Stretchable ionic hydrogels with superior all-round properties that can detect multimodal sensations with high discriminability to decouple multiple stimuli and high robustness against external disturbances are highly required for artificial electronic skin applications. However, some of the critical material parameters exhibit intrinsic tradeoffs with each other for most ionic hydrogels. Here, we demonstrate a microphase-separated hydrogel design by combining three strategies: (1) the use of a low crosslinker/monomer ratio to obtain highly entangled polymer chains as the first network; (2) the introduction of zwitterions into the first network; (3) the synthesis of a ultrasoft polyelectrolyte as the second network. This approach creates an all-round elastic ionic hydrogel with a skin-like Young’s modulus (< 60 kPa), large stretchability (> 900%), high resilience (> 95%), low hysteresis (< 5%), unique strain-stiffening behavior, excellent fatigue tolerance, high ionic conductivity (> 2.0 S/m), and anti-freezing capability, which were not achieved with previous ionic hydrogels. These comprehensive properties allow the ionic hydrogel to operate as a stretchable multimodal sensor that can detect and decouple multiple stimuli (temperature, pressure, and proximity) with both high discriminability and sensitivity. It also shows strong sensing robustness against large strains and subzero temperature perturbations. The ionic hydrogel sensor exhibits great potential for intelligent electronic skin applications such as reliable health monitoring and accurate object identification.
Article
WITHDRAWN: A Microphase-Separated Design Toward an All-Round Ionic Hydrogel with Discriminable and Anti-Disturbance Multisensory Functions
https://doi.org/10.21203/rs.3.rs-3143087/v1
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Skin, the body’s all-round and most versatile organ, provides protection and receives external sensory stimuli1. Artificial electronic skin is designed to mimic the multisensory functions and unique mechanical properties of natural skin and has been applied in advanced robotics, intelligent medical diagnostics, skin-attachable wearables, and human-machine interfaces2. As the primary part of the somatosensory system, the human skin can feel multiple sensations with high discriminability to decouple multiple stimuli without interference via different mechanoreceptors, which are composed of ion conductors in the skin3. Moreover, the skin has intriguing mechanical properties, including softness, elasticity, fatigue resistance, and strain-stiffening behavior, which allow the embedded mechanoreceptors to perceive with high sensing accuracy, reliability, and superior robustness against mechanical disturbances. Despite these impressive characteristics, its physiological functions are susceptible to harsh climatic conditions4. For instance, the human skin lacks an anti-freezing capability, and low environmental temperatures can decrease its sensing and protective functions and increase its susceptibility to mechanical stresses4. Thus, artificial electronic skins have been developed to mimic skin’s comprehensive sensory and mechanical properties while also adding new desired properties such as freezing tolerance5.
Inspired by the ion-conducting and water-containing nature of natural skin, stretchable ionic conductors based on polymer hydrogels have been developed, which show great potential for developing artificial sensory skin6. Various ionic hydrogels have been applied in the design of wearable sensing devices that can detect stimuli such as strain, pressure, or temperature7. Despite notable advances in the development of ionic hydrogel sensors, there has been no report of a stretchable ionic hydrogel that simultaneously possesses robust and discriminable multisensory functions, skin-like mechanical properties, and skin-absent, yet highly desirable, anti-freezing capability. Previous studies have realized multisensory functions in one stretchable ionic hydrogel, but multiple stimuli were usually converted into one type of electrical signal output, such as a resistance or capacitance change3,6−8. Thus, they could not decouple the interference of multiple signals and lacked stimuli discriminability and sensing robustness8.
While stretchable and ionic-conductive hydrogels have been frequently reported, most cannot combine skin-like mechanical and physical properties and anti-freezing properties since some of the critical material parameters exhibited intrinsic tradeoffs with each other. First, conventional polymer hydrogels show a tradeoff between their stretchability and hysteresis9–11. High resilience and low hysteresis are essential for obtaining stable and durable stretchable materials where repetitive movements or loadings are required10. However, polymer hydrogels with good stretchability typically have difficulty achieving a low hysteresis and high resilience due to polymer chain entanglements and chemical bond breakage11. Second, polymer hydrogels show another tradeoff between their stretchability and ionic conductivity12–14. A key property of polyelectrolyte hydrogels is their ionic conductivity15, but they are often mechanically weak due to their high swelling ratios and lack of stress dissipation mechanisms16. Double-network toughening strategies include the introduction of a dense neutral polymer into the polyelectrolyte matrix or the use of another polyelectrolyte with equivalent opposite charges to form a dense polyelectrolyte complex16. These approaches have been developed to improve the stretchability and toughness of polyelectrolyte hydrogels, but they typically employ a polyelectrolyte as the brittle and sacrificial first network to dissipate energy upon stretching, thus inevitably leading to irreversible mechanical degradation, large hysteresis, and a decrease in ionic conductivity16. Third, there is a tradeoff between the anti-freezing performance and mechanical properties of conventional hydrogels17. Almost all reported anti-freezing gels are either organogels (including organo/hydrogels) or hydrophilic hydrogels that contain anti-freezing additives such as ionic compounds, natural proteins, and carbon nanomaterials18. However, the mechanical properties of hydrogels with anti-freezing additives significantly degraded as the additive content increased19–21. Till now, there have been no reports on stretchable ionic hydrogels with skin-like distinguishable multisensory functions, superior mechanical properties, and skin-absent anti-freezing properties.
Herein, we report the design and fabrication of a highly elastic, superb robust, and anti-freezing artificial multisensory skin based on an interpenetrating network (IPN) ionic polymer hydrogel with a microphase-separated microstructure by combining three strategies: (1) the use of a highly-entangled copolymer as the first network, (2) the introduction of zwitterions into the first network, and (3) the use of an ionic conductive polyelectrolyte as the second network. The highly-entangled structure together with strong intermolecular interactions between the interpenetrating networks facilitated the formation of microphase-separated domains, which greatly improved the elasticity and robustness of the hydrogel. The resultant ionic hydrogel showed a high ionic conductivity (> 2.0 S/m), large stretchability (> 900%), superb elasticity (resilience > 95%, hysteresis < 5%), skin-like softness (Young’s modulus < 60 kPa), unique strain-stiffening behavior, excellent fatigue resistance, and desirable moisture-retention and anti-freezing properties. Similar to skin tactile, the ionic hydrogel skin could detect and decouple multiple stimuli (temperature, pressure, and proximity) via an ionic thermoelectric effect, piezo-capacitive effect, and fringe field effect, respectively. The combination of mechanical and physical effects enabled the multimodal hydrogel sensor to exhibit high-capacitive pressure sensitivity (up to 7.5 kPa− 1), high temperature sensing resolution within 0.1°C, and excellent sensing robustness against large strain (> 200%) and subzero temperature perturbations. This ionic hydrogel showed great promise in reliable health monitoring and accurate object identification applications.
