interface Fig. 5. Shear strain and failure distribution through the specimen thickness It is further noticed that as compared to the reference composite test specimens (FDV1, FDV2), the maximum displacement is significantly increased (by approximately 100%) and the maximum load significantly decreased (by approximately -30%), cf. Table 1 The initial bending stiffness of the composite embedding two layers of flat electrical cable varies between 5,000 and 6,000 N/mm which is lower than the bending stiffness of the blank specimens which was measured at 7,700 N/mm. The second main observation that can be drawn is the non-influence of the orientation of the wires on the mechanical response of the structure. This further confirms the assumption that the out-of-plane deformations are mainly transmitted through the insulator resin of the layer within the cables. Indeed, this is consistent with the observation made in tensile characterisation [START_REF] Walrick | Influence sur la tenue mécanique de l'insertion d'une fonction de protection électromagnétique au sein de composites à renforts de fibres de verre = Strength mechanical influence of electromagnetic shield function embedded in laminated glass composite[END_REF] where through-the-thickness tensile tests had revealed that the weakest interface within the structure was the insulator resin.

Dynamic short 3 points bending tests

The dynamic bending tests are performed on similar both blank and multifunctional test specimens. The bottom support was kept at 30 mm each other. A specific impact bending test setup was designed, figure 6. The setup was directly mounted on an Imatek IM10-30 drop-weight impact tester with two screwed connectors. The impactor and cylindrical test supports had cylindrical heads of 10 mm diameter, and the impactor total weight was 11,07 kg. Fig . 6. Impact bending device on the droop test In the literature, such test setups are not often encountered as impact analysis of composite laminates which are mainly performed on fully clamped specimens during drop weight impact tests [START_REF]AITM1-0010 -Airbus Test Method[END_REF]- [START_REF] Mathivanan | Experimental investigation of woven E-glass epoxy composite laminates subjected to low-velocity impact at different energy levels[END_REF]. Yet, some studies deal with the study of the dynamic flexural behaviour of woven composites [START_REF] Topac | Experimental and Numerical Investigation of Damage Process in Composite Laminates under Low-Velocity Impact[END_REF], [START_REF] Ullah | Dynamic bending behaviour of woven composites for sports products: Experiments and damage analysis[END_REF], [START_REF] Ivañez | FEM analysis of dynamic flexural behaviour of composite sandwich beams with foam core[END_REF]. Such a configuration is especially expected to provide a better understanding of the influence of boundary conditions on the dynamic response of the structure [START_REF] Ullah | Evolution and interaction of damage modes in fabric-reinforced composites under dynamic flexural loading[END_REF], and to allow performing Digital Image Correlations (DIC).

The impact energies were chosen based on the damage threshold which is considered as the impact energy at which first ply failure occurs on the back face of the test specimens: this resulted in impact tests between 12 Joules and 13 Joules. As the weight of the impactor was kept constant, these energies were achieved by adjusting the height of the impactor. For instance, to obtain an impact energy of 12 Joules with a mass of 11,07kg the droop height of the striker needs to be at 0,11m. Forcedisplacement curves for each test are recorded: the displacement is the striker displacement link with the mobile carriage coupled with piezoelectric force sensor (30 KN Kistler dynamic sensor). The impact events were recorded in 2D via a ultra-high speed camera from the side section of the specimens. The force-displacement curves of impact tests are performed on blank tests specimens (a) and multifunctional structure (b) are presented in Figure 7. The effective impact energies were measured between 12 and 13 joules. For each configuration, test specimens exhibited a similar behaviour as regards to the forcedisplacement curves. For blank specimens, all test specimens fail at lower peak contact loads (8kN) than the quasi-static test specimens (8,75 kN). The same downward trend is present for the multifunctional specimen like for the quasi-static campaign. For the blank specimens, the main failure mode is a ply failure under tension on the bottom face under bending. For multifunctional test, similarly to the quasi-static measurements, the force-displacement curves can be divided into two parts: the initial loading up to slipping/cohesive damage within the cables, and loading up to ultimate failure. The experimental load-displacement curve seems to be altered due to a non-homogeneous distribution of the load up to 1.0 as sown by DIC analyse.

Associated numerical progressive model

A model referenced MAT162 coded in LS-DYNA [START_REF]MAT162 User's Manual Version 15A-2015[END_REF] and implemented in ABAQUS via VUMAT is used for correlation with the which is based on the principle of progressive failure of Hashin [START_REF] Hashin | Failure criteria for unidirectional fibre composites[END_REF] and damage mechanics of Matzenmiller et al. [START_REF] Matzenmiller | A constitutive model for anisotropic damage in fiber-composites[END_REF]. The complete model uses a set of seven quadratic failure criteria related to fibre fracture, fibre crush, fibre shear, in-plane matrix crack and delamination. Among these seven failure criteria, only 5 are retained here:

Fibre tension-shear along direction 1 (Eq.1)

Fibre tension-shear along direction 2 (Eq.2)

Crush failure under compressive pressure (Eq.3)

In-plane matrix crack (Eq.4)

Through-the-thickness matrix crack & delamination (Eq.5)

With (Eq.6)

SSR stands for the compressive normal strain dependence of the shear strengths (this can be assimilated to the Mohr-Coulomb theory with the Coulomb's friction angle).

A positive value of a function fi denotes the initiation of a specific damage mode with a damage threshold ri. Damage propagation and post-damage softening behaviour is then controlled by an exponential function associated with a softening parameter mi (m1 controls the softening of tensile failure in direction 1, m2 the softening of tensile failure in direction 2, m3 the softening related to fibre crush and m4 the softening due to both in-plane and throughthe-thickness matrix cracking). A set of damage variables are then introduced to relate the onset and growth of damage to stiffness losses in the material according to a coupling matrix that relates a mechanical property to the five modes of damage considered. For instance, damage mode 1 (fibre tension-shear along direction 1) is related to E1, G12 and G13 while the delamination criteria is related to E33, G13 and G23. Experimentally, quasi-static short 3 point bend tests revealed that sliding occurs inside the cables under inter-laminar shear strains. To reproduce this behaviour, cohesive law interactions are defied at the interface between the cables and the composite plies.

