Soft, Conductive, and Anti‐Freezing Conducting Polymer Organohydrogels

Soft and conductive materials are highly desirable for wearable electronics. In particular, anti‐freezing, long‐water retention, and highly conductive gels with Young's modulus matching that of biological tissues, show promise in bioelectronics. Herein, soft organohydrogels obtained by mixing poly (3,4‐ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS), ethylene glycol (EG), and tannic acid (TA) are reported. The PEDOT/EG/TA organohydrogels exhibit a low compressive Young's modulus of ≈20 kPa, a conductivity of ≈6 S cm−1, as well as anti‐freezing and water retention properties. Epidermal patch electrodes prepared using the PEDOT/EG/TA gel exhibit low skin–electrode impedance at low frequency (1–100 Hz) and high‐quality electrocardiography (ECG) and electromyography signal recordings. Moreover, these gels demonstrate long‐term stability with high ECG recording quality after being placed under ambient conditions for seven days.


Introduction
Soft and conductive materials with long-term stability have attracted interest in the field of epidermal bioelectronics, due to their conformal contacts with skin and the ability to acquire biosignals.3][4][5][6] ECG, EMG, and EEG are typically achieved with metal (ground strap, bar, or cup) or, more often, with Ag/AgCl gel electrodes. [7]OI: 10.1002/adsr.202300072][9] Therefore, there is a strong demand for soft conductive electronics that can form conformal interfaces with biological tissues for long-term use, without leading to skin damage and irritation.
The conductive hydrogels reported so far might not be suitable for long-term use in dry and cold environments due to water evaporation or freezing, which affects both mechanical and electrical properties. [31][33][34][35] However, these materials show rather low conductivity owing to the insulating matrix, typically made of PVA, [34] PAAM, [36] and PAA. [35]PVA/glycerol/polyaniline gels based on the glycerol-water binary mixture, [34] exhibited antifreezing properties and an electrical conductivity of ≈0.003 S cm −1 .A glycerol-water hydrogel with polydopamine (PDA)decorated CNTs conducting nanofillers and PAAM and PAA as the polymer matrix demonstrated anti-freezing and anti-drying properties, and a conductivity of ≈0.08 S cm −1 . [35]As low conductivity limits applications in digital circuits, bioelectronics, electromagnetic interference shielding (EMI), thermoelectric, and energy storage, [31,[37][38][39] the preparation of highly conductive hydrogels with anti-freezing, long-water retention properties, and Young's modulus matching that of biological tissues is in high demand.
Herein, we report conductive organohydrogels obtained from aqueous mixtures of PEDOT:PSS, EG, and TA, which showed a low compressive Young's modulus of ≈20 kPa, conductivity of ≈6 S cm −1 , as well as anti-freezing and water retention properties.Our gels were used to prepare epidermal electrodes for ECG and EMG, which showed similar performance to that of commercial Ag/AgCl gel electrodes, even after storage in ambient air for 1 week.We also demonstrated the use of gel electrodes as humanmachine interfaces to control the opening and closing of a robotic hand in real-time.The simple fabrication, high conductivity, and mechanical properties make our gels highly promising for applications in bioelectronics electronics.

Experimental Section
The protocol for human experiments was approved by the ethical committee of Polytechnique Montréal (approval number CER-2021-04-D1).All the measurements (except for Zeta potential) in this study were conducted under ambient condition.