1. Design and fabrication of the hydrogels
In contrast to the conventional double-network toughening strategy22, we combined three approaches to fabricate our ionic hydrogel skin to overcome the tradeoff issues. First, a single-network copolymer hydrogel was synthesized by using neutral acrylamide (AM) and zwitterionic sulfobetaine methacrylate (SBMA) as monomers and N, N′-methylene bis(acrylamide) (MBAA) as a cross-linker in a molar ratio of AM: SBMA: MBAA = 2175: 188: 1. Due to the low value of the crosslinker/monomer molar ratio (0.00042) of poly(AM-co-SBMA), this first network hydrogel was highly-entangled and was mainly crosslinked by these entanglements (Fig. 1a), giving it a high degree of elasticity23, 24. Second, the poly(AM-co-SBMA) hydrogel was immersed in excess diallyldimthyl-ammonium chloride (DADMAC) monomer solution, and the first network was swollen in the DADMAC solution with a swelling ratio of about 200% due to the polyelectrolyte nature and osmotic pressure of SBMA (Figure S1)15,16,25. After swelling, DADMAC was polymerized to form poly(diallyldimthyl-ammonium chloride (PDADMAC) in the presence of a diluted first network to obtain a poly(AM-co-SBMA)/PDADMAC hydrogel with an IPN structure (denoted as PAS/PD-IPN hydrogel). The molar ratio of AM: SMBA: DADMAC was optimized to be 2175: 188: 3760 (discussed below). PDADMAC was chosen as the polyelectrolyte second network because of its high ionic conductivity15,26, ultrasoft nature16, and antibacterial quaternary ammonium structure27, which are advantageous for wearable applications. Third, the zwitterionic SBMA in the highly-entangled first network formed strong intermolecular bonds (SBMA-DADMAC ionic complex) with PDADMAC (Fig. 1a)28,29. These allowed the formation of microphase-separated domains inside the hydrogels, which endowed the hydrogel with excellent elasticity and robustness30, 31. For comparison, a poly(AM-co-SBMA) single network hydrogel (denoted PAS-SN), poly(acrylamide) (PAM) single network hydrogel (denoted PA-SN), PDADMAC single network hydrogel (denoted PD-SN), and poly(acrylamide)/PDADMAC IPN hydrogel (denoted PA/PD-IPN) were also fabricated.
2. Characterization
The highly entangled structure and strong intermolecular interactions together contributed to the formation of microphase-separated domains, as reflected by the macroscopic morphological change from transparent to opaque. Figure 1b presents photos of the PAS/PD-IPN hydrogel showing its opacity, which was caused by interfacial reflections and a biphase refractive index difference due to the existence of phase-separated regions32. In contrast, the isotropous PAS-SN and PD-SN with uniform microstructures showed a greater light penetration depth and were entirely transparent. To further verify microphase separation, scanning electron microscopy (SEM) was conducted to dissect local structural differences between the PAS-SN and PAS/PD-IPN hydrogels. The SEM image in Fig. 1c shows that PAS/PD-IPN presented a microporous and sturdy skeleton, while PAS-SN in Fig. 1d possessed a loose and porous network, which confirmed the internal attraction-induced structural changes.
To accurately investigate this phenomenon from a microscopic perspective, the internal microstructure of phase-separated hydrogel was visualized using in situ nondestructive nano-computed tomography (Nano CT) scanning. In the 3D reconstruction (Fig. 1e), the light element of the solvent phase (colored grey) and the heavy element of the polymer phase (colored blue) in the PAS/PD-IPN hydrogel exhibited a clear solid-liquid interphase boundary and a denser solid polymer-rich region. The binary images in Figure S2a reveal that, similar to traditional microphase separation, the polymer-rich region and solvent-rich region formed a bicontinuous, microscale structure. The distance between adjacent polymer-rich regions calculated from Figure S2b increased from 25 µm (PAS-SN) to 61 µm (PAS/PD-IPN), which qualitatively indicates phase separation in the PAS/PD-IPN hydrogel33.
According to our initial design, interchain interactions and a highly-entangled network are both necessary for jointly accelerating the formation of a microphase-separated structure. Therefore, to verify the interchain interactions between PAS-SN, PD-SN, and PAS/PD-IPN, Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) characterizations were applied. For PAS-SN hydrogel, the characteristic absorption bands, which were assigned to the S = O asymmetric stretching bands of -SO3− in SBMA, shifted from 1180 cm− 1 to 1186 cm− 1 after forming the IPN hydrogel with PDADMAC (Fig. 1f)25. This shift implies a strong attraction between the anionic sulfonate group of SBMA and the cationic quaternary ammonium group of PDADMAC. Moreover, the results in Fig. 1g indicated that due to the strong interactions between the quaternary ammonium groups and -SO3−, the spin-orbit doublet of S2p of -SO3− species from the zwitterionic moiety of PAS-SN shifted from 168.68 eV and 167.46 eV to 168.5 eV and 167.3 eV, respectively34,35. These suggest the existence of the SBMA-DADMAC ionic complex in the PAS/PD-IPN hydrogel.
