Ultra-stretchable, tough, and crack-resistant polyoxometalates-composited hydrogels for wearable sensors
0 Abstract
Hydrogels with good ionic conductivity and high stretchability hold great promise for flexible sensors, but are challenged by the low toughness and poor crack resistance, which severely limit their performance under complex mechanical conditions. Herein, we report an ionically conductive polyoxometalates (POMs)-composited hydrogel fabricated by incorporating chitosan oligosaccharide-modified silicotungstic acid (COS@SIW) nanocomplexes into a polyacrylamide (PAM) network. The incorporation of COS effectively enhances the interfacial bonding between COS@SIW and the PAM matrix through abundant electrostatic and hydrogen-bonding interactions among COS, SIW, and PAM chains, facilitating efficient stress transfer and energy dissipation. As a result, the obtained POM-composited hydrogel exhibits integrated mechanical properties, including ultrahigh stretchability (2,423%), high toughness (3.77 MJ·m-3), and excellent crack resistance (fracture energy of 8.3 kJ·m-2). Moreover, the hydrogel demonstrates a high ionic conductivity of 0.17 S·m-1, attributed to the intrinsic proton mobility of SIW. The resulting hydrogels exhibit superior strain sensitivity with a wide working range, rapid response, and excellent reliability, enabling their application as wearable sensors for monitoring diverse human motions. Furthermore, the hydrogel can function as a bioelectrode for accurate and reliable detection of electrocardiogram signals. This work provides a new strategy for designing ionically conductive hydrogels with high stretchability, toughness, and superior crack resistance, offering promising opportunities for advanced wearable sensing platforms.
Keywords
INTRODUCTION
Technological advances in healthcare and the rise of personalized health management have highlighted the importance of real-time monitoring of human motion[1-3] and physiological signals in chronic disease diagnosis and health assessment[4,5]. Flexible and wearable sensors capable of converting external stimuli, such as strain and stress, into electrical signals have been extensively developed to meet these needs[6,7]. In practical applications, there flexible sensors usually from large deformation and thus are susceptible to crack propagation, which in turn drives the demand for advanced sensing materials[8,9]. Hydrogels, composed of three-dimensional polymer networks and water, have emerged as promising candidates for flexible sensors owing to their excellent biocompatibility, tissue-like softness, and structural tunability[10,11]. However, conventional hydrogels are typically electrically insulating and mechanically weak, which significantly limits their further application in flexible sensing technologies[12].
Recently, various conductive hydrogels (including ionic and electronic conductivity) have been developed through the incorporation of ionic salts[13], conductive fillers[14], or conducting polymers[15]. Among these, ionically conductive hydrogels offer the advantages of biomimetic ion transport and stable conductivity, making them highly suitable as bioelectrical materials for wearable sensors[16]. However, common single-network ionically conductive hydrogels are inherently brittle with low elongation and toughness[12]. To enhance the mechanical performance, different strategies - such as the introduction of double-network (DN) structures[17,18], nanocomposite reinforcement[19,20], multiple dynamic crosslinking[21,22], and other energy dissipation mechanisms - have been employed to create highly stretchable and tough hydrogels. Despite these advances, most tough hydrogels remain flaw-sensitive, leading to a marked decrease in stretchability and mechanical integrity upon the formation of small cracks, which are unavoidable in practical use. To overcome this limitation, researchers have been devoted to improving the crack-resistance ability of hydrogels by forming high entanglements[23], constructing anisotropic structures[24] or introducing fiber reinforcement[25]. Nevertheless, these approaches often yield hydrogels with either high stiffness (elastic modulus > 100 kPa, exceeding that of soft tissues) or limited elongation (< 1,000%). More recently, some highly stretchable and crack-insensitive hydrogels have been achieved by strengthening the entanglements and interactions between polymer chains and porous nanopillars[26]. Yet, these systems rely on specialized nanofillers and typically lack ionic conductivity. Therefore, developing ionically conductive hydrogels that simultaneously combine high stretchability, toughness, and crack-resistance remains a significant challenge.
Polyoxometalates (POMs)[27-29] are a class of nanoscale metal-oxygen clusters featuring well-defined molecular structures, high electronegativity, and diverse physicochemical properties. Notably, they exhibit excellent proton conductivity owing to their low proton dissociation energy and abundant proton-hopping sites on the surface[30]. These unique features make POMs versatile and promising building blocks for constructing composite materials with polymeric components. Lately, POMs have been incorporated into hydrogel systems to construct conductive hydrogels[31,32]. The introduction of POMs has largely improved the stretchability and toughness of the obtained POMs-composited hydrogels by forming multiple electrostatic or hydrogen-bonding interactions with polymer matrixes[32]. Unfortunately, the previously reported POMs-composited hydrogels barely possess flaw-tolerant and crack-resistant capacity, primarily due to weak interfacial interactions between POMs and the hydrogel network. Therefore, strengthening the interfacial interactions between POMs and polymer chains may offer a promising approach to achieve crack-insensitive POMs-composited hydrogels.
Herein, we report an ionically conductive POMs-composited hydrogel constructed by incorporating chitosan oligosaccharide (COS)-modified Keggin-structure POM (silicotungstic acid, SIW) into polyacrylamide (PAM) network. The protonated amino and hydroxyl groups on the COS interact strongly with SIW through electrostatic and hydrogen-bonding interactions, forming stable COS@SIW nanocomplexes. When embedded within the PAM network, abundant hydrogen bonds between COS@SIW and PAM chains provide efficient energy dissipation during deformation. Meanwhile, the robust interfacial bonding between COS@SIW and the hydrogel matrix effectively relieves stress concentration at crack tips via stress transfer. As a result, the obtained POMs-composited hydrogel exhibits excellent stretchability (2,423%), high toughness (3.77 MJ·m-3), and superior insensitivity toward different types of notches (fracture energy of 8.3 kJ·m-2). Moreover, owing to the intrinsic high proton mobility of SIW, the hydrogel also has good ionic conductivity of 0.17 S·m-1. Benefiting from these superior performances, the hydrogel demonstrates high strain sensitivity with a broad working range, rapid response and good reliability. This hydrogel can serve as a wearable sensor to accurately monitor subtle and intense human movements, as well as flexible electrodes to record electromyography (EMG) and electrocardiogram (ECG) signals. This study offers a new paradigm for developing highly stretchable, crack-resistant, and ionically conductive hydrogels, holding significant promise for applications in flexible electronics, health monitoring, and human-machine interfaces.
EXPERIMENTAL
Methods
Materials
SIW (H4SiW12O40, ≥ 99.9%) was purchased from Sigma. COS was provided by Shandong Haidebei Biotechnology Co., Ltd. Acrylamide (AM), and ammonium persulfate (APS) were obtained from Aladdin Biochemical Technology Co., Ltd. All chemicals were of analytical grade and used without further purification.
