Soft Electronic Block Copolymer Elastomer Composites for Multi‐Material Printing of Stretchable Physiological Sensors on Textiles

Soft and stretchable electronic materials have a number of unique applications, not least within sensors for monitoring human health. Through development of appropriate inks, micro‐extrusion 3D printing offers an appealing route for integrating soft electronic materials within wearable garments. Toward this objective, here a series of conductive inks based on soft thermoplastic styrene–ethylene–butylene–styrene elastomers combined with silver micro‐flakes, carbon black nanoparticles, or poly(3,4‐ethylenedioxythiophene) (PEDOT) conducting polymer additives, is developed. Their electrical and mechanical properties are systematically compared and found to be highly dependent on additive amount and type. Thus, while silver composites offer the highest conductivity, their stretchability is far inferior to carbon black composites, which can maintain conductivity beyond 400% strain. The PEDOT composites are the least conductive and stretchable but display unique properties due to their propensity for ionic conductivity. To integrate these inks, as well as insulating counterparts, into functional designs, a multi‐material micro‐extrusion 3D printing routine for direct deposition onto stretchable, elastic fabrics is established. As demonstration, prototypes are produced for sensing common health markers including strain, physiological temperatures, and electrocardiograms. Collectively, this work demonstrates multi‐material 3D printing of soft styrene–ethylene–butylene–styrene elastomer composites as a versatile method for fabricating soft bio‐sensors.


Introduction
Soft and stretchable electronics are attractive for applications that require integration of electrical sensing or stimulation modalities within mechanically active environments, for instance within biomedical implants, [1] laboratory models of mechanically active tissues, [2,3] robotics, [4] and wearable health sensors. [5] Independent of the intended application, stretchable electronics rely on patterning conducting materials into stretchable geometries or on applying intrinsically compliant conducting materials. Thus, while metal and semiconductors are inherently rigid, they can still serve as basis for stretchable electronics in the form of micro-structured thin films, generated through lithographical techniques. [5,6] Intrinsically stretchable electronic materials and composites, on the other hand, do not require intricate structuring to achieve soft stretchable devices but often offer inferior electronic performance (e.g., conductivity).
Intrinsically stretchable electronics are highly diverse and span across several different types of materials [5] including elastic and self-healing hydrogels, [7,8] ionic liquids, [9] liquid metals, [10] conducting polymers, [11] and elastomeric composites. [2,[12][13][14] While several of these have unique benefits such as extreme toughness, fabrication of functional devices can pose a challenge. To overcome such issues, several intrinsically conductive materials have been formulated for automated deposition using additive manufacturing techniques, including inkjet, [15][16][17][18] screenprinting, [13,19] and micro-extrusion/direct ink writing (DIW) 3D printing. [2,10,12,14] To achieve reliable high-resolution deposition using such techniques, ink rheology, carrier solvents, and particle additives must be tailored to the printing method. Equally important, the fabrication procedure should consider not only the deposition of a single active material but also the integration and registration of several conductors/semiconductors as well as flexible electrical insulation. DIW is particularly interesting in this regard as it allows for single-procedure integration of a wide range of inks, as well as hybrid manufacturing that combines printing with pick-and-place integration Soft and stretchable electronic materials have a number of unique applications, not least within sensors for monitoring human health. Through development of appropriate inks, micro-extrusion 3D printing offers an appealing route for integrating soft electronic materials within wearable garments. Toward this objective, here a series of conductive inks based on soft thermoplastic styrene-ethylene-butylene-styrene elastomers combined with silver micro-flakes, carbon black nanoparticles, or poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer additives, is developed. Their electrical and mechanical properties are systematically compared and found to be highly dependent on additive amount and type. Thus, while silver composites offer the highest conductivity, their stretchability is far inferior to carbon black composites, which can maintain conductivity beyond 400% strain. The PEDOT composites are the least conductive and stretchable but display unique properties due to their propensity for ionic conductivity. To integrate these inks, as well as insulating counterparts, into functional designs, a multi-material micro-extrusion 3D printing routine for direct deposition onto stretchable, elastic fabrics is established. As demonstration, prototypes are produced for sensing common health markers including strain, physiological temperatures, and electrocardiograms. Collectively, this work demonstrates multi-material 3D printing of soft styrene-ethylene-butylene-styrene elastomer composites as a versatile method for fabricating soft bio-sensors.
of surface mount electrical components. [12] Moreover, similar to other digitally controlled printing methods, it allows for rapid design alterations, which are appealing for biomedical applications.