Apart from the above-mentioned mutual interactions, chain entanglement of the first network was another integral element for the formation of phase-separated domains in this work. We investigated how different AM-to-SBMA ratios contributed to this microphase separation. First, AM and SBMA ratio adjustments were essential to the construction of the PAS-SN (Figure S3). The molar ratio of AM and SBMA was changed from 2894: 0 to 719: 548 (denoted as PA, PA2175S188, PA1678S308, PA1199S428, and PA719S548, respectively). After soaking in DADMAC monomer solutions (at two extreme contents), the extent of phase separation of both PA/PD and PA719S548/PD was inferior to that of PA2175S188/PD (Figure S3). This suggests that single chain-entanglement or SBMA-DADMAC interactions did not lead to phase separation and that the optimal AM-to-SBMA ratio was 2175: 188. Based on the above experiments, all PAxSy-SN samples were transparent, and phase separation only occurred after introducing PDADMAC. This reconfirmed that strong interactions between the two skeletons and the high entanglement of the first PAS-SN network were jointly responsible for phase separation. These microphase-separated domains can work as strong crosslinking points in the IPN hydrogels, which made it possible to improve the mechanical properties upon stress or compression loading30,31.
Mechanical and anti-freezing properties
Tensile stress-strain tests were performed to quantitatively examine the mechanical properties of hydrogel samples. As shown in Fig. 2a, the PD-SN electrolyte hydrogel showed a low Young’s modulus, tensile strength, strain at break, and toughness (9.6 kPa, 18.3 kPa, 250%, and 22.4 kJ/m3, respectively. After forming an interpenetrating network with poly(AM-co-SBMA), the PAS/PD-IPN hydrogel exhibited a significantly higher Young’s modulus, tensile strength, strain at break, and toughness (60.2 kPa, 220.2 kPa, 900%, and 1023 kJ/m3, respectively) than the PD-SN hydrogels (Fig. 2a). Moreover, compared with the PAS-SN hydrogel, the PAS/PD-IPN hydrogel was softer yet tougher (Table S1). While PAS/PD-IPN and PAS-SN hydrogels showed similar strain at break values, the higher toughness of the PAS/PD-IPN hydrogel was attributed to its unique strain-stiffening behavior36. The differential modulus first decreases from 60.2 to 14.7 kPa in the strain range of 0-350%, and then increases to 27.9 kPa at the maximum 900% strain, corresponding to the typical characteristic of stiffness variation. Similar to natural skin, the PAS/PD-IPN hydrogel was soft to the touch due to its low Young’s modulus at low strains6. However, it could stiffen due to a rapid increase in its Young’s modulus at large strains, which may help prevent injury37. The initial softness at a low strain state was mainly due to the low modulus of the PDADMAC network and the microphase-separated microstructure16,33. After stretching to a large strain, the alignment and subsequent fragmentation of fragile PDADMAC resulted in stiffening behavior under a large strain28.
We next performed dynamic mechanical analysis (DMA) on the hydrogels to study their viscoelastic properties. As shown in Fig. 2b, the storage modulus (G’) was about one order of magnitude larger than the loss modulus (G”) over a frequency range of 1-100 rad/s at ambient temperature for PAS/PD-IPN hydrogel. This indicates the elastic-dominated mechanical behavior of the ionic hydrogel38,39. The weak frequency dependence of both values reveals the stability of the PAS/PD-IPN hydrogel, which was attributed to the strong microphase-separated domains and highly entangled polymer networks in the hydrogels40.
Hysteresis and resilience are two of the most prominent parameters used to assess the viscoelasticity of hydrogels and elastomers10. Hysteresis is defined as the energy lost during energy storage and dissipation during material stretching and release10. Resilience is defined as the ratio of energy recovered during unloading to the work done to the material during loading23. We measured the cyclic loading-unloading tensile curves of the PAS/PD-IPN hydrogel under different strains and deformation ratios to demonstrate its elasticity. The resilience values of the PAS/PD-IPN hydrogel were in the range of 95-99.5% up to a strain of 400% (Fig. 2c). These values were much higher than those for the PAS-SN, PA/PD-IPN, and PA-SN hydrogels (Fig. 2d). This comparison indicates that the design containing zwitterionic monomers and the formation of microphase-separated domains played critical roles in improving the elasticity of the ionic hydrogel. Moreover, the PAS/PD-IPN hydrogel showed no hysteresis loops under a cyclic strain of 100% and exhibited low hysteresis (< 5%) when the strain increased to 300% and 400% (Figure S4). The tensile response of the PAS/PD-IPN hydrogel was nearly independent of the stretching rate, in contrast to conventional polymer hydrogels (Fig. 2e)41,42. Moreover, the PAS/PD-IPN hydrogel exhibited small hysteresis loops (< 3%), with a resilience higher than 94% at strain rates in the range of 20–200 mm/min at 200% strain (Fig. 2f). These resilience and hysteresis values of PAS/PD-IPN hydrogel belong to one of the best values for polymer hydrogels and elastomers (Table S2).
With low energy dissipation and excellent resilience, the IPN ionic hydrogel was considered to exhibit good mechanical fatigue resistance10. We demonstrate this fatigue tolerance behavior by performing cyclic tensile tests at a fixed strain of 100% under a deformation rate of 100 mm/min. As displayed in Fig. 2g, the loading-unloading tensile curves were almost unchanged, with low hysteresis and high resilience for more than 1000 strain cycles. In comparison, the loading-unloading tensile curves of the PD-SN hydrogel exhibited large residual strain for more than 1000 strain cycles, and hysteresis increased with the number of strain cycles for the PAS-SN hydrogel (Figure S5). The PAS/PD-IPN hydrogel also exhibited excellent resilience and fatigue resistance during consecutive compressive testing. Cyclic stress-strain tests under gradually increasing compressive strains from 20–80% (Fig. 2h) and consecutive compressive cycles at a strain of 80% (Fig. 2i) were conducted for PD-SN, PAS-SN, and PAS/PD-IPN hydrogels. The loading-unloading curves of the PAS/PD-IPN ionic hydrogel during different cycles almost overlapped with each other after the first ten cycles and exhibited little residual strain during 200 cycles (Fig. 2i). In contrast, the PD-SN hydrogel broke after one compressive cycle at a strain of 80% (Figure S6). The PAS-SN hydrogel exhibited large residual strain in its compressive curves during multiple loading-unloading cycles (Figure S6). The excellent elasticity of the PAS/PD-IPN hydrogel was strongly related to the microphase-separated domains within the hydrogel30,31.