The cohesive constitutive law used here is a classical bi-linear mixed-mode tractionseparation relationship [START_REF] Song | Guidelines and parameter selection for the simulation of progressive delamination[END_REF] which is already available within Abaqus CAE, Damage initiation is taken into account with a maximum stress criterion (Eq.7), and damage propagation is modelled with an energy-based Benzeggagh-Kenane (BK) criterion (Eq.8).

(Eq. 7)

(Eq. 8)

Where η is the BK material parameter, , and

The properties used to define the cohesive interactions are listed in Table 1

Correlation with experimental bend tests

Numerical models are achieved for short 3 points bend test in an explicit solver with the experimental setup. Solid finite elements are used for composite plies, embedded layers and interfaces between composite plies and embedded layer. Mat 162 as describe previously is associated for composite plies, with cohesive model for interface elements. Numerical results show below correspond to an impact of 13 Joules. Numerical correlations with experimental data help to define the best set of parameters to be used in cohesive zones. Under dynamic short 3 point bend test, as well quasi-static, the main strains fields through the sample thickness is mainly in tension at the bottom and inter-laminar shear between plies focused at the embedded cable layer interfaces. The strain gradient lead to local matrix damage initiation and delamination distribution similar to the experimental observations. The level of maximum inter laminar shear strain computed between -3.7% and 5% for displacement close to 2 mm seems similar to the level measured by DIC for experimental tests. Numerical Force history curves are compared to the experimental for 13 Joules impact bending. Some fluctuations are present in the dynamic experimental curves due to periodic oscillations along the striker at impact and transmitted to the force sensor. So, this phenomenon is due to the influence of a striker resonant frequency. To prevent loss of information for comparison, the curve is not filtered. The numerical force follows similar progression regarding experimental behaviour, but seems underestimate of 10 % the maximal amplitude. Global collapse occurs at the same time, more softly for experimental behaviour. 13). These tests are conducted with a constant impact mass of 9.067Kg at impact energies of 10J, 15J and 20J dropped from a height between 0,11m and 0,22m to reach the impact energies. The hemispherical hardened steel impactor has a diameter of 20mm. The contact force history of three test specimens with two layers of embedded cables impacted at 10J, 15J and 20 Joules are plotted in figure 13. As compared to the test specimens without embedded cables, these test specimens show a much more dissipative behaviour with lower peak contact and higher impact durations for the three impact energies. In particular, the multifunctional test specimens seem to exhibit a tri-linear behaviour, with two noticeable load drops. This tri-linear behaviour is most probably associated to both slipping and interlaminar damage among the two embedded cables similar to quasi-static short 3 point bend tests, and to initiation of matrix cracking and delamination among the host composite structure as observed previously.

As predict in numerical model, transverse shear strains seem to initiate inter-laminar damage and delamination at the interface of the flat cable layers (Fig. 15.a) and identify in the contact force histories (Fig. 15.b).

It is observed that the fibre tension-shear failure which seems to initiate at the interface with the embedded cables is a consequence of the sliding that occurs inside the embedded cables. Indeed, sliding among the cables first occurs and is well reproduced by the cohesive surface to-surface interaction. This triggers the first load drop observed experimentally at 0,8 ms. It induces higher interlaminar and tensile strains on the adjacent composite plies, which leads to tension-shear fibre fracture captured at approximately 1,6ms

Post impact dielectric strength tests.

In order to study the influence of such damage mechanisms on the electrical functionality of the structure, post-impact dielectric strength tests were performed. In addition to the impact tests presented above, some additional test specimens were also impacted at 25J. Dielectric strength measurements are performed on electrical components to determine the effectiveness of their insulation [START_REF]Aerospace series -Cables, Electrical, Aircraft use[END_REF][START_REF] Esa | ESCC Detail Specification No. 3901/001, Polyimide insulated wires and cables[END_REF]. The basic principle is to apply an increasing voltage between two electrical conductors until dielectric breakdown of the material occurs. The higher the dielectric strength the better the quality of the insulation.

Reference dielectric strength tests were first performed on non-embedded cables with dielectric strength tester. No electrical shorting occurs between any conductors up to 10kV, which is the maximum capacity of the machine.

Post-impact dielectric strength tests were then performed on every impacted test specimens [START_REF] Vogel | Investigations on the damage of multifunctional structures under low energy impacts[END_REF]. Different injection configurations were tested (between adjacent conductor on same layer and between superposed conductors), resulting in 13 dielectric strength tests per test specimens according to figure These considerations led to the results presented in figure 16.a. where the dieletric strength (breakdown voltage in kV) is plotted versus the impact energy. So, no electrical shorting ever occurred on any test specimens impacted at 10J and 15J. For test specimens impacted at higher impact energies, electrical shorting did occur among the embedded cables (figure 16.a). It always occurred between two conductors of the same cable (adjacent conductors of the top or bottom cable).

Conclusion

Representative multifunctional composite structure specimens embedding two layers of flat cables are achieved for experimental quasi-static and dynamic test. Experimental tests aim for study the impact response of these structures and particularly influence of the embedded layers on failure.

The structure appeared to be particularly weakened when loaded in bending static and dynamic due to the low shear stiffness of the embedded cables which induced higher internal shear strains. Ultimately, these additional shear strains led to induced both internal fiber failure of composite plies at interface with embedded cables, and de-cohesions within the cables. In addition they may result in insulation degradation between adjacent wires, and alter electrical performances.

A progressive damage model coupled to cohesive elements is defined for correlation with test results.

The model has the capability to get damage and failure mechanisms in accordance with experimental observation.