PEDOT/EG/TA Organohydrogel Preparation
A concentrated PEDOT:PSS suspension was obtained by evaporating ≈2/3 of the weight of water from a Clevios PH1000 suspension.Specifically, 150 g of Clevios PH1000 were boiled on a hot plate until the weight of the suspension was reduced to ≈50 g.To determine the weight percentage of PEDOT:PSS in the concentrated suspension, we measured the mass of solid films obtained by casting 2 g of the concentrated suspension and baking at 140 °C for 6 h.The weight percent of PEDOT:PSS (W % ) was determined to be ≈3, using the following Equation: Where m R is the mass of the films after baking, and m o is the mass of the concentrated suspension used for processing.
PEDOT/EG/TA gels were prepared from a mixture of concentrated PEDOT:PSS, EG, and TA.The concentrated PEDOT:PSS suspension was used to facilitate the gelation.For instance, the sample PEDOT/EG/TA-2 contained 10 g of concentrated PE-DOT:PSS (62.5 wt.%), 4 g of EG (25 wt.%), and 2 g of TA (12.5 wt.%) (Table 1).After mixing the components at 2000 rpm for 10 min in a planetary mixer (ARM-310, Thinky Co.), the mixture was filtered (CHROMSPEC UV Syringe Filters, Nylon, 25 mm diameter, 5.0 μm pore size) and cast into a covered mold or vial to prevent solution (water and EG) evaporation.The gels were obtained by heating at 90 °C for 30 min.PEDOT/TA hydrogels (without EG) were obtained from a mixture of 10 g of concentrated PEDOT:PSS (83.3 wt.%) and 2 g of TA (16.7 wt.%) using the same procedure (Table 1).The mixture of concentrated PE-DOT:PSS (12.5 wt.%), EG, and TA(12.5 wt.%) could not form a gel when the amount of EG was 75 wt.%.

Preparation of Gel Electrodes
Gel electrodes were prepared as follows (Figure S2, Supporting Information): a strip of copper tape was placed on a pre-cut PET sheet, and a snap button was attached at one end of the copper tape (Figure S2a, Supporting Information).A double-sided tape with a round hole (8 mm in diameter) at the center was placed on the copper/PET strip (Figure S2b, Supporting Information).Tegaderm tape (with a central hole 8 mm in diameter, Figure S2c, Supporting Information) was attached to the other side of the adhesive tape.A gel disc (diameter of 18 mm and thickness of ≈1 mm, Figure S2d, Supporting Information) was placed on the center of a Tegaderm tape and connected to the copper tape (Figure S2e, Supporting Information).

Conductivity Measurement
The conductivities of the gels (samples of rectangular geometry, ≈30 mm × ≈10 mm × ≈0.5) were measured using a Biomomentum four-point probe system (Figure S3, Supporting Information) connected to a source measurement unit (B2902A, Agilent).The conductivity () of the gels was calculated using the Equation (2).
where R A is the average resistance of the gels and L and S are the length and section of the gels, respectively.Five samples were measured for each gel.The reported conductivity values are the average of five measured samples and the error corresponds to the standard deviation.

Mechanical Characterization
Compressive stress-strain tests were performed using a Mach-1 V500cst MA009 mechanical tester (Biomomentum Inc., Canada).Cylindrical gels with a height of ≈5 mm and a diameter of ≈12 mm were compressed at a speed of 12 mm min −1 under a 70 N load cell.Three samples of each type of gel were tested.
The reported compressive Young's modulus corresponds to the average of three measured samples, and the error corresponds to the standard deviation.Rheological measurements were performed using a stresscontrolled rheometer (MCR 502 MultiDrive, Anton Paar) and a parallel plate geometry (25 mm diameter).The rheological behavior of the PEDOT/EG/TA mixture and gels was studied at a shear rate of 0.1-10 s −1 to identify the change in viscosity.The frequency sweep measurements were conducted in the range of 0.1-100 rad s −1 angular frequencies at a constant strain of 0.25%.

Fourier-Transform Infrared (FTIR) Spectroscopy
Fourier-transform infrared (FT-IR) spectroscopy in absorption mode was performed using a Perkin Elmer Spectrum 65 spectrophotometer, with a scan range spanning from 500 to 4000 cm −1 .
FTIR spectroscopy (Figure S4, Supporting Information) was performed to characterize the functional groups of the PEDOT/EG/TA-2 gel.The gels made of TA, EG, and PEDOT:PSS showed the characteristic peaks of the functional groups of these species.The broad peak at 3000-3700 cm −1 observed for all samples was attributed to the stretching vibration of the ─OH groups.The characteristic peaks at ≈1620 and ≈1380 cm −1 , corresponding to the C═C and C─C bonds of the thiophene ring, [47] and the band at ≈1120 cm −1 , corresponding to the ─SO 3 H group of PSS, [47] indicate the presence of PEDOT:PSS.The peaks at ≈2938 and ≈2875 cm −1 correspond to C−H stretching, while those at 1084 and 882 cm −1 correspond to C─O stretching and C─C─O symmetric stretching, respectively, [48] confirming the presence of EG.A peak at ≈1452 cm −1 was observed, corresponding to the stretching vibrations of the-C─C aromatic groups in TA, [49] indicating the presence of TA.