In addition to its superior elasticity, the ionic hydrogel skin also maintained its mechanical flexibility at subzero temperatures43. Temperature-dependent dynamic mechanical analysis (DMA) in both tensile and compressive modes was conducted to investigate the anti-freezing property of the PAS/PD-IPN hydrogel. Figure 2j and 2k show that the storage modulus (E’) and loss modulus (E”) of the PAS/PD-IPN hydrogel remained stable when the temperature decreased from 50 to -10 oC. Further decreasing the temperature only slightly increased E’ and E”, indicating that the freezing point of the PAS/PD-IPN hydrogel was about − 16 oC. This is consistent with the differential scanning calorimetry (DSC) results in Fig. 2l. The exothermic peaks corresponding to water crystallization during the cooling process appeared at -20 oC and 0 oC for the PAS/PD-IPN and PAS-SN hydrogels, respectively, indicating that the PAS-SN hydrogel did not possess anti-freezing properties. Interestingly, negligible heat flow could be detected in the PD-SN hydrogel, indicating no ice formation43,44. Thus, the anti-freezing property of the PAS/PD-IPN hydrogel mainly arose from the polyelectrolyte in the IPN hydrogel. The PDADMAC network contained numerous ionic groups, which can strongly bind to water via ion-induced solvation18,45. These associated ions (salts) in the hydrogel depressed the freezing point of water, behaving similarly to an ion-induced anti-freezing mechanism reported for salt-added hydrogels46,47. Due to this anti-freezing property, the PAS/PD-IPN hydrogel showed similar stress-strain behavior when measured at ambient temperature (20 oC) and at a subzero temperature of -10 oC (Fig. 2m). The PAS/PD-IPN hydrogel remained mechanically elastic and could be reversibly bent and twisted without fracturing at -20 oC (Fig. 2n). Moreover, the PAS/PD-IPN hydrogels exhibited superior water retention to the PAS-SN hydrogel (Figure S7).
4. Ionic conductivity and electrophysiological signal sensing
To investigate the ion-transport properties of the PAS/PD-IPN hydrogel, the ionic conductivity and ionic Seebeck coefficient were measured. Figure 3a demonstrates the ionic conductivity of PAS/PD-IPN hydrogel measured at temperatures ranging from 25 oC to -10 oC. The ionic conductivity was obtained by fitting the electrochemical impedance spectroscopy (EIS) spectra to an equivalent circuit (Figure S8). The PAS/PD-IPN hydrogel exhibited an ionic conductivity of 2.3 S/m at 25 oC, surpassing most previously reported conductive hydrogels (Table S3). While the ionic conductivity showed a gradual decrease along with the temperature, it remained at 0.9 S/m at -10 oC, indicating its anti-freezing property. The ionic conductivity of the PAS/PD-IPN hydrogel under different stretching states was also characterized (Fig. 3b). The results showed that the tensile strain had little impact on the ionic conductivity, and the PAS/PD-IPN hydrogel still delivered an ionic conductivity of about 2.0 S/m at a tensile strain of 200%, revealing its resistance to strain perturbations. This was mainly attributed to the microphase-separated domains that acted as a hard phase that maintained the polymer’s structural and functional integrity inside its domains upon stretching30.
The combination of high ionic conductivity and superior elasticity makes the PAS/PD-IPN hydrogel an ideal electrode material for measuring electrophysiological signals (Fig. 3c), such as electrocardiograms (ECGs) and electromyograms (EMGs). By using a commercial three-lead ECG sensing setup, the ECG waveforms were first recorded with our PAS/PD-IPN hydrogel electrode and then with a commercial Ag/AgCl gel electrode as a comparison. As shown in Fig. 3d, the PAS/PD-IPN hydrogel electrode delivered high-quality ECG signals with characteristic P, Q, R, S, and T peaks identical to those obtained from a commercial Ag/AgCl gel electrode. The P wave, QRS complex, and T wave correspond to the activation of the atria, activation, and depolarization of the ventricles, and repolarization of the ventricles, respectively. A signal-to-noise ratio (SNR) of 36.9 dB was calculated from the ECG signals recorded by the PAS/PD-IPN hydrogel (calculation details in the Methods section), which was higher than that of the commercial Ag/AgCl gel electrode (24.3 dB).
The mechanical robustness, low Young’s modulus, and adhesion enabled the PAS/PD-IPN hydrogel to be firmly attached to the skin to measure ECG signals during exercise and long-term monitoring. Figure 3e shows that the heart rates of a human volunteer increased from 80 beats/min at rest to 120 beats/min after exercise. The ECG signals measured using the PAS/PD-IPN hydrogel electrode during skin stretching and squeezing all showed higher SNR values than those obtained from the commercial Ag/AgCl electrode (Fig. 3f and 3g). During long-term monitoring, ECG signals collected from the PAS/PD-IPN hydrogel electrode showed only slightly lower SNR values after 12 hours of continuous recording. These values were still much higher than the initial SNR value obtained using the commercial Ag/AgCl electrode (Fig. 3h and 3i). In addition, the PAS/PD-IPN hydrogel electrodes were adhered to the skin of a volunteer’s forearm to monitor the EMG signals generated from muscle fibers. The hand motions of clenching and loosening a dynamometer with 5, 10, 20, 40, and 60 kg force were characterized. The corresponding intensity of measured EMG signals increased with the gripping force (Fig. 3j). The PAS/PD-IPN hydrogel electrode also produced higher-quality EMG signals than the commercial Ag/AgCl electrode (Fig. 3j).
5. Ionic thermoelectric thermal sensing performance
Because the PAS/PD-IPN hydrogel skin is an ion conductor, it can form a concentration gradient of ions in response to a temperature differential (ΔT) to generate thermopower based on the Soret effect48. When ΔT formed in the PAS/PD-IPN hydrogel skin, the primary mobile ion diffused from the hot side to the cold side through the immobile polymer chains (Fig. 4a)49. The mobile ions were mainly transported through the immobile dimethyl diallyl ammonium groups of PDADMAC26,49. Therefore, we evaluated the ionic thermoelectric thermal-sensing properties of the PAS/PD-IPN hydrogel under different conditions. Two Peltier modules were connected in parallel to either heat or cool the PAS/PD-IPN hydrogel skin and were applied on opposite ends of the hydrogel film to generate a temperature gradient across it (Fig. 4a). According to ionic thermoelectric mechanisms, the ionic Seebeck coefficient (Si) of the ionic hydrogel was obtained by fitting the slope of open-circuit voltage (Vtherm) vs. temperature difference (ΔT) plots50,51. One Peltier module was used to control one side of the hydrogel film at a fixed initial temperature (T0) and one Peltier module was used to apply a temperature (heating or cooling) on the other side of the hydrogel (Ts), thus generating ΔT across the hydrogel sample. As shown in Fig. 4b inset, at a fixed T0 of 20 oC and 50% relative humidity (RH), various ΔT of 14.0, 8.1, 3.1, -3.1, -8.7, and − 13.8 oC were generated across the PAS/PD-IPN hydrogel. Their corresponding Vtherm reached stable values of 16.03, 10.02, 3.33, -3.78, -11.08, and − 17.90 mV, respectively. The maximum Si of the PAS/PD-IPN hydrogel was calculated to be 1.21 mV/K (Fig. 4b), which showed no major change (Si = 1.17 mV/K), even when T0 was decreased to -10 oC (Fig. 4c). This indicates that the IPN ionic hydrogel skin had an anti-freezing property and exhibited a temperature sensitivity at both ambient and subzero temperatures.
