Tailored wrinkles for tunable sensing performance by stereolithography

Conducting polymer hydrogel can address the challenges of stricken biocompatibility and durability. Nevertheless, conventional conducting polymer hydrogels are often brittle and weak due to the intrinsic quality of the material, which exhibits viscoelasticity. This property may cause a delay in sensor response time due to hysteresis. To overcome these limitations, we have designed a wrinkle morphology three‐dimensional (3D) substrate using digital light processing technology and then followed by in situ polymerization to form interpenetrating polymer network hydrogels. This novel design results in a wrinkle morphology conducting polymer hydrogel elastomer with high precision and geometric freedom, as the size of the wrinkles can be controlled by adjusting the treating time. The wrinkle morphology on the conducting polymer hydrogel effectively reduces its viscoelasticity, leading to samples with quick response time, low hysteresis, stable cyclic performance, and remarkable resistance change. Simultaneously, the 3D gradient structure augmented the sensor's sensitivity under minimal stress while exhibiting consistent sensing performance. These properties indicate the potential of the conducting polymer hydrogel as a flexible sensor.


| INTRODUCTION
3][4][5] Flexible pressure sensors, which incorporate flexible substrates and conductive fillers, exhibit changes in resistance in response to structural deformation caused by external stress, thereby presenting significant application prospects by improving the rigidity, hardness, and intransigency of traditional rigid semiconductor, metal, and ceramic sensors. [6,7]Currently, a range of flexible sensors have been fabricated, including those that combine flexible substrates such as polydimethylsiloxane (PDMS [7] ), thermoplastic polyurethane (TPU [8,9] ), and hydrogels, [10,11] conductive fillers such as carbon materials include carbon nanotubes graphene [12,13] , metals, [14,15] and metal oxides, [16,17] and conducting polymers [18] such as poly (3,4-ethylenedioxythiophene)  (PEDOT [19][20][21][22] ), polypyrrole (PPy [23] ), and polyaniline (PANI [11] ).As compared to traditional electronic inorganic fillers, conducting polymer possesses inherent conductivity and flexibility, making it well-suited for the preparation of flexible sensors. [18]However, it remains a challenge to achieve biocompatibility, combine the filler within the flexible substrate, decrease the intrinsic viscoelasticity of polymer substrate, and maintain high elastic compressibility for flexible sensors. [24]onducting polymer hydrogels become candidates for flexible sensors, [10,11,13,25] biosensors, [26,27] and electronic skin [7,[28][29][30][31][32] due to their flexibility, biocompatibility, and specific conductor electronic properties.For conducting polymer hydrogels, mechanical mixing [20] or in situ polymerization [23] is typically used to bind the conductive material to the hydrogels.However, the poor compatibility between conductive polymers and the hydrogel substrate, [24] in many cases, will lead to nonuniform electron transport and mechanical discrepancy of conducting hydrogels, compromising the mechanical and electrical properties.So it is a promising method to guide conducting polymers grow into hydrogel network to form interpenetrating polymer network (IPN) hydrogels by in situ polymerization. [33]lso, most polymer elastomers such as hydrogels have severe viscoelasticity, [34,35] which will increase the hysteresis and reduce the response speed of the sensors. [36,37]High hysteresis may fail to accurately reflect the sensing signal.To improve the sensing properties and reduce the hysteresis caused by viscoelasticity of flexible pressure sensors, [38] surface microstructure design plays a significant role.In recent years, [39] micro-structures such as micro-cracks [40] and wrinkles [41,42] have been successfully designed on the surface of sensors.Surface wrinkling is generally the simplest method, providing a low-cost, large-scale, and efficient strategy for adjusting sensor performance. [42]Various methods have been developed