In addition, a methodology of characterization was developed to take the temperature-dependent stiffness through dynamic mechanical analyses into account [START_REF] Vogel | Investigations on the thermo-mechanical behaviour of a complex assembly through dynamic mechanical analyses[END_REF]. This work help to define both design and electrical degradation criteria for the development of multifunctional sandwich architecture with embedding electrical cables for aeronautic specifications.

Fig. 2 .

 2 Fig. 2. Configuration of the Short 3 Point Bend Tests

Fig. 3 .

 3 Fig. 3. Two configurations of multifunctional S3PB test specimens: (a) transverse and (b) longitudinal

Fig. 4 .

 4 Fig. 4. Short 3 Point Bend for cyclic Test

  (a) Without cables layer (blank tests) (b) With cables layer Fig. 7. Contact force-displacement curves for blank test specimen (a) and functional specimen (b)

  (a) Interlaminar shear strains at d=0.7 mm (b) Interlaminar shear strains at d=2.0 mm Fig . 8. Digital Image Correlations on (a) multifunctional and (b) blank test specimens (a) Transverse specimen (b) Longitudinal specimen Fig .9. Damage repartition for multifunctional specimen (a) transverse

  and have been determined by fitting the experimental results of the Short 3 Point Bend (S3PB) tests with numerical simulations.

Fig. 10 .

 10 Fig. 10. Bending multifunctional sample model with stacking sequence of solid elements

Fig . 12 .

 12 Fig . 12. Contact force history curves comparison

Fig 13 .

 13 Fig 13. Droop weight impact test setup and impacted specimen with 20 Joules

Fig . 14 .

 14 Fig . 14. Contact force histories of multifunctional and blank test specimens

Fig . 15 .

 15 Fig .15. Damage influence take into account by the numerical model

  16.b. (8 tests on adjacent conductors and 5 on superposed conductors). a. b.Measures are made between two flat cables. Fig . 16. (a) Results of the post-impact dielectric strength tests, (b) configuration of the post impact dielectric strength.

Table 1 :

 1 . Results of the S3PB test on the reference test specimens

	specimen	FDV1 FDV2 Trans 1 Trans 2 Long 1 Long 2
	Maximum Load (N)	8 600	8 900	5 991	5 912	5 360	5 841
	Maximum Displacement (mm)	1.20	1.25	2.44	2.36	2.44	2.40

Table 2 .

 2 Mechanical properties used to model the glass/epoxy woven fabric

	E11	E22	E33	v12	v13	v23	G12	G13	G23
	12 GPa	12 GPa	5,6 GPa	0,18	0,40	0,40	2,4 GPa	1,6 GPa	1,6 GPa
	X1T	X2T	X3C	S12	S23	S13	SFS	φ	m1
	650 MPa	650 MPa	750 MPa	85 MPa 65 MPa 65 MPa 250 MPa	10	0,5
	m2	m3	m4
	0,5	1	1
	E11	E22	E33	v12	v13	v23	G12	G13	G23
	19.5GPa	1.1 GPa	0,48 GPa	0,48	0,48	0,48	0,43 GPa 0,16 GPa 0,16 GPa

Table 3 .

 3 Mechanical properties of an electrical cable

Table 4 .

 4 Properties used to define the cohesive interactions at interface

Acknowledgements

The authors would like to thank Airbus Safran Launchers (ASL), Les Mureaux, France, for the funding of the research.


Jean-François Lutz
email: jflutz@unistra.fr

The Future of Sequence-defined Polymers

Keywords: Sequence-controlled polymers, Informational Polymers, Precision Polymers, Sequencing, Polymer Materials

The objective of this perspective article is to critically analyze the emerging domain of sequence-defined polymers and to discuss future challenges and trends in the field. Sequencedefined polymers are macromolecules, in which the monomer arrangement is perfectly controlled from one chain-end to the other. They therefore have a defined chain-length and primary structure. This article focuses mainly on synthetic polymers and does not provide an in-depth discussion about the chemical synthesis of biopolymers (i.e. nucleic acids and proteins) and biopolymer mimics (i.e. peptidomimetics and xeno-nucleic acids). Although the synthesis of non-biological sequence-defined polymers was initially a challenge, it has grown over the last fifteen into a mature discipline. Indeed, major progress has been made for the synthesis, characterization and application of these manmade macromolecules. Thus, in the first section of this article, a brief summary of the most important achievements is provided. For instance, synthesis by multistep-growth "polymerization", sequencing (i.e. complete sequence determination) and emerging applications (e.g. data storage, anti-counterfeiting technologies, catalysis, selective drug carriers) are discussed. The second section of this article provides an in-depth discussion about the future of the field. Despite enormous progress, there are still major limitations regarding the synthesis and applicability of sequence-defined polymers. Moreover, some important properties of sequence-defined polymers still have to be discovered. All these aspects are discussed in details herein and some priority topics are suggested for the future.