When a tensile strain was applied, the Vtherm values of the PAS/PD-IPN hydrogel slightly increased. The Si values of the IPN ionogel skin at strains of 50% and 100% slightly increased to 1.70 mV/K and 1.64 mV/K, respectively, from an initial value of about 1.21 mV/K at 0% strain (Fig. 4d). The stability of the Si upon cyclic stretch-release tests at 200% strain was also evaluated (Fig. 4e). Remarkably, after subjecting the hydrogel to 200 stretch-release cycles, the Si value remained as high as 1.43 mV/K. These results indicate the ability of the PAS/PD-IPN hydrogel to retain its temperature sensitivity under large strain deformation, which mainly stems from its superior elasticity.
Temperature resolution and response time are two other critical parameters for gauging the performance of the sensing material. As shown in Fig. 4f, the PAS/PD-IPN hydrogel demonstrated a detection accuracy of ΔT = 0.1°C with Vtherm = 0.10 mV and 0.12 mV under 0% and 100% strain, respectively. This temperature resolution was attributed to the high Si value of PAS/PD-IPN hydrogel in both the original and stretched state. Moreover, at a ΔT of 0.1°C, the PAS/PD-IPN hydrogel exhibited a response time of about 20 s under 0% and 100% strain (Fig. 4f inset). To evaluate the practical thermal sensing behavior of the PAS/PD-IPN hydrogel, we measured the time-dependent variation in Vtherm by dropping water droplets at different temperatures (10, 15, 20, 25, and 30°C) onto the PAS/PD-IPN hydrogel at 20°C. Vtherm rapidly dropped by -1.21 and − 0.20 mV after contacting the cold-water droplets (10 oC and 15°C) due to the Soret effect. Then, it gradually increased due to the warming of the cold-water droplet over time and stabilized at 0 mV upon reaching thermal equilibrium (Fig. 4g and 4h). When warm-water droplets (25 oC and 30°C) were dropped onto the PAS/PD-IPN hydrogel, the instantaneous values of Vtherm increased to 1.01 mV and 2.03 mV. Thus, variations in thermal stimuli from water droplets could be detected by the different instantaneous Vtherm values.
6. Capacitive pressure sensing performance
Next, to evaluate the electromechanical characteristics of the PAS/PD-IPN hydrogel when used as a capacitive pressure sensor, it was sandwiched between two flexible electrodes, which were both gold/polyimide (Au/PET) films (Fig. 5a). Because the sensitivity of a capacitive pressure sensor mainly depends on the dielectric constant and deformation of the dielectric material, the high dielectric constant and low Young’s modulus of the PAS/PD-IPN hydrogel can help increase the capacitive pressure sensitivity52. To further improve the sensitivity, the PAS/PD-IPN hydrogel was micropatterned (Fig. 5b). In most conventional capacitive pressure sensors, the baseline capacitance (initial capacitance, C0) is usually on the order of tens of femtofarads due to the low dielectric constant of the sensing material52. Such a low baseline capacitance leads to a low signal-to-noise ratio, which makes the capacitive sensor highly vulnerable to nearby parasitic effects and/or in the readout circuitry52. As shown in Fig. 5c, the measured initial value of the PAS/PD-IPN hydrogel sensor was on the order of microfarads as the applied pressure increased from 1 Pa to 400 Pa. These high capacitance values can be easily measured by using low-cost electronics53. To ensure reliable measurements, the C0 was set at 1 µF to measure the capacitive pressure sensing performance.
The relative capacitance change (ΔC = Cp-C0, Cp corresponding to the capacitance under pressure) as a function of applied pressure of the PAS/PD-IPN hydrogel sensor was measured over a pressure range of 0–10 kPa, as shown in Fig. 5d. The capacitive response curves contained three linear regions: 0-0.2 kPa, 0.2-2 kPa, and 2–10 kPa. The corresponding linearity and sensitivity (S = δ(ΔC/C0)/δP, P denotes the applied pressure) were calculated to be 0.94, 0.96, and 0.97, and 7.92 kPa− 1, 6.10 kPa− 1, and 0.45 kPa− 1, respectively. A higher sensitivity was obtained in the low-pressure region because the micropatterned surface of the PAS/PD-IPN hydrogel was more prone to a low pressure and the modulus of the hydrogel was low at small compressive strains54, 55. Significantly, the sensitivity of 7.92 kPa− 1 within the 0-0.2 kPa range and 6.10 kPa− 1 within the 0.2-2 kPa range of the PAS/PD-IPN hydrogel sensor surpassed those of most previously reported capacitive pressure sensors in the pressure range below 2 kPa (Table S4).
To evaluate the dynamic response of the hydrogel sensor, the PAS/PD-IPN hydrogel was subjected to cyclic transient pressure by increasing the loading from 0.1 kPa to 10 kPa. As illustrated in Fig. 5e, the capacitance response remained stable under various pressure loading and unloading cycles. To measure the sensing reliability of the hydrogel sensor, its sensing curves under various compression rates were measured (Fig. 5f). Under a loading pressure of 0.1 kPa, the peak values of Cp were unchanged upon increasing the loading rate from 1 to 10 mm/min. Moreover, the sensing stability of the PAS/PD-IPN hydrogel was investigated by subjecting the hydrogel sensor to long-term loading-unloading cycles (Fig. 5g). The hydrogel sensor exhibited a stable and reliable capacitive response and withstood more than 1000 repeated compressions up to 0.4 kPa. The response time to pressure stimuli was about 150 ms (Fig. 5h). These results reveal the sensing reliability, durability, stability, and accuracy of the PAS/PD-IPN hydrogel sensor. We further demonstrated the capacitive pressure sensing performance of the PAS/PD-IPN hydrogel sensor by successively dropping water droplets onto it. The results showed that the sensor could differentiate pressures induced by six water droplets applied one after another (Fig. 5i).