Preparation of PAM/COS@SIW hydrogel
The PAM/COS@SIW hydrogel was prepared via a one-pot synthesis method. First, COS was dissolved in
Characterization
Zeta potentials of SIW and COS aqueous dispersions were measured using a laser particle size analyzer. Fourier transform infrared spectroscopy (FTIR) were recorded over 400-4,000 cm-1 to characterize the chemical structures of the samples. The microstructure of the hydrogel samples was observed by scanning electron microscopy (SEM, S-4800II) and transmission electron microscopy (TEM, HT7700). For SEM observation, the hydrogel specimens were first immersed in deionized water for 24 h, followed by freeze-drying to remove water prior to imaging. The freeze-dried samples were sputter-coated with a thin Au layer before SEM measurement. Raman spectra were obtained using a Raman microscope (Horiba Jobin Yvon Xplora PLUS).
Mechanical test
The mechanical performance of the hydrogels was evaluated using a multifunctional mechanical testing machine (Shimadzu AGS-X, Japan) equipped with a 100 N load cell. Cylindrical hydrogel samples (8 mm diameter, 10 mm height) were subjected to uniaxial and cyclic compression at a crosshead speed of
The tear resistance of the hydrogels was assessed using a trouser-tear test. Rectangular hydrogel strips
where F is the maximum tearing force, t is the thickness of hydrogel, L is the tearing displacement, and Lbulk is the initial notch length.
The notch insensitivity of PAM/COS@SIW hydrogels was evaluated using a pure tear test. Rectangular samples (20 mm × 10 mm × 2 mm) were notched into three geometries - circular, triangular, and 1-shaped - with a width of 2 mm (one-fifth of the sample width). Uniaxial tensile tests were conducted at a rate of
where W was the toughness calculated by integration of the stress vs. strain of unnotched specimens, c was the length of the notch, λ was the fracture strain of the notched specimen.
The fatigue threshold of the hydrogels was tested using the single-notch method. At first, unnotched dumbbell-shaped samples were used to perform the cyclic tensile test at an applied stretch of λA. The strain energy density (W) of the unnotched sample under the Nth cycle of applied stretch λA can be calculated by the following equation[35]:
in which S is the measured nominal stress. In this work, the unloading part of the cyclic tensile loading-unloading curves was used to calculate the W. Thereafter, the same stretch λA is applied on the notched sample with an initial crack length c0 (less than 1/5 of the sample width) with the cycle number N. The applied energy release rate (G) in the notched can be calculated as[35]:
in which κ is a varying function of the applied stretch as κ = 3/
Electrochemical test
The conductivity of the PAM/COS@SIW hydrogels was measured using an electrochemical workstation (CHI760E, CH Instruments, China) via electrochemical impedance spectroscopy (EIS). The conductivity (σ) was calculated according to[36]:
where L, R, and S represent the sample length (cm), resistance (Ω), and cross-sectional area (cm2) of the hydrogel sample, respectively.
Biocompatibility evaluation
Hematoxylin and eosin (H&E) staining was performed to evaluate in vivo tissue responses to the PAM/COS@SIW hydrogel. Male mice (6-8 weeks old) were obtained from Beijing HFK Bioscience Co., Ltd., and all animal procedures were approved by the Ethics Committee of Yanshan University. Sterilized hydrogel samples were implanted subcutaneously into the left flank of mice, with normal skin tissues serving as controls. After 3 days, implants with surrounding tissues were harvested, fixed in 10% neutral buffered formalin, paraffin-embedded, sectioned, and stained with H&E following standard histological procedures. Images were acquired using an optical microscope (BX51, Olympus, Japan). A preliminary human skin patch test was further conducted by attaching a hydrogel sample to the forearm of a healthy volunteer for 24 h. After removal, the contact area was visually inspected for signs of erythema or edema. This test served as a short-term qualitative assessment and was not intended as a comprehensive biocompatibility evaluation.
Sensing performance and application
The strain-sensing performance of PAM/COS@SIW hydrogels was evaluated by integrating a universal testing machine (Shimadzu AGS-X, Japan) with an LCR meter (E4980AL, Keysight, USA), enabling simultaneous mechanical loading and electrical signal acquisition. The relative resistance change (∆R/R0) was calculated as[36]:
where R is the real-time resistance during deformation and R0 is the initial resistance of the hydrogel. The gauge factor (GF), representing strain sensitivity, was determined by[37]:
where ε is the strain of the hydrogel.
Wearable sensor was assembled by connecting copper wires to both ends of the hydrogel specimen, which was then encapsulated between two layers of VHB tape to reduce environmental impact. For human motion monitoring, the sensors were directly attached to the skin of a volunteer, and their sensing performance was evaluated using an electrochemical workstation.
Physiological signal monitoring
For practical applications, the PAM/COS@SIW hydrogel was cut into rectangular pieces (20 mm × 20 mm × 2 mm) and employed as flexible bioelectrodes for electrophysiological signal recording. EMG signals were acquired using a multichannel physiological signal acquisition and processing system (RM 6240 XC, Chengdu Instrument Factory, China). During EMG measurements, the hydrogel electrodes were attached to the volunteer’s skin in a three-electrode configuration. Connecting wires were secured with insulating tape to minimize motion-induced artifacts, and the opposite ends were connected to the signal acquisition system, which was interfaced with a computer for real-time data recording. As no specific national or institutional guidelines currently exist for motion and electrophysiological signal experiments in the country where this study was conducted, all participants provided informed consent prior to testing.
All quantitative results are presented as mean ± standard deviation (SD), obtained from at least three independent measurements. Error bars shown in bar graphs represent the SD, reflecting experimental repeatability.
RESULTS AND DISCUSSION
Preparation and characterization of PAM/COS@SIW hydrogels
The preparation process and network structure of the PAM/COS@SIW hydrogel are illustrated in Figure 1A. Keggin-type POM, SIW, was employed owing to its unique polyanionic nanocluster structure, uniform size distribution, and high ionization propensity in aqueous media[38]. Initially, SIW was added into a predetermined concentration of COS solution to prepare COS@SIW nanocomplexes. The precursor solution was acidic, under which COS and SIW carried opposite surface charges, as confirmed by their zeta potentials [Supplementary Figure 1]. Consequently, strong electrostatic interactions and hydrogen bonds occurred between COS and SIW, resulting in the formation of stable COS@SIW nanocomplexes. The strong interfacial bonding between COS and SIW was further verified by FTIR. Comparison between COS, SIW and COS@SIW revealed that the characteristic amide I and amide II bands at 1,622 and 1,512 cm-1 in COS disappeared with the emergence of a new peak at 1,636 cm-1 corresponding to the bending vibration of –NH3+ in COS@SIW[39], and meanwhile the characteristic peaks of SIW at 976 and 910 cm-1, attributed to W=O stretching and W–O–W framework vibrations respectively, shifted to 970 and 913 cm-1 in COS@SIW, confirming the formation of strong hydrogen bonds and electrostatic interactions between COS and SIW [Supplementary Figure 2][40]. TEM images reveal characteristic nanoscale COS@SIW domains [Supplementary Figure 3]. Subsequently, the monomer AM and the thermal initiator APS were introduced, and the mixture was polymerized at 60 °C for 12 h to obtain the PAM/COS@SIW hydrogel. In this system, the COS@SIW serviced as multifunctional cross-linkers to crosslink PAM chains via chain entanglements and hydrogen bonds, forming a dense supramolecular network.