Introduction

The idea of placing each monomer at a defined position in a man-made macromolecule has emerged in the early 21 th century as an exciting new challenge in polymer chemistry. [START_REF] Lutz | Sequence-controlled polymerizations: the next Holy Grail in polymer science?[END_REF] Indeed, most of the synthetic copolymers prepared in the 20 th century contained uncontrolled (e.g. random) or poorly-controlled (e.g. blocky) monomer sequences. [START_REF] Lutz | Sequence-Controlled Polymers[END_REF] In fact, absolute sequence control was achieved by chemical means in the 20 th century but mostly for recreating in the laboratory sequence-defined biopolymers such as nucleic acids and proteins. [START_REF] Caruthers | Gene synthesis machines: DNA chemistry and its uses[END_REF][START_REF] Merrifield | Solid phase synthesis[END_REF] Some bioinspired sequenced-defined oligomers such as peptidomimetics [START_REF] Avan | Peptidomimetics via modifications of amino acids and peptide bonds[END_REF] and xeno-nucleic acids [START_REF] Chaput | What Is XNA?[END_REF] were also developed but only few attempts were made to develop other families of abiotic sequence-defined polymers (SDPs), as described in early reviews on the topic. [START_REF] Badi | Sequence control in polymer synthesis[END_REF][START_REF] Brudno | Recent Progress Toward the Templated Synthesis and Directed Evolution of Sequence-Defined Synthetic Polymers[END_REF] Starting about 15 years ago, research on non-biological SDPs has become more prominent. A broad variety of oligomers and polymers have been reported, as described in a number of recent reviews. [START_REF] Trinh | Synthesis of Monodisperse Sequence-Defined Polymers Using Protecting-Group-Free Iterative Strategies[END_REF][START_REF] Solleder | Recent Progress in the Design of Monodisperse, Sequence-Defined Macromolecules[END_REF][START_REF] Alabi | Iterative Synthetic Methods for the Assembly of Sequence-Controlled Non-Natural Polymers[END_REF][START_REF] Shi | Engineering precise sequence-defined polymers for advanced functions[END_REF] Additionally, some innovative properties as well as some emerging applications were rapidly identified for SDPs. [START_REF] Shi | Engineering precise sequence-defined polymers for advanced functions[END_REF][START_REF] Meier | A New Class of Materials: Sequence-Defined Macromolecules and Their Emerging Applications[END_REF][START_REF] Aksakal | Applications of Discrete Synthetic Macromolecules in Life and Materials Science: Recent and Future Trends[END_REF][START_REF] Destefano | Where Biology and Traditional Polymers Meet: The Potential of Associating Sequence-Defined Polymers for Materials Science[END_REF][START_REF] Van Genabeek | Properties and applications of precision oligomer materials; where organic and polymer chemistry join forces[END_REF] One of the major advantages of SDPs as compared to ill-defined copolymers is the possibility to include a functional monomer at a specific position. This can be used as a powerful strategy to tailor the intramolecular conformation (e.g. folding) as well as the intermolecular interactions (e.g. selfassembly, host-guest interactions, catalytic activity) of a macromolecule. Furthermore, a defined set of monomers can be used as a molecular alphabet to write and store information in polymer chains. [START_REF] Lutz | Coding Macromolecules: Inputting Information in Polymers Using Monomer-Based Alphabets[END_REF] Although such a concept is central in molecular biology, it was unexplored in synthetic polymer chemistry until 10 years ago. [START_REF] Lutz | Sequence-Controlled Polymers[END_REF][START_REF] Colquhoun | Information-containing macromolecules[END_REF] Its development has therefore rapidly enabled the discovery of new types of applied polymer materials.

Yet, the synthesis of SDPs remains overall very demanding. Indeed, it relies on multistep growth mechanisms, [START_REF] Nerantzaki | Multistep Growth "Polymerizations[END_REF] which are time-consuming and limited in yields, scale and chain-length. In this context, the aim of this short article is to critically analyze the possible future of this emerging area. Recent achievements will be described in a first section, followed by a discussion about future challenges and prospects in the field.

Emerging trends

Important achievements have been made for the preparation of SDPs during the last 15 years.

It shall be first reminded to the readers that the terms "sequence-defined" imply an absolute sequence control; meaning that the positioning of every monomer unit in a linear chain is perfectly controlled from one chain-end to the other. [START_REF] Lutz | Defining the Field of Sequence-Controlled Polymers[END_REF] It implies molecular uniformity and shall therefore be distinguished from the broader category of sequence-controlled polymers, [START_REF] Lutz | Sequence-Controlled Polymers[END_REF] which may exhibit some dispersity. Consequently, SDPs are generally synthesized using multistep growth "polymerizations", for example using stepwise approaches such as linear iterative growth (performed in solution or using soluble-or solid-supports), iterative exponential growth or dendrimer synthesis. These mechanisms have been described in details in a recent review and will not be re-explained in details herein. [START_REF] Nerantzaki | Multistep Growth "Polymerizations[END_REF] In brief, these syntheses are not performed in one-pot like classical chain-growth or step-growth polymerizations but using a series of discrete reactions steps. [START_REF] Lutz | From Precision Polymers to Complex Materials and Systems[END_REF] Thus, their syntheses are limited by several aspects. Since they rely on successive reactions and purifications, they usually require more time (e.g. from a few hours to several weeks depending on the chosen chemistry) than classical polymerizations. Furthermore, the global yield for polymer synthesis depends on the number of performed steps and therefore drops down with chain-length. It is therefore challenging to synthesize very large macromolecules in these approaches. In addition, the typical laboratory scale for the synthesis of these polymers is generally quite low (i.e. from mg to g). All these aspects also reflect on the cost of the formed polymers, which are much more expensive than classical commodity polymers. Despite all these limitations, significant progress has recently been made in the field as described in the following sections.

