Sampada Bodkhe

Gabrielle Turcot

Frederick P Gosselin

Daniel Therriault

ONE-STEP SOLVENT EVAPORATION-ASSISTED 3D PRINTING OF PIEZOELECTRIC PVDF NANOCOMPOSITE STRUCTURES

Keywords: 3D printing, barium titanate nanoparticles, piezoelectric, polyvinylidene fluoride, sensors

The development of a 3D printable material system, possessing inherent piezoelectric properties, to fabricate integrable sensors in a single-step printing process without poling is of importance to the creation of a wide variety of smart structures. Here, we study the effect of addition of barium 2 titanate nanoparticles in nucleating piezoelectric β-polymorph in 3D printable polyvinylidene fluoride (PVDF) and fabrication of the layer-by-layer and self-supporting piezoelectric structures at micro to millimeter scale by solvent evaporation-assisted 3D printing at room temperature.

The nanocomposite formulation, obtained after a comprehensive investigation of composition and processing techniques, possesses a piezoelectric coefficient, d31, of 18 pC N -1 which is comparable to typical poled and stretched commercial PVDF film sensors. A 3D contact sensor that generates up to 4 V upon gentle finger taps demonstrates the efficacy of the fabrication technique. Our one-step 3D printing of piezoelectric nanocomposites can form ready-to-use complex-shaped, flexible and lightweight piezoelectric devices. When combined with other 3D printable materials, they could serve as stand-alone or embedded sensors in aerospace, biomedicine and robotic applications.

INTRODUCTION

Polyvinylidene fluoride (PVDF) is the most piezoelectric polymer. Although PVDF's piezoelectric properties are lower than those of ceramics, it scores over them in terms of the inherent advantages of biocompatibility, toughness, flexibility, formability, and higher fatigue life. [START_REF] Pan | Design and Fabrication of Self-Powered Micro-Harvesters: Rotating and Vibrated Micro-Power Systems[END_REF] Among the various crystalline phases of PVDF, the electroactive β-phase (all trans) with a planar zig-zag form 2 imparts the highest dipole moment, resulting in higher piezoelectric and ferroelectric constants. Unfortunately, PVDF commonly crystallizes into the thermodynamically stable non-polar α-phase upon solidification, which does not possess the desired piezoelectric properties. The attainment of β-phase thus requires either one or more physical transformations via application of heat (annealing), [START_REF] Seok Ju | Spin Cast Ferroelectric Beta Poly(Vinylidene Fluoride) Thin Films Via Rapid Thermal Annealing[END_REF] large mechanical strain (stretching), [START_REF] Sajkiewicz | Phase Transitions During Stretching of Poly(Vinylidene Fluoride)[END_REF][START_REF] Sencadas | Α to Β Phase Transformation and Microestructural Changes of Pvdf Films Induced by Uniaxial Stretch[END_REF][START_REF] Zhang | Phase Transformation Mechanisms and Piezoelectric Properties of Poly(Vinylidene Fluoride)/Montmorillonite Composite[END_REF] large electric fields (poling), [START_REF] Neumann | Polarization Distribution in Pvdf Obtained by Poling under Constant Current Condition, Electrical Insulation and Dielectric Phenomena[END_REF][START_REF]Poling and Characterization of Piezoelectric Polymer Fibers for Use in Textile Sensors[END_REF] or filler addition. [START_REF] Bodkhe | Beta-Phase Enhancement in Polyvinylidene Fluoride through Filler Addition: Comparing Cellulose with Carbon Nanotubes and Clay[END_REF][START_REF] Rajesh | Enhancing Beta-Phase in Pvdf through Physicochemical Modification of Cellulose[END_REF] Fillers like carbon nanotubes, [START_REF] Huang | Nanocomposites of Poly(Vinylidene Fluoride) with Multiwalled Carbon Nanotubes[END_REF][START_REF] Liu | Piezoelectric Properties of Pvdf/Mwcnt Nanofiber Using near-Field Electrospinning[END_REF] clay, [START_REF] Dillon | On the Structure and Morphology of Polyvinylidene Fluoride-Nanoclay Nanocomposites[END_REF] cellulose, 10 magnetic [START_REF] Andrew | Enhanced Ferroelectric Phase Content of Polyvinylidene Difluoride Fibers with the Addition of Magnetic Nanoparticles[END_REF] and piezoelectric nanoparticles (NPs) [START_REF] Jing | Magnetic and Dielectric Properties of Barium Ferrite Fibers/Poly(Vinylidene Fluoride) Composite Films[END_REF] have proven to be very efficient in preventing the reversion of β-phase to α-phase upon solidification. Addition of eco-friendly and biocompatible [START_REF] Ni | Flexible Nanogenerator Based on Single Batio3 Nanowire[END_REF][START_REF] Yuan | Biocompatible Nanogenerators through High Piezoelectric Coefficient 0.5ba(Zr0.2ti0.8)O3-0.5(Ba0.7ca0.3)Tio3 Nanowires for in-Vivo Applications[END_REF] piezoelectric ceramic filler -barium-titanate (BaTiO3 NPs) [START_REF] Mofokeng | Preparation and Properties of Pvdf Based Batio3 Containing Nanocomposites[END_REF][START_REF] Ye | Tetradecylphosphonic Acid Modified Batio3 Nanoparticles and Its Nanocomposite[END_REF][START_REF] Luo | Preparation and Electromechanical Properties of Pvdf Matrix Piezoelectric Composites Containing Highly Oriented Batio3 Whiskers[END_REF][START_REF] Zhao | High Output Piezoelectric Nanocomposite Generators Composed of Oriented Batio3 Nps@Pvdf[END_REF][START_REF] Nunes-Pereira | Energy Harvesting Performance of Batio3/Poly(Vinylidene Fluoride-Trifluoroethylene) Spin Coated Nanocomposites[END_REF] into PVDF has also shown to improve the ferro-and piezoelectric properties of PVDF without compromising its flexibility or strength. The incorporation of BaTiO3 NPs into PVDF has been attempted so far via sonication [START_REF] Zhao | High Output Piezoelectric Nanocomposite Generators Composed of Oriented Batio3 Nps@Pvdf[END_REF][START_REF] Valiyaneerilakkal | Poly (Vinylidene Fluoride-Trifluoroethylene)/Barium Titanate Nanocomposite for Ferroelectric Nonvolatile Memory Devices[END_REF][START_REF] Rajib | Temperature Influence on Dielectric Energy Storage of Nanocomposites[END_REF] and ball-milling [START_REF] Zhang | Dielectric Properties of Pvdf/Ag/Batio3 Composites[END_REF][START_REF] Lee | Enhanced Dielectric Constant of Polymer-Matrix Composites Using Nano-Batio3 Agglomerates[END_REF] approaches.

Commercially available piezoelectric sensors essentially possess flat or fiber-like configurations and hence are limited to the use of single piezoelectric coefficient only for electromechanical coupling. Three-dimensional (3D) printing of piezoelectric materials benefits from improved electromechanical response extractable from 3D designs. Such an improvement stems from the synergistic augmentation of both accessible strain and coupling between both the in-plane and out-of-plane piezoelectric coefficients, thereby increasing the voltage output and consequently the sensitivity of the device. [START_REF] Yiin-Kuen | All-Fiber Transparent Piezoelectric Harvester with a Cooperatively Enhanced Structure[END_REF] For a piezoelectric sensor to be efficient, it will have to be integrated into the end-component. A multimaterial 3D printing process can result in elimination of the additional manufacturing step of sensor attachment, thereby serving as a long-term solution. In all the previously developed techniques for 3D printing of piezoelectric polymers such as fused deposition modelling of PVDF, [START_REF] Lee | Electric Poling-Assisted Additive Manufacturing Process for Lead-Free Piezoelectric Device Fabrication[END_REF] optical printing of barium titanate (BaTiO3) in a photoliable polymer, [START_REF] Kim | 3d Optical Printing of Piezoelectric Nanoparticle-Polymer Composite Materials[END_REF] near-field electrospinning of PVDF, [START_REF] Luo | Direct-Write, Self-Aligned Electrospinning on Paper for Controllable Fabrication of Three-Dimensional Structures[END_REF][START_REF] Pu | Piezoelectric Actuation of Direct-Write Electrospun Fibers[END_REF] poling still dominates as an obligatory treatment. Further, the requirement of poling can be envisaged as a hindrance to achieve simultaneous integration of piezoelectric sensors during a multimaterial 3D printing process. The concern is aggravated when the adjacent material or substrate properties are adversely affected by the poling conditions (e.g., degradation or melting of the polymer structure). Therefore, the removal of the poling step will not only simplify the printing process but also at the same time allow: (i) multimaterial printing, (ii) conformal printing on sensitive materials or systems, (iii) new opportunities for creating piezoelectric structures with complex configurations.