Figure 1. (A) Schematic illustration of the synthesis process and crosslinked structure of the PAM/COS@SIW hydrogel; (B) FTIR spectra of AM, COS@SIW, PAM and PAM/COS@SIW hydrogels; (C) SEM image and (D) the corresponding EDS mapping of the PAM/COS@SIW hydrogel; (E) HRTEM image of the PAM/COS@SIW hydrogel; (F-H) Schematic representations of the remarkable properties of PAM/COS@SIW hydrogel, including excellent toughness (F), outstanding stretchability (G), and pronounced notch insensitivity (H); (I) Schematic application of the PAM/COS@SIW hydrogel in human motion monitoring. PAM: Polyacrylamide; COS: chitosan oligosaccharide; SIW: silicotungstic acid; FTIR: Fourier transform infrared spectroscopy; AM: acrylamide; SEM: scanning electron microscopy; EDS: energy-dispersive X-ray spectroscopy; HRTEM: high-resolution transmission electron microscopy; APS: ammonium persulfate.
To reveal the microstructure and internal interactions in PAM/COS@SIW hydrogel, a series of characterizations are conducted. FTIR spectra of AM, COS@SIW, PAM and PAM/COS@SIW hydrogels are presented in Figure 1B. The absorption peaks of –CH=CH2 at 1,605 cm-1 for AM completely disappeared in the FTIR spectra of PAM and PAM/COS@SIW hydrogels, confirming the successful polymerization of hydrogel precursors[41]. In addition, AM exhibited characteristic peaks at 3,335, 3,162 and 1,656 cm-1, corresponding to the symmetric and asymmetric stretching vibrations of N–H and C=O stretching vibration, respectively[42]. Compared with AM, the N–H and C=O stretching peaks in PAM hydrogel shifted to 3,333, 3,185 and 1,648 cm-1, respectively[43], demonstrating hydrogen bonds among PAM chains. Upon incorporation of COS@SIW into the PAM network, the characteristic peaks of COS@SIW at 970 cm-1 (W=O), 913 cm-1 (W–O–W) and 1,636 cm-1 (–NH3+) shifted to 973, 922 and 1,600 cm-1,
Mechanical properties of PAM/COS@SIW hydrogels
The incorporation of COS@SIW markedly enhanced the mechanical properties of PAM/COS@SIW hydrogels. As shown in Figure 2A(i) and (ii), the strip-shaped sample could be stretched to ten times its original length, exhibiting large elongation. The hydrogel sample with a thickness of 1 mm and a width of
Figure 2. (A) Photos of the striped hydrogel sample: (i and ii) sustaining large elongation and (iii) lifting a 200 g weight. All optical photographs were taken by the authors using personal mobile devices; (B) Typical tensile stress - strain curves and (C) the corresponding elastic modulus and toughness of PAM, PAM/SIW, PAM/COS, and PAM/COS@SIW hydrogels. Data are presented as mean values ± SD, n = 3 independent samples; (D) Tensile loading - unloading curves and (E) the corresponding dissipated energies and hysteresis ratios of the PAM/COS@SIW hydrogel at different strains; (F) Ten consecutive loading - unloading cycles and (G) the corresponding dissipated energies and maximum stresses of the PAM/COS@SIW hydrogel at a maximum strain of 600%; (H) Compression curves of the PAM/COS@SIW hydrogel at different strains; (I) Ten consecutive cyclic compression curves of the PAM/COS@SIW hydrogel at a compressive strain of 80%. PAM: Polyacrylamide; SIW: silicotungstic acid; COS: chitosan oligosaccharide; SD: standard deviation.
The influence of COS@SIW content on the mechanical properties of the PAM/COS@SIW hydrogel was investigated [Supplementary Figure 6]. As the COS@SIW content increased, the tensile strain and toughness of the PAM/COS@SIW hydrogel first increased and then decreased. An appropriate increase in COS@SIW content improved the crosslinking density, whereas excessive incorporation induced aggregation, which in turn compromised the mechanical performance[51]. The COS-to-SIW ratio also influenced the mechanical performance of the PAM/COS@SIW hydrogel [Supplementary Figure 7]. Increasing this ratio enhanced the tensile strain and toughness by strengthening the interfacial interactions between COS@SIW and the polymer network. However, further increases led to a slight reduction in tensile strain, which can be attributed to the decreased segmental mobility of polymer chains[52]. Additionally, the mechanical properties of the PAM/COS@SIW hydrogel could be further regulated by adjusting the AM content [Supplementary Figure 8]. When the AM content increased from 33 wt% to 42 wt%, both the tensile strain and toughness were markedly enhanced. However, at higher AM content, the excessive network densification restricted polymer chain mobility, leading to a reduction in tensile strain[53]. Based on the above comprehensive evaluation, the PAM/COS@SIW hydrogel prepared with 42 wt% AM, a COS-to-SIW ratio of 1:3 and 4 wt% COS@SIW exhibited the superior combination of tensile strength (324 kPa), elastic modulus (62 kPa), stretchability (2,423%) and toughness (3.77 MJ·m-3), which was selected for subsequent research.
Cyclic tensile loading - unloading tests were conducted to evaluate the mechanical energy dissipation and cycling stability of the PAM/COS@SIW hydrogel. Successive cyclic tensile tests under different maximum strains revealed pronounced hysteresis behavior, with the hysteresis area increasing as the maximum strain rose [Figure 2D]. The dissipated energy improved from 43.37 to 171.52 kJ·m-3 with the increase of strain from 200% to 1,000%, while the hysteresis ratio maintained a stable value of 26% [Figure 2E]. These findings suggest that PAM/COS@SIW hydrogel can dissipate substantial energy under large strains to withstand applied loads, while sustaining good resilience upon unloading. Furthermore, ten successive loading - unloading cycles under 600% strain were performed [Figure 2F and G]. In the first cycle, a large hysteresis loop was observed with a dissipated energy of 192 kJ·m-3, manifesting the effective rupture of noncovalent interactions within the network to dissipate mechanical energy. After the first cycle, the subsequent nine cycles showed only minor hysteresis loops and negligible stress reduction[54]. These results confirm the good cycling stability of the crosslinked network structure even as partial crosslinking points were destroyed.
The compressive performance of PAM/COS@SIW hydrogels was also examined. Comparison of the compression curves of different hydrogels showed that the PAM/COS@SIW hydrogel had the highest compressive stress (420 kPa) at 80% compressive strain [Supplementary Figure 9]. The compressive loading - unloading cycles at different strain levels demonstrated that the PAM/COS@SIW hydrogel could dissipate more mechanical energy under larger strain [Figure 2H]. Moreover, ten cyclic compressive tests at a maximum strain of 80% showed the overlapping hysteresis loops [Figure 2I], further demonstrating the excellent toughness and resilience[55].