Inks based on thermoplastic elastomers in carrier solvents are attractive for multi-material DIW printing of flexible electronics as they enable thin flexible designs and do not require post-printing curing or printing at elevated temperatures. Thermoplastic inks based on styrene-ethylene-butylene-styrene (SEBS) have been formulated in solvents such as toluene and dichlorobenzene, [19][20][21] while thermoplastic polyurethane (TPU) inks have been printed using polar organic solvents such as tetrahydrofuran and dimethylformamide. [2,12] Aside from health concerns, these solvents can be problematic for printing flexible electronics directly onto textiles as they can also dissolve common stretchable fabrics such as Elastane. Here, we have formulated a family of inks based on soft, low styrene-content SEBS in butylacetate (ButAc) to enable printing of thin and soft multimaterial electronics directly onto elastic fabrics. In addition to insulating SEBS inks, we tailored three classes of composites based on carbon black (CB) nanoparticles, silver (Ag) microflakes, or poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymers. Each composite carries unique advantages in terms of strain stability, conductivity, and bio-sensing, and may be combined for optimal, multifunctional performance; see Figure 1. As demonstration, we integrate several SEBS inks in direct-onfabric multi-material print procedures, and demonstrate prototypes for monitoring strain, physiological temperatures, and electrocardiograms.

Formulation of Soft SEBS Composite Inks
For practical applications within stretchable electronics and wearables, it is advantageous that material traces are both soft and thin. As the stiffness of SEBS thermoplastic elastomers increases with the styrene fraction of the block co-polymer, we chose to base our inks on a soft SEBS with a low styrene content of ≈12% and applied ButAc as volatile carrier solvent to enable thin traces. We found ButAc to be an excellent replacement for, for example, toluene, which has been applied in earlier studies on SEBS inks. [19,20] As expected, we found the rheology of the SEBS in ButAc to be highly dependent on concentration. At SEBS:ButAc ratios (wt:wt) of 1:2 and below, shear-thinning solutions with concentration-dependent viscosities were formed, while at higher concentrations, viscoelastic gels were approached; see Figure 2a,c,e; Figure S1, Supporting Information. As basis for thin compliant traces, we found lower viscosity inks based on 1:5 (wt:wt) to be superior. Notably, for such low viscosity formulations, the shape fidelity is ensured by solvent evaporation. Thus, across formulations, the 1:5-based inks gave rise to well-defined traces with thickness well below 100 µm when applying a 200 µm nozzle.
To create conductive composite inks, we introduced Ag microflakes, CB nanoparticles, or PEDOT conducting polymers in organic carrier solvents, to a 1:5 (wt:wt) SEBS:ButAc stock. The particle additives notably increased the viscosity and shear thinning properties of the ink, while generally still enabling consistent extrusion of thin traces without notable particle aggregation, Figure 2a-d. In general, the CB nanoparticles had a stronger rheological effect than Ag microflakes. For instance, gels with a defined yield stress were observed at ≈40% (v/v) Ag loading relative to the solid content, while a similar behavior was observed at ≈25% (v/v) solid loading for CB, Figure 2a,c. Moreover, the notable thixotropic effect of CB meant that for ≈20% (v/v) CB, shear-thinning gels were formed within the first hour after mixing; see Figure  S2, Supporting Information. While we found inks based on 1:5 (wt:wt) SEBS:ButAc stocks to be superior for printing thin film traces, we were also able to formulate stackable inks enabling in-air printing of free-standing strings spanning several centimeters by applying a higher solid content 1:2 (wt:wt) SEBS:ButAc as stock; see Figure 2e,f; Figure S3, Supporting Information.