In this work, we engineered a nanocomposite formulation to attain a combination of printability and piezoelectric properties close to that of commercial PVDF films without the requirement of poling. To achieve the nanocomposite formulation, we compared three different methods: ballmilling, extrusion mixing, and sonication, to incorporate varying concentrations of BaTiO3 NPs in PVDF. The ability of the nanoparticles to nucleate and further retain the β-phase in PVDF is studied through custom designed piezoelectric tests and available characterization techniques: scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier transform infrared photo-acoustic spectroscopy (FTIR-PAS), and impedance spectroscopy. The formulation showing the best piezoelectric properties is then utilized to form micro-to millimeter scale 3D features via solvent evaporation-assisted 3D printing. The process involves the dissolution of the nanocomposite into a highly volatile solvent followed by extrusion through a small nozzle under applied pressure. [START_REF] Guo | Solvent-Cast Three-Dimensional Printing of Multifunctional Microsystems[END_REF][START_REF] Guo | Properties of Polylactide Inks for Solvent-Cast Printing of Three-Dimensional Freeform Microstructures[END_REF] The rapid evaporation of the solvent results in retention of the desired shapes: layer-by-layer, self-supporting, and even freestanding structures. [START_REF] Guo | Solvent-Cast Three-Dimensional Printing of Multifunctional Microsystems[END_REF] The application of our work is highlighted in the form of a ready-to-use millimeter scale 3D contact sensor fabricated in a single printing step.

EXPERIMENTAL SECTION

Methods

A mixture of acetone (BDH) and dimethylformamide (DMF; Alfa Aesar) was chosen as the solvent system for polyvinylidene fluoride (PVDF; Sigma-Aldrich). DMF is considered the least hazardous among the solvents used for PVDF and has a boiling point of about 153 °C. 34 On the other hand, acetone has a low boiling point (~56 C) but cannot dissolve PVDF. The ratio of DMF:acetone was experimentally optimized to 40:60 to minimize the amount of DMF while facilitating a quicker shape retention during printing. Dimethyl sulfoxide (DMSO; Sigma-Aldrich; 65 g L -1 ) was added as a β-phase initiating agent. 35 PVDF (2 g) was sonicated with DMF (4 mL), acetone (6 mL), and DMSO (0.6 mL) for 20 min to form a 0.2 g mL -1 solution for printing. A similar solution was also used to fabricate a solution cast film for comparison.

Barium titanate nanocomposite preparation (0.2 g mL -1 nanocomposite in the solvent mixture)

Ball-mill mixing: A mixture of Barium titanate NPs (BaTiO3; 99.9% purity, 100 nm;

Nanostructured & Amorphous Materials Inc.; 0.2 g) and PVDF (2 g) was sonicated in DMF for 2 h. The solution was then mixed in a high energy shaker ball-mill (SPEX SamplePrep 8000 Series Mixer/Mill) at a rate of 1080 cycles per minute for 20 minutes. The number of zirconia balls for milling was chosen to match the weight of the solution. The solution was dried in a vacuum oven at 60 C for 12 h to remove all the DMF. The dry nanocomposite (2 g) was later sonicated with DMF (4 mL), acetone (6 mL), and DMSO (0.6 mL) until complete dissolution of the nanocomposite was achieved. For apparent viscosity tests and printing 3D structures, 2.5 g and 3.0 g of the nanocomposite were dissolved to form 0.25 and 0.3 g mL -1 solutions, respectively. Extrusion mixing: A mixture of BaTiO3 NPs (0.2 g) and PVDF (2 g) was fed into a 5 mL twin screw microextruder (Xplore) at 185 C and mixed for 5 min. A temperature of 185 C was maintained above the melting point of PVDF (Tm = 171 C) 36 at both the inlet and exit of the extruder. The screw speed was 200 rpm. The mixture of PVDF and the NPs (2 g) obtained in the form of the filaments from the extruder was sonicated with a mixture of DMF (4 mL), acetone (6 mL), and DMSO (0.6 mL) until complete dissolution of the nanocomposite was achieved. Sonication: BaTiO3 NPs (0.18 g) were added to DMF (4 mL) and sonicated for 3 h to achieve complete dispersion. PVDF (1.8 g) was completely dispersed separately in acetone (6 mL) via sonication. The two solutions were mixed, and DMSO (0.6 mL) was added and sonicated for 20 min.

Solvent evaporation-assisted 3D printing

PVDF and its nanocomposite solutions were poured into a syringe (3 mL) and placed into a pneumatically operated dispensing system (HP-7X, EFD) to apply precise pressures for printing, as depicted in Figure 1a. This dispensing system was mounted on a robotic head (I&J2200-4, I&J Fisnar Inc.) controlled by a commercial software (JR Points for Dispensing, Janome Sewing Machine) that further enabled the deposition of the inks on a movable stage. The geometry of the profile to be printed was communicated to the robotic arm as a series of coordinates through the software. The air pressure from the dispenser forced the material out of the syringe through a nozzle. As the material escaped the nozzle, the evaporation of the solvents provides rigidity to retain the designed geometry. To fabricate one-dimensional (1D) filaments, two-dimensional (2D) films, and 3D layer-by-layer and self-supporting structures, different solution concentrations (0.2, 0.25, and 0.3 g mL -1 ) were used with a metallic nozzle (inner diameter = 100 µm) and varying extrusion pressures, P (0.1 to 2 MPa) and robot speeds (0.5 -20 mm s -1 ). While fabricating films, the concentration of nanocomposite in the solvent mixture was kept at 0.2 g mL -1 and the printing parameters were as follows, unless mentioned otherwise: nozzle inner diameter = 100 µm; P ~ 1 MPa; robot speed = 20 mm s -1 .

Characterization

To fabricate the sensors, a thin conductive electrode layer was painted on both sides of the printed nano-composite films and the internal and external surfaces of the 3D cylinder with commercially available conductive silver paste (Sigma-Aldrich). Electrical connections were made using a conducting aluminum tape (3M).

To determine the linearity of the sensors, a dynamic mechanical analyzer (DMA; Q800; TA instruments) was further used in a single frequency (45 Hz) tension mode with a multistress force ramp of 0.1 N until fracture (rate of 0.5 N min -1 ). The sensors consisting of films (20  5  0.1 mm) with electrodes (10  5 mm) were used with a grip separation of ~ 10 mm. Three samples were tested for each concentration of 5, 10, and 15 wt.% of NPs in the nanocomposites.

Piezoelectric charges obtained from all the sensors were converted into voltages using a charge amplifier (Piezo lab amplifier, MEAS Specialties) in charge mode acquired with an NI-9239 data acquisition systemattached to a USB carrier NI-9162 (National Instruments) and recorded using a LabVIEW interface.

To obtain the diffractogram patterns of the films, a Philips X'pert diffractometer was used with scan angles from 10° to 50°. A scan rate of 0.4 ° min -1 was used with a Cu target and K radiation of 50 kV and 40 mA. FTIR-PAS (FTS 6000 spectrometer, Biorad) was used to obtain the absorption spectra of PVDF and its nanocomposite films in the range of 500 -4000 cm -1 . 128 scans were conducted on each sample with a resolution value of 8 cm -1 .