to produce wrinkles on hydrogels over the past few decades, including using different thermal annealing, [43] solvent immersion leading to irregular swelling, [44] mechanical pre-straining, [45] and template method. [46]he presence of air in the wrinkles can reduce the effective mechanical modulus, [47] which will improve sensor sensitivity. [48]Also, it enables the surface to elastically deform when pressure is applied, which can store and release energy reversibly, thereby greatly reducing the effect of the viscoelasticity of the matrix and helping the sensors recover quickly during the load-release process, enhancing the deformation ability.But the wrinkle size uncontrollable is still a burning question. [49]o achieve better mechanical performance, wrinkle morphology combined with high-resolution threedimensional (3D) structure has become an emerging method.The presence of microwrinkles can facilitate the formation of numerous hollow 3D structures with ample free volume.As a result of the significant amount of air trapped within these porous structures, the elasticity of the pressure sensor experiences a notable enhancement compared to its bulk counterpart.Furthermore, the interaction between the wrinkles creates supplementary conductive pathways upon the application of external stress, leading to evident improvements in the sensor's sensitivity and threshold value.The remarkable compressibility of these microstructures allows them to undergo deformation even under minimal applied pressure.However, it is a challenge to design.To prepare 3D structure, additive manufacturing provides new possibilities in addressing these challenges. [50]54][55] The high resolution (27 μm) and photopolymerized manufacturing process offer higher structural complexity, unit fineness, and printing efficiency than fused deposition modelling and direct ink writing.Enhanced printing precision enables the creation of intricate structures capable of detecting subtle pressure or deformation changes, thereby playing a pivotal role in the design of high-precision flexible sensors.This advancement positions it as a promising technology to fulfill the requirements of intricate and sophisticated flexible sensors.
In this study, we present a novel approach to fabricating 3D structure IPN conducting polymer hydrogel elastomer with controllable surface morphology using DLP and in situ polymerization.By introducing a microstructure throughout the material, the high hysteresis caused by the inherent viscoelasticity of the polymer has been significantly reduced, exhibiting excellent mechanical properties and cyclic stability.Additionally, the introduction of a 3D gradient structure has enhanced the sensor's sensitivity under minimal stress while maintaining consistent sensing performance.The wrinkles observed are a result of the cross-linking gradient in poly(ethylene glycol) diacrylate (PEGDA).This gradient refers to the varying degrees of cross-linking from the bottom to the top during the UV curing process.When UV light is irradiated from the bottom of the ink, light intensity loss occurs, resulting in different cross-linking degrees of the photocuring ink, known as the crosslinking gradient.And into hydrogel network, we grew PEDOT via a simple chemical catalytic [56] polymerization process.The resulting conducting polymer hydrogel elastomer exhibited wrinkle morphology without the need for a separate wrinkle design step, thus eliminating the uneven distribution of conductive fillers that can occur with traditional mechanical mixing processes during stretching and compression.Additionally, the intrinsic wrinkles on the PEGDA substrate were regulated by adjusting the treating time, enabling us to modify the surface morphology of the conducting polymer hydrogel and improve its performance.The resulting samples exhibited stable sensing properties and antifatigue behavior, making them as promising candidates for wearable sensors.By combining DLP and in situ polymerization, we have developed a versatile approach to controlling the surface morphology of conducting polymer hydrogels, opening up new possibilities for their use in a variety of applications.