We next evaluated conductivity as a function of additive filling percentage. For silver micro-flake fillers, we did not observe percolation at 10% (v/v) of solid content. However, from 20% (v/v), the conductivity increased proportionality to filler content, reaching >1 kS cm −1 for 30% (v/v) silver and ≈50 kS cm −1 for 40% (v/v); see Figure 2g. For CB nanoparticle additives, we observed a lower percolation threshold of ≈10% (v/v) of solid content, while the conductivity of the composites was markedly lower, reaching ≈4 S cm −1 for 30% (v/v) CB, Figure 2h. PEDOT composites had even lower conductivities, for example, below ≈0.05 S cm −1 for 30%, Figure 2i. We therefore explored whether a synergistic effect in conductivity could be found between CB and PEDOT, combining 30% (wt/wt) PEDOT and 20% CB (wt/wt). These reached a conductivity of ≈3 S cm −1 , indicating only minor synergy, Figure 2i.

Mechanical and Electrical Properties of Printed SEBS Composites
Given the variation in type and size of the conducting additives, we anticipated that the composites would have markedly different responses to strain. We evaluated their mechanical and electrical performance under uniaxial strain by straining 3D printed dogbone samples and simultaneously recording stress and electrical resistance (Figure 3).
Consistent with prior reports, for pure SEBS, we observed a failure strain of more than 1000% and an apparent Young's modulus of 1.2 MPa, in the low strain regime (0-3% strain) ( Figure S4, Supporting Information). We investigated CB and Ag composites above the v/v% required for electronic percolation (Figure 3a,b). For both CB and Ag composites, expectedly, an increase in additive content resulted in an increase in Young's modulus and a decrease in elongation at break. Compared to Ag, the CB particles had a more pronounced effect on Young's modulus, which at 30 vol% loading was 46 MPa for CB additives on average while only 13 MPa on average for Ag flakes.
However, while being stiffer at comparable loadings, conducting CB:SEBS composites could be formulated at lower concentrations and generally showed greater resilience of electronic conductivity under strain compared to composites based on Ag:SEBS (Figure 3a,b). Thus, at 10% strain Ag:SEBS composites with 20 vol% and 30 vol% additive had average relative resistance increases of 8 and 2, respectively and exhibited loss of measurable conductivity at less than 150% strain. Contrastingly, at 10% strain 20 vol% and 30 vol% CB:SEBS composites had average relative resistances below 0.3 and measurable conductivities could be maintained to at , modulus (below 3% strain), and mechanical failure strain (middle), and ΔR/R 0 at 10% strain and strain to electrical failure indicated by loss of conductivity (right) for SEBS + 20 and 30% v/v Ag micro-flakes relative to total solid content (a), SEBS + 20 and 30% v/v CB nanoparticles relative to total solid content (b), and 70/30 w/w PEDOT/SEBS and 70/30 w/w PEDOT/SEBS + 20% v/v CB nanoparticles relative to total solid content (c). All samples were printed as two to three layers, with thicknesses between 0.12 and 0.21 mm, excluding + 30 vol% CB (thicknesses between 0.03 and 0.05 mm), which could not be printed beyond one layer without significant crack formation during drying. For all bar graphs, bars represent average values and open circles represent individual data points.
www.advelectronicmat.de least 470% strain, or to the point of tensile failure (Figure 3b). Cyclic strain studies also indicated less plastic deformation for CB:SEBS than for Ag:SEBS, comparing the most stretchable composites in each class; see Figure S6, Supporting Information.
For Ag:SEBS, resilience of electronic conductivity was improved by increasing the additive concentration significantly above the percolation threshold. When increasing from 20 vol% to 30 vol% Ag, the average strain to electronic failure increased from 30% to 110%. In the case of CB:SEBS, increasing the additive concentration was less advantageous because mechanical failure occurred before electronic failure in samples with 30 vol% CB, indicating a tradeoff in overall conductivity and mechanical stability for these composites. For composites including 30% w/w PEDOT, mechanical failure strain was ≈300% (Figure 3c). Moreover, for this PEDOT formulation, measurable conductivity was lost at relatively low strain (45%). Given the comparatively low initial electronic conductivity of PEDOT compared to the other additives used in this study (i.e., Ag micro-flakes and CB nanoparticles), we propose that SEBS composites based on PEDOT are most relevant to low strain applications where ionic conductivity may be useful.