For viscosity characterization, five identical filaments were printed (nozzle diameter = 100 µm; speed = 0.5 mm s -1 ; printing time =120 s) at different pressures (0.1 -2 MPa) using 0.20, 0.25, and 0.30 g mL -1 of ball-milled PVDF/ BaTiO3 nanocomposite in the solvent mixture. End effects were neglected as the length to diameter (L/D) ratio of the nozzle was greater than 50. [START_REF] John | Elements of Rheology[END_REF] The weights of the filaments were determined after 48 h to ensure complete evaporation of solvents. Mass flow-rates were calculated by reverting to the original mass before solvent evaporation.

The apparent viscosity calculation details are provided in the Supporting Information and in Bruneaux et al. [START_REF] Bruneaux | Micro-Extrusion of Organic Inks for Direct-Write Assembly[END_REF] To image structures printed with the nanocomposites, optical microscopy (BX-61 Olympus microscope; Image-Pro plus V5, an image processing software from Media Cybernetics) was used. The morphology and cross-sections of the 3D printed films and structures were observed using a field emission scanning electron microscope (FE-SEM; JEOL JSM-7600TFE). A SEM (JEOL JSM 840) was used to obtain the images of the 3D printed structures at lower magnifications. Both SEMs were operated at an accelerating voltage of 10 kV. For viewing the cross-sections, the fibers and films were fractured in liquid nitrogen. All of the structures and films were gold-coated before imaging.

RESULTS AND DISCUSSION

Figure 1a is a schematic of the one-step solvent evaporation-assisted 3D printing of a piezoelectric nanocomposite contact sensor. The 70-layer cylindrical sensor shown in Figure 1b was printed with 0.25 g mL -1 of 10 wt.% BaTiO3 NP ball-milled nanocomposite solution. The programmed diameter and height of the cylinder were 5 and 7 mm, respectively. After solvent evaporation, the printed cylinder shrunk to ~ 3.1 mm in both height and outer diameter. The 3D contact sensor was used right after printing without the need for a poling step. The sensor was gently tapped with a forefinger (Figure 1b) five times consecutively, and the generated signal can be seen in Figure 1c. The sensor produced a consistent maximum voltage output of ~ 4 V for touching and ~ -2 V upon releasing (see Figure S7 for the detailed mechanism). The 3D sensor produced voltage approximately ten times higher than that of a film sensor of the same electrode surface area (Figure S6). The high voltage outputs from the 3D sensor presented here result from the improved inherent piezoelectric properties of PVDF by extrusion-based printing and the addition of BaTiO3 NPs. The contribution from interfacial charges to the response of the 3D sensor can be neglected as both the filler and the polymer used in this case are dielectrics, [START_REF] Kontos | Electrical Relaxation Dynamics in Tio2 -Polymer Matrix Composites[END_REF] and nanoscale particles have a negligible contribution towards Maxwell-Wagner effects. [START_REF] Roy | Polymer Nanocomposite Dielectrics-the Role of the Interface[END_REF] Literature also suggests that addition of pristine BaTiO3 nanoparticles to PVDF does not result in interfacial polarization. [START_REF] Zhang | Enhanced Dielectric Properties of Poly(Vinylidene Fluoride) Composites Filled with Nano Iron Oxide-Deposited Barium Titanate Hybrid Particles[END_REF] In the following paragraphs, the effect of four parameters on the piezoelectric properties of PVDF are investigated: (i) extrusion-based printing, (ii) filler addition, (iii) processing techniques, and finally (iv) filler concentration. We use XRD and FTIR-PAS to study the effect of solvent evaporation-assisted 3D printing of PVDF on the conversion of its α-phase into βphase. [START_REF] Bodkhe | In Three-Dimensional Printing of Polyvinylidene Fluoride Nanocomposites[END_REF] The XRD patterns of solution-cast and 3D printed PVDF films prepared by using 0.20 g mL -1 PVDF in the solvent mixture can be observed in Figure 2a. Peaks pertaining to the α-phase of PVDF at 18.70º, 20.04º and 26.50º are clearly visible in case of the film fabricated via solution casting route, [START_REF] Dillon | On the Structure and Morphology of Polyvinylidene Fluoride-Nanoclay Nanocomposites[END_REF][START_REF] Martins | Electroactive Phases of Poly(Vinylidene Fluoride): Determination, Processing and Applications[END_REF] whereas a single distinct peak at 20.26º corresponding to the β-phase confirms the effect of the extrusion pressures in the attainment of β-phase. As the polymer chains come out of the nozzle, the extrusion pressure of ~1 MPa leads to the significant phase transformation, and the passage through the relatively long slender nozzle (12.7 mm; L/D ratio = 127) can align the polymer chains. We then carried out FTIR on the two kinds of films discussed above. Bands at 763 and 974 cm -1 are known to represent the presence of α-phase, and those at 512, 840, and 1279 cm -1 the β-phase in PVDF. [START_REF] Huang | Electrospinning of Polyvinylidene Difluoride with Carbon Nanotubes: Synergistic Effects of Extensional Force and Interfacial Interaction on Crystalline Structures[END_REF][START_REF] Yu | Formation Mechanism of Β-Phase in Pvdf/Cnt Composite Prepared by the Sonication Method[END_REF][START_REF] Shah | Dramatic Enhancements in Toughness of Polyvinylidene Fluoride Nanocomposites Via Nanoclay-Directed Crystal Structure and Morphology[END_REF] As seen from a large drop in absorbance at ~763 cm -1 in Figure 2b, the fraction of β-phase, F(β), considerably increased upon 3D printing.

The F(β) in the 3D printed film (64%), calculated using Beer-Lambert's law (Equation S2), was found to be ~30 % higher than that in the solution-cast film (49%).

To further improve the F(β) in PVDF, we incorporated 10 wt.% BaTiO3 NPs into the PVDF matrix. We compared three mixing methods-ball-milling, extrusion, and sonication-to determine which one had the most favorable impact on the piezoelectric voltage outputs resulting from the application of a dynamic mechanical load using the DMA. As the film-based sensors were subjected to longitudinal loads (direction 1), charges were generated across the thickness (direction 3) of the film (Figure S2c). The maximum voltage output was obtained with the sensors fabricated from the ball-milled nanocomposites (Figure S3a). For these sensors, the peak-to-peak voltage saturated at 6 V for frequencies higher than 35 Hz. In comparison, the sensors fabricated by extrusion and sonication failed to give outputs distinguishable from the noise. In addition, there was no voltage output from a sensor fabricated by ball-mill mixing of 10 wt.% BaTiO3 NPs with polylactic acid (PLA), which ascertains the fact that this process works due to the enhancement of β-phase in PVDF and not due to the piezoelectric properties of the NPs. Additionally, BaTiO3 is ferroelectric in nature and rendered piezoelectric only upon electric poling. This strengthens the fact that the piezoelectric response obtained in our material is purely from PVDF and has no contribution from the nanoparticles. [START_REF] Dineva | Dynamic Fracture of Piezoelectric Materials: Solution of Time-Harmonic Problems Via Biem[END_REF] We employed SEM and XRD characterization to understand why solely the ball-milled nanocomposites exhibited piezoelectricity. Figure 3a shows SEM image of BaTiO3 NPs. The size of the NPs varies between 80 and 100 nm. The SEM images in Figure 3b-d show the morphology of the nanocomposites films. In the case of ball-milled nanocomposites (Figure 3b), large crystallites and pore sizes reveal higher crystal growth; a uniform dispersion of nanoparticle agglomerates (highlighted by circles) is also seen. It has been reported that dispersed agglomerates of NPs lead to an increased space charge density which in turn leads to higher piezoelectric properties. [START_REF] Niu | Enhanced Dielectric Performance of Batio3/Pvdf Composites Prepared by Modified Process for Energy Storage Applications[END_REF] Non-uniformly distributed NP clusters are observed in the extruded nanocomposite (Figure 3c). There is more nucleation of crystallites in the sonicated nanocomposite (Figure 3d) as smaller spherulites are evident. Higher nucleation prohibits the growth of crystallites. [START_REF] Bodkhe | Beta-Phase Enhancement in Polyvinylidene Fluoride through Filler Addition: Comparing Cellulose with Carbon Nanotubes and Clay[END_REF] The uniform dispersion of nanoparticles (seen as tiny white dots) in the sonicated nanocomposite is possibly the reason for this higher nucleation. Large micrometersized pores are present throughout the surface (Figure 3b) and cross section (Figure S4) of ballmilled nanocomposite films, whereas internal pores are very small or absent in the extrusionmixed and sonicated nanocomposites. The highly porous structure of ball-milled nanocomposites resulted in lower modulus as found by the mechanical tests (Figure S1b). Increased flexibility allows for soft sensor applications and reduced interference with sensed structures.