Crack-resistance of PAM/COS@SIW hydrogels
The crack-resistance ability is a critical characteristic that allows materials to resist external mechanical damage, prolonging their service lifetime[56]. The PAM/COS@SIW hydrogel exhibited excellent toughness in terms of tearing and fracture energy. To investigate tear resistance, trouser-tear tests were conducted on four types of hydrogels: PAM, PAM/SIW, PAM/COS, and PAM/COS@SIW. As shown in Figure 3A, a trouser-shaped notched PAM/COS@SIW hydrogel sample can be stretched to a displacement of more than 190 mm, which is much larger than that of PAM (39 mm), PAM/SIW (48 mm), PAM/COS (75 mm) hydrogel samples. Figure 3B shows that the tearing energy of the PAM/COS@SIW hydrogel (8.3 kJ·m-2) was 3.6, 1.5, and 1.4 times that of the PAM, PAM/SIW, and PAM/COS hydrogels, respectively, demonstrating strong tear tolerance. Additionally, the single-edge notched method was employed to evaluate the crack propagation insensitivity of the PAM/COS@SIW hydrogel. Rectangular samples with notches corresponding to one-fifth of the sample width were subjected to uniaxial tensile tests under three different notch geometries. As shown in Figure 3C, hydrogel samples with different types of notches (circular, triangular, and 1-shaped) can be highly stretched, with circular-notched sample achieving tensile strain of ca. 1,000%. Based on the crack propagation strains of the notched samples and the stress - strain curves of the unnotched samples, the calculated fracture energies of different notched samples were larger than 4 kJ·m-2 [Figure 3D]. Particularly, the PAM/COS@SIW hydrogel with a circular notch had the highest fracture energy of 8.3 kJ·m-2. As visually displayed in Figure 3E, when the notched PAM/COS@SIW sample was subjected to loading perpendicular to the notch direction, it was stretched to more than 1,000% without propagation of the crack tip, showing outstanding crack propagation insensitivity. The notched PAM/COS@SIW sample exhibited similar stress - strain curves at different stretching rates [Figure 3F and Supplementary Figure 10], further demonstrating high crack propagation insensitivity. Figure 3G presents the continuous cyclic tensile tests at 100% strain for 100 cycles under straight-notch conditions, confirming remarkable fatigue resistance of the PAM/COS@SIW hydrogel. To further quantify the fatigue resistance, a single-notch cyclic loading protocol was employed to determine the fatigue threshold, defined as the critical energy release rate below which crack propagation ceases. As shown in Supplementary Figure 11, the fatigue threshold of the PAM/COS@SIW hydrogel was determined to be 57.58 J·m-2, providing quantitative evidence for its strong resistance to cyclic crack growth. The above results indicate that the rigid COS@SIW nanocomplexes effectively mitigate stress concentration at crack tips, thereby preventing crack propagation[26]. This arises from the fact that individual COS@SIW nanocomplexes act as multifunctional crosslinkers capable of bridging multiple polymer chains. Owing to the dense electrostatic and hydrogen-bonding interactions between PAM chains and COS@SIW nanocomplexes, applied stress can be efficiently transmitted across the polymer network. Meanwhile, these reversible interactions facilitate mechanical energy dissipation and delocalization of stress concentrations, particularly near crack tips, thereby markedly enhancing the toughness and crack insensitivity of the hydrogel network. Compared with the previously reported notch-insensitive hydrogels[30,57-63], the PAM/COS@SIW hydrogel exhibited the best combination of ultra-stretchability and fracture energy [Figure 3H]. Taken together, with the high stretchability and excellent mechanical durability, the PAM/COS@SIW hydrogel is regarded as promising materials for flexible and wearable electronics.
Figure 3. (A) Tearing curves (tearing force vs. displacement) and (B) the corresponding tearing energies of PAM, PAM/SIW, PAM/COS, and PAM/COS@SIW hydrogels. Data are presented as mean values ± SD, n = 3 independent samples; (C) Stress - strain curves and (D) the corresponding fracture energies of the PAM/COS@SIW hydrogel with different notch types: unnotch, circular notch, triangular notch, and 1-shaped notch. Data are presented as mean values ± SD, n = 3 independent samples; (E) Optical images of the PAM/COS@SIW hydrogel with a circular notch stretched to 500% and 1,000% strains. All optical photographs were taken by the authors using personal mobile devices; (F) Stress - strain curves of the PAM/COS@SIW hydrogel with a circular notch under four different strain rates; (G) Successive loading - unloading curves of the PAM/COS@SIW hydrogel with a single-edge notch under a fixed strain of 100% for 100 cycles; (H) Comparison of the fracture energy and fracture strain of the present hydrogel with those of previously reported hydrogel systems. PAM: Polyacrylamide; SIW: silicotungstic acid; COS: chitosan oligosaccharide; SD: standard deviation.
Electrical and sensing properties of PAM/COS@SIW hydrogels
Different from typical inert nanofillers, SIW as a heteropoly-acid/POM introduces abundant Brønsted-acid sites and enriches mobile protons in hydrated domains, thereby increasing the ionic carrier density of the PAM/COS@SIW hydrogel[64]. Moreover, proton transport proceeds via the coexistence of vehicular diffusion and Grotthuss-type hopping along hydrogen-bonded water networks[65], which are strengthened by the highly hydrophilic, hydrogen-bond-rich interfacial microenvironment created around uniformly dispersed and effectively connected COS@SIW nanocomplexes, forming continuous “proton-rich” pathways with reduced tortuosity (percolation-like transport)[66]. Notably, these ionic pathways are coupled with the mechanically dissipative supramolecular network: dynamic ionic/hydrogen-bond interactions simultaneously promote stress transfer/energy dissipation and help maintain pathway continuity under large deformation, enabling reliable wearable sensing[67]. EIS was performed to quantitatively evaluate the ionic conductivity [Supplementary Figure 12]. The Nyquist spectra were interpreted using an equivalent circuit model of R1 - (CPE1//R2), where R1 corresponds to the bulk ionic resistance, while the low-frequency response is mainly associated with interfacial polarization described by the R2 - CPE1 element; a pronounced Warburg-type diffusion tail was not observed within the measured frequency window. Compared with PAM, PAM/COS and PAM/SIW hydrogels, the PAM/COS@SIW hydrogel exhibited the highest conductivity of ~0.17 S·m-1 [Supplementary Figure 13A]. The conductivity of the PAM/COS@SIW can also be regulated by COS due to its ion-adsorption capability [Supplementary Figure 13B]. Apparently, the conductivity of the PAM/COS@SIW hydrogel gradually increased with the increase of the COS@SIW content [Supplementary Figure 13C]. When a rectangular strip of the PAM/COS@SIW hydrogel was connected in a closed circuit with a red light-emitting diode (LED), the LED lit up smoothly upon applying power, demonstrating the good ionic conductivity [Figure 4A]. To benchmark the overall performance of PAM/COS@SIW in a broader context, we compared it with previously reported ionically conductive hydrogels that exhibit comparable mechanical robustness and stretchability. As summarized in Supplementary Table 1[30,57-63,68-74], the PAM/COS@SIW hydrogel achieved a competitive ionic conductivity while retaining ultrahigh stretchability and mechanical robustness, positioning it among the high-performance ionically conductive hydrogel systems reported to date. This balanced combination of conductivity and deformability is particularly advantageous for large-strain sensing and wearable applications.
