English

• In the past few years, flexible wearable sensors have attracted significant attention across various applications owing to their excellent flexibility and high sensitivity, such as biomedical applications [1, 2], electronic skin [3, 4], soft robots [5, 6], human-computer interfaces [7, 8], and supercapacitors [9, 10]. As a kind of soft matter, hydrogels are currently proving to be desirable materials for flexible wearable sensors [11]. To meet the requirement for practical application, hydrogels should possess certain physicochemical properties, including high electrical conductivity, satisfactory mechanical performances, strong adhesion, antibacterial activity, and anti-freezing performance, which ensures the accuracy and stability of signal detections and long-term service under complicated environments [12-14]. More specifically, the high electrical conductivity facilitates the conversion of external signals into recognizable electrical signals easily [15, 16]; the satisfactory mechanical strength guarantees integrity and continuity of electrical signals during repeated compression or stretching [17-19] the strong adhesion allows hydrogels to attach to human skin directly and improves the detection sensitivity of human motions [20-22] the antibacterial activity contributes to the long-term stability of hydrogels when used in human or marine environments [23, 24]; the anti-icing capability assures the practical application of hydrogels when using below sub-zero temperatures [25-27].

  Recently, many efforts have sprung up to develop hydrogel-based flexible sensors with multifunctional properties to meet the application demands in various scenarios [28-35]. For example, Gao et al. [32, 33] introduced natural antifreeze proteins to construct a customizable hydrogel sensor successfully with toughness, adhesiveness and anti-icing properties. Zhao et al. [34] developed a hydrogel nanofiber composite via chemical cross-linking of poly(vinyl alcohol)-Borax and by forming an interpenetrating network of thermoplastic urethanes nanofibers, which exhibited good mechanical and antibacterial characteristics and electrical conductivity when used as a strain sensor. Sun et al. [35] reported an underwater instant adhesive hydrogel that was obtained by copolymerization of various acrylic monomers with MXene nanosheets and the sensors can maintain sensitive and stable signal detection in the underwater environment benefitted from the strong adhesion. However, most of the current hydrogel-based sensors have achieved a part of the properties while are difficult to simultaneously possess all the above properties, severely hampering their practical applications. Based on this, developing a hydrogel sensor integrating all of these properties is imperative.

  Herein, inspired by the cation-π structure of cement proteins in the barnacle [35-37], a simple and effective approach is proposed to develop a multifunctional hydrogel with satisfactory mechanical strength, adhesiveness, antibacterial activity, and anti-icing performance to assemble into the wearable sensor. The poly(DMAPA-co-PHEA) hydrogels (CP hydrogels) were fabricated by cationic-π interactions as physical crosslinks in the presence of Ag nanoparticles and NaCl additives. Based on strong multiple physical interactions, the hydrogels exhibited high mechanical strength and strong adhesion to different substrates. Moreover, the multifunctional hydrogels showed satisfactory antibacterial activity and long-term antifouling stability owing to the broad-spectrum antibacterial performance of Ag nanoparticles (AgNPs). Notably, high concentrations of NaCl solution conferred high conductivity, anti-freezing and anti-icing ability on the hydrogels and the salt concentration can be adjusted according to different requirements. Taken together, the multifunctional hydrogels with high sensitivity were suitable as flexible wearable strain sensors directly adhered to the skin to detect human movements (Scheme 1), which have promising applications for wearable devices and soft robots in complex environments.

  Scheme 1

  

  Scheme 1.  Schematic representation for the fabrication, gelation mechanism, and applications of the bio-inspired multifunctional CP hydrogels.

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  In the first place, the CP hydrogels, which consist of a copolymer of (3-acrylamidopropyl)trimethyl-ammonium chloride (DMAPA) and ethylene glycol phenyl ether acrylate (PHEA) monomers, were prepared via a simple free-radical copolymerization of monomers in DMSO solution. A prescribed amount of AgNPs was added into the precursor solution, irradiated ultraviolet lights (UV) for 20 min and then the polymer solution was transferred into the reaction cell. The CP hydrogels were readily obtained after UV light irradiation for 10 h and a solvent exchange process in NaCl solution (Scheme 1 and Fig. S1 in Supporting information). In the CP hydrogels, the DMAPA (cationic, C) and PHEA (π, P) segments mimic the cationic and phenyl-involved hydrophobic amino acids in cement proteins respectively, which form strong π-π and cation-π interactions as physical crosslinks to enhance the toughness and cohesion strength of the hydrogels [38, 39]. Besides, with the cooperative role of neighboring aromatic groups, high amounts of cationic groups form strong cation-π and electrostatic interactions with substrates, and therefore the hydrogels achieve robust adhesion with different substrates [40, 41]. At the same time, the hydrogels exhibit antibacterial, anti-icing, and sensing multi-functions with the addition of AgNPs and NaCl.