| Preparation of PEDOT-PEGDA hydrogel
PEGDA is a biocompatible and biodegradable hydrogel that can be polymerized using an initiator under specific conditions such as heating, light, or radiation.Figure 1A illustrates the preparation process and experimental procedures of PEDOT-PEGDA hydrogel.Firstly, a PEGDA hydrogel substrate was printed using DLP technology.A UV-curable ink was formulated using PEGDA monomer (A-i) and photoinitiator diphenyl (2,4,6 trimethyl benzoyl) phosphine oxide (TPO) (A-ii).When the ray of UV light shines through the photocurable resin, with the light intensity decrease, there generated a cross-linking gradient in the PEGDA layer.This phenomenon creates a slight strain mismatch between adjacent layers, resulting in structural instability and deformation.The difference in shrinkage can cause compressive stress, resulting in the wrinkled patterns (A-iii).Next, the conducting polymer poly(3,4-ethyldioxythiophene) (PEDOT) was deposited on the pre-printed 3D PEGDA hydrogel substrate through chemical vapor deposition in the reactor by oxidation-reduction.The oxidative radical polymerization and acid-catalyzed polymerization were initiated using two polymerization initiators, oxidant (FeCl 3 ) and acid (HCl).Subsequently, the PEDOT-PEGDA hydrogel was prepared, and a remarkable change was observed in the hard PEGDA hydrogel substrate, which transformed into a compressible elastomer after PEDOT growth (A-iv).The swelling degree Q of PEGDA and PEDOT-PEGDA was 54.69% and 75.78%, respectively (see Supporting Information S1: Table S1).The swelling degree is the ratio of the swelling equilibrium volume of polymer in the solvent divided by the initial volume, which can be indicative of the degree of crosslinking of the polymer.A higher swelling degree reflects a lower cross-linking degree, which makes the molecular chain more flexible, resulting in a more flexible polymer.Based on the change in the swelling degree, it was evident that the cross-linking degree of PEGDA decreased after the growth of PEDOT.The degree of cross-linking affects both the glass transition temperature (T g ) and the decomposition temperature.The glass transition temperature has a direct impact on the mechanical properties of the material.Generally speaking, a higher glass transition temperature corresponds to a higher modulus for the material.Below this temperature, the molecular arrangement is relatively orderly, with limited movement between molecules.However, when the temperature surpasses the glass transition point, molecular movement becomes more active, and the molecular arrangement becomes more disordered.This results in the material being more prone to deformation, ultimately leading to an increase in its flexibility.To verify this, differential scanning calorimetry (DSC) tests were conducted on PEGDA, HCl-treated PEGDA, and PEDOT-PEGDA, which revealed a sequential decrease in T g (Supporting Information S1: Figure S1).The results suggest that the degree of cross-linking of PEGDA decreased after HCl treatment and PEDOT growth, leading to increased elasticity.The change of T g can infect many.Additionally, thermogravimetric analysis (TGA) tests were performed on the samples (Supporting Information: Figure S2).The thermal decomposition temperatures of HCl-treated PEGDA and PEDOT-PEGDA hydrogels were determined using TGA and DTG (thermogravimetric differential curve) analyses.DTG curves are the curves made by differentiating the time of each point on the TG curve, representing the weight change rate with temperature or time.The peak point on the DTG curve signifies the temperature or time at which the rate of weight change is the fastest for each step of weight loss or gain.Analyzing the DTG curves reveals that the rapid mass loss temperature of PEDOT-PEGDA is lower compared to that of PEGDA, indicating that the decomposition temperatures decreased in relation to the degree of cross-linking, where lower cross-linking led to lower decomposition temperatures.Fourier transform infrared (FT-IR) spectroscopy analysis (Supporting Information S1: Figure S3) showed the appearance of a Cl element peak in HCltreated PEGDA and PEDOT-PEGDA samples, as well as a red shift in the -OH peak, indicating the generation of intramolecular hydrogen bonds in the polymer, that will also cause decomposition temperature decrease.Also, we did the Raman characterization to analyse the functional group change of PEGDA, HCl-treated PEGDA, PEDOT-PEGDA hydrogel, and commercial PEDOT:PSS solution, shown in Supporting Information S1: Figure S4.In comparison of PEGDA with HCl-treated PEGDA, most peaks are one-to-one correspondence, except the peak in 2100 cm −1 , which represents the ─C═C═O bond.Same as HCl-treated PEGDA, the PEDOT-PEGDA hydrogel also exhibits this peak.In view of that the PEDOT-PEGDA preparation process also involves HCl, we consider this is caused by HCl.In comparison of PEDOT-PEGDA hydrogel to commercial PEDOT:PSS, the characteristic peak of PEDOT existed, which means PEDOT has grown in the PEGDA network successfully.To investigate the elasticity of rigid PEGDA hydrogels, EDOT monomer was removed from the system and the PEGDA sample was treated with HCl, resulting in increased elasticity.It was inferred that HCl can de-crosslink the PEGDA network, as demonstrated by Supporting Information S1: Figure S5.Additionally, PEGDA samples were treated with HCl for varying holding times (as shown in Figure 1B), and it was observed that as holding time increased, sample elasticity also increased.SEM analysis revealed changes in the volume and shape of wrinkles on the surface of PEGDA with increasing treating time, with the morphology of PEGDA treated for 16 h displaying excellent wrinkle shape.Therefore, 16 h were selected for EDOT polymerization (shown in Figure 1C).