SEBS Composites Printed onto Textiles in Adjustable Strain Patterns
As a first step in formulating wearable sensors, we printed traces of SEBS composites directly onto stretchable textiles based on bamboo/Elastane, or polyamide, and studied their immediate adhesion. The composite inks printed well onto these substrates, without dissolving the textiles and without delamination. Next, to demonstrate the versatility of the procedure, we printed in straight lines and strain-relieving meandering patterns of SEBS-insulated SEBS:CB and SEBS:Ag traces onto bamboo/elastane textiles and compared their resistive response to mechanical strain; see Figure 4. For both types of composites, we found that the strain of the traces and resultant increase in resistance could be drastically improved by simply introducing meander patterns. Thus, for Ag:SEBS with 30% (v/v) loading as well as for CB:SEBS with 20% (v/v) loading, the resistance increase introduced by cyclic 10%, 1% and 0.1% strains was diminished more than ten times Figure 4a-f. Equally important, the hysteresis associated in particular to larger strains was dramatically diminished; see Figure 4a,d. These data demonstrate how the composites can be optimized for strain sensing applications by tailoring the strain experienced by the wires traces through path design. Importantly, the basis for this approach is that both thickness and stiffness of SEBS composite traces are sufficiently low. If not, the strain may be absorbed completely by the textile backing and signal minimal. On the other hand, if more dramatic strain-relief is required, changes in geometrical patterns can be combined with local adjustments in the amount of SEBS insulation printed below and on top of the traces. This will lead to stiffer traces and to the textile backing absorbing a larger fraction of the strain. Indeed, we observed that the local stiffness of the textiles could be adjusted by increasing the number of SEBS traces; see Figure S7, Supporting Information.

Printed Physiological Sensors
In addition to strain sensing, we hypothesized that the fundamental differences in the composites may give rise to distinct sensory responses to temperature. We thus compared the resistive temperature response for Ag:SEBS, CB:SEBS, and PEDOT/ CB:SEBS to temperatures spanning 20 °C to 50 °C sweeping high-to-low and low-to-high; see Figure 5a-c. We observed the largest difference in resistance of up to 20% for Ag:SEBS when decreasing from 50 °C to 20 °C. However, the variance as well as hysteresis was very high for the response of these composites, indicating that they are not suited for thermal sensing in this range. For CB:SEBS on the other hand, we observed a reversible and almost linear resistance increase in response to temperature increase across the range. Moreover, the effect showed minimal hysteresis when sweeping high to low and low to high; see Figure 5a. Potentially in combination with strain relief geometries, CB:SEBS traces may serve as simple resistive thermal sensors. Interestingly, for PEDOT/CB:SEBS composites, the resistance was largely unchanged in response to temperature changes in the range 20 °C to 50 °C (see Figure 5a), making it appealing as reference in resistive temperature sensing.