  After having fulfilled the preparation of CP hydrogels, the features of various hydrogels, including swelling ratios, solvent contents, and surface contact angle were further systematically investigated. For C: P = 8:2, the gel highly swelled in NaCl solution because of comparatively a few aggregations of hydrophobic groups and cation-π interactions. As the molar fraction of PHEA increased, the π-π and cation-π attractions of CP hydrogels strengthened and the solvent contents were descended from 95.9% to 73.4% (C: P = 3:7) in 3 wt% NaCl solution. When in 20 wt% NaCl solution, the gels still manifested a high solvent content though slightly decreased, which was important for versatile applications of electrochemical performance and soft wearable devices (Fig. 1a and Fig. S2 in Supporting information). In addition, the surface of the hydrogels became more hydrophobic and the water contact angle changed significantly from 84.6° to 102.1° with increasing the hydrophobic PHEA fraction and NaCl concentration (Fig. 1b and Fig. S3 in Supporting information). Interestingly, further addition of AgNPs/CuNPs (3 mg/mL) also led to an increase (108.3° ± 14°/106.4° ± 7.5°) in the water contact angle of C: P = 1:1–20 wt% NaCl gel. Moreover, to achieve a uniform distribution of nanoparticles, the nanoparticles were dispersed in the precursor solution for 10 min by ultrasonic machine and irradiated UV lights. From the result of energy dispersive spectrometer analyses (Fig. 1c and Fig. S4 in Supporting information), different elements of hydrogels were distributed uniformly and no obvious agglomeration of the nanoparticles occurred on hydrogels, which ensured the stable performance of hydrogels in the tests. According to the element analysis, the proportion of Ag was 2.024% ± 0.69%, which is in good agreement with the 3 mg/mL feed ratio of AgNPs in the hydrogels.

  Figure 1

  

  Figure 1.  (a) Water content and NaCl solution content of various CP hydrogels in 3 wt% NaCl solution. (b) Water contact angles of CP hydrogels. (c) Energy dispersive spectrometer (EDS) images of the hydrogel (C: P = 1:1–3 mg/mL Ag-20 wt% NaCl). The influence of chemical composition on the mechanical performance of CP hydrogels: uniaxial stress-strain curves of hydrogels about different (d) molar feed ratios, (e) additive amounts of Ag and (f) concentrations of NaCl solution (Ag/Cu). Data are presented as mean ± standard deviation (n = 5).

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  Since the mechanical properties are critical in determining the practical application of hydrogel sensors, they are given priority in the subsequent investigations. Figs. 1d–f and Fig. S5 (Supporting information) show the tensile stress-strain curves and Young's modulus (E) of the various hydrogels. Firstly, different monomer feed ratios of CP hydrogels in 20 wt% NaCl solution had a significant influence on the mechanical performance. Compared with the C: P = 8:2 gel which was mechanically weak and fragile, the other hydrogels (C: P = 6:4, 1:1, 4:6, 3:7, 2:8) had enhanced cohesion strength remarkably and the strain of gels increased at first and then descended with the increase of the hydrophobic monomer ratio (Fig. 1d). For example, the elongation of C: P = 4:6 gel (1430%) was 17 times higher than that of C: P = 8:2 gel because of the coupling effect of cationic-π and π-π interactions while the strain of the gels rapidly decreased to 835% of C: P = 6:4 as the hydrophobic monomers increased and the cationic-π interactions decreased. Especially, the tensile strength of the C: P = 2:8 gel achieved 1.21 MPa with a uniaxial elongation of 860% and an elastic modulus of 0.15 MPa (Fig. S5a), exhibiting satisfactory mechanical performance.

  In addition, different additive amounts of AgNPs and concentrations of NaCl solution influenced the mechanical properties of the CP hydrogels and were further researched. As shown in Fig. 1e and Fig. S5b, the elastic modulus of hydrogels increased from 6.85 kPa (0 mg/mL) to 24.87 kPa (4.5 mg/mL) with the increasing addition of AgNPs though the elongation decreased slightly. The fracture stress also reached a maximum of 0.15 MPa at 3.0 mg/mL AgNPs, indicating the toughening of CP hydrogels by the addition of AgNPs. Meanwhile, hydrogels with various kinds of additives and concentrations of NaCl solution were fabricated to fully evaluate other factors affecting the mechanical properties (Fig. 1f and Fig. S5c). From the results, with the incorporation of CuNPs rather than AgNPs, the obtained hydrogels in 20 wt% NaCl showed a higher strain (1300%) while the modulus was lower than 10 kPa. Not surprisingly, a lower concentration of NaCl solution (3 wt%) led to a monotonous decrease in mechanical performance (Young's modulus, strain, and fracture stress) of the materials. Since the concentration of electrolytes significantly affects the hydration level with osmotic pressure, the hydrogels manifest an increase in polymer content with increasing NaCl content. As a result, the elastic modulus of gels gradually enhanced with the rise in effective polymer chain content. Evidently, the strong π-π and cation-π interactions, addition of AgNPs, and high concentration of NaCl solution enhanced mechanical properties significantly.