Figure 4a shows the X-ray diffractogram of the BaTiO3 NPs with characteristic peaks of BaTiO3 at 22.15º, 31.53º, 39º and 45º. [START_REF] Nayak | Facile Preparation of Uniform Barium Titanate (Batio3) Multipods with High Permittivity: Impedance and Temperature Dependent Dielectric Behavior[END_REF] The splitting of peaks belonging to the (200) and (002) planes at ~45º indicates the presence of tetragonal crystal structure, which is essential for spontaneous polarization and thus, piezoelectricity. [START_REF] Zhao | High Output Piezoelectric Nanocomposite Generators Composed of Oriented Batio3 Nps@Pvdf[END_REF][START_REF] Dineva | Dynamic Fracture of Piezoelectric Materials: Solution of Time-Harmonic Problems Via Biem[END_REF][START_REF] Yamashita | Synthesis of Ruo2-Loaded Batino2n + 1 (N = 1, 2 and 5) Using a Polymerizable Complex Method and Its Photocatalytic Activity for the Decomposition of Water[END_REF] Figure 4b shows the X-ray diffractogram of PVDF nanocomposites prepared by ball-milling, extrusion, and sonication. In the nanocomposite prepared by the ball-milling approach, the peaks at 18.70º, 20.04º and 26.50º belonging to αphase of PVDF are absent. [START_REF] Dillon | On the Structure and Morphology of Polyvinylidene Fluoride-Nanoclay Nanocomposites[END_REF][START_REF] Martins | Electroactive Phases of Poly(Vinylidene Fluoride): Determination, Processing and Applications[END_REF] The presence of a single broad peak at 2-theta value of 20.26º corresponding to the (200) and (110) planes [START_REF] Dillon | On the Structure and Morphology of Polyvinylidene Fluoride-Nanoclay Nanocomposites[END_REF][START_REF] Martins | Electroactive Phases of Poly(Vinylidene Fluoride): Determination, Processing and Applications[END_REF] of β-phase underscores the inference that nanocomposite films prepared by ball-milling approach crystallized only in β-phase. In the case of the samples prepared by sonication, visible α-peaks are found at 18.7º. [START_REF] Dillon | On the Structure and Morphology of Polyvinylidene Fluoride-Nanoclay Nanocomposites[END_REF][START_REF] Martins | Electroactive Phases of Poly(Vinylidene Fluoride): Determination, Processing and Applications[END_REF] A prominent αpeak at 40º and a γ-peak close to 26º is seen in the extrusion-mixed sample. [START_REF] Thakur | Novel Polymer Nanocomposites from Bioinspired Green Aqueous Functionalization of Bnnts[END_REF][START_REF] Gaur | Thermally Stimulated Discharge Current (Tsdc) Characteristics in Β-Phase Pvdf-Batio3 Nanocomposites[END_REF][START_REF] Pramoda | Crystal Transformation and Thermomechanical Properties of Poly(Vinylidene Fluoride)/Clay Nanocomposites[END_REF] Uniformly dispersed individual nanoparticles in sonicated films were probably not as efficient as the agglomerates present in the ball-milled nanocomposites in preventing the reversion to α-phase during the crystallization process. High temperature shear during extrusion shifted the BaTiO3 tetragonal peak at 45º, depicting a change from the tetragonal structure of BaTiO3 to a nonpiezoelectric cubic structure. [START_REF] Lin | Formation of Pvdf-G-Hema/Batio3 Nanocomposites Via in Situ Nanoparticle Synthesis for High Performance Capacitor Applications[END_REF][START_REF] Sánchez | Influence of Batio3 Submicrometric Particles on the Structure, Morphology, and Crystallization Behavior of Poly(Vinylidene Fluoride)[END_REF] This may be linked to the temperature used for melt-mixing, i.e. 185 ºC, which is higher than the BaTiO3's Curie temperature of 120 ºC. [START_REF] Fu | Role of Ca Off-Centering in Tuning Ferroelectric Phase Transitions in Ba(Zr,Ti)O3system[END_REF] As there is a change from noncentrosymmetric to a symmetric crystal lattice, the melt-mixed nanocomposites could not exhibit piezoelectricity. The properties of BaTiO3 NPs seem to be unaltered by the ball-milling process as no change in their peak locations is observed.

After determining that ball-milling is the most appropriate mixing method in terms of piezoelectric properties, three different nanoparticle loadings (5, 10, and 15 wt.%) were investigated to determine the optimum filler weight fraction to fabricate 3D sensors. The voltage responses from the single frequency scans observed in Figure 5 are linear with the force until fracture, as should be the case with piezoelectric materials. The voltage outputs initially increased from 5 wt.% of BaTiO3 to 10 wt.%. However, the nanocomposite sensors with 15 wt.% of the fillers exhibited no considerable output. A maximum peak-to-peak voltage around 2.8 V was obtained at a force of 2.7 N for the sensor with 10 wt.% fillers.

Impedance spectroscopy was used to calculate the dielectric constants of the sensors. The nanocomposites with 10 wt.% NP loading possessed the highest dielectric constant of ~210 (Table S2). The piezoelectric coefficient is the ratio of electric field produced in a piezoelectric material to the applied stress. Average piezoelectric coefficients d31 = 2.5 and 18 pC N -1 were obtained for ball-milled nanocomposite sensors with 5 and 10 wt.% BaTiO3 NPs, respectively (calculated using the voltage values as in Figure 5 and Equation S1). The value of 18 pC N -1 , obtained here without any poling is comparable to that of typical commercial stretched and poled PVDF-based film sensors (23 pC N -1 ). [START_REF] Division | Piezo Film Sensors Technical Manual[END_REF] We further use the results from SEM, XRD, and FTIR to understand the performance of our ballmilled nanocomposites. Figure 6a-c are SEM images of 5, 10, and 15 wt.% of BaTiO3 NPs in PVDF via ball-milling. Fewer number of agglomerates are found in the case of the 5 wt.% loading compared to that found in the 10 wt.%. Although the number of agglomerates is increased in the 15 wt.% loading, their average size is almost halved in comparison to the 10 wt.% loading. This establishes that there has to be an optimum size and number of agglomerates to effectively prevent the reversion to α-phase. The XRD results (Figure S5) depicted that the films prepared with 10 wt.% NP loading exhibited no other phases than β-phase, while the other two films crystallized additionally in α-or γ-phase or both. FTIR-PAS was employed to explain the difference between the piezoelectric constants of the 5 and 10 wt.% nanocomposites. Figure 6d shows the FTIR spectra of the 5 and 10 wt.% ballmilled nanocomposite films. A clear decrease in the content of α-phase can be observed in the curve pertaining to 10 wt.% nanoparticle loading. Beer-Lambert's law was used to quantify this difference in the amount of the two phases in the nanocomposites. The F(β) present in the 5 and 10 wt.% ball-milled nanocomposites was 68 and 78%, respectively (Equation S2). The higher value of F(β) in the 10 wt.% nanocomposites explains the improved response and higher piezoelectric coefficient value. The Beer-Lambert law could not be applied to the 15 wt.% loaded nanocomposite (data not shown) due to the presence of additional γ-phase as seen from the XRD (Figure S5). Piezoelectric fillers do not contribute to the overall piezoelectricity of the nanocomposite when used in very small quantities, which was the case until 10 wt.% loading. [START_REF] Gan | Effect of Tio 2 on Enhanced Pyroelectric Activity of Pvdf Composite[END_REF] Hence, the increased presence of β-phase in PVDF upon the addition of 10 wt.% NPs via ball-milling is attributed to the mechanical activation of the NPs and their dispersed agglomerates, leading to an increased dipole moment. [START_REF] Lee | Enhanced Dielectric Constant of Polymer-Matrix Composites Using Nano-Batio3 Agglomerates[END_REF][START_REF] Niu | Enhanced Dielectric Performance of Batio3/Pvdf Composites Prepared by Modified Process for Energy Storage Applications[END_REF] Further, given that the piezoelectric coefficients of the filler and the polymer have different signs, poling in a single direction would cancel out the d31 coefficients. [START_REF] Ploss | Poling Study of Pzt/P(Vdf-Trfe) Composites[END_REF] Hence, our approach to orient dipoles without the application of any electric field serves to improve the inherent piezoelectric properties in PVDF at low mass fraction. Lower effective Young's modulus in the case of the ball-milled nanocomposites as compared to that in the other two nanocomposites (Figure S1b) resulted in increased sensitivity. [START_REF] Sirohi | Fundamental Understanding of Piezoelectric Strain Sensors[END_REF] However, as the process relies on improving the inherent piezoelectric properties of the polymer, factors such as humidity, temperature, print directions, speeds, and pressures can have a strong effect on the properties and hence reproducibility. Thus, efforts are being made to optimize these parameters and will form a part of our future work.

