Figure 1a shows the preparation process and sandwich structure of PVA-PAA-PANI hydrogel. Randomly distributed acrylic monomers were first triggered to polymerize and form a double network by intertwining with PVA macromolecular chains. The subsequent soaking process induced the formation of hydrophobic cross-linking of the macromolecular chains, which is the key to enhance the mechanical strength of the hydrogel material. Simultaneously, aniline was anchored in the surface network of the hydrogel and polymerized in situ with the help of ammonium persulfate. The appearance of the hydrogel changed significantly before and after soaking, from colorless and transparent to dark green and opaque (Fig. 1b). The cross-section of the material shows that PANI is uniformly distributed on both sides of the PVA-PAA hydrogel, and forms a perfect sandwich structure with the PVA-PAA hydrogel (Fig. 1c). PVA-PAA-PANI hydrogel can be easily bent and twisted and still remain intact state (Fig. 1c). Moreover, after experiencing compression and stretching by external forces, the hydrogel can return to its original state immediately (Fig. 1d, e). These phenomena visually reflect the strong flexibility and good elasticity of hydrogel.
Hydrophobic cross-linking is an important reason for the strength enhancement of PVA-PAA-PANI hydrogels. The strength of the hydrogel was significantly enhanced with the increase of soaking time (Fig. 2a). When the soaking time was 3 h, the fracture strength of the hydrogel increased to 0.82 MPa, 1200% that of the original PVA-PAA hydrogel, and when the time was extended to 12 h, the hydrogel strength reached 3.5 MPa, which was an increase of more than 5800% (Fig. 2b). Moreover, the elongation at break and toughness of the PVA-PAA hydrogel after 12 h soaking reached 700% and 11.5 MJ/m3, respectively, which are significantly superior to those reported in the past papers [22–33] (Fig. 2c). The tensile-recovery cycle curve is an important data to study the energy dissipation and fatigue resistance of the material. From Fig. 2d, the hysteresis phenomenon of the tensile-recovery curves could be clearly observed at deformation variables ranging from 10–200%, which indicated that the PVA-PAA-12PANI hydrogel can perform effective energy dissipation during the stretching process [34]. What’s more, all tensile-recovery curves of the PVA-PAA-12PANI hydrogel overlapped well with each other at 50% fixed deformation, except for the first cycle, implying satisfactory fatigue resistance (Fig. 2e). In addition, a much higher remarkable energy dissipation of 9.1 kJ/m3 could be achieved in the first tensile-recovery cycle when 50% strains are loaded. The energy dissipation then stabilized at 5 kJ/m3 during the second to 300th cycles (Fig. 2f), meaning that the hydrogel possessed excellent resilience and shape recovery performance. And, even when the fixed deformation is expanded to 200%, the tensile-recovery curve still maintained a high degree of overlap (Fig. S1). Fig. S2 shows the FTIR spectra of wet PVA-PAA and PVA-PAA-12PANI hydrogels. Compared with PVA-PAA hydrogels, PVA-PAA-12PANI hydrogels did not exhibit distinct characteristic peaks attributed to quinoid ring and benzene ring [35], which might be due to the overlap of absorption peaks caused by the existence of large amount of water. However, a clear doping peak attributed to sulfuric acid could still be observed at 1110 cm− 1, signifying the successful polymerization of PANI [36]. The mixing of PANI during soaking process disturbed the molecular arrangement in the PVA-PAA hydrogel, which in turn reduced the crystalline properties of the material (Fig. S2). SEM, as an effective method to observe the microscopic morphology of the material, can visually analyze the microstructure of the hydrogel and the distribution of PANI. As can be seen from Fig. 2g, the PVA-PAA hydrogel showed a uniform pore structure and the pore size was about 2 µm. This unique structure created the possibility of solvent exchange and the entry of aniline monomer during the soaking process. In contrast, the PVA-PAA-12PANI surface was covered by scale-like PANI and part of the pores were blocked, which marked the success of in situ PANI polymerization.
In order to investigate the electrochemical performance of PVA-PAA-12PANI hydrogel, a supercapacitor was prepared by sandwiching a hydrogel with a thickness of about 1 mm between two pieces of carbon cloth. Then, the comprehensive electrochemical performance was evaluated in a two-electrode system. Clearly, a distinct redox peak could be found in the CV curve of the scan rate from 5 mV/s to 100 mV/s, which arose from the pseudocapacitive property of PANI (Fig. 3a). Besides, the shape of the CV curve did not appear to change significantly with the increase of the scan rate. In addition, the GCD curve shown in Fig. 3b had a good symmetrical shape and showed a clear redox process, which was typical of the charge/discharge mode of supercapacitors and highly consistent with the CV curves. The Nyquist plot in Fig. 3c demonstrates that the EIS curves of PVA-PAA-PANI had a distinct radial shape and were approximately parallel to the imaginary axis, which indicated their ideal capacitive performance. As can be judged from the intersection of the curve and the real axis, the equivalent series resistance of the device was about 174 Ω, which was a low value and ensured the free movement of ions in the device. The specific capacitance calculated from the GCD discharge curve is plotted in Fig. 3d. The specific capacitance of the PVA-PAA-PANI-based supercapacitor reached 32.4 mF/cm2 at a current density of 0.03 mA/cm2. Even after increasing the current density to 0.6 mA/cm2, the specific capacitance of the device still approached 30 mF/cm2, signifying an unparalleled rate performance. Further, the continuous constant current charge/discharge stability of the device in the voltage range of 0-0.8 V was tested and analyzed. It is clear from the test results that even after 5000 charge/discharge cycles, the device maintained 82% of its original specific capacitance, implying surprising operating stability and long service life (Fig. 3e).