Beyond sensing of common physiological markers such as strain and temperature, which is of relevance in the broadest sense of monitoring human health, a core aim of wearable sensors is to provide minimally obstructive readouts of clinically relevant risk indicators. Perhaps the most important example is electrocardiograms (ECG) where continuous monitoring can enable detection of rare arrhythmic events and patient response to medication. To demonstrate the relevance of our inks and printing procedure, we printed several stretchable textile-backed ECG-sensing electrode-arrays, where Ag:SEBS provided high-conductivity connections, and CB:SEBS or PEDOT/CB:SEBS where applied as sensing pads. We adopted this layout to increase the total electrode area without compromising overall stretchability. Each connected array thus served as a single electrode in a classical threepoint ECG setup (on each arm and left leg); see Figure 5b. The textile-printed electrodes enabled us to resolve the full ECG including P waves, QRS complex, and T wave, to enable straight forward monitoring of, for example, Q-T interval; see Figure 5c. Still, in all cases, notable noise was observed. Part of this noise may have originated from electromagnetic radiation from wireless devices and electronic installations because the recordings were performed in a regular lab setting. However, a large part of the noise likely originated from the impedance between the printed electrodes and the skin. In line with this, we observed that PEDOT/CB:SEBS-based sensors provide a much clearer signal in the wet state as compared to dry; see Figure 5d. We propose that this difference is caused by the ionic conductivity of PEDOT. The impedance thus drops when the PEDOT electrodes are wet and better electrical contact is established between electrode and skin. Notably, this effect was not observed for CB:SEBS-based sensors without PEDOT. www.advelectronicmat.de

Discussion
Several properties are relevant to consider when evaluating stretchable conducting materials for biomedical applications. Most obviously, the mechanical and conductive properties are critical, but so are the means of processing, dimensional restrictions, possibility for integration with other materials, stability, and biocompatibility. In this study, we demonstrated a robust framework for multi-material printing thin compliant conducting and insulating soft SEBS composites directly on textiles. As basis for high conductivity and sensory modalities, we formulated and compared the electrical and mechanical properties of composites based on soft SEBS using three diverse types of fillers. As expected, the conductivity of the composites was determined by the conductivity of the fillers, once a percolation threshold was surpassed. For ≈30 vol% fillers, the conductivity of Ag micro-flake composites (≈2kS cm −1 ) was 500 times higher than that of the CB nanoparticles (≈4 S cm −1 ), and 40 000 times higher than that of PEDOT composites (0.05 S cm −1 ). Despite being more conductive, the conductivity of Ag flake-SEBS composites was less resilient to strain than composites based on CB nanoparticles, which we hypothesize is a result of their greater chemical compatibility with the SEBS matrix. Specifically, for 30 vol% filler, the conductivity of Ag:SEBS was lost prior to mechanical failure, between 79% and 150% strain, whereas the most robust CB:SEBS composites remained conductive above 500% strain. Earlier reported TPU-based composites using similar Ag particles and loading degrees remained conductive  6 mm). a-c) SEBS + 30% v/v Ag micro-flakes relative to total solid content strained cyclically 10 times to 10% at 20% per min (a), 100 times to 1% at 2% per s (b), and 100 times to 0.1% at 0.2% per s (c). d-f) SEBS + 20% v/v CB nanoparticles relative to total solid content strained cyclically 10 times to 10% at 20% per min (d), 100 times to 1% at 2% per s (e), 100 times to 0.1% at 0.2% per s (f).
beyond 200% strain. [12] We therefore suggest that the lower electrical robustness of our SEBS:Ag composites is likely due to a limited bonding between the Ag flakes and the SEBS matrix, the ethylene-butylene fraction in particular. Further, as conductivity is lost for Ag:SEBS well before mechanical failure, micro-crack and/or domain separation seem to be likely mechanisms for www.advelectronicmat.de disruption of the percolating Ag particle network. Interestingly, in earlier studies on quite similar printable Ag:SEBS composites in the form or thin wires embedded in polydimethylsiloxane (PDMS), strains of several hundred percent did not break up the composite conductivity. [21] This could indicate that encapsulation can counteract domain separation or micro-crack formation. For practical purposes, limitations in intrinsic material stretchability could be overcome by introducing geometrical strain-relief patterns, as means of generating highly stretchable systems. Here, we demonstrated this approach using multi-material, direct-onfabric printing, to generate robust wires with minimal or tailored resistive responses to strain.