| Different methods in creating wrinkles
Recently, people have summarized several approaches for fabricating wrinkling structures on polymeric elastomers (shown in Supporting Information S1: Table S2), such as mechanical pre-straining, chemical swelling, and thermal annealing.All those methods are developed by applying a modulus mismatch between the top layers and the bottom soft substrates. [42]That means it is essential to prepare more than two kinds of materials when creating wrinkles that will cause the complexity of the process (Figure 2).In this work, we create wrinkles not only on the surface but also on the interior of samples, with only one material through digital light printing.The wrinkles were generated from different cross-linking degrees when UV light through PEGDA, the intensity of UV light will attenuate and generate remnant stress in each layer, which is much easier than other methods.

| Analysis of PEDOT-PEGDA hydrogel
To characterize the cross-sectional microscopic morphology of the PEDOT-PEGDA hydrogel, we prepared section samples and characterized them by scanning electron microscopy (SEM), as shown in Figure 3E.Our observations revealed that wrinkles not only exist on the surface of the PEDOT-PEGDA hydrogel but also on the crosssection, with no differences in their size and morphology.This suggests that the effect of wrinkles can influence the entire sample.To characterize the distribution of elements in the PEDOT-PEGDA hydrogel, we conducted energy dispersive spectroscopy (EDS) mapping, as shown in Figure 3F.The results of EDS mapping revealed a uniform distribution of PEDOT in PEGDA hydrogel.The typical element of PEDOT is sulfur, while chlorine represents HCl.We did not observe the uneven distribution of either chlorine or sulfur elements in the PEDOT-PEGDA hydrogel.In situ SEM observations were conducted on the honeycomb primitive gradient structure of PEDOT-PEGDA hydrogels during compression (as shown in Figure 3B-D).Optical images of the compression process (Figure 3B) revealed that the structure can quickly recover to its initial shape after removing stress, indicating good elasticity.In situ SEM images during compression showed that the unit cell of the structure changed gradually from rhomboid to gyroid-like with increasing strain degree (Figure 3C).Furthermore, in situ SEM images (Figure 3D) showed that the wrinkles on the PEDOT-PEGDA hydrogel became increasingly compact with increasing strain.As the compression pressure increased, the wrinkles became more compact, resulting in an increase in contact area.
Benefiting from the wrinkle structure, under low levels of stress, the deformation of the sensor is primarily caused by the deformation of the unit cell, while the sensitivity is mainly provided by the variation in the connecting area of these wrinkles during compression.

| The sensitivity and mechanical performance of PEDOT-PEGDA hydrogel
Sensitivity (S) in the sensor was defined as the slope of the curve between ΔR/R 0 and applied pressure (P).It is a key parameter to evaluate the sensing performance.Structure design plays a decisive role in optimizing the sensitivity of sensor.More specifically, three different structures were selected for investigation, namely honeycomb primitive, honeycomb gyroid, and honeycomb primitive gradient (with density gradient 30%-40%, shown in Supporting Information S1: Figure S6).The density gradient pertains to the phenomenon where the porosity of a material gradually varies with increasing depth in a particular direction (from top to bottom), resulting in a stepwise change in porosity along the direction of the gradient, also known as the density gradient.PEDOT-PEGDA hydrogel sensors, measuring 1.5 × 1.5 × 0.6 (mm), were fabricated using these three structures, and their sensitivity was evaluated.The relative resistance variation (ΔR/R 0 ) of the sensors with the three structures was measured when stress was increased, as shown in Figure 4A-C.Our results demonstrate that the honeycomb primitive gradient exhibited the highest sensitivity (0.4 kPa −1 ), followed by honeycomb primitive (0.197 kPa −1 ) and honeycomb gyroid (0.14 kPa −1 ), with a stress range of approximately 20 kPa.As microstructures, wrinkles are instrumental in enhancing the sensitivity of the sensor under small stresses.As the stress increases, the unit cells with thinner wall thicknesses become connected with each other and compressed tightly.Upon further stress increase, the unit cells with thicker walls begin to deform and become compressed tightly.Once the unit cells are completely compressed, the wrinkles are also compacted, which increases the sensitivity of the sensor once again.Different levels of stress cause cells with different volume ratio deformation successively and connect with each other, allowing the gradient structure to maintain considerable sensitivity under different stresses.Thus, the gradient structure can provide a sensor with higher sensitivity under small stress.To evaluate the antifatigue performance of the material and structure, cyclic compression tests were conducted.Figure 4D illustrates the compression curve of PEDOT-PEGDA hydrogel with a honeycomb primitive gradient structure under a maximum strain of 50% for 600 cycles.The cyclic hysteresis loops from the 2nd to the 600th cycle were almost overlapping completely, indicating a good fatigue resistance.During periodic elastic deformation, the portion surrounded by the stress-strain curve is known as the hysteresis ring, which represents energy lost during the deformation process.Honeycomb primitive structure and honeycomb gyroid structure have a hysteresis loop area of about 0.3144 and 0.42635, respectively (Supporting Information S1: Figures S7 and  S8), while the honeycomb primitive gradient structure hysteresis loop area is only 0.08607.A small hysteresis ring area signifies that PEDOT-PEGDA hydrogel with the honeycomb primitive gradient structure has the least energy loss during deformation, and that the stress-strain is almost synchronized, indicating good sensor measurement accuracy.Figure 4E is the loading test for 4300 s.The changeless compression stress can prove our material has considerable good elasticity and antifatigue property, just as Figure 3D,E show, the wrinkles contribution on the surface and inner part of the hydrogel are beneficial to improve the material elasticity.During this test, the maximum value of compression stress remained relatively stable, reflecting the robustness and remarkable rebound and compressibility of the honeycomb primitive gradient structure and PEDOT-PEGDA hydrogel.