We propose the following hypothesis to summarize the process of formation of aligned β-phase chains during 3D printing. As seen in Figure 7, the addition of BaTiO3 NPs to PVDF via ballmilling led to increased activation sites [START_REF] Pavlović | Structural Properties of Composites of Polyvinylidene Fluoride and Mechanically Activated Batio 3 Particles[END_REF][START_REF] Pavlović | Influence of Mechanical Activation on the Structure of Ultrafine Batio3 Powders[END_REF] on the NPs acting as β-phase nucleating sites because of the strong O _ H … F _ C hydrogen interaction at the PVDF/BaTiO3 interfaces. [START_REF] Mendes | Effect of Filler Size and Concentration on the Structure and Properties of Poly(Vinylidene Fluoride)/Batio3 Nanocomposites[END_REF] These interactions combined with the dipolar interactions of polar DMF and PVDF resulted in CH2 _ CF2 dipoles as in the case of all trans structure of β-phase. Moreover, the dipole alignment in our work is attained through mechanical means and not from conventional electrical poling. [START_REF] Nunes-Pereira | Energy Harvesting Performance of Piezoelectric Electrospun Polymer Fibers and Polymer/Ceramic Composites[END_REF] The extensional flow of the solution resulted in the required chain and dipole alignment. [START_REF] Kraft | Polymer Orientation[END_REF][START_REF] Yao | High Strength and High Modulus Electrospun Nanofibers[END_REF] Indeed, the high pressures (~1 MPa) during the passage through the fine nozzle with a high L/D ratio (~120) and post-extrusion stretching due to higher velocities of the robotic motion (~20 mm s -1 ) contributed to align the PVDF polymer chains. The sub-micron sized agglomerates further prevented any chances of reversion to α-phase. [START_REF] Roy | Polymer Nanocomposite Dielectrics-the Role of the Interface[END_REF] In the absence of electric poling, the ferroelectric BaTiO3 NPs could not contribute to the improved piezoelectricity.

The shape and thickness of the extruded material from the nozzle during the printing process predominantly depends on the relationship between the applied pressure and the relative velocity of the robotic arm over the stage. Thus, to attain the required geometry, it is essential to arrive at the optimal relationship between the extrusion pressure and the velocity of the arm movement via a rheological analysis. [START_REF] Guo | Properties of Polylactide Inks for Solvent-Cast Printing of Three-Dimensional Freeform Microstructures[END_REF] Rheological characterization allows us to establish the dependence of apparent viscosity on the extrusion pressure and the printing speed. The former is proportional to the shear stress at the wall, whereas the latter relates to the shear rate during the printing process. Figure 8 shows the process-related viscosity as a function of the applied shear rates. The viscosity range for the three different concentrations (0.2, 0.25, and 0.3 g mL -1 of ball-milled PVDF/ BaTiO3 nanocomposite) is found to be between 0.1 and 2.7 Pa s. The viscosity decreases with increasing shear rate, characteristic of a shear thinning behavior. [START_REF] Carreau | Rheology of Polymeric Systems: Principles and Applications[END_REF] This property is essential for a material to be employed in extrusion-based printing processes. The optimal viscosity for a material is low enough to be extruded out of the small nozzle but at the same time high enough for shape retention after extrusion. [START_REF] Compton | 3d-Printing of Lightweight Cellular Composites[END_REF] Thus, the rheological properties of the inks are well-suited for 3D printing. Moreover, it can be observed that the viscosity of the inks significantly increases with the concentration of the nanocomposites in the solutions aiding the fabrication of 3D selfsupporting structures due to the increased rigidity.

The viscosity study aided in determining the optimum concentrations of the ball-milled PVDF nanocomposite required to obtain 1D, 2D, and 3D shapes, as shown in Figure 9. We can see a SEM image of the cross-section of a 1D filament (Figure 9a) extruded using 0.3 g mL -1 nanocomposite solution with a 100 µm nozzle at ~2.6 MPa. The average diameter of the fiber after complete evaporation of the solvents was about 56 ± 6 µm. The reduction in diameter of the fiber and the rough surface upon extrusion is attributed to evaporation of the solvents postextrusion. A loading of 0.2 g mL -1 of nanocomposite in the solution mixture was observed to be optimal for printing 2D films and patterns without voids or gaps. A 2D pattern (LM2, Figure 9b; the abbreviation of our laboratory) was printed using 0.2 g mL -1 solution with a pressure of ~0.11 MPa and robot velocity of 10 mm s -1 . A concentration of 0.25 g mL -1 facilitated the formation of 2.5D layer-by-layer structures: a circular cylinder (Figure 9c) printed at ~0.48 MPa and 1 mm s -1 . The cylinder has 70 layers and a programmed diameter and height of 5 and 7 mm, respectively. The concentration of 0.3 g mL -1 of the nanocomposite was best to print spanning or self-supported structures such as 3D scaffolds. A 9-layer scaffold (Figure 9d) was printed at ~1.3 MPa and 13 mm s -1 using a 100 µm nozzle with clearly distinguishable self-supporting filaments and individual layers. The spanning distance between two consecutive parallel filaments is 10 mm.

CONCLUSION

Fusion of the solvent evaporation-assisted 3D printing and our piezoelectric material results in two critical achievements: (i) genesis and stabilization of electroactive β-phase, leading to remarkable sensing and actuation characteristics at room temperature, and (ii) optimized rheology to fabricate self-supporting structures with satisfactory mechanical robustness, all without the need for additional electromotive (poling) treatment. High dielectric and piezoelectric properties were obtained via 3D printing of a nanocomposite solution of 10 wt.% BaTiO3 NPs ball-milled with PVDF due to the prevalence of β-phase. A piezoelectric coefficient of 18 pC N -1 comparable to that found in commercial PVDF sensors was obtained. A fully functional 3D printed sensor generating a maximum voltage output of up to 4 V upon finger tapping was fabricated and demonstrated. The one-step 3D printing of piezoelectric materials opens up a new demographic of sensor designs that is more integrable, conformable, scalable, versatile, and application-oriented. Apart from sensing, 3D piezoelectric scaffolds fabricated by the shown process could be used as electroactive scaffolds in tissue engineering, [START_REF] Correia | Strategies for the Development of Three Dimensional Scaffolds from Piezoelectric Poly(Vinylidene Fluoride)[END_REF] providing a biomimetic environment for cell culture. Given the limitation of the conventional electrode deposition techniques (forming planar electrodes), there is a need to develop a hybrid process to print the electrodes along with the polymer, thereby, delivering the entire sensor in a single step.

The sensors produced in the single step could be used as hair-like sensors for aerodynamics, as embedded sensors in aerospace or biomedical prosthesis, and as filaments for smart textiles.

Figure 1 .Figure 2 .

 12 Figure 1. (a) Solvent evaporation-assisted 3D printing process for PVDF nanocomposite-based

Figure 3 .

 3 Figure 3. (a) SEM image of BaTiO3 NPs (scale bar = 100 nm). SEM images of surfaces of films fabricated from the 0.2 g mL -1 solutions of nanocomposites prepared by (b) ball-milling, (c)

Figure 4 .