The electromechanical robustness of the PAS/PD-IPN hydrogel sensor against large strains and low-temperature perturbations was further investigated. The electromechanical characteristics of the hydrogel sensor were measured by applying a tensile strain to the PAS/PD-IPN hydrogel. As shown in Fig. 5j, the sensitivity of the hydrogel sensor in the stretched state only slightly decreased and remained high over a wide pressure range. The sensitivity of the PAS/PD-IPN hydrogel sensor under 100% strain was calculated to be 5.65 kPa− 1 within the range of 0-0.2 kPa, 3.61 kPa− 1 within 0.2-2 kPa, and 0.93 kPa− 1 within 2–10 kPa (Table S5). These values were still higher than those of most reported unstretched capacitive pressure sensors (Table S4). The sensitivity of the hydrogel sensor even exhibited a mild increase to 8.68 kPa− 1 within the range of 0-0.2 kPa after being subjected to 200 stretching-releasing cycles to a strain of up to 200% (Fig. 5k, Table S5). Compared with the test performed at ambient temperature (25 oC), the hydrogel sensor at 0 and − 5 oC showed a lower capacitive response under stimulation by a high pressure (Fig. 5l). However, the sensitivity of the hydrogel sensor remained as high as 6.12 kPa− 1 and 4.42 kPa− 1 within the low-pressure range of 0-0.2 kPa at 0 and − 5 oC, respectively (Table S5). Such high sensitivity in the low-pressure range under subzero temperature was due to the excellent anti-freezing property of the PAS/PD-IPN hydrogel.
7. Proximity sensing performance
In addition to capacitive pressure sensing and ionic thermoelectric thermal sensing capabilities, the PAS/PD-IPN hydrogel skin could discern an approaching object, if the object is conductive or has a dielectric constant different from that of the air56. To determine the proximity sensing ability, a fringe field sensor was constructed by placing a pair of parallel electrodes on one surface of the PAS/PD-IPN hydrogel with an elastic insulating tape (VHB 4905, 3M Inc.) laminated between one electrode and the hydrogel (Fig. 6a). In theory, the capacitance change during proximity sensing originates from a disturbance in the fringing electric field. When an object approaches a fringing field sensor, the fringing electric field is intercepted, leading to a drop in mutual capacitance (Cm)38. Figure 6b exhibits continuous Cm changes as a conductive Cu bulk (10 × 3 × 0.2 cm) repeatedly approached from an initial distance (D0) of 30 cm to various detection distances (Dt) from the sensor and then moved away back to D0. Interestingly, the hydrogel-based proximity sensor detected the conductive object as far away as 15 cm from the sensor. Cm showed a clear drop as Dt decreased. The proximity sensor could also detect dielectric objects such as solvents. Figure 6c illustrates the relative Cm changes as a function of Dt when a glass tube containing water (30 mL) approached the sensor. The relative Cm change became more pronounced and reached − 1.65% at Dt = 0.5 cm (Fig. 6c and Figure S9). Moreover, this proximity-sensing performance exhibited a very slight increase as the hydrogel sensor was stretched to 100% strain (Fig. 6c and Figure S9), indicating its robust sensing performance against strain perturbation. The proximity-sensing ability of the PAS/PD-IPN hydrogel sensor was independent of the movement speed of target objects (Fig. 6d). The response time to an approaching water droplet was as fast as ∼70 ms (Fig. 6e). In addition to distance, the fringe field sensor was sensitive to changes in the dielectric constant (ε) of objects (57). Continuous changes in Cm were recorded when 11 types of solvents, including water, DMF, and ether, with different dielectric constants and a fixed volume, repeatedly approached and moved away from the PAS/PD-IPN sensor, as shown in Fig. 6f and Figure S10. The Cm changes of water (ε = 80.1), DMF (ε = 38.3), and ether (ε = 4.27) at Dt = 1 cm were measured to be -0.125, -0.029, and − 0.0074, respectively, demonstrating that the sensor could differentiate solvents with different ε values in a non-contact mode. The ε and Cm relationship for 11 typical solvents (Figure S10) is summarized in Fig. 6g. The measured Cm changes of most solvents were well fitted to their intrinsic ε, except for solvents with very similar ε values, such as cyclohexane (2.02) and dioxane (2.22).
8. Discriminable multiple sensing performance
Our PAS/PD-IPN hydrogel sensor can perceive proximity, temperature, and pressure stimuli with high sensitivity, high accuracy, fast response, and good reliability. Different from traditional multimodal sensing arrays constructed by integrating various individual sensing units with a complex but low-resolution layout58, our ionic hydrogel sensor incorporated multimodal sensory functions in one sensing material, and thus has a simple structure, low cost, and easy operation58,59. As shown in Fig. 7a, a multimodal sensor was fabricated by simply patterning three pairs of electrodes onto the PAS/PD-IPN hydrogel, which endowed the sensing signals with three distinguishable outputs: a capacitive change for pressure sensing, a fringing field change for proximity sensing, and a thermal voltage change for temperature sensing. The uniqueness of our ionic hydrogel sensor is its ability to differentiate material types with different physical properties (i.e., density (ρ), ε, or temperature). To demonstrate this, we used solvent identification as an example. Figure 7b shows the quantification process and different output signals when a solvent droplet with a fixed volume was dropped from a fixed distance onto the surface of the PAS/PD-IPN multimodal sensor. When the solvent droplet began to approach the sensor surface, the Cm value monitored by the fringing field sensing unit in the multimodal sensor sharply decreased and finally reached a stable value immediately after the solvent contacted the sensor. When the solvent contacted the sensor, the capacitance monitored by the pressure-sensing unit instantaneously increased and stabilized due to the pressure applied by the solvent droplet. The thermal voltage output of the thermal-sensing unit gradually changed due to heat transfer between the solvent droplet and the sensor.