Figure 4. (A) Images showing the luminance of LEDs when the PAM/COS@SIW hydrogel was used as a conductor in series circuits. All optical photographs were taken by the authors using personal mobile devices; (B) Relative resistance variation (ΔR/R0) of a PAM/COS@SIW hydrogel as a function of tensile strain and the corresponding GF; (C) Stepwise relative resistance response of the PAM/COS@SIW hydrogel during incremental strain loading (0%-100%) and subsequent unloading to 0%, with each strain maintained for 10 s; ΔR/R0 of the PAM/COS@SIW hydrogel under five repeated stretching - releasing cycles under (D) small strain levels (20%, 40%, 60%, and 80%) and (E) large strain levels (100%, 300%, 500%, and 700%); (F) ΔR/R0 of the PAM/COS@SIW hydrogel under five repeated stretching - releasing cycles at different stretching rates (50, 100, 200, and 300 mm·min-1) at 100% strain; (G) Response and recovery times of the PAM/COS@SIW hydrogel; (H) ΔR/R0 of the PAM/COS@SIW hydrogel under 100% strain for 100 consecutive cycles. LEDs: Light-emitting diodes; PAM: polyacrylamide; COS: chitosan oligosaccharide; SIW: silicotungstic acid; GF: gauge factor.
The strain sensing performance of the PAM/COS@SIW hydrogel was then investigated. As shown in Figure 4B, the relative resistance variation (ΔR/R0) of the hydrogel sample increased dramatically with the increase of tensile strain. Gauge factor (GF), defined as the ratio of ΔR/R0 to the applied strain (ε), was used to evaluate the sensitivity. The GF was 1.01 below 250% strain, and then increased to 3.14 and 8.97 in the strain ranges of 250%-1,000% and 1,000%-2,300%, indicating the high sensitivity of the PAM/COS@SIW hydrogel sensor within a large sensing range. With the stepwise increase of the applied strain from 0% to 100% and then released to zero (10 s interval each step), the ΔR/R0 exhibited distinct stepwise increments with excellent segmental stability [Figure 4C], indicating that the hydrogel strain sensor can reliably monitor multi-cycle strains in real time. Moreover, as shown in Figure 4D and E, the hydrogel strain sensor exhibited stable and repeatable sensing responses under cyclic loading - unloading processes at both small (20%, 40%, 60%, 80%) and large (100%, 300%, 500%, 700%) strains, demonstrating its capability to accurately detect a wide range of deformations. At a fixed strain of 100%, ΔR/R0 exhibited negligible variation with increasing stretching rate, confirming the rate-independent sensing characteristics of the hydrogel [Figure 4F]. Additionally, instantaneous stretching - releasing tests revealed a rapid response time of 330 ms and a recovery time of
Application as wearable sensors and bioelectrodes
Given its excellent stretchability, toughness, and strain sensitivity, the PAM/COS@SIW hydrogel was assembled into a wearable sensor for monitoring a wide range of human motions. First, biocompatibility of the PAM/COS@SIW hydrogel was preliminarily evaluated to confirm the suitability of wearable applications [Supplementary Figure 15]. Histological examination of mouse dorsal skin after 3 days of contact, as assessed by H&E staining, revealed that the epidermal and dermal architectures in the hydrogel-contact region were comparable to those of the control, without noticeable tissue damage or inflammatory infiltration. Furthermore, a short-term human skin patch test (24 h) showed no obvious redness or swelling after removal of the hydrogel. These results provide supporting evidence for the suitability of the PAM/COS@SIW hydrogel in epidermal-contact wearable sensing and bioelectrode applications. As shown in Figure 5A, when the sensor is attached to the finger joint, variations in electrical signals can clearly distinguish different bending angles. Moreover, the sensor reliably captured the corresponding signal variations during repeated finger bending [Supplementary Figure 16A]. Likewise, when attached to the wrist, elbow, or knee, the sensor effectively monitored motion changes with stable and reproducible response signals [Figure 5B, Supplementary Figure 16B and C]. Except for large joint movements, the high sensitivity makes the wearable sensor capable of detecting subtle facial movements. For instance, when attached to a volunteer’s cheek, periodic mouth opening and closing were clearly recorded, and speech-related facial movements were similarly detected [Figure 5C and Supplementary Figure 16D]. Moreover, when the sensor was positioned on a volunteer’s laryngeal prominence, laryngeal movements were effectively captured [Supplementary Figure 16E]. More importantly, when the notched hydrogel was attached to a finger, it also reliably monitored periodic finger bending, indicating that the conductive network was not compromised by the presence of macroscopic defects and retained robust functionality under practical, non-ideal conditions [Supplementary Figure 17]. In addition, the sensor functioned as a pressure detector, producing distinct signal changes in response to external loading [Supplementary Figure 18].
Figure 5. (A-C) ΔR/R0 of the PAM/COS@SIW hydrogel-based wearable sensors in response to (A) finger bending at different angles, (B) cyclic wrist bending at the same angle, and (C) cyclic mouth opening and closing at the same degree; (D) Optical photograph of ECG monitoring using a three-electrode configuration; ECG signals recorded by commercial Ag/AgCl electrodes and PAM/COS@SIW hydrogel electrodes during (E) the static state and (F) motion; (G) Optical photograph of EMG monitoring using a three-electrode configuration. All optical photographs (A-D and G) were taken by the authors using personal mobile devices; (H) EMG signals recorded under different grip forces (5, 15 and 30 kg) by commercial Ag/AgCl electrodes and PAM/COS@SIW hydrogel electrodes; (I) EMG signals recorded by commercial Ag/AgCl electrodes and PAM/COS@SIW hydrogel electrodes when lifting different loads (1, 3.5 and
To further evaluate its practical applicability, the PAM/COS@SIW hydrogel was integrated into skin-mounted electrodes for the detection of electrophysiological signals, including ECG and EMG. To ensure accurate bioelectrical signal acquisition, a widely used three-electrode configuration was employed for ECG monitoring [Figure 5D]. Figure 5E and F present a comparison of ECG signals recorded with commercial electrodes and the PAM/COS@SIW hydrogel electrodes. Distinct PQRST waveforms were clearly observed with the hydrogel electrodes, and their signal quality closely matched that of commercial Ag/AgCl electrodes during rest and exercise. Similarly, a three-electrode configuration was used to record EMG signals generated by muscle fibers during voluntary movements [Figure 5G]. Figure 5H and I further demonstrate that the hydrogel electrodes reliably recorded EMG signals during arm movements, with signal quality comparable to that of commercial electrodes. EMG analysis showed that increasing grip strength from 5 to 30 kg resulted in corresponding increases in both the peak amplitude and duration of individual EMG signals, indicating that greater grip force requires stronger muscle contraction and prolonged recovery. During dumbbell lifting, EMG recordings revealed distinct muscle activity during the lifting, holding, and lowering phases. As the dumbbell weight increased, EMG amplitudes progressively rose, reflecting the greater muscular effort needed to overcome heavier loads. Importantly, the EMG patterns allowed clear delineation of the muscle exertion process, underscoring the potential of PAM/COS@SIW hydrogels as sensitive and reliable electrodes for motion detection and exercise monitoring.