  Furthermore, the cyclic tensile performance of the hydrogels was investigated to further characterize their mechanical performance. As shown in Fig. S6a (Supporting information), the CP hydrogels could easily recover to their original state in the loading-unloading cycles with a maximum strain of 200% with a 10 s waiting time, suggesting the very good self-recovery property of hydrogels at room temperature. Additionally, the hydrogels possessed good fatigue resistance under repeated loading-unloading cycles (Fig. S6b in Supporting information). Taken together, considering the combined mechanical properties and application for complex environments of multifunctional hydrogels, the hydrogels (C: P = 1:1–3 mg/mL AgNPs) in different concentrations of NaCl solution are especially investigated in the following.

  The CP hydrogels not only possess good mechanical properties but also demonstrate satisfactory adhesion strength with various substrates. As shown in Fig. 2a, a small piece of gel (D = 15 mm) exhibited strong adhesion to stainless weights (100 g), copper, petri dish, plastic cap, steel, composite stone, red wax, bakelite, silicone rubber, glass, notes under only 1 N pressure. To better understand the effect of pressure change on the diameter and adhesive properties, we accurately measured the diameter and adhesion strength of the gel by applying different pressures. As a result, the diameter and adhesion strength of the gel increased with increasing pressure while the adhesion strength began to decrease after the pressure reached about 10 N (Figs. S7a and b in Supporting information). Therefore, we selected a pressure of 3 N to quantify the adhesion behavior of the various CP hydrogels through a lap shear test (Fig. 2b and Figs. S7c and d in Supporting information). The hydrophobic interactions played a major influence at a low concentration of 3 wt% NaCl, the adhesion strength of the gel increased gradually with the increase of hydrophobic monomers (PHEA) fraction. However, the adhesion strength of the gel did not decrease significantly with the decrease of the PHEA fraction in 20 wt% NaCl solution and remained above 21 kPa (C: P = 6:4), indicating cation-π interactions also play important roles in adhesion. These results indicate that the hydrogels have strong adhesion effects for the abundant presence of hydrophobic interactions and cation–π interactions.

  Figure 2

  

  Figure 2.  Adhesive performance and antibacterial activity of the CP hydrogels. (a) Digital photos of hydrogel (C: P = 1:1-Ag-20 wt% NaCl) adhered to different objects. (b) The adhesion strength of the various hydrogels to glass. (c) The antibacterial ratio and (d) images of hydrogels loaded with AgNPs incubated with E. coli and S. aureus for 12 h. (e) Photos of inhibition zones of hydrogels against E. coli and S. aureus for 24 h. Scale bar: 1 cm. Data are presented as mean ± standard deviation (n = 5).

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  Benefitting from the addition of AgNPs, the CP hydrogels not only enhanced the mechanical and adhesive properties but also showed adequate antibacterial activity and antifouling performance which derived from the bactericidal capacities of AgNPs. Herein, Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were used to evaluate the antibacterial properties of the CP hydrogel loaded with AgNPs. Figs. 2c and d showed the antibacterial activities of hydrogels loaded with various amounts of AgNPs incubated with E. coli and S. aureus for 12 h, and the antibacterial ratio was elevated with the increase of AgNPs amount. In particular, the hydrogels with 4.5 mg/mL of AgNPs manifested excellent antibacterial effects with no bacteria survival. In addition, we also investigated the antibacterial activity of the hydrogels in a solid environment by agar diffusion tests. As shown in Fig. 2e and Table S2 (Supporting information), except for the hydrogel without AgNPs, the hydrogels showed good antibacterial activity against E. coli and S. aureus. After 24-h co-incubation, the inhibition zone diameters of hydrogels with AgNPs were as high as 29 mm. Moreover, we used E. coli to evaluate the antifouling performance of multifunctional hydrogels and characterized quantitatively the colony coverage area ratio on the surface of the gels. As shown in Fig. S8 (Supporting information), in comparison to no addition of nanoparticles, the colonies of the CP-AgNPs hydrogels were significantly reduced. When adding 3 mg/mL AgNPs, the colony coverage area ratio was only 0.08% after incubation for 7 days and in the same order of magnitude as 0.03% of 4.5 mg/mL AgNPs. Furthermore, the hemocompatibility and cytocompatibility tests showed that the hydrogel had good biocompatibility (Fig. S9 in Supporting information). All the above results indicated that the CP hydrogels exhibited fair antibacterial activity and long-term antifouling stability properties, creating a good condition for practical applications. For example, it is effective for hydrogel sensors to avoid bacterial infections and inflammations.

  The CP hydrogels achieved superior anti-freezing, anti-icing, and anti-frosting abilities on account of the introduction of high concentrations of salts, indicating that hydrogels are unfrozen and can be used in cold conditions. Firstly, differential scanning calorimeter (DSC) measurements were employed to investigate the anti-freezing ability of hydrogels. As shown in Figs