| The sensing performance of PEDOT-PEGDA hydrogel
The sensing performance of a PEDOT-PEGDA hydrogel with the honeycomb primitive gradient structure was investigated using dynamic cyclic loading experiments.Sensing tests were initially conducted under small strains of 1%-5% (Figure 5A), where the relative resistance (ΔR/R 0 ) of the sensor increased from 0.10 to 0.26.The gradient structure enabled the PEDOT-PEGDA sensor to detect deformation signals even at small strains, and the signal was stable and significant.Subsequently, sensing tests were performed under larger strains of 10%-50%, where the relative resistance (ΔR/R 0 ) changed from 0.3 to 0.81, and the signal remained stable and clear (Figure 5B).The PEDOT-PEGDA hydrogel sensor displayed reliable sensitivity and sensing signals under 1%-50% strain.Additionally, relative resistance variation was examined at different compression rates under 50% strain, and the sensing signal remained stationary (Figure 5C).The PEDOT-PEGDA hydrogel sensor exhibited stable sensing performance under different compression rates of 50-300 mm/min.Response and recovery time curves were also plotted under high compression rates (Figure 5D), with the honeycomb primitive gradient structure displaying short response and recovery times of 110 and 100 ms, respectively.The cyclic stability of the PEDOT-PEGDA hydrogel sensor was tested by recording the ΔR/R 0 signal during 600 compression cycles at the compression rate of 200 mm/min and the strain between 0% and 50%, which reveals that no increase in resistance due to the elastic and robust properties of the PEDOT-PEGDA hydrogel that enabled it to recover quickly under high compression rates and long-term stress cycles (Figure 5E).

| The application of PEDOT-PEGDA hydrogel sensor
3D printing can fabricate multitudinous structures sensors to fit human body for use in wearable devices.To demonstrate the application of the PEDOT-PEGDA hydrogel sensor in wearable technology and daily life, various samples were prepared.As motion tracking devices, a finger cot was used to measure the force when the finger pressed an object, and the sensor showed a consistent signal when volunteers applied the same force to a table while wearing the cot (Figure 6A).The half-finger cot structure was selected to measure the resistance change when a force was applied from soft to hard, and the results showed a significant difference with an increase in force (Figure 6B).We can also attach them on human fingers; the resistance change when the finger was straight, bent at 45°, and bent at 90°was recorded accurately by fixing the film sample to the index finger joint (Figure 6C), where the change in resistance was due to the increase in resistance when the PEDOT-PEGDA film was stretched by the finger.The greater the bending angle, the greater the resistance change.Moreover, the film sample was attached to a stereo and human neck.When the stereo played the drum sound, the volunteer said "hello" and "good," with the PEDOT-PEGDA hydrogel film successfully perceiving and recording the tiny vibration of the sound from the regularity ΔR/R 0 signal (Figure 6D).As a health monitoring, the prepared PEDOT-PEGDA film sample was affixing to the wrist artery (Figure 6E), and the pulse of volunteers was monitored in real-time through the resistance change, which was 60 bpm (rest) and 93 (after exercise) bpm per minute.Additionally, in daily life, the PEDOT-PEGDA hydrogel could be used for humidity monitoring due to its good water absorption ability.The hydrogel was placed in a confined space, and a small humidifier was used to control the air humidity (Figure 6F).The resistance decreased significantly when the humidity increased from 58% to 88%, due to the water molecules entering the conducting polymer hydrogel, which caused an increase in conductivity.Upon entering the conducting polymer hydrogel, ions facilitate an increase in its conductivity.Nevertheless, owing to the rapid absorption rate of water in comparison to that of drying, resistance alterations during the drying process do not align with those occurring during absorption.However, the resistance eventually reverts to its pre-absorption state.In conclusion, we use a novel method to fabricate PEDOT-PEGDA conducting polymer hydrogel elastomer with controllable wrinkle morphology by using DLP and in situ polymerization and have developed a versatile approach to controlling the surface morphology of conducting polymer hydrogels, giving them an even conducting polymer distribution and excellent elasticity, that can provide samples with stable sensing properties and antifatigue behavior, making them promising candidates for wearable sensors.By changing the structure, we can open up new possibilities for their use in a variety of applications.