Zeta Potential Measurements
Zeta potential was measured using a Zetasizer Nano ZSP instrument (Malvern.Instruments Ltd., Worcestershire, UK).Ten gram Clevios PH1000 suspension were diluted for ten times.Ten gram the diluted suspension containing ≈0.01 g PEDOT:PSS (corresponding to 1 mg mL −1 PEDOT:PSS aqueous solution) and 10 g the diluted suspension containing ≈0.01 g PEDOT:PSS and ≈0.06 g TA powder were used for the tests, and the measurements were performed at 25 °C.The amounts of PEDOT:PSS and TA for the tests were chosen taking into account the zeta potential testing conditions (maximum concentration of the particles is 10 mg mL −1 ) and the actual amounts of PEDOT:PSS and TA used to prepare the gels (Table 1).

Water Retention Measurements
The water retention properties of the PEDOT/EG/TA-2 and PE-DOT/TA gels were evaluated by aging them under ambient conditions for seven days.The mass change () of the gels over time is determined as follows: where m and m 0 are the mass of the gel at a given time and initial mass of the gel, respectively.

Anti-Freezing Measurements
PEDOT/EG/TA-2 and PEDOT/TA gels were stored at a temperature of −24 °C in a freezer for one day.The compressive stressstrain tests of the gels were carried out immediately after taking them out of the freezer.The times to carry out the tests were ≈30 s and ≈50 s for the frozen PEDOT/TA gels and PEDOT/EG/TA-2 gels, respectively.

Swelling Properties
The swelling properties of the PEDOT/EG/TA-2 gels were evaluated by measuring the changes in the height and diameter of the gel.The volumetric swelling ratio J(c) was calculated using Equation (4): Where V denotes the volume of the gel at a given soaking time and V i is the volume of the gel after baking for 6 h.

Characterization of Epidermal Electrodes
Skin-electrode impedance, ECG, and EMG measurements were carried out following our prior research protocols. [21]To record EMG signals, positive (PE) and negative electrodes (NE) were placed on the flexor carpi muscle of the right arm, and a ground electrode (GE) was attached to the elbow.Twelve signal regions and 13 noise regions were selected to calculate the root-meansquare (RMS) signal and RMS noise values, respectively.
For the EMG-controlled robotic hand (Bionic Robot Hand, Robot shop Inc., Canada), the GE, PE, and NE were placed on the right forearm of the volunteer at an equal distance of 5 cm.The signals were collected using a biosensing board (Cyton, Open-BCI, USA) and transmitted to an Arduino UNO controlling the arm.The codes are available upon request to the corresponding authors.

Statistical Analysis
The acquired images were drawn using 2019 Microsoft Power-Point version 16.22 and origin 2022 software.The chemical structures were draw by ChemDraw Ultra 8.0 software.Data was presented as means ± standard deviation, and statistical analysis was conducted using origin 2022 software.