Limitations and outlook
Despite the promising mechanical robustness, crack resistance, and sensing performance of the developed PAM/COS@SIW hydrogel, several limitations should be acknowledged. First, the current hydrogel architecture has not yet been optimized toward ultrathin or highly breathable configurations, which may influence long-term wearing comfort in practical epidermal applications. Second, although stable sensing performance was achieved under repeated deformation, the long-term durability of the hydrogel under prolonged use and complex environmental conditions requires further systematic investigation. Third, mechanical hysteresis during cyclic loading remains non-negligible, and further reduction of hysteresis would be beneficial for improving signal stability in continuous and long-term sensing scenarios.
Future efforts may therefore focus on structural engineering and interfacial optimization to realize ultrathin and breathable hydrogel architectures, enhancing long-term stability under practical service conditions, and minimizing hysteresis through refined network design. In addition, extending the COS@SIW supramolecular-node strategy to other polymer matrices may further broaden the applicability of this approach for advanced wearable bioelectronics.
CONCLUSIONS
In summary, we have developed an ionically conductive POMs-composited hydrogel (PAM/COS@SIW) that combines exceptional stretchability, toughness, and crack resistance through the incorporation of COS-modified SIW nanoclusters into a PAM network. COS molecules established strong electrostatic and hydrogen-bonding interactions with SIW, resulting in the formation of stable COS@SIW nanocomplexes. Upon incorporation into the PAM network, multiscale hydrogen bonds significantly reinforced the interfacial bonding between COS@SIW and the polymer matrix, facilitating efficient stress transfer and energy dissipation. As a result, the resulting hydrogel exhibited remarkable mechanical robustness - combining high stretchability (2,423%), superior toughness (3.77 MJ·m-3), and pronounced crack insensitivity (fracture energy of 8.3 kJ·m-2) - together with good ionic conductivity attributed to the intrinsic proton mobility of SIW. PAM/COS@SIW hydrogel exhibited reliable and sensitive strain-sensing capability over a broad deformation range, along with rapid response and outstanding durability. The hydrogel further demonstrates reliable performance as both a wearable strain sensor and a skin-mounted bioelectrode for monitoring human motions and electrophysiological signals, demonstrating great potential as a multifunctional platform for wearable sensing and bioelectronic interfaces.
DECLARATIONS
Authors’ contributions
Conceptualization, methodology, investigation, formal analysis, software, data curation, validation, visualization, writing - original draft, review and editing: Liang, Y.
Methodology, investigation, formal analysis, software: Zhang, Y.
Investigation, data curation, visualization: Li, N.; Shi, X.; Qiao, Y.
Conceptualization, funding acquisition, methodology, project administration, resources, supervision, writing - review and editing: Jiao, T.; Qin, Z.
Availability of data and materials
All data are available in the main text or the Supplementary Materials. Information requests should be directed to the corresponding authors upon reasonable request.
AI and AI-assisted tools statement
Not applicable.
Financial support and sponsorship
We greatly appreciate the financial support of the National Natural Science Foundation of China (Nos. 22102139, 22372143), the Hebei Natural Science Foundation (Nos. B2025203022 and B2025203050), the Science Research Project of Hebei Education Department (No. JCZX2026028).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
All physiological electrical signal tests involving human participants were performed with the consent of the volunteer (one author of the study). The experiments involved only non-invasive monitoring of physiological electrical signals on the surface of the human body, in full compliance with relevant ethical guidelines and local regulations and did not cause any risk or discomfort to the participants. All animal procedures were approved by the Ethics Committee of Yanshan University (No. 2022001) and conducted in accordance with relevant ethical guidelines.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2026.
Supplementary Materials
REFERENCES
1. Li, Z.; Song, P.; Li, G.; et al. AI energized hydrogel design, optimization and application in biomedicine. Mater. Today. Bio. 2024, 25, 101014.
2. Wang, K.; Zhang, J.; Li, H.; et al. Smart hydrogel sensors for health monitoring and early warning. Adv. Sens. Res. 2024, 3, 2400003.
3. Zhang, C. W.; Chen, C.; Duan, S.; Yan, Y.; He, P.; He, X. Hydrogel-based soft bioelectronics for personalized healthcare. Med. X. 2024, 2, 20.
4. Liu, Q.; Xu, X.; Zhang, Y.; Liang, L.; Zhang, B.; Chen, S. Robust accurate fatigue assessment enabled by an ultrasoft and super-adhesive low-impedance conducting polymer hydrogel. Chem. Eng. J. 2025, 509, 161207.
5. Shadab, A.; Farokhi, S.; Fakouri, A.; et al. Hydrogel-based nanoparticles: revolutionizing brain tumor treatment and paving the way for future innovations. Eur. J. Med. Res. 2025, 30, 71.
6. Apoorva, S.; Nguyen, N. T.; Sreejith, K. R. Recent developments and future perspectives of microfluidics and smart technologies in wearable devices. Lab. Chip. 2024, 24, 1833-66.
7. Haghayegh, F.; Norouziazad, A.; Haghani, E.; et al. Revolutionary point-of-care wearable diagnostics for early disease detection and biomarker discovery through intelligent technologies. Adv. Sci. 2024, 11, e2400595.
8. Li, W.; Li, L.; Zheng, S.; et al. Recyclable, healable, and tough ionogels insensitive to crack propagation. Adv. Mater. 2022, 34, e2203049.
9. Xu, S.; Zhou, J.; Pan, P. Strain-induced multiscale structural evolutions of crystallized polymers: from fundamental studies to recent progresses. Prog. Polym. Sci. 2023, 140, 101676.
10. Han, S.; Wu, Q.; Zhu, J.; et al. Tough hydrogel with high water content and ordered fibrous structures as an artificial human ligament. Mater. Horiz. 2023, 10, 1012-9.
11. Xiang, C.; Zhang, X.; Zhang, J.; et al. A porous hydrogel with high mechanical strength and biocompatibility for bone tissue engineering. J. Funct. Biomater. 2022, 13, 140.
12. Sha, Z.; Chen, X.; Song, H.; Zheng, Y.; Cui, W.; Ran, R. Toughening hydrogels with small molecules: tiny matter, big impact. Mater. Horiz. 2025, 12, 7244-76.
13. Zhang, C.; Wang, J.; Li, S.; et al. Construction and characterization of highly stretchable ionic conductive hydrogels for flexible sensors with good anti-freezing performance. Eur. Polym. J. 2023, 186, 111827.
14. Ni, Q.; He, X.; Zhou, J.; et al. Mechanical tough and stretchable quaternized cellulose nanofibrils/MXene conductive hydrogel for flexible strain sensor with multi-scale monitoring. J. Mater. Sc. Technol. 2024, 191, 181-91.
15. Wang, H.; Zhuang, T.; Wang, J.; et al. Multifunctional filler-free PEDOT:PSS hydrogels with ultrahigh electrical conductivity induced by lewis-acid-promoted ion exchange. Adv. Mater. 2023, 35, e2302919.
16. Tang, H.; Li, Y.; Liao, S.; Liu, H.; Qiao, Y.; Zhou, J. Multifunctional conductive hydrogel interface for bioelectronic recording and stimulation. Adv. Healthc. Mater. 2024, 13, e2400562.