F I G U R E 1
The preparation of PEDOT-PEGDA hydrogel.(A) The schematic of DLP 3D printing technique and the preparation of PEDOT-PEGDA hydrogel.(A-i) The PEGDA cross-linked network.(A-ii) The monomers and photoinitator of photocurable resin.(A-iii) The optic images of 3D PEDOT-PEGDA hydrogel during compressing.(A-iv) Wrinkled film was generated by different cross-linking degrees in layers.(B) The optic images of HCl-treated PEGDA in different time.(C) The SEM photos of HCl-treated PEGDA morphology in different time.

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I G U R E 2 Brief schematics of traditional wrinkles fabrication methods and the method in this work.(A) A wrinkled film was generated by pre-straining and depositing different modulus material through chemical vapor deposition (CVD).(B) A wrinkled film was generated by pre-straining and UVO ultraviolet ozone (UVO) radiation.(C) A wrinkled film was generated by chemical swelling.(D) A wrinkled film was generated by thermal annealing.

F I G U R E 3
Analysis of PEDOT-PEGDA hydrogel.(A) The schematic of the samples morphology change during compression; (A-i) without wrinkles and (A-ii) with wrinkles.(B-D) The in situ SEM images of honeycomb primitive gradient structure during compression (strain from 10% to 50% and 1% to 5%).(E) The SEM images of PEDOT-PEGDA hydrogel, (E-i) side and (E-ii) cross section.(F) The EDS mapping images of PEDOT-PEGDA hydrogel, (F-i) the SEM image and (F-ii-F-vi) the elements distribution.

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I G U R E 4 The sensitivity and mechanical performance.The sensitivity of the PEDOT-PEGDA hydrogel in different structure: (A) honeycomb primitive structure.(B) honeycomb gyroid structure.(C) honeycomb primitive gradient structure.(D) Strain-stress curves of the honeycomb primitive gradient structure sensor at 50% strain for 600 compression cycles.(E) The compression stress-time curves of the honeycomb primitive gradient structure sensor at 50% strain for 4300 s cycles.

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I G U R E 5 The sensing performance of PEDOT-PEGDA hydrogel in honeycomb primitive gradient structure.(A) Under small compression strains (1%-5% with a 2% increment) and (B) under larger strains (10%-50% with a 20% increment).(C) Resistance variation of the sensor at 50% strain under different compressive rates.(D) Response time and recovery time in high-speed dynamic loading.(E) Cyclic stability tests of the sensing property under repeated compression up to 4300 s at 50% strain.

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I G U R E 6 The application of PEDOT-PEGDA sensor.(A) Resistance change can detect the pressure from finger press finger cot.(B) When press half of the finger cot from soft to hard, different resistances can detect different pressures.(C) The resistance change of PEDOT-PEGDA film can reflect the finger bending angle.(D) The PEDOT-PEGDA film can detect voice from the stereo and the throat.(E) The PEDOT-PEGDA film structure can detect human's wrist pulse.(F) In different humidity, the PEDOT-PEGDA film has different resistance, which can reflect the humidity change.