To demonstrate the material identification capability of our hydrogel-based multimodal sensor, we measured time-dependent variations in Cm, Cp, and Vtherm by dropping two types of solvent droplets (0.5 mL) onto the sensor at different temperatures. The first group dropped onto PAS/PD-IPN sensor at T0 = 30oC included two types of solvents with similar ε values but very different ρ values: cyclohexane (ε = 2.02 F/m, ρ = 0.77 g/mL, 20oC) and dioxane (ε = 2.22 F/m, ρ = 1.03 g/mL 25oC) (Fig. 7c). While the output signals of Cm from the fringing field sensing unit could not identify cyclohexane or dioxane due to their very similar ε values (Fig. 7d), they could be differentiated by changes in the output signals of ΔCp from the pressure-sensing unit because of their large differences in ρ (Fig. 7e). Moreover, variations in the initial temperature of the solvent droplets could also be detected by the difference in the instantaneous and equilibrated Vtherm values. As shown in Fig. 7f, when cyclohexane (20 oC) and dioxane (25 oC) droplets were dropped onto the sensor at 30oC, the Vtherm values decreased to -0.8 mV and − 0.6 mV after 30 seconds, respectively. The other two solvents chosen for identification in the second group were cyclohexane (ε = 2.02 F/m, ρ = 0.77 g/mL, 30 oC) and acetone (ε = 21.01 F/m, ρ = 0.79 g/mL, 35 oC), which have similar ρ values but had an order of magnitude difference in their ε values (Fig. 7g). When cyclohexane and acetone droplets were dropped onto the sensor at 20 oC, their respective ultimate ΔCm values reached − 0.45 pF and − 0.75 pF, exhibiting a large difference in output signals, thus allowing them to be identified (Fig. 7h). The output ΔCp signals from the pressure-sensing unit of both solvents were identical (Fig. 7i) due to their similar ρ values. Moreover, variations in thermal stimuli from cyclohexane and acetone could be detected by differences in their Vtherm values (Fig. 7j). These results show that our PAS/PD-IPN hydrogel sensor could detect and discriminate different types of materials without crosstalk due to its decoupled multimodal sensing ability.
Here, we proposed a unique strategy for designing and fabricating an all-round multisensory ionic hydrogel with a microphase-separated structure. The highly-entangled first network copolymerized with zwitterions, which allowed it to form strong interactions with the polyelectrolyte second network. These interactions facilitated the formation of a microphase-separated microstructure in the ionic hydrogel, which contributed to its excellent mechanical properties including skin-like Young’s modulus (< 60 kPa), high stretchability (> 900%), superior elasticity (resilience > 95%, hysteresis < 5%), strain-stiffening behavior, and excellent fatigue tolerance. The presence of zwitterions and a polyelectrolyte also gave the ionic hydrogel desirable moisture-retention and anti-freezing properties. The ionic hydrogel retained a high ionic conductivity and ionic Seebeck coefficient under large tensile strain and subzero temperatures. Most importantly, the ionic hydrogel skin could detect and decouple multiple stimuli (temperature, pressure, and proximity) via an ionic thermoelectric effect, piezo-capacitive effect, and fringing field effect, respectively. The synergistic mechanical and physical effects gave the ionic hydrogel sensor a high capacitive pressure sensitivity of up to 7.5 kPa-1 with a detection limit of 1 Pa, a temperature sensing resolution of 0.1 °C, and excellent sensing robustness against large strain (> 200%) and subzero temperature perturbations. This ionic hydrogel sensor shows great potential for intelligent electronic skin applications, such as reliable health monitoring and accurate object identification.
Materials.
Acrylamide (AM, 98%) was purchased from Energy Chemical, China. [2-(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) (SBMA, 98%) was purchased from Picasso (Shanghai, China). Diallyldimethylammonium chloride solution (DADMAC, 60 wt.% in water) was purchased from Shanghai Macklin Biochemical Co., Ltd, China. N, N′-Methylene bis(acrylamide) (MBAA, 99%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd, China, and the photoinitiator (2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, I2959, 98%) was purchased from Shanghai Yuanye Bio-Technology Co., Ltd, China. All the other reagents were of analytical grade and were used without any further purification.
Characterization
Scanning electron microscopy (SEM, Phenom ProX, Netherlands) equipped with a temperature-controlled sample holder (-25-50℃) was used to characterize the morphologies of samples. Nano computed tomography (Nano CT, Bruker SkyScan 2211, USA) was applied to visualize the solid-liquid phase distribution. Fourier-transform infrared (FT-IR) spectroscopy was performed using a Thermo Scientific Nicolet IS50 FT-IR spectrometer in the wavenumber range of 4000-400 cm-1 to characterize the formation of a second network and interchain interactions inside the interpenetrating network. The interchain interactions between the dual-network were also confirmed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, USA). To evaluate the viscoelastic properties of the dual-network structure, dynamic mechanical analysis (DMA, TA Q800, USA) was conducted at a heating rate of 5℃/min and a frequency of 1 Hz. Differential scanning calorimetry (DSC, DSC25, TA Instruments, USA) was used to determine the boundaries of the phase regions. The microstructure of hydrogel was observed using an ultra-depth three-dimensional microscope (LY-WN-YH8500, Chengdu Liyang, China).
Preparation of PAS-SN hydrogel and PD-SN hydrogel.
Single-network hydrogels were prepared using a photo-polymerization method using I2959 as the initiator. First, 9.0 g AM and 3.0 g SBMA were dissolved in 18.0 mL water, followed by the addition of 9.0 mg MBAA as the crosslinker and 270.0 mg I2959 as the photoinitiator. After ultrasonic dissolution, the solution was poured into a glass container and then sealed. After 365 nm UV irradiation for 1 h, the PAS-SN hydrogel was obtained. Similarly, to prepare PD-SN hydrogel, 20.0 g DADMAC, 100.0 mg MBAA, and 600.0 mg I2959 were dissolved in 24.4 g ultrapure water, and the method for preparing the PD-SN hydrogel was the same as above.
Preparation of PAS/PD-IPN hydrogel.