Fabrication and Characterization
Soft gels were fabricated from mixtures of PEDOT:PSS, EG, and TA (chemical structures shown in Figure 1a).PEDOT:PSS is a conducting polymer widely used in bioelectronics because of its ease of processing from aqueous solutions, mixed electronic and ionic conductivity, and biocompatibility. [26]EG, in addition to acting as a conductivity enhancer for PEDOT:PSS, [50] is known to be a component of hydrogels with anti-freezing and moisturizing properties. [32,33,51]TA was added to promote gel formation, owing to its acidic properties, i.e., the ability to release H + in aqueous environment (a pH value of 3.17 was observed for 12.5 wt.%TA aqueous solution).
We observed that adding TA to the PEDOT:PSS suspension led to a decrease in the zeta potential from −32 to −9 mV (Figure S5, Supporting Information).In agreement with literature reports on PEDOT:PSS gelation induced by H 2 SO 4 , [30] this result may indicate a partial protonation of the PSS chains, which weakens the electrostatic attraction between PEDOT and PSS, thus exposing PEDOT chains to each other and promoting their interchain interactions via - stacking and hydrophobic attractions, to form a connected 3D gel network. [30,52]Because these processes are likely thermally activated, heating at moderate temperatures (for example, 90 °C) could accelerate the gel formation. [30]Besides crosslinking, other possible interactions are hydrogen bonding between the OH groups of TA and EG and the sulfonate groups of PEDOT:PSS, - stacking and hydrophobic attractions between TA and PEDOT (Figure 1b).
According to the literature, physically crosslinked hydrogels are characterized by a decrease of viscosity upon increasing shear rate. [30]We, therefore, performed viscosity measurements to further confirm the presence of a crosslinked network.In agreement with the literature, [30,52] we observed that the viscosity of our gel decreased linearly with the shear rate (Figure S6a, Supporting Information).The storage modulus (G') was approximately ten times the loss modulus (G'') at angular frequencies of 0.1-100 rad s −1 , which implies that the elastic response is predominant (Figure S6b, Supporting Information).
It is interesting to note that the mixture of concentrated DOT:PSS, EG, and TA, used to prepare the gels, showed shearthinning behavior (Figure S7, Supporting Information), that is, it became less viscous (i.e., flow more easily) under an applied shear stress.This property makes the mixture modelable, printable, and injectable.For instance, PEDOT/EG/TA-2 gels with arbitrary shapes (e.g., round (Figure 1c 1 ) and ring (Figure S8a, Supporting Information) shapes) could be obtained by casting the PEDOT/EG/TA-2 mixture into a mold.The mixture could also be printed into curves (Figure 1c 2 ) and letters (Figure S8b, Supporting Information) and injected into molds with a heart (Figure 1c 3 ; Movie S1, Supporting Information) or strip (Figure S8c, Supporting Information) shape.

Mechanical Properties
Figure 2a shows the compressive stress-strain of the gels with different amounts of TA.The plots exhibit a linear behaviour at low and moderate strains, while at higher strains the relationship becomes nonlinear, likely owing to the deformation of the gel.The stress rapidly decreased at ≈45%, due to the fracture of the gels.The Young's moduli of the TA-containing gels (Figure 2b and Table 2) are very similar and show a moderate increase upon increasing the amount of TA.Table 2. Compressive Young's moduli in the 0-10% strain range and gel conductivities.Data for the compressive Young's moduli (n = 3) and conductivities (n = 5) are reported as the mean ± standard deviation.The compressive strain at fracture increased with increasing the amount of EG (Figure 2c).The compressive Young's modulus of the gels displayed a gradually decrease when increasing the amount of EG (Figure 2d and Table 2).This clearly indicates that the addition of EG to PEDOT/TA results in softer materials, likely owing to the weakening of the interaction between the PEDOT and PSS chains, which the effect is similar to that of polyethylene glycol (PEG). [53]