 4 Figure 4. X-ray diffractogram of (a) BaTiO3 NPs and (b) nanocomposites of 10 wt.% BaTiO3

Figure 5 .

 5 Figure 5. Peak-to-peak voltage as a function of peak force at an excitation frequency of 45 Hz

Figure 6 :

 6 Figure 6: SEM image of surfaces of films fabricated from the 0.2 g mL -1 ball-milled

Figure 7 .Figure 8 .

 78 Figure 7. Schematic of the proposed process leading to increase in β-phase fraction in PVDF due

Figure 9 .

 9 Figure 9. Structures fabricated using 10 wt.% BaTiO3 ball-milled nanocomposite solution. (a)

Table of Contents Graphic

				β-PVDF
			P
				v
				t
	hydrogen	carbon	fluorine	BaTiO3 nanoparticle


Sampada Bodkhe

Prof. F Clara Noonan

Frederick P Gosselin

Daniel Therriault
email: daniel.therriault@polymtl.ca

Prof P D Gosselin

Therriault

Coextrusion of Multifunctional Smart Sensors

Keywords: coextrusion, nanocomposite, piezoelectric, sensor, wearables

Three-dimensional (3D) printing of a piezoelectric sensor conventionally involves a minimum of three steps: fabrication of the sensor structure, electrode deposition to collect the generated charges and electrical poling. Here, we report a novel approach to fabricate a working piezoelectric sensor with its electrodes in a single step. We optimize the rheological characteristics of a piezoelectric nanocomposite ink, formulated and processed to work without the need for poling, and a metallic conductive paint for coextrusion. We then employ solvent evaporation-assisted 3D printing to coextrude ready-to-use sensors. The process fabricates conformal sensors, 3D self-supported cat whiskers with aspect ratios over 15, and filaments spanning over 2 cm. We present potential applications in the form of aero-elastic sensors and smart active thread for wearable electronics. We print sensors directly on fused deposition modelling printed miniature wings to monitor aero-elastic stability. In another application, we use the coextruded filament in the form of a piezoelectric thread for wearable sensors for knee-joint and respiration monitoring. The self-powered piezoelectric sensing elements are an attractive alternative for customized, multi-material applications where each watt and each gram counts such as wearables, and micro-drones.The process is adaptable to other multi-material and multi-component printing needs beyond piezoelectric materials.

Introduction

Remote monitoring is imperative to extract information about the environment, flight conditions and structural damage to complement the performance of air-vehicles and automobile. The weight and aerodynamics are paramount to the structures, along with mechanical durability and smooth functioning in isolated locations inaccessible to power. [1,2] 3D printing of piezoelectric materials serves as an apt tool to integrate sensors directly on the surface or embed them inside the parts to be monitored.

On the other hand, wearable sensors are increasingly sought after over conventional ambulatory diagnostics for monitoring vital signs in military, sports, and rehabilitation prosthetics owing to their foremost advantage of real-time unobtrusive electrophysiological analysis combined with low cost (over 10 times lower), portability, and versatility. [3,[START_REF] Koncar | Smart Textiles and their Applications[END_REF] Present day wearable sensors rely on resistive [START_REF] Beker | [END_REF]6,7,8] or capacitive [9,10] technology operating with bulky batteries that need to be frequently recharged and replaced, whereas piezoelectric sensors serve as self-powered alternatives. [11,12,13] The componentselectronics and multifunctional materials (material-making the clothing intelligent), are either embedded or woven in the fabric or directly embedded into the fibers themselves. [14] Piezoelectric sensors generate electric signals linearly proportional to mechanical deformation and come with the advantages of larger band-width, low-response time, and low operational costs. [15] Ceramics have dominated the piezoelectric market for decades due to the inefficiency of the piezoelectric polymers. Biocompatible, compliant, conformal and tough piezoelectric ceramic composites with polymers now overcome the limitations of brittle and rigid ceramics. [11,13,16,[START_REF] Ponnamma | Smart Polymer Nanocomposites: Energy Harvesting, Self-Healing and Shape Memory Applications[END_REF][START_REF] Wu | [END_REF] However things have not changed much in the form in which sensors continue to be fabricated: fibers [19,20] or films. [13,21] The sensors are then either bonded, [22,23] or strapped [24] at the sensing locations increasing non-conformability with the substrate, thus leading to inconsistent and erroneous results. Furthermore, electrodes to collect the generated charges are typically coated, [25] screen-printed, [23] or deposited [26] which is multi-stepped, time-consuming, and arduous for miniature slender structures. Conventional piezoelectric sensor fabrication processes entail additional poling steps, and often necessitate clean-room environment, which form a bottleneck in an industry setting. It is thus exigent to devise a process adept to industrial production. To apply the piezoelectric fiber into smart textiles, its diameter needs to match that of the fabric's thread. Unlike the metallic resistive sensors, the piezoelectric sensor requires two electrodes to collect and transport the generated charges. However, coating, and depositing electrodes form additional processing steps increasing the complexity and costs.

We proffer an unprecedented technique to three-dimensionally (3D) print flexible piezoelectric sensors with two co-fabricated electrodes in a single step quintessential for integrated and complex-shaped sensors and present their applications on two fronts: as active threads to study knee-motion and respiration in wearable electronics and aero-elastic monitoring of 3D printed wings. We investigate the physical properties affecting the printability and piezoelectricity of the coextruded system and show the adeptness of the process through various 3D printed structures.

Results and Discussion

We have recently demonstrated successful 3D printing of a piezoelectric nanocomposite ink with 10 wt. % barium titanate nanoparticles (BaTiO3 NPs) in polyvinylidene fluoride (PVDF) matrix. [27] The as-fabricated 3D structures possessed a d31 constant of ~18 pC N -1 without any poling. In this work, we coextrude (see details in SI and Supplementary Movie 1) this piezoelectric ink with a commercial silver ink to form piezoelectric sensors in a single step (see Figure 1a). We printed various kinds of piezoelectric structures: a filament (see Figure 1b andc), freestanding whiskers of a cat with aspect ratio of 15 (see Figure 1d), conformal features on a hemisphere (see Figure 1e) and filaments spanning up to 20 mm (print diameter = 0.84 mm, see Figure 1f). The SEM image of the cross-section reveals that the silver electrodes were formed on a small region on the outer surface of the filament (EDS details in SI). We believe uneven evaporation rates of the two inks turn the expected circular crosssection closer to an ellipse (see Section S3).

For coextrusion printing with interlayer stability and layer uniformity the two fluids should have similar rheological behavior [START_REF] Chown | Google Patents[END_REF] , and exhibit shear thinning properties, [START_REF] Therriault | [END_REF] and finally equal flow velocities for both the inks are achievable only when the outer layer has a lower viscosity. [START_REF] Dooley | Determining the Processability of Multilayer Coextruded Structures[END_REF] We optimized the concentration of the two inks in order to achieve similar rheological properties at the same time without compromising the compliance and thus, the mechanical sensitivity [21] of the printed filament (see Section S4 and S5). Piezoelectric materials are shown to measure the strain up to 1.2 % [21] and most human activities fall in the low frequency range of 0.25 -2 Hz. [25] Our piezoelectric thread completely reproduces the applied mechanical cyclic loading with 1 % strain at a frequency of 2 Hz (see Section S6).

To demonstrate the 3D printability of the current system, an aero-elasticity monitoring sensor in the shape of a typical strain gauge was printed on top of a fused deposition modeling (FDM)-printed beam shaped-like an insect wing (see Figure 2a). We assessed the sensitivity of the sensor to: (i) the magnitude and frequency of the applied forces, (ii) mechanical obstructions, and (iii) any changes in the surrounding wind flow, by mounting the wing on an electromagnetic shaker (see Figure S7) and exciting it at various forcing frequencies and amplitudes. The insets in Figure 2a confirm that there was no shorting between the electrodes at the turns and corners of the sensor. The response of the sensor (see Figure 2b) to the applied sinusoidal mechanical input (4 Hz) to the shaker was mostly linear as expected of piezoelectric materials. The sensor further reproduced the sinusoidal input very well for all the frequencies tested between 0.2 Hz and 15 Hz; a few of them are presented in Figure 2c.