Chemodiversity

Multistep growth mechanisms allowing the synthesis of SDPs have been identified long time ago. [START_REF] Nerantzaki | Multistep Growth "Polymerizations[END_REF] However, as mentioned in the introduction, there were initially principally used for the synthesis of biopolymers or bio-related polymer mimics. This implies that most SDPs had phosphate-sugar or amide-based backbones. [START_REF] Brudno | Recent Progress Toward the Templated Synthesis and Directed Evolution of Sequence-Defined Synthetic Polymers[END_REF] One important step-forward of the last fifteen years has been the identification of a wide variety of chemistries that were previously not considered for the synthesis of sequence-defined polymer backbone. It is not the purpose of the present article to describe all these main-chain chemistries in details and the reader is guided towards recent reviews for an exhaustive description. [START_REF] Trinh | Synthesis of Monodisperse Sequence-Defined Polymers Using Protecting-Group-Free Iterative Strategies[END_REF][START_REF] Solleder | Recent Progress in the Design of Monodisperse, Sequence-Defined Macromolecules[END_REF][START_REF] Alabi | Iterative Synthetic Methods for the Assembly of Sequence-Controlled Non-Natural Polymers[END_REF] As selected examples, one may mention the use of: azide-alkyne cycloadditions, [START_REF] Pfeifer | Liquid-Phase Synthesis of Block Copolymers Containing Sequence-Ordered Segments[END_REF] thiolactone chemistry, [START_REF] Espeel | Multifunctionalized Sequence-Defined Oligomers from a Single Building Block[END_REF] Michael additions, [START_REF] Porel | Sequence-Defined Polymers via Orthogonal Allyl Acrylamide Building Blocks[END_REF][START_REF] Huang | Combining Orthogonal Chain-End Deprotections and Thiol-Maleimide Michael Coupling: Engineering Discrete Oligomers by an Iterative Growth Strategy[END_REF] multicomponent reactions, [START_REF] Solleder | Sequence Control in Polymer Chemistry through the Passerini Three-Component Reaction[END_REF] nitroxide radical coupling, [START_REF] Roy | Design and synthesis of digitally encoded polymers that can be decoded and erased[END_REF] activated carbonate chemistry, [START_REF] Gunay | Chemoselective Synthesis of Uniform Sequence-Coded Polyurethanes and Their Use as Molecular Tags[END_REF] Sonogashira coupling, [START_REF] Szweda | Synthesis of oligoarylacetylenes with defined conjugated sequences using tailor-made soluble polymer supports[END_REF] silane chemistry, [START_REF] Matsumoto | One-Pot Sequence-Controlled Synthesis of Oligosiloxanes[END_REF] Menshutkin reaction [START_REF] Zhao | Scalable Synthesis of Positively Charged Sequence-Defined Functional Polymers[END_REF], single unit radical addition, [START_REF] Xu | Single Unit Monomer Insertion: A Versatile Platform for Molecular Engineering through Radical Addition Reactions and Polymerization[END_REF] and carbone disulfide chemistry. [START_REF] Nanjan | Sequence-Defined Dithiocarbamate Oligomers via a Scalable, Support-free, Iterative Strategy[END_REF] This backbone chemodiversity has opened up new horizons in the field. Most of these chemistries were selected because they lead to high yields coupling steps, which is a prerequisite for multistep growth "polymerizations". [START_REF] Nerantzaki | Multistep Growth "Polymerizations[END_REF] However, some of them are also orthogonal to other functional groups and can therefore be combined to achieve protecting-group-free iterative synthesis. [START_REF] Trinh | Synthesis of Monodisperse Sequence-Defined Polymers Using Protecting-Group-Free Iterative Strategies[END_REF] The tolerance to other functional groups is also an advantage for side-chain functionalization. For example, as reported in this journal, [START_REF] Boukis | Data storage in sequence-defined macromolecules via multicomponent reactions[END_REF] multicomponent reactions allow combination of a broad variety of functional building blocks. Yet, one drawback of this increased chemodiversity is that it gives access to an extremely large pool of possible sequences. Consequently, the assessment of structure-properties relationships is an enormous task, which is by far much more complex than for simple copolymers such as block copolymers. [START_REF] Bates | Multiblock Polymers: Panacea or Pandora's Box?[END_REF] In an excellent recent review, Perry and Sing have discussed the emerging physics of SDPs and pointed out the challenge that constitutes such a vast parameter space. [START_REF] Perry | 100th Anniversary of Macromolecular Science Viewpoint: Opportunities in the Physics of Sequence-Defined Polymers[END_REF] It is obvious that, in addition to current synthetic efforts, theoretical studies and chemoinformatics will be mandatory in the future to address this broadening chemodiversity.

Characterization

During the last ten years, important progress has been made regarding the characterization of SDPs. As discussed above, there are two main features that SDPs do not share with classical copolymers: (i) an exact chain-length and (ii) a perfectly-controlled monomer sequence. [START_REF] Lutz | Aperiodic Copolymers[END_REF] All other molecular features such as chain-ends, tacticity and comonomer composition can be characterized using standard polymer analytics, for example using NMR. [START_REF] Miyoshi | Characterization of polymers by NMR[END_REF] For the characterization of chain-length and molecular uniformity, two main techniques are typically used namely size exclusion chromatography and mass spectrometry (electrospray or MALDI).