Water Retention and Anti-Freezing Properties
Water retention ability is an important factor that affects the mechanical and electrical properties of gels.In the absence of EG, the gels showed fast shrinkage and deformation even after one day (Figure 4a; Figure S9, Supporting Information), whereas those containing EG maintained their shape even after seven days (Figure 4a,b).Mass change versus time plots (Figure 4c) showed that both gels experienced a substantial weight loss after one day, likely due to the evaporation of free water from the surface and the bulk.After this loss, their weight remained relatively stable, with PEDOT/TA gels experiencing an overall higher loss.The effect of EG on water retention is due to the formation of hydrogen bonds between water and EG, which helps stabilize the gel network and prevent water evaporation. [33,60,61]This longlasting water-holding ability of organohydrogel was also reported for glycerol-water binary solution system, due to the formation of hydrogen bonds between glycerol and water. [35,62]he EG-containing gels also exhibited stable mechanical and electrical properties over time, which are important for their long-term use.Compressive strain measurements (Figure 4d; Figure S10a, Supporting Information) indicate that the dehydrated PEDOT/EG/TA-2 gel (with 25 wt.%EG) exhibited a moderate increase of compressive stress with increasing strain and could withstand high strain (60%) without fracture.In contrast, the dehydrated PEDOT/TA gel (without EG) showed low compression resistance.Dehydrated PEDOT/TA gels showed a few drops on the compressive stress-strain (Figure 4d; Figure S10a, Supporting Information), which can be attributed to the shrinkage and formation of voids in the gels (Figure S10b,c).A video taken to visualize the compressive mechanical properties (Movie S2, Supporting Information) shows that the dehydrated PEDOT/EG/TA-2 gel remained soft and could be manually compressed without fracture.In contrast, the dehydrated PE-DOT/TA gel became harder owing to water loss and resisted the compression without deformation.It is interesting to note that the pristine and dehydrated PEDOT/EG/TA-2 gels showed different compressive behavior (Figures 2a,c and 4d).The dehydrated PEDOT/EG/TA-2 gels could withstand higher compressive strain (>60% strain), which may be due to the stronger interactions between EG, PEDOT:PSS, and TA.
Conductivity and mass change over time (Figure S11, Supporting Information) show that PEDOT/EG/TA-2 gels experienced a substantial conductivity increase from ≈ 6 to ≈17 S cm −1 and weight loss (≈58% of its initial weight) after one day (Table 3), which can be attributed to water loss, leading to an increased PE-DOT:PSS concentration.The weight of the gels further dropped to ≈29% after one week and ≈21% after twelve days, and their corresponding conductivity increased to ≈27 and ≈36 S cm −1 (Figure S11, Supporting Information and Table 3).
It was not possible to measure the conductivity of the dehydrated PEDOT/TA gel using four-point probe system, owing to its irregular shape resulting from water loss.However, using a digital multimeter, we determined that after 7 days the resistance of the PEDOT/TA gels increased by about three orders of magnitude, while that of PEDOT/EG/TA-2 gels remained relatively stable.Conductive gels with stable electrical and mechanical properties at low temperatures are desirable for applications in cold environments.Measurements of the gel resistance before and after freezing them at −24 °C for one day showed a change from 0.2 to ≈15 Ω for the PEDOT/EG/TA-2 gel, and from 0.3 to ≈149 Ω (≈500 times than its initial value) for the PEDOT/TA gel (Figure S12, Supporting Information).This shows that the PEDOT/EG/TA-2 gel may maintain good electrical conductivity at low temperatures (<−20 °C).
To better understand the effect of EG on the mechanical properties of the gels at low temperature, we performed compressive measurements immediately after removing them from the freezer.The compressive strength-strain curves (Figure S13, Supporting Information) showed that the PEDOT/TA gel became hard because of the frozen water, resulting in increased compressive stress, whereas the PEDOT/EG/TA-2 gels preserved their softness with low compressive stress and could be compressed by hand (Movie S3, Supporting Information).The anti-freezing properties of PEDOT/EG/TA-2 gels can be attributed to EG, which can form a variety of molecular clusters with H 2 O, leading to a decrease in water vapor pressure.As a result, the gel does not readily crystallize and the freezing point is decreased. [32]

Swelling Ability of PEDOT/EG/TA Gels
The swelling properties of the gels were investigated, as they significantly affect mechanical properties, [63] nutrient transport, and drug delivery throughout the hydrogels. [64]Digital images show that the cylindrical gel (Figure 5a 1 ) shrunk and deformed after baking for 2 h (Figure 5a 2 ), and then remained relatively stable (Figure 5a 3-4 ).Volumetric shrinkage ratio versus baking time plots (Figure 5b) reveal a large shrinkage after baking for 2 h, followed by a stability region.The shrinkage was ≈75% after baking for 6 h.In contrast, the dried gel (Figure 5a 4 ,c 1 ) retained its shape even after rehydration in water for 24 h (Figure 5c 1-4 ).Volumetric swelling ratio versus soaking time plots showed that the gel presented the largest volumetric swelling ratio after a rehydration of 8 h and then the change remained stable with a volumetric swelling ratio of ≈135% (with respect to the volume of the dehydrated sample) after two days (Figure 5d); thus, it showed slow swelling ability and only partially recovered its volume.