The PAS/PD-IPN hydrogel was prepared based on the first-prepared PAS-SN hydrogel. Briefly, the precursor solution of the second network was prepared with 80.0 g DADMAC, 0.4 g MBAA, 2.4 g I2959, and 97.6 g ultrapure water. Then, the PAS-SN hydrogel was immersed in the prepared precursor solution for 24 hours. Finally, the soaked hydrogel was polymerized under 365 nm ultraviolet irradiation for 1 hour.
Mechanical property tests.
The mechanical properties under stretching and compression were tested by using a universal tensile testing machine (AGS-X, Shimadzu, Japan). For the stretching test, the hydrogel samples were stretched until fracture under a constant rate of 100 mm/min with an initial distance of 10 mm while the stress-strain curves were recorded at the same time. The conditions for the compression test were identical to those of stretching, except the rate was 20 mm/min. The conditions of other relevant tests during the stretching or compression process are illustrated in the manuscript. Due to its anti-freezing properties, we also evaluated the mechanical performance of PAS/PD-IPN hydrogel at 20℃ and -10℃, and the test conditions were the same as those above.
Measurement of the ionic conductivity of PAS/PD-IPN hydrogel.
The ionic conductivity was evaluated under dynamic conditions, including different environmental temperatures (-10℃, -5℃, 0℃, 5℃, 15℃, and 25℃) and different strains (0%, 50%, 100%, and 200%). The electrochemical impedance spectroscopy (EIS) analysis of the PAS/PD-IPN hydrogels was performed in a frequency range of 1-1 × 106 Hz using a CHI660 electrochemical workstation (Chenhua Co., China). The size of the PAS/PD-IPN hydrogel used for EIS detection was 10 × 10 × 2.6 mm. The ionic conductivity (σ) was calculated using the following equation51:
where d is the thickness of the hydrogel sample (cm); R is the body impedance of the measured sample (ohm), which was obtained from the intersection of the semi-arc and oblique lines in the Nyquist plot of the EIS spectrum; S is the effective cross-section area between the electrodes and hydrogel (cm2).
The ionic conductivity of the PAS/PD-IPN hydrogel at different temperatures was measured using a Peltier temperature control unit. To investigate the conductivity at different strains, the hydrogels were stretched to 0%, 50%, 100%, and 200% from an initial length of 2 cm under a constant temperature.
Fabrication of microstructured IPN hydrogel and its pressure-sensing performance.
To amplify the pressure-sensing performance, a microstructure was introduced into the dual-network hydrogels using the same fabrication process as for the PAS/PD-IPN hydrogels. The only difference was that the prepared precursor solution was transferred to a glass container with one side pasted with sandpaper (120 mesh). The sensing performance was dramatically enhanced due to the more-sensitive structure deformation at the same pressure. The pressure-sensing properties of the hydrogel were characterized by applying an external load using a compression testing machine (AGS-X, Shimadzu, Japan), in combination with an LCR meter (TH2838H, Tonghui Electronic, China). This setup was used to record the capacitance values at a maximum voltage of 1 V and a sinusoidal signal of 1 kHz. The recognition of liquids was similar to the temperature difference detection. Water was selected for a demonstration and dropped onto the silver electrode. A dramatic increase in the capacitance was recorded using a TH2838H LCR with a frequency of 1 kHz.
Measurement of non-contact proximity sensing performance.
Solvents with different dielectric constants were encapsulated into identical glass test tubes and then fixed onto a motorized linear stage with a built-in controller from Zolix Inc. The solvents were cyclically moved over the hydrogel at a distance of 1 cm at a speed of 10 mm/s. For the cycling test, the distance and speed were set to 1 cm and 20 mm/s, respectively. For liquid sorting applications, different liquids encapsulated in glass tubes were moved circularly at a speed of 20 mm/min with a distance of 1 cm between the tube and electrodes. The capacitance was recorded using an LCR meter (TH2832, Tonghui Electronic, China) with a maximum voltage of 1 V and a 100 kHz sinusoidal signal.
Sensing performance of PAS/PD-IPN hydrogel under dynamic conditions.
The evaluation of sensing performance under dynamic conditions included different environmental temperatures and different stretched states. The hydrogel-based sensors were characterized using three parameters (temperature, pressure, and proximity) at different temperatures (-10, -5, 0, 5, 15, and 25 ℃) and strains (0%, 50%, 100%, and 200%). For the temperature-variable sensing measurements, the hydrogel was first placed onto a temperature-control module, which was assembled and adjusted using a Peltier module to precisely control the temperature. For the sensing tests under stretching, the hydrogel was fixed onto a tensile platform. The thermovoltage and capacitance signals versus temperature or tensile condition were recorded, and the other test conditions were kept constant to provide a comparison.
Demonstration of real-time solvent-recognition ability.
For a real-time sensing demonstration, we compared dioxane, cyclohexane, and acetone because they possess different densities, dielectric constants, and temperatures. The scheme of the testing device is shown in Figure 7a. First, for proximity sensing, glass droppers containing 1 mL of different liquids at the same temperature were moved to approach the electrode quickly, and then the distance was kept at 1 cm. The capacitance was recorded by a TH2832 LCR with a frequency of 100 kHz. Then, for pressure sensing, 0.5 mL of the liquids were dropped, and the capacitance was recorded by a TH2838H LCR with a frequency of 1 kHz. Finally, during temperature sensing, the temperature was kept constant on one side, and then 0.5 mL of liquids at different temperatures were dropped onto the other side to form a temperature gradient. Then, the thermovoltage was recorded by a Keithley 2000 multimeter. The distance between the two electrodes and thermocouples was 5 mm.
Acknowledgement
This work reported here was supported by the National Natural Science Foundation of China (52173238), the Municipal Natural Science Fund of Tianjin (20JCJQJC00010 and 21JCQNJC00170), the Ministry of Science and Technology of China (2022YFA1203304), and the Fundamental Research Funds for the Central Universities of Nankai University (63191520).
Author contributions:
J. L. supervised the project. J. L. conceived and designed the research. X. L., J. G., R. Z., and X. J. participated in materials preparation, device fabrication, device test, or interpretation of results. J. L. and X. L. analyzed the data and co-wrote the manuscript. All authors analyzed and discussed the results.
Competing interests:
There are no conflicts to declare.
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There is NO Competing Interest.