In addition, ion-exchange chromatography and gel electrophoresis can be used for the characterization of charged SDPs. [START_REF] Ouahabi | Synthesis of Monodisperse Sequence-Coded Polymers with Chain Lengths above DP100[END_REF][START_REF] Mondal | Large Sequence-Defined Supramolecules Obtained by the DNA-Guided Assembly of Biohybrid Poly(phosphodiester)s[END_REF] It shall be specified that polymers prepared by multistep growth "polymerizations" do not exhibit a molecular weight distribution like polymers prepared by chain-growth and step-growth polymerizations. When not perfectly uniform, these polymers exhibit a low molecular weight tailing that corresponds to incomplete reactions steps. Still, the most significant progress has been made for the sequencing of synthetic polymers. As underlined previously, [START_REF] Mutlu | Reading Polymers: Sequencing of Natural and Synthetic Macromolecules[END_REF] sequencing, which is the complete determination of a comonomer sequence from one chain-end to the other, shall be distinguished from classical sequence analysis that only gives a rough estimation of sequence motifs (e.g. dyads or triads). Initially, sequencing methods were only developed for the characterization of nucleic acids and proteins. Thus, one major achievement of the last decade has been the development of sequencing techniques allowing decryption of abiotic polymers. Comparing the content of the review recently published by Du Prez and co-workers [START_REF] Soete | Reading Information Stored in Synthetic Macromolecules[END_REF] to the one published by our group in 2014 [START_REF] Mutlu | Reading Polymers: Sequencing of Natural and Synthetic Macromolecules[END_REF] gives a good measure of what has been achieved. Currently, the most widespread analytical technique for the sequencing of non-biological polymers is tandem mass spectrometry (MS/MS). [START_REF] Wesdemiotis | Multidimensional Mass Spectrometry of Synthetic Polymers and Advanced Materials[END_REF] In this approach, sequence-defined macromolecules are subjected to collision-induced fragmentations and the resulting MS/MS spectra allow primary structure determination. [START_REF] Charles | MS/MS Sequencing of Digitally Encoded Poly(alkoxyamine amide)s[END_REF] One important feature that was highlighted during the last decade is that the broad diversity of reactions and building blocks that can be used for the synthesis of SDPs (see previous section about chemodiversity) allows the intentional design of polymers that can be easily sequenced by MS/MS. [START_REF] Cavallo | Orthogonal Synthesis of "Easyto-Read" Information-Containing Polymers Using Phosphoramidite and Radical Coupling Steps[END_REF] It is even possible to induce stepwise controlled fragmentations (e.g. MS 3 ) that allow deciphering of long polymer chains. [START_REF] Ouahabi | Mass Spectrometry Sequencing of Long Digital Polymers Facilitated by Programmed Inter-byte Fragmentation[END_REF] Furthermore, MS/MS and MS 3 spectra may be rapidly interpreted with the help of decoding softwares. [START_REF] Burel | MS-DECODER: Milliseconds Sequencing of Coded Polymers[END_REF][START_REF] Martens | Multifunctional sequence-defined macromolecules for chemical data storage[END_REF] As an interesting alternative to MS/MS fragmentations, SDPs may also be pre-fragmented by chemical degradation and the formed fragments may be analyzed using by simple MS [START_REF] Soete | Sequencing of Uniform Multifunctional Oligoesters via Random Chain Cleavages[END_REF] or LC-MS. [START_REF] Dahlhauser | Sequencing of Sequence-Defined Oligourethanes via Controlled Self-Immolation[END_REF] Another powerful method for the sequencing of non-biological polymers is nanopore sequencing. [START_REF] Hu | Nanopore Deciphering Single Digital Polymers Towards High-Density Data Storage[END_REF] It has been recently shown for the first time that non-biological oligomers can be decoded by this technique (Figure 1). [START_REF] Cao | Aerolysin nanopores decode digital information stored in tailored macromolecular analytes[END_REF] Yet, this promising approach is still underexplored.

Properties and applications

Perhaps one of the most important achievements of the last decades has been the identification of reliable applications for SDPs. These emerging applications have been reviewed and only a brief summary will be provided herein. [START_REF] Meier | A New Class of Materials: Sequence-Defined Macromolecules and Their Emerging Applications[END_REF][START_REF] Aksakal | Applications of Discrete Synthetic Macromolecules in Life and Materials Science: Recent and Future Trends[END_REF][START_REF] Van Genabeek | Properties and applications of precision oligomer materials; where organic and polymer chemistry join forces[END_REF][START_REF] Yang | Geared Toward Applications: A Perspective on Functional Sequence-Controlled Polymers[END_REF] First, very interesting discoveries have been made regarding the catalytic activity, [START_REF] Chandra | Sequence and Surface Confinement Direct Cooperativity in Catalytic Precision Oligomers[END_REF] molecular-recognition properties [START_REF] Celasun | Digging into the Sequential Space of Thiolactone Precision Polymers: A Combinatorial Strategy to Identify Functional Domains[END_REF] and guestinclusion properties [START_REF] Chandramouli | Iterative design of a helically folded aromatic oligoamide sequence for the selective encapsulation of fructose[END_REF] of SDPs. In such applications, not only the monomer sequence matters (i.e. the primary structure) but sometimes also the folding of the chain into a defined threedimensional shape (i.e. secondary or tertiary structure). Although the field of synthetic foldamers has been widely explored, [START_REF] Guichard | Synthetic foldamers[END_REF] there are few studies describing the folding of Another major development in the field has been the discovery of so-called informational polymers, in which the monomer sequence stores a readable message. [START_REF] Colquhoun | Information-containing macromolecules[END_REF] The type of information stored in the polymer can be computer data (e.g. text, image, video, audio files, program) or security data (e.g. identification number, security key, pin code, encrypted sequence). Thus, this new family of functional polymers has opened up applications in the field of data storage but also in the areas of anti-counterfeiting technologies, product identification, plastics recycling and steganography. For data storage applications (mostly cold storage and long-term archiving), the stored information is a binary sequence and the formed macromolecules are therefore named digital polymers. It can be written with an alphabet of two monomers but also using expanded alphabets containing more symbols. [START_REF] Lutz | Coding Macromolecules: Inputting Information in Polymers Using Monomer-Based Alphabets[END_REF] Since the first description of a binary-encoded abiotic oligomer in 2014, [START_REF] Trinh | Synthesis of Molecularly Encoded Oligomers Using a Chemoselective "AB + CD" Iterative Approach[END_REF] various types of digital polymers have been reported as described in a recent review. [START_REF] Yu | Digital synthetic polymers for information storage[END_REF] The decryption of digital polymers has also been significantly explored and improved during the last ten years (see section 2.2). [START_REF] Soete | Reading Information Stored in Synthetic Macromolecules[END_REF] Approaches for erasing, [START_REF] Roy | Design and synthesis of digitally encoded polymers that can be decoded and erased[END_REF] editing [START_REF] König | Photo-editable macromolecular information[END_REF][START_REF] Soete | Rewritable Macromolecular Data Storage with Automated Read-out[END_REF] digital polymers have also been reported. Here again, the chemodiversity of SDPs (see section 2.1) brings many opportunities for adjusting storage properties. Last but not least, since digital polymer cannot be of infinite chain-length (the current highest reported single chain capacity is 440 bits/chain [START_REF] Laurent | Storing the portrait of Antoine de Lavoisier in a Single Macromolecule[END_REF]), the spatial organization of library of digital polymers has also been investigated, as summarized in a recent overview published in this journal. [START_REF] Steinkoenig | Molecular access to multi-dimensionally encoded information[END_REF] For example, Zhang and coworkers have recently reported the preparation of a 2D microarray of digital polymers (Figure 3). [START_REF] Shi | Fabrication and Decryption of a Microarray of Digital Dithiosuccinimide Oligomers[END_REF] Besides data storage, various concepts have recently been reported for cryptography, [START_REF] Boukis | Multicomponent reactions provide key molecules for secret communication[END_REF] product identification [START_REF] Shi | Digital micelles of encoded polymeric amphiphiles for direct sequence reading and ex vivo label-free quantification[END_REF] and anti-counterfeiting applications. [START_REF] Gunay | Chemoselective Synthesis of Uniform Sequence-Coded Polyurethanes and Their Use as Molecular Tags[END_REF][START_REF] Holloway | From Sequence-Defined Macromolecules to Macromolecular Pin Codes[END_REF][START_REF] Zwillinger | Isotope Ratio Encoding of Sequence-Defined Oligomers[END_REF] In the latter case, sequencedefined oligomers are used as taggants that can be dispersed as traces in solid, gel or liquid materials. They can be extracted from the host material and decoded using MS/MS or other analytical techniques. A sequence-defined polymer taggant was recently commercialized under the name Poltag ® . The oligomers are loaded on the substrate using a robotic arm and decoded on the surface by MALDI MS/MS.