Skin-Electrode Impedance, ECG, and EMG Measurements
The skin-electrode impedance was studied for PEDOT/EG/TA-2 gel electrodes in a three-electrode configuration (Figure S14a, Supporting Information).The impedance versus frequency plots (Figure S14b, Supporting Information) show that the electrodes performed slightly better than the commercial Ag/AgCl gel electrodes in the relevant frequency range up to 100 Hz (e.g., ≈130  kΩ vs ≈220 kΩ for the Ag/AgCl gel electrodes at 10 Hz, Table S2, Supporting Information), whereas both electrodes performed similarly overall at higher frequencies.The lower impedance at low frequencies might be due to the high conductivity and softness of the PEDOT/EG/TA-2 gels.
ECG and EMG are diagnostic tools commonly used in human healthcare, and the quality of their recording is essential for accurate clinical diagnosis.To record ECG biopotentials, we attached ground (GE) and positive (PE) electrodes to the left forearm and negative electrode (NE) to the right forearm (Figure 6a).The voltage versus time plots (Figure 6b) show that the PEDOT/EG/TA-2 gel electrodes present well-defined ECG characteristic peaks, such as the P wave, QRS complex, and T wave, indicating the suitability of these electrodes for monitoring bio-signals.We compared the ECG signals recorded using PEDOT/EG/TA-2 electrodes with those recorded using three different types of commercial Ag/AgCl gel electrodes (Figure S15a-c, Supporting Information).The results indicate that the ECG recording quality of the PEDOT/EG/TA-2 electrodes was comparable to that of commercial Ag/AgCl gel electrodes.
Similarly, to evaluate the EMG recording ability of the PEDOT/EG/TA-2 gel electrodes, we performed EMG tests by placing PE and NE on the right forearm and GE on the elbow of a volunteer (Figure 6c), and recorded voltage changes produced by switching between relaxed and tense states of the flexor carpi muscle for over 90 s.The voltage versus time plots (Figure 6d) indicate that the PEDOT/EG/TA-2 gel electrodes and three different types of Ag/AgCl gel electrodes show similar potential changes when switching the state of the muscle (Figure S15d-f, Supporting Information).Notably, the PEDOT/EG/TA-2 gel electrodes show a higher SNR of EMG signals (40.0 ± 2.2 dB) compared to those commercial Natus (34.1 ± 0.9 dB), 3m (38.7 ± 0.8 dB), and Ambu (36.2 ± 0.3 dB) Ag/AgCl gel electrodes (Figure S15g, Supporting Information).
The detection of EMG signals for muscle motion can have essential applications in human-machine interfaces.For example, the PEDOT/EG/TA-2 gel electrodes can capture EMG signals generated by opening and closing a hand, and transmit them to control an anthropomorphic robotic hand in real-time (Figure 6e,f; Movie S4, Supporting Information).Therefore, the gel electrodes can serve as a user interface, which may be due to the softness and high conductivity of the PEDOT/EG/TA-2 gels electrodes.The EMG signal could also be used in combination with an EMG-controlled prosthetic device to enhance the strength of movements such as walking or lifting objects, or replace missing limbs, and therefore improve quality of life for patients with musculoskeletal impairments. [65]he long-term use of gels is important for continuous monitoring of vital signs, such as in the intensive care units and sleeping monitoring.Here, we evaluated the long-term stability of PEDOT/EG/TA-2 gels by skin-electrode impedance measurement and ECG recording after aging the gels (without encapsulation) at ambient temperature for seven days.The impedance versus frequency plots (Figure S16a, Supporting Information) indicate that the aged PEDOT/EG/TA-2 gels and commercial Ag/AgCl electrodes performed similarly (values listed in Table S3, Supporting Information).The aged gel electrodes (Figure S16b, Supporting Information) show well-defined ECG characteristic peaks, indicating the suitability of the PEDOT/EG/TA-2 gel electrodes for the long-term monitoring.