17. Xu, X.; Jerca, V. V.; Hoogenboom, R. Bioinspired double network hydrogels: from covalent double network hydrogels via hybrid double network hydrogels to physical double network hydrogels. Mater. Horiz. 2021, 8, 1173-88.
18. Zhang, M.; Zhang, D.; Chen, H.; et al. A multiscale polymerization framework towards network structure and fracture of double-network hydrogels. npj. Comput. Mater. 2021, 7, 39.
19. Kumar, R.; Parashar, A. Atomistic simulations of pristine and nanoparticle reinforced hydrogels: a review. WIREs. Comput. Mol. Sci. 2023, 13, e1655.
20. Li, B.; Chen, Y.; Han, Y.; Cao, X.; Luo, Z. Tough, highly resilient and conductive nanocomposite hydrogels reinforced with surface-grafted cellulose nanocrystals and reduced graphene oxide for flexible strain sensors. Colloids. Surf. A. Physicochem. Eng. Aspects. 2022, 648, 129341.
21. Lu, C. H.; Yu, C. H.; Yeh, Y. C. Engineering nanocomposite hydrogels using dynamic bonds. Acta. Biomater. 2021, 130, 66-79.
22. Qiao, L.; Liang, Y.; Chen, J.; et al. Antibacterial conductive self-healing hydrogel wound dressing with dual dynamic bonds promotes infected wound healing. Bioact. Mater. 2023, 30, 129-41.
23. Zhu, R.; Zhu, D.; Zheng, Z.; Wang, X. Tough double network hydrogels with rapid self-reinforcement and low hysteresis based on highly entangled networks. Nat. Commun. 2024, 15, 1344.
24. Lin, S.; Liu, J.; Liu, X.; Zhao, X. Muscle-like fatigue-resistant hydrogels by mechanical training. Proc. Natl. Acad. Sci. U. S. A. 2019, 116, 10244-9.
25. Kent, R. N. 3rd; Huang, A. H.; Baker, B. M. Augmentation of tendon and ligament repair with fiber-reinforced hydrogel composites. Adv. Healthc. Mater. 2024, 13, e2400668.
26. Li, W.; Wang, X.; Liu, Z.; et al. Nanoconfined polymerization limits crack propagation in hysteresis-free gels. Nat. Mater. 2024, 23, 131-8.
27. Baroudi, I.; Simonnet-Jégat, C.; Roch-Marchal, C.; et al. Supramolecular assembly of gelatin and inorganic polyanions: fine-tuning the mechanical properties of nanocomposites by varying their composition and microstructure. Chem. Mater. 2015, 27, 1452-64.
28. Du, D. Y.; Qin, J. S.; Li, S. L.; Su, Z. M.; Lan, Y. Q. Recent advances in porous polyoxometalate-based metal-organic framework materials. Chem. Soc. Rev. 2014, 43, 4615-32.
29. Gumerova, N. I.; Rompel, A. Synthesis, structures and applications of electron-rich polyoxometalates. Nat. Rev. Chem. 2018, 2, 0112.
30. Cao, Q.; Shu, Z.; Zhang, T.; Ji, W.; Chen, J.; Wei, Y. Highly elastic, sensitive, stretchable, and skin-inspired conductive sodium alginate/polyacrylamide/gallium composite hydrogel with toughness as a flexible strain sensor. Biomacromolecules 2022, 23, 2603-13.
31. Huang, S.; Xia, X.; Fan, R.; Qian, Z. Programmable electrostatic interactions expand the landscape of dynamic functional hydrogels. Chem. Mater. 2020, 32, 1937-45.
32. Wei, X.; Ma, K.; Cheng, Y.; et al. Adhesive, conductive, self-healing, and antibacterial hydrogel based on chitosan–polyoxometalate complexes for wearable strain sensor. ACS. Appl. Polym. Mater. 2020, 2, 2541-9.
33. Liu, D.; Cao, Y.; Jiang, P.; et al. Tough, transparent, and slippery PVA hydrogel led by syneresis. Small 2023, 19, e2206819.
34. Li, Y.; Qin, Z.; He, P.; et al. Fully degradable protein gels with superior mechanical properties and durability: regulation of hydrogen bond donors. Adv. Mater. 2025, 37, e2506577.
35. Su, G.; Zhang, X.; Zhou, Y.; et al. Biomimetic all-weather strong, tough, and fatigue-resistant composite organohydrogels for electronic artificial ligaments. Small 2025, 21, e04139.
36. Ou, K.; Wang, M.; Meng, C.; et al. Enhanced mechanical strength and stretchable ionic conductive hydrogel with double-network structure for wearable strain sensing and energy harvesting. Compos. Sci. Technol. 2024, 255, 110732.
37. Tang, L.; Wu, S.; Qu, J.; Gong, L.; Tang, J. A review of conductive hydrogel used in flexible strain sensor. Materials 2020, 13, 3947.
38. Huang, S. C.; Zhu, Y. J.; Huang, X. Y.; Xia, X. X.; Qian, Z. G. Programmable adhesion and morphing of protein hydrogels for underwater robots. Nat. Commun. 2024, 15, 195.
39. Duong, N. B.; Trang, Q. T. T.; Bich, P. T. N.; Lam, P. V. Preparation and ammonium adsorption behavior of interpenetrating polymer networks hydrogel of chitosan‐g‐poly(acrylic acid)/bentonite and chitosan‐glutaraldehyde. Vietnam. J. Chem. 2024, 62, 179-86.
40. Bielański, A.; Datka, J.; Gil, B.; Małecka-Lubańska, A.; Micek-Ilnicka, A. FTIR study of hydration of dodecatungstosilicic acid. Catal. Lett. 1999, 57, 61-4.
41. Hu, H.; Xin, J. H.; Hu, H. PAM/graphene/Ag ternary hydrogel: synthesis, characterization and catalytic application. J. Mater. Chem. A. 2014, 2, 11319-33.
42. Krakovský, I.; Hanyková, L.; Štastná, J. Phase transition in polymer hydrogels investigated by swelling, DSC, FTIR and NMR. J. Therm. Anal. Calorim. 2024, 150, 1245-62.
43. Athokpam, B.; Ramesh, S. G.; Mckenzie, R. H. Effect of hydrogen bonding on the infrared absorption intensity of OH stretch vibrations. Chem. Phys. 2017, 488-9, 43-54.
44. He, L.; Xue, R.; Yang, D.; Liu, Y.; Song, R. Effects of blending chitosan with peg on surface morphology, crystallization and thermal properties. Chin. J. Polym. Sci. 2009, 27, 501.
45. Wiercigroch, E.; Szafraniec, E.; Czamara, K.; et al. Raman and infrared spectroscopy of carbohydrates: a review. Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 2017, 185, 317-35.
46. Kolli, H. K.; Jana, D.; Kumar, M. P.; Das, S. K. Giant polyoxometalate {W72Fe30} into pure inorganic gel and xerogel: rheology and proton conduction. Chempluschem 2025, 90, e202500084.
47. Nochi, M.; Ozaki, Y.; Sato, H. Water-induced conformational changes in the powder and film of ε-poly(L)lysine studied by infrared and Raman spectroscopy. Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 2021, 260, 119900.