For printing health-monitoring sensors directly on elastic fabrics, we formulated the SEBS inks using butylacetate as carrier solvent, avoiding solvents such as dimethylformamide (DMF) or tetrahydrofuran (THF). While DMF and THF have been successfully applied in earlier reported 3D printable conductive inks based on thermoplastic elastomers, [12,21] these solvents are problematic here because they dissolve common elastic textiles based on polyurethanes (e.g., Elastane). Further, to ensure that textile-backed printed sensors are comfortable and do not restrict the mobility of the user, we optimized our inks for minimal stiffness and thicknesses below 100 µm by relying on dilute inks and SEBS with minimal styrene content. For sensing purposes, we found that the CB-based composites displayed a linear and reversible increase in resistance to elevated physiological temperatures, and low noise in ECG monitoring, in the dry state. On the contrary, PEDOT-based composites displayed only minor sensitivity to temperature changes and low noise in ECG monitoring in wet state. We ascribe this to the propensity of PEDOT for ionic conductance.
Beyond wearable sensors, the SEBS-based inks are of relevance for various other soft sensor applications, not least within laboratory models of mechanically active tissues, for example, in the form of instrumented micro-physiological systems and organs on chips. [2,3,22] In this regard, SEBS has been highlighted as a promising alternative elastomer to widely used PDMS, [22,23] which is hampered by unwanted side-effects such as adsorption of hydrophobic drugs. For such applications, the ability to print micrometer scale traces, as demonstrated here, is of particular relevance. In such applications, inks based on higher SEBS content are also of interest as we found these to be applicable not only for printing of high-aspect ratio structures but also for in-air printing of freestanding ribbons spanning up to several centimeters.

Conclusion
Our study illustrates how a range of 3D printable soft conductive SEBS composites can be formulated using butylacetate as a carrier solvent. By adjusting the type (CB nanoparticles, PEDOT conducting polymers, or Ag micro-flakes) and content of conductive additives, we formulate composites with tailored rheology, conductivity, mechanical properties, and sensory capabilities. Our findings highlight the importance of considering the interaction between matrix and particle additives, as well as total particle loading, when designing printable soft electronically active composites.
We moreover demonstrate how the individual strengths of several SEBS composites can be combined in a single multimaterial 3D printing route to generate textile-based sensors arrays for monitoring markers such as physiological temperature, strain, and ECGs. Future work on integrating and anchoring microelectronic components during the on-textile print procedure will enable continuous wireless sensing of, for example, health markers without compromising a patient's mobility. Beyond wearables, the SEBS composite inks presented here are of interest to future research in a variety of application spaces, such as bio-sensory microsystems that mimic native tissue in vitro. For future biological applications, whether in vivo or in vitro, cell-material interactions and biocompatibility will be important variables to consider.

Experimental Section
Ink Preparation: SEBS Kraton G1645, (PS 11.5-13.5%) was used for all inks. SEBS stock solutions (without conductive additives) were first prepared by dissolving SEBS in ButAc using magnetic stirring until a visually homogenous mixture was formed. 1:2 (SEBS:ButAc, wt:wt) stock solutions were generally made and then diluted to the concentration needed for desired experiments (e.g., 1:5 wt/wt for the majority of the DIW experiments). Unless otherwise noted, prints were done using inks based on 1:5 (wt:wt) SEBS:ButAc. To generate conducting composites, designated amounts of particles additives Carbon Black (Vulcan XC72R, CABOT), or Silver Flake, APS 2-5 microns, 99.95%, (Inframat 47MR-10F) were mixed into SEBS stocks using a planetary mixer (Hauchchild Speedmixer 150.1 DAC FVZ, GER) at 2500 rpm for 3 × 5 min and loaded into syringes. To formulate PEDOT-SEBS composites, an organicsolution formulation of PEDOT (RD Clevios P SB 6, Heraeus, GER) was used. The PEDOT came as a 2% (w/w) suspension in Butyl Acetate and Anisol. Prior to use, the solution was concentrated to 6% (w/w) at ≈50 °C using stirring. SEBS:PEDOT inks were formulated by mixing the 6% stock with a SEBS stock in appropriate ratios.