Conclusion
In summary, we demonstrated a soft, conductive, and antifreezing conductive polymer organohydrogel obtained from PE-DOT:PSS, EG, and TA.Physical crosslinking between the PE-DOT polymer chains endowed the PEDOT/EG/TA gel with an electrical conductivity of ≈6 S cm −1 and a low compressive Young's modulus of ≈20 kPa.The resulting PEDOT/EG/TA-2 gels exhibited anti-freezing properties and long-water retention properties.Using the PEDOT/EG/TA-2 gel, we demonstrated epidermal patch electrodes that showed low skin-electrode impedance at low frequency (1-100 Hz) and high quality for ECG and EMG signal recording.The PEDOT/EG/TA-2 gel electrodes show high potential for applications in human-machine interfaces.In addition, the PEDOT/EG/TA-2 gels showed outstanding long-term usability for ECG recording after being placed under ambient conditions for seven days.Moreover, the PEDOT/EG/TA-2 mixture exhibited good processability and could be printed or injected to prepare gels with the desired sizes and shapes for practical applications.This study provides an effective fabrication method for soft epidermal electronics.We believe that the challenges in gel-based soft electronics for longterm use include fine-tuning of mechanical, conductivity, and water-retention ability and the integration of the gel into adhesive patch electrodes.

Figure 2 .
Figure 2. Compressive stress-strain and Young's modulus of gels.a) Compressive stress versus strain with different amounts of TA (EG content is 25 wt.%), b) average compressive Young's modulus extracted from the slope of the stress-strain relationship in the range of 0-10% strain of gels with different amounts of TA (EG content is 25 wt.%), c) compressive stress versus strain with different amounts of EG (TA content is 12.5 wt.%),and d) average compressive Young's modulus extracted from the slope of the stress-strain relationship in the range of 0-10% strain of gels with different amounts of EG (TA content is 12.5 wt.%).Data for Young's modulus (n = 3) is reported as the mean ± standard deviation.

Figure 3 .
Figure 3. Conductivities of the gels with different amounts of a) TA and b) EG.Data for the conductivities (n = 5) is reported as the mean ± standard deviation.

Figure 4 .Table 3 .
Figure 4. a) Digital images of the fresh PEDOT/EG/TA-2 and PEDOT/TA gels.b) Digital images of PEDOT/EG/TA-2 and PEDOT/TA gels stored at ambient temperature for seven days.c) Mass change of PEDOT/EG/TA-2 and PEDOT/TA gels over seven days.d) Compressive stress versus strain of PEDOT/EG/TA-2 and PEDOT/TA gels stored at ambient temperature for seven days.Data for mass change (n = 3) are reported as mean ± standard deviation.

Figure 5 .
Figure 5. Swelling properties of PEDOT/EG/TA-2 gels.a) Digital images showing the volumetric shrinkage of the gel (cylindrical geometry) dehydrated at 140°C for a 1 ) 0, a 2 ) 2, a 3 ) 4, and a 4 ) 6 h.b) Volumetric shrinkage ratio versus baking time plots of PEDOT/EG/TA-2 gel dehydrated at 140 °C for 6 h.c) Digital images showing the volumetric swelling of the dried gel rehydrated in water at ambient temperature for c 1 ) 0, c 2 ) 3, c 3 ) 6, and c 4 ) 24 h.d) Volumetric swelling ratio versus soaking time plots of the dried gel rehydrated at ambient temperature for 48 h.V denotes the volume of the gel at a given baking or soaking time.V 0 is the initial volume of the gel before baking and V i represents the volume of the gel after baking for 6 h (shown in Figure 5a 4 ).

Figure 6 .
Figure 6.ECG and EMG signal recordings of PEDOT/EG/TA-2 gel electrodes.a) Configuration of ECG measurement in a volunteer.Positive (PE) and ground (GE) electrodes were attached to the left forearm, and a negative electrode (NE) was placed on the right forearm.b) ECG biopotentials (voltage vs time) were recorded using the PEDOT/EG/TA-2 gel electrodes.c) Configuration of EMG measurements on a volunteer.The EMG biopotentials between the electrodes were recorded using PE and NE on the right forearm and GE on the elbow.d) EMG monitoring using PEDOT/EG/TA-2 gel electrodes: EMG signals (PEDOT/EG/TA-2 gel electrodes) were used to control the motion of the robotic hand, that is, e) opening and f) closing.

Table 1 .
Summary of PEDOT/EG/TA gel samples used in this work.