The entry of glycerol during the soaking process gave the gel excellent low-temperature resistance, allowing it to maintain flexibility and good electrical conductivity at low temperatures (Fig. 4a). The DSC data confirmed that the PVA-PAA-12PANI hydrogel did not show any heat absorption or exothermic peaks in the range of -60-20°C, indicating that no phase transition process occurred within the material (Fig. S4). Visually, the surface of PVA-PAA-12PANI remained soft and moist state after being frozen in a refrigerator at -20°C for 24 h and could be bent and stretched arbitrarily (Fig. 4b). In addition, the EIS curve of the device at cold environment maintained almost the same shape as that of room temperature, but a slight shift of the intercept, which indicated that the equivalent series resistance of the device did not undergo a sharp drop at low temperatures and implied normal movement of ions within the material (Fig. S5). To investigate the low temperature resistance of PVA-PAA-12PANI-based supercapacitors, the electrochemical performance of the device at -20°C was systematically assessed. As shown in Fig. 4c, the redox peak belonging to PANI could still be found on the CV curve of the device at -20°C, but the area of the curve slightly decreased, a phenomenon that could be attributed to the increased resistance of ion movement at low temperatures [37]. In addition, the area of the CV curve of the device increased significantly with increasing scan rate, but the curve also gradually deviated from the original shape because of the inclusion/ejection and diffusion of counter ions (Fig. 4d) [22]. It could be found from the GCD curves at different temperatures that the charging and discharging times of the device became shorter at -20°C, but the curves still had a symmetric shape (Fig. 4e). Further, the increase in current density did not significantly change the symmetric shape of the GCD curve (Fig. 4f). Notably, the specific capacitance of PVA-PAA-12PANI hydrogel-based supercapacitor could reach 20 mF/cm2 (a current density of 0.03 mA/cm2) when the temperature dropped to -20°C, which was about 61.7% of that at room temperature. Even the current density increases to 0.6 mA, the specific capacity can still approach 12.8 mF/cm2, reaching 63.5% of the original value and proving a good rate performance (Fig. 4g). As a key criterion for low temperature electrochemical performance, the charge/discharge cycles under − 20°C were tested at a current density of 0.1mA/cm2. It was clear from Fig. 4h that after enduring 5000 charge/discharge cycles, the supercapacitor exhibited a capacitance retention close to 79.2%, which proved the possibility of long-term use at low temperatures.
Due to the abundant presence of Na+, H+ and SO42−, the PVA-PAA-12PANI hydrogel exhibited satisfactory electrical conductivity. Besides, there was a significant positive correlation between the resistance of the hydrogel and the deformation, and the linear range could reach about 80% (Fig. 5a). What’s more, the PVA-PAA-PANI hydrogel showed almost the same resistance change under constant deformation stretching at different frequencies, indicating that the deformation was the only factor determining the hydrogel resistance (Fig. 5b). This phenomenon and conclusion could be further supported by Fig. 5c and d. From Fig. 5c and d, it could be found that whether under the peak-like or step-like program settings, the changes in hydrogel resistance under the same deformation are essentially the same, indicating that a fixed deformation could be judged from the resistance change rate of the hydrogel. Attractively, the hydrogel could send out the corresponding electrical signal feedback in about 0.3 s after external stimulation, which marked that signal feedback and external stimulation occur almost simultaneously. Moreover, when unloading the external force, the resistance decreased to the original state within 0.38 s, implying a negligible hysteresis (Fig. 5e). Further, after 500 stretch-recovery cycles with a fixed large deformation of 100%, the PVA-PAA-12PANI hydrogel could still capture the external stimulus and gave the corresponding electrical signal change in time, indicating excellent reproducibility (Fig. 5f).
The extremely short response time of the hydrogel is comparable to the human reaction time, indicating that it can capture the stress stimulus generated by human motion and monitor it in a timely manner. As shown in Fig. 6a-d, the PVA-PAA-12PANI hydrogel was attached to different joint parts of the model to confirm its performance in sensing and detecting human motion behavior. The PVA-PAA-12PANI could promptly capture the human body's joint movements and accurately delivers significant resistance signal changes when fixed to the model's knee (Fig. 6a), elbow (Fig. 6b) and fingers (Fig. 6c, d). Moreover, the frequency of electrical signal change could be well matched with the frequency of joint motion. The high frequency of joint motion led to high frequency of electrical signal changes (Fig. 6c). More interestingly, a change in the finger bending angle also led to a different change rate of hydrogel resistance, for example, a 30° bending yielded a 10% change in resistance, while a 90° bend corresponded to 35% (Fig. 6d). The PVA-PAA-12PANI hydrogel was also sensitive to subtle body movements. As depicted in Fig. 6e, the PVA-PAA-12PANI hydrogel anchored in the model's larynx could well sense the vibrations caused by cough and swallowing, and generate obvious electrical signal changes. Amazingly, the vibration signals generated by different words could also be captured and distinguished. From Fig. 6f, it was clearly seen that the pronunciation of “DLUT” produced a high and sharp signal peak of more than 20% in resistance change, while “LOVE” gave a short and wide.