Reprinted with permission from reference [START_REF] Shi | Fabrication and Decryption of a Microarray of Digital Dithiosuccinimide Oligomers[END_REF]. Copyright Wiley VCH 2022.

Future vision

Challenges in synthesis

Despite the significant progress described in section 2, many challenges lie ahead for the design and synthesis of SDPs. First of all, there was no real mechanistic breakthrough during the last 15 years.

Step-by-step (or piece-by-piece) iterative synthesis is known for decades [START_REF] Nerantzaki | Multistep Growth "Polymerizations[END_REF] and is still the hard way to achieve absolute sequence-control. Interesting alternatives have been proposed such as template polymerizations, [START_REF] Samokhvalova | Macromolecular Information Transfer[END_REF] DNA-guided synthesis [START_REF] O'reilly | The Evolution of DNA-Templated Synthesis as a Tool for Materials Discovery[END_REF] and processing molecular machines. [START_REF] Van Dongen | Processive Catalysis[END_REF] However, these approaches remain currently even more complicated than stepwise synthesis and limited to the synthesis of oligomers. Using natural biocatalysts (e.g. enzymes, ribosome) and cell machinery is also an interesting strategy to synthesize SDPs. Protein engineering allows the use of non-canonical monomers but remains restricted to the synthesis of natural protein backbones. [START_REF] Miserez | Protein-Based Biological Materials: Molecular Design and Artificial Production[END_REF] To bypass such limitations, there was recently some interesting efforts for achieving ribosomal synthesis of other polymers than α-amino acidsbased proteins. [START_REF] Rogers | Ribosomal synthesis and folding of peptide-helical aromatic foldamer hybrids[END_REF] Still, these approaches remain for the moment limited to the synthesis of oligoamides.

Another major aspect that shall be improved in the future is the chain-length of SDPs.

As aforementioned, multistep growth approaches are limited by the yields of repeating steps. Thus, current iterative methods mainly allow the synthesis of oligomers or short polymer chains (i.e. between 2 and 20 monomer units). Although such sizes are sufficient for some applications (see section 2.3.), it would be important for the future of polymer chemistry to go beyond current boundaries. With the help of automated synthesis, capping chemistries and column purification, chain-length above hundred monomer units can be achieved [START_REF] Ouahabi | Synthesis of Monodisperse Sequence-Coded Polymers with Chain Lengths above DP100[END_REF] but it seems conceptually difficult to go above using a pure step-by-step approach. Thus, alternative concepts shall also be explored. One of them is the controlled ligation of preformed sequencedefined macromolecular segments. This is rather difficult to achieve by covalent chemistry because of the accessibility of reactive chain-ends in macromolecular reagents. However, some non-covalent strategies can also be considered. For instance, our group has recently reported the synthesis of large sequence-defined supramolecules by DNA-guided assembly of preformed bio-hybrid segments. [START_REF] Mondal | Large Sequence-Defined Supramolecules Obtained by the DNA-Guided Assembly of Biohybrid Poly(phosphodiester)s[END_REF] Another important aspect that has not been sufficiently explored is the tacticity of SDPs. [START_REF] Li | Discrete multifunctional sequence-defined oligomers with controlled chirality[END_REF][START_REF] Mertens | Stereocontrolled, multi-functional sequence-defined oligomers through automated synthesis[END_REF] The majority of non-biological SDPs that have been recently reported are either achiral or synthesized using racemic building-blocks, thus atactic. Although stereocontrol is not mandatory for some applications described above (e.g. information storage), it could be of major importance in other areas, in which precise chain folding is needed (e.g. enzyme mimics). Progress for preparing abiotic polymers that are both sequence-defined and stereocontrolled were described in a very recent review. [START_REF] Szweda | Sequence-and stereo-defined macromolecules: properties and emerging functionalities[END_REF] Another important progress would be to achieve absolute sequence-definition in supramolecular copolymers. [START_REF] Lehn | Dynamers: dynamic molecular and supramolecular polymers[END_REF] There were recently some interesting studies for rationalizing and controlling non-covalent copolymerizations. [START_REF] Adelizzi | Future of Supramolecular Copolymers Unveiled by Reflecting on Covalent Copolymerization[END_REF] However, the dynamic rearrangement of supramolecular backbones renders sequence control particularly challenging.