48. Huang, S.; Wang, R.; Lei, M.; et al. Preparation of silk fibroin-carboxymethyl cellulose composite binder and its application in silicon-based anode for lithium-ion batteries. Nanomaterials 2025, 15, 1509.
49. Mensch, C.; Bultinck, P.; Johannessen, C. The effect of protein backbone hydration on the amide vibrations in Raman and Raman optical activity spectra. Phys. Chem. Chem. Phys. 2019, 21, 1988-2005.
50. Tang, Y.; Ye, L.; Zhang, Z.; Friedrich, K. Interlaminar fracture toughness and CAI strength of fibre-reinforced composites with nanoparticles - a review. Compos. Sci. Technol. 2013, 86, 26-37.
51. Vishvanathperumal, S.; Kannan, A. Effect of graphene oxide (GO) on the mechanical properties of ethylene-propylene-diene monomer/acrylonitrile butadiene rubber (EPNBR) blend composites. J. Polym. Res. 2025, 32, 145.
52. Du, H.; Chen, X.; Gong, H.; et al. Bioinspired anti‐freezing hydrogel with localized ice regulation for subzero soft robotics. Angew. Chem. 2025, 137, e202512142.
53. Li, Z.; Zhang, Q.; Zhu, J.; Xu, W.; Gong, B.; Lin, J. Atomistic insights into the effect of bacterial cellulose and water content on the mechanical properties of the bacterial cellulose/polyvinyl alcohol (BC/PVA) composite hydrogel. Mech. Mater. 2025, 211, 105494.
54. Ugrinovic, V.; Markovic, M.; Bozic, B.; Panic, V.; Veljovic, D. Physically crosslinked poly(methacrylic acid)/gelatin hydrogels with excellent fatigue resistance and shape memory properties. Gels 2024, 10, 444.
55. Xu, L.; Wang, C.; Cui, Y.; Li, A.; Qiao, Y.; Qiu, D. Conjoined-network rendered stiff and tough hydrogels from biogenic molecules. Sci. Adv. 2019, 5, eaau3442.
56. Hao, Y.; Ren, W.; Zhou, Q.; et al. Skin-mimicking soft strain sensor with elastic resilience, crack tolerance, and amphibious self-adhesion. ACS. Sens. 2025, 10, 3180-8.
57. Li, R.; Xu, Z.; Li, L.; et al. Breakage-resistant hydrogel electrode enables ultrahigh mechanical reliability for triboelectric nanogenerators. Chem. Eng. J. 2023, 454, 140261.
58. Zhang, M.; Yu, J.; Shen, K.; et al. Highly stretchable nanocomposite hydrogels with outstanding antifatigue fracture based on robust noncovalent interactions for wound healing. Chem. Mater. 2021, 33, 6453-63.
59. Liu, X.; Chen, D.; Feng, S.; Yang, M.; Yang, M. Super-stretchable hydrogel films with high fracture energy enabled by coordination nanoparticles as multifunctional wound dressings. ACS. Appl. Polym. Mater. 2023, 5, 7318-27.
60. Yu, J.; Xu, K.; Chen, X.; et al. Highly stretchable, tough, resilient, and antifatigue hydrogels based on multiple hydrogen bonding interactions formed by phenylalanine derivatives. Biomacromolecules 2021, 22, 1297-304.
61. Narita, T.; Hsieh, W. C.; Ku, Y. T.; Su, Y. C.; Inoguchi, H.; Takeno, H. Fracture behavior and biocompatibility of cellulose nanofiber-reinforced poly(vinyl alcohol) composite hydrogels cross-linked with borax. Biomacromolecules 2025, 26, 374-86.
62. Shi, Y.; Yuan, Z.; Cao, Z.; et al. Robust, notch-insensitive and impact-resistance physical hydrogels with homogeneous topologic network enabled by partial hydrolysis and metal coordination. Polymer 2025, 317, 127913.
63. Sun, M.; Qiu, J.; Lu, C.; Jin, S.; Zhang, G.; Sakai, E. Multi-sacrificial bonds enhanced double network hydrogel with high toughness, resilience, damping, and notch-insensitivity. Polymers 2020, 12, 2263.
64. Zhang, H.; Wang, Y.; Shi, Y.; et al. Recent advances in polyoxometalate-based proton conducting materials: design strategies, conduction mechanisms, structure–function relationships and future perspectives. Nano. Res. 2025, 18, 94907743.
65. Zhai, L.; Li, H. Polyoxometalate-polymer hybrid materials as proton exchange membranes for fuel cell applications. Molecules 2019, 24, 3425.
66. Ureña, N.; Pérez-Prior, M. T.; Levenfeld, B.; García-Salaberri, P. A. On the conductivity of proton-exchange membranes based on multiblock copolymers of sulfonated polysulfone and polyphenylsulfone: an experimental and modeling study. Polymers 2021, 13, 363.
67. Mo, F.; Lin, Y.; Liu, Y.; et al. Advances in ionic conductive hydrogels for skin sensor applications. Mater. Sci. Eng. R. Rep. 2025, 165, 100989.
68. Wang, R.; Jin, B.; Li, J.; et al. Bio-inspired synthesis of injectable, self-healing PAA-Zn-silk fibroin-MXene hydrogel for multifunctional wearable capacitive strain sensor. Gels 2025, 11, 377.
69. Fang, T.; Zhu, J.; Xu, S.; Jia, L.; Ma, Y. Highly stretchable, self-healing and conductive silk fibroin-based double network gels via a sonication-induced and self-emulsifying green procedure. RSC. Adv. 2022, 12, 11574-82.
70. Di, X.; Li, L.; Jin, Q.; et al. Highly sensitive, degradable, and rapid self-healing hydrogel sensor with semi-interpenetrating network for recognition of micro-expressions. Small 2024, 20, e2403955.
71. Huang, X.; Wang, C.; Yang, L.; Ao, X. Highly stretchable, self-adhesive, antidrying ionic conductive organohydrogels for strain sensors. Molecules 2023, 28, 2817.
72. Ran, Z.; Xu, J.; Zeng, W.; et al. An orange peel-based hydrogel composite for touch-responsive electronic skin. Commun. Mater. 2024, 5, 97.
73. Fu, Q.; Tang, J.; Wang, W.; Wang, R. Biocomposite polyvinyl alcohol/ferritin hydrogels with enhanced stretchability and conductivity for flexible strain sensors. Gels 2025, 11, 59.
Cite This Article
Open Access
How to Cite
Download Citation
Export Citation File:
Type of Import
Tips on Downloading Citation
Citation Manager File Format
Type of Import
Direct Import: When the Direct Import option is selected (the default state), a dialogue box will give you the option to Save or Open the downloaded citation data. Choosing Open will either launch your citation manager or give you a choice of applications with which to use the metadata. The Save option saves the file locally for later use.
Indirect Import: When the Indirect Import option is selected, the metadata is displayed and may be copied and pasted as needed.
About This Article
Copyright
Data & Comments
Data
0
Comments
Comments must be written in English. Spam, offensive content, impersonation, and private information will not be permitted. If any comment is reported and identified as inappropriate content by OAE staff, the comment will be removed without notice. If you have any queries or need any help, please contact us at [email protected].