We then applied a sinusoidal input of 1 Hz to the wing (see Figure 2d and2e). The response in the absence of any external perturbations is a regular sine curve. The sensor picked up physical obstructions (see Figure 2e) and flow perturbations (see Figure 2f) to the wing as seen by the disturbances in the electrical output signal establishing its applicability for monitoring the aero-elastic behavior of the wing.

We wove the sensors into a knee-stabilizer (see Figure 3a) that offered extensive contact with the joints aiding firm movements. A single unpoled filament sewn into the knee support detected and distinguished joint movements. We studied the response of the sensor during dynamic activities while cycling (Figure 3b) which is a combination of the continuous form of the two static motions (see Section 8 in SI). The voltage response curve is a resultant of the affect of the motion of all the parts: hip, knee and ankle. [31] The origin of the system is when the leg is fully stretched. The superposed red curve over the pictures corresponds to the anklemotion in Cartesian coordinates. A and B are the points at which the thigh forms a right angle with the calf. The first positive peak in the piezoelectric response curve (Figure 3c) corresponds to the bending of the leg behind the knee making a right angle. The negative peak occurs when the leg comes in the forward position and the thigh is parallel to the ground making the second right angle. Our sensor accurately detected the cycling frequency and the response is in complete accordance with the angular velocity (see Figure 3c). The small lag seen in the signals is mostly due to the lower sampling frequency of the video recording (59.9 fps) as compared with the voltage acquisition (2000 Hz). The noisy peaks in the voltage signal can be attributed to wrinkling of the filaments which are not well supported as compared to those printed directly on the wings. The variation of maximum voltage in each cycle with the cycling rate (see Figure S9) for two different set of tests (Figure 3c andanother cycle not shown here) ascertains that higher the cycling speed larger will be the voltage obtained from the sensor.

After studying the sensors for large-scale motion, we studied their sensitivity to detect smallscale motion as in respiration. Respiration is a physiological phenomenon involving the expansion of the chest cavity during inhalation, and its contraction during exhalation forming one respiration cycle. For normal breathing at rest, the respiration rate is around 12-20 cycles per min. [START_REF] Kumar | [END_REF] A sensor sewn into a t-shirt on the side of the thorax, normal to the axillary lines (see Figure 3d), tracked the breathing rhythm of the wearer. During breathing tests the amplitude of the voltage response from the sensor (see Figure 3e andf) corresponded to the breathing volume while the frequency to the respiration rate. The breathing sensor measured respiration rates very well (see Supplementary Table 1 and2). The sensor aided in differentiating between the types of breathing (see Figure 3e andf): slow or fast, and shallow or deep based on the frequency and amplitude of the voltage, respectively.

Conclusions

We have demonstrated a novel technique to form ready to use piezoelectric sensors with their electrodes in a single step. Our piezoelectric sensors successfully detect the frequency and the generated voltage is linearly proportional to the applied strain. Sensors determining the speed of the motion and distinguishing one movement from the other when directly woven into the clothing are convenient and comfortable to wear and wash without adversely affect the skin.

Additional piezoelectric filaments embedded in the textiles could act as a power source to the attached electronics [21] avoiding batteries and entangling wires. [25] Our technique demonstrates the feasibility of a multi-material printing approach where the structure and the sensor could be printed in a single facility at the same time. 3D printability of these sensors would allow conformability to various automobile and aerospace components for structural health monitoring. The process promises potential towards a series of other set of materials and applications that benefit from coextrusion. [33]

Experimental Section

Materials: Preparation of PVDF/BaTiO3 nanocomposite: We ball-milled (1080 cycles per min; SPEX SamplePrep™ 8000 Series Mixer/Mill™) a mixture of 0.31 g of BaTiO3 nanoparticles (10 wt.%, 99.9 % purity, 100 nm; Nanostructured & Amorphous materials Inc) and 3.1 g of PVDF (Sigma Aldrich) with 4 mL dimethyl formamide (DMF, Alfa Aesar), 6 mL acetone (BDH) and 0.6 mL (DMSO, Sigma Aldrich) for 20 min. Silver print conductive liquid paint (0.2 Ohm x mil resistivity; MG Chemicals) formed the electrodes.

Printing: We loaded the piezoelectric and the silver inks into 3 mL syringe barrels (see Figure 1a-d) and placed the syringe barrels into a pneumatically operated dispensing system (HP-7X, EFD) to apply precise pressures (0.1 -4.0 MPa) to coextrude the filaments through tapered nozzles (diameter, d = 410 µm -1.6 mm). We used a 840 µm nozzle to print all the structures.

We coextruded piezoelectric threads with 0.1-0.15 MPa extrusion pressures (P). An interfacing software transferred structure geometries in the form of Cartesian coordinates and print speeds to a robotic dispensing system (I&J2200-4, I&J Fisnar Inc.) via (JR points for dispensing, Janome Sewing Machine). We printed cat whiskers at P = 0.04 MPa and stage speeds (v) from 2 -4 mm s -1 , piezoelectric strain sensors on FDM (FDM details in SI) printed wings with P = 0.08 -0.12 MPa and v = 35 mm s -1 , conformal sensors with P = 60 kPa and v = 10 mm s -1 and spanning filaments with P = 150 kPa and v = 20 mm s -1 .

Characterization: Optical microscopy (BX-61 Olympus; Image-Pro plus V5 an image processing software from Media Cybernetics) permitted the measurement of the thickness and morphology of the filaments and 3D printed structures. A field emission scanning electron microscope (FE-SEM; JEOL JSM-7600TFE) with a low angle back-scattered electron detector obtained high contrast images of the coextruded filament cross-section.

Data Acquisition: For all the sensors, we attached copper tape (3M Canada) at the ends of the piezoelectric thread to form an interface for charge collection. A charge amplifier (Measurement Specialties) with an NI-9239 data acquisition system attached to a USB carrier NI-9162 (National Instruments) converted the charges collected from the sensors into readable voltages to read with a LabVIEW interface.

Aero-elastic monitoring:

We coextruded the sensors along with their electrodes and 3D printed them on top of a FDM printed insect wing in the shape of a strain gauge (Figure 4a). We screwed the wing on an electromagnetic shaker (The Mode Shop Inc. Model K2007E01).

A waveform generator (TE GAM, Model 2720), excited the shaker with input voltage (0.02 to 0.12 V) in the frequency range (0.2 to 20 Hz). A copper mesh covered the entire test set up for electro-magnetic shielding. We made mechanical perturbations to the wing by hitting it with a slender object, and simulated gusts using a hair dryer (Revlon, Ionic ceramic), at its highest speed, held vertically above the wing.

Wearable sensors: Piezoelectric filaments were woven into a clinical elastic knee stabilizer (see Figure 3a) (Formedica®). A commercial freeware, Kinovea (0.8.15), extracted motion data from the recorded videos (60 fps; Canon EOS Rebel T4i) for comparison with the response from our piezoelectric sensors during the static tests. In case of cycling, Image J was used to track the motion and determine the angular frequency. Motion extraction details are provided in the SI. Cycling tests were performed on a clinical bike (Sport Excaliber, Lode BV Medical Technology) at a mechanical power of 80 W. To monitor respiration, we wove the thread into a tight t-shirt on the left side of the thoracic cavity, parallel to the ribs (see Figure 3d). Conducting threads embroidered into the t-shirt formed the electrical connections from the sensor to avoid any noise from moving wires. We tested for two breathing types at rest: quick/shallow and slow/deep. The rate of evaporation of the solvents (see Figure S2) in the two inks dominates the shape of the cross-section of the printed filament or structure. Though the evaporation rates are similar at the onset of printing, in the final structure the piezoelectric ink shrinks the most (more than twice) as compared to the silver ink owing to the presence of a significant amount of solvents.