Applicability

In addition to the synthesis issues described in the previous section, the applicability and industrialization of SDPs remains an open challenge. Although some SDPs have already been commercialized, their production is still time-consuming and costly. It is probably clear to all readers that SDPs are not meant to become commodity polymers, and probably also not largescale specialty polymers. The commercial applications that can absorb the high cost of SDPs are more dealing with specialty additives, pharmaceutical industry (e.g. drug carriers) and information technologies. In such applications, the scale of synthesis is not a limiting factor. As discussed in a prior text, [START_REF] Nerantzaki | Multistep Growth "Polymerizations[END_REF] some iterative syntheses can be performed at the multi-kilo scale, which is sufficient for some specialty additives production. Yet, any development that shall simplify the production and characterization of SDPs will, of course, be beneficial for the field.

Such improvements may come from chemistry as discussed above but also from engineering (e.g. automation, parallelization, nanofabrication, microfluidics), which is currently an aspect that has been scarcely investigated. For instance, interesting strategies such as monomer recycling [START_REF] Shamout | Recovery, Purification, and Reusability of Building Blocks for Solid Phase Synthesis[END_REF] and fueled one-pot processes [START_REF] Cwynar | One-pot approach for multi-step, iterative synthesis of sequence-defined oligocarbamates[END_REF] (Figure 4) have already been reported for reducing the amount of reagents and solvents in iterative syntheses. In addition, automation is a crucial aspect to simplify the synthesis of SDPs. As long as SDPs will be prepared by stepwise chemical synthesis, it is hard to imagine industrial applications based on manual production.

Although sequence-defined peptides and oligonucleotides are routinely synthesized by automated synthesis and sold by numerous companies, only a minority of polymers described in section 2.1 were prepared using robotic instruments. Therefore, research on the automated synthesis of SDPs has to be significantly intensified. It may require the development of specific instruments allowing fast parallel synthesis. To this end, a synergy between synthetic chemists and chemical engineer seems mandatory. Similarly, the spatial organization of coded matter has to be further explored. This may include studies about the (co)crystallization and selfassembly of SDPs but also regarding micro-fabrication (e.g. development of 2D-or 3Dorganized nanomaterials). [START_REF] Steinkoenig | Molecular access to multi-dimensionally encoded information[END_REF]

Still to be discovered…

There are probably many more applications for SDPs than the ones described in Section 2.3.

Based on a simple analogy with biopolymers such as nucleic acids and proteins, it seems obvious that all the properties of manmade SDPs have not been discovered yet. Indeed, in biology, nucleic acids and proteins have many other properties and functions besides molecular recognition, biocatalysis and information storage. For instance, DNA can self-replicate, selfrepair, undergo Darwinian evolution and sustain life. RNA can act as a messenger and transfer its information to proteins. It cannot be excluded that man-made polymers may also exhibit comparable properties. This has been discussed in details in two recent essays. [START_REF] Lutz | Can Life Emerge from Synthetic Polymers?[END_REF][START_REF] Lutz | 100th Anniversary of Macromolecular Science Viewpoint: Toward Artificial Life-Supporting Macromolecules[END_REF] One important message of these prior texts is that a complex property such as artificial life can probably not be created by pure rational design and will require the development of directed out-of-equilibrium systems. However, the basic mechanisms of life-supporting polymers are pure fundamental polymer chemistry problems. For instance, mutations, which are the basic molecular events for Darwinian evolution, are simple monomer alterations in polymer chains.

Similarly, replication, translation and transcription, which are the basis for life transmission and biomaterials synthesis, are simple questions of macromolecular information transfer. [START_REF] Samokhvalova | Macromolecular Information Transfer[END_REF] All these events might be recreated in synthetic polymers and some rudimentary proofs-of-principles of artificial mutations, [START_REF] König | Photo-editable macromolecular information[END_REF] repair, [START_REF] Mondal | Damage and Repair in Informational Poly(N-substituted urethane)s[END_REF] hybridization, [START_REF] Leguizamon | Sequence-selective dynamic covalent assembly of informationbearing oligomers[END_REF] replication [START_REF] Núñez-Villanueva | Replication of Sequence Information in Synthetic Oligomers[END_REF] and translation [START_REF] Niu | Enzyme-free translation of DNA into sequence-defined synthetic polymers structurally unrelated to nucleic acids[END_REF] have already been reported. Hence, research on all these aspects shall be strengthened in the future. Accessing such properties in synthetic polymers could open up interesting perspectives for xenobiology (e.g. for understanding if other polymers than nucleic acids could sustain life) but also for human applications such as data storage.

Figure 1 .

 1 Figure 1. Nanopore sequencing of digitally-encoded SDPs. (a) Schematic representation of the single-channel recording experiment using a K238A aerolysin pore. (b) Molecular structure of the digital polymers. The color code correlates with the levels of current blockade shown in c. The letter A denotes adenine nucleotides. (c) Typical current recorded for the oligomer shown in b. (d) Raw current traces are first filtered (left) and then classified using a deep neural network (middle). The resulting confusion matrix for polymers encoding 4 bits is reported on the right. Adapted from ref [52].

Figure 2 .

 2 Figure 2. (a) Selective encapsulation of guest molecules (schematized in yellow) in a folded helical SDP (drawn in dark blue). (b) Molecular structure of the monomer units. (c) Molecular structures of monosaccharide guests. (d) Different monomer sequences that were studied. (e) Association constants measured for different host-guest combinations.Redrawn with permission of the authors.[START_REF] Chandramouli | Iterative design of a helically folded aromatic oligoamide sequence for the selective encapsulation of fructose[END_REF]

Figure 3 .

 3 Figure 3. Fabrication of microarrays of digital SDPs. (a) Schematic representation of DNA microarrays used in genomics. (b) Concept for the elaboration of microarrays of 4-bits digitally-encoded dithiosuccinimide oligomers.

Data availability.

Not applicable.