Printing Procedures: A multi-material 3D Discovery extrusion printer from RegenHU (CH) was used for printing. Designated BioCAD software was applied for defining prints. 3CC cartridge (Nordson EFD) and metal or plastic cononical nozzles with a nominal inner diameter of 200 µm (Cellink SWE) were used. Print speeds were generally 5-10 mm s −1 . Pneumatic pressure was applied for dispensing and adjusted according to rheological properties of the ink being used. In general, pressures were on the order of hundreds of kPas. For printing onto textiles, the textile was first temporarily attached and flattened on a PS or PMMA plate using a water soluble gelatin-based adhesive (one part gelatin, one part glycerol, one part acetic acid (4-8%), and four parts water, all by weight). After printing, on-textile prints were released by immersion in water. Textile-backed Instron samples printed onto Bamboo:Elastane (96:4) textiles (STOF 2000, DEN) were insulated using two layers of pure SEBS, one below the print and one on top of the conductive composite trace(s). To ensure complete drying, ≈10 min wait was included between each layer in the multi-material prints on textiles. For printing freestanding dog-bone structures, a water soluble poly(vinyl alcohol) release layer was applied.
Stretchable sensing arrays were printed onto Bamboo:Elastane (96:4) textiles. The arrays contained 42 sensing pads, each 5 mm in diameter, spaced by 10 mm. Sensing pads were based on either SEBS:CB with 30% (v/v) CB, or SEBS:PEDOT:CB with 30% PEDOT and 20% CB, respectively. Sensing arrays were connected by meanders based on SEBS:Ag with 30% (v/v) Ag. Entire arrays were insulated on the back with two layers of pure SEBS and SEBS:Ag wiring was further insulated with one layer of SEBS on top. A push-bottom was inserted into a SEBS:Ag connection pad, which served as a connector to the ECG recording wiring and hardware.

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Rheological Studies: The rheology of each ink was analyzed using a Discovery Hybrid Rheometer (TA instruments, DE, USA) equipped with a Peltier plate thermal controller and a parallel plate geometry with a diameter of 40 mm equipped with a solvent trap filled with Butyl Acetate during measurements.
Uniaxial Strain Studies: 3D printed dogbones were used for uniaxial strain studies. The print file was a 2/3 scaled-down version of the ASTM D412 Die C dogbone commonly used for evaluation of elastomers. Due to ink spreading during printing, gauge length and width were slightly larger than the CAD file, and were measured manually. Typical gauge length and width were 25 and 4.5 mm, respectively. Thickness of each sample was measured with a Digital Vernier Caliper Micrometer before performing a uniaxial tension test with an Instron 5967 using a 500 N load cell. All samples were printed as two to three layers, with thicknesses between 0.12 and 0.21 mm, excluding +30 vol% CB (thicknesses between 0.03 and 0.05 mm), which could not be printed beyond one layer without significant crack formation during drying. For strain to failure tests, strain rates of: 5-20%/min were applied. A Digital multimeter (Keithley DMM34461A Tektronix) was used to measure two-wire resistance as a function of time by clamping Copper foil electrodes flush with the dogbone samples between the instron grips. Additional insulation was not used and conductivites were deemed "unmeasurable" in post processing when recorded values exceeded 1 × 10 6 Ω. Reported modulus values were extracted by performing linear regression on each stress-strain curve between 0% and 3% of strain in Microscoft Excel.
Temperature Response Studies: To investigate the temperature response, a simple linear line (0.5 mm wide and 10 mm in length) of SEBS with 30% (v/v) Ag, SEBS with 20% (v/v) CB, and 70/30 SEBS/PEDOT with 20% CB was printed on a polystyrene substrate and insulated with one layer of pure SEBS ink. For electrode connections, both ends of this line were joined with a 5 mm radius semicircle printed with Ag ink. The samples were placed within a Weiss Technik climatic test chamber and resistance changes were measured using a digital multimeter (Keysight 34461A). All samples were evaluated at temperatures ranging from 20 °C to 50 °C with a constant relative humidity of 55%. At each temperature, 5 min were allowed for stabilization.
ECG Studies: Textile-backed ECG sensing arrays were fabricated as described above. A sensing array was applied to each arm and one leg of the test subject (Male 29 year), as the basis for a three-point ECG. The ECG recordings were acquired using a PowerLab 26T data acquisition system from AD Systems and analyzed using designated LabChart software.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.