We believe that this turns the expected circular cross-section closer to an ellipse (see Figure 1g). Rapid solvent evaporation renders the sensors ready to use right after extrusion. This technique fabricates sensors with feature sizes: 250 µm -1.5 mm, without intermixing the piezoelectric and the electrode materials. For coextrusion without mixing or diffusion of one ink into the other, and to allow continuous extrusion of the two inks at any pressure, the two fluids should have similar rheological behavior. [START_REF] Chown | Google Patents[END_REF] Further, shear thinning behavior in both the inks is significant to attain shape fidelity post-extrusion. [START_REF] Therriault | [END_REF] Though the viscosities of the two materials vary at the given shear rates (see Figure S3), the curves for the two materials have similar slopes and comparable power law indices, n. Rheometer tests on the piezo and silver inks provide the power law indices as 0.77 and 0.79, respectively. Interlayer stability and layer uniformity, i.e., equal flow velocities for both the inks are achievable only when the outer layer has a lower viscosity. [START_REF] Dooley | Determining the Processability of Multilayer Coextruded Structures[END_REF] It is desirable to extrude the material system at low flow-rates to be in the laminar flow regime, thus avoiding any turbulent mixing of the two materials.

S5. Tensile tests

Uniaxial tensile tests (Insight electromechanical testing machine), performed at a strain rate = 48 mm min -1 with distance between the grips = 20 mm using a 5 N load cell according to ASTM D3822 standard evaluated the Young's moduli, strength and strain at break of the filaments. The reported values are an average of five successful samples (diameter ~500 µm) tested until failure. Keeping silver to a minimum avoids its detrimental effect on the mechanical properties of the fiber, confirmed by mechanical tests. The coextruded filament exhibits a decrease in tensile strength (see Figure S4 for the stress -strain curve) from 8.1 MPa ± 0.9 MPa to 5.6 MPa ± 0.5 MPa (average of 5 values) and strain at break from 422 % ± 24 % to 296 % ± 20 % as compared with the extruded piezoelectric filament with similar dimensions. The Young's modulus of the piezoelectric filament is 19.8 MPa ± 2.9 MPa and the one for the coextruded filament is 20.0 MPa ± 3.0 MPa. Both filaments present moduli, implying that electrodes do not significantly affect the compliance of the thread. This low value of elastic modulus results in increased mechanical sensitivity. 21

S6. Piezoelectric tests

S8. Static motion analysis

Preliminary tests with the knee-movement sensors involved the wearer raising and lowering the lower limb (from the knee to the foot) both upon sitting and while standing (see Figure S7a andb). Each exercise was filmed and then, analyzed using Kinovea (0.8.25 x 64) to monitor the movement of the ankle. The software included a motion tracker, that allowed the user to select a specific point in the video to monitor its movement along X and Y axes. The X and Y coordinates could be used to calculate the R values for each movement, R being the displacement of the ankle from the origin (the point at which the leg was at rest).

S9. Dynamic motion analysis

For the cycling tests, the interior angle at the knee was obtained using Image J (win64), calculated for each frame (processed using MATLAB) from the recorded video (60 fps).

Figure 1 .Figure 2 .

 12 Figure 1. a) Schematic of the coextrusion process, inset: cross-section of the coextruded filament showing the piezoelectric and conductive inks) b) SEM image of cold fractured cross-section of the coextruded filament (scale bar = 100 µm), major NP agglomerates are indicated with red arrows, and the silver electrode is highlighted with a red border; photograph of coextruded c) microscopic image of the piezoelectric thread (scale bar = 500 µm);d) freestanding whiskers printed on a FDM printed cat (scale bar = 10 mm); e) conformal sensors printed on a hemisphere (scale bar = 5 mm),; and f) spanning filaments (scale bar = 10 mm). (Printing parameters: nozzle diameter = 840 µm, extrusion pressure = 0.04 -0.2 MPa, speed = 2 -20mm s -1 ).

Figure 3 .

 3 Figure 3. Wearable sensors tests: a) Schematic depicting the sensor sewn on a knee-stabilizer; b) photograph of the cycling test: red plot shows the path of the ankle during cycling as extracted from the video frames and the yellow arrow shows the cycling direction; c) plot of angular velocity obtained from the recorded video and voltage output of the sensor during cycling; d) schematic of the piezo thread woven into a T-shirt; breathing sensor output during e) shallow/quick breathing, and f) deep/slow breathing for 1 min.

Figure S2 .

 S2 Figure S2. a) SEM image of the cold fractured cross-section of the coextruded filament (scale bar = 250 µm); energy-dispersive spectroscopy (EDS) results for the regions selected in 'a': b) silver region, and c) BaTiO3 agglomerates.

Figure S3 .

 S3 Figure S3. Volumetric shrinkage during solvent evaporation of piezoelectric and silver inks.

S4:

  Rheological characterizationA rotational rheometer (Anton Parr, Physica MCR 501) provided the variation of viscosity versus shear rate for both the piezoelectric and silver inks. We used a rough concentric cylindrical geometry (CC 55) with a shear ramp rate of 0.1 s -1 . Oil poured on the top of the inks during the tests prevented solvent evaporation.

Figure S4 .

 S4 Figure S4. Log-log plots of viscosity versus shear rate.

Figure S5 .

 S5 Figure S5. Representative tensile stress versus strain curves of the coextruded and piezoelectric filaments, inset: zoomed view of the elastic region.

Figure S7 .

 S7 Figure S7. Schematic of the FDM printed wing mounted on an electromagnetic shaker and subjected to sinusoidal mechanical input.

  and d show the comparison of R values of the ankle with the voltage response form the piezoelectric thread for the two tests. The piezoelectric signal of the instrumented knee stabilizer measured as the wearer raised the leg compared well with visual recordings analysed using Kinovea software.

Figure S8 .

 S8 Figure S8. Simple leg raising tests a) sitting and b) standing. Red plots show the movement of the ankle from its initial to final position. Correlation between the output voltage versus ankle displacement (R) for five consecutive cycles c) sitting and d) standing.

Figure S9 .

 S9 Figure S9. Peak voltage output per cycle as a function of cycling rate for two separate tests.

Acknowledgements

The authors acknowledge the financial support from Natural Sciences and Engineering Research Council of Canada (NSERC, #RGPIN312568-2013) and Canada Research Chair (#CRC950-216943). The authors thank Prof. Jonathan Tremblay, Kinesiology department, University of Montreal, for the access to the research bike and breathing monitoring equipment and Mr. Nicolas Leduc-Savard for assistance with the logistics. The authors also thank Ms. Isabelle Nowlan, and the Laboratory for Acoustics and Vibration Analysis and the Laboratory for Electrical Energy at Polytechnique Montreal for the electromechanical shaker and the function generator, respectively.

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Supporting Information

Supporting Information is available from the Wiley Online Library or from the author. We first insert a commercial silver ink (MG -842) along the edges of a syringe (3 mL) using a clinical syringe-needle system (1 mL) to form thin lines of silver on the walls as in Figure S1. We then quickly pour a solution of PVDF/ BaTiO3 nanocomposite into the syringe (see Figure S1c) to minimize the drying of the silver ink. Then we coextrude the two inks in the required design to attain the designed shapes.

S2. SEM

To determine the piezoelectric properties, we measured the piezoelectric response from the filaments (distance between the grips = 12-15 mm, 3 samples) using a dynamic mechanical analyzer (DMA Q800, TA Instruments) in multi-strain mode (1 % strain, 2 Hz; preload of 0.1 N) for 5 min each. Piezoelectric materials are shown to measure the strain up to 1.2 % 21 and most human activities fall in the low frequency range of 0.25 -2 Hz. 25 We tested the piezoelectric thread under cyclic tensile loading with 1 % strain at a frequency of 2 Hz using a dynamic mechanical analyzer. Figure S5b is a representative curve of the piezoelectric output from the sensor with respect to time. As the fiber is stretched along its length, charges are generated across its thickness (see schematic in Figure S5a). Figure S5c is an enlarged view of the piezoelectric response and the output signal clearly resembles the input sine wave.

S7. Vibration tests for the aero-elasticity monitoring sensor S10. Respiration monitoring

S11. FDM printed structures

We designed a hemisphere, a cat and a U-shaped structure for printing the spanning filaments in Catia V5. We obtained the cicada wing image from stillblog.net. We acquired the geometry of the image using Sketch Tracer tool in Catia V5. We acquired the design of the cat from Thingiverse.com. We then converted all the part files into .stl files. We used Leapfrog FDM printer to print all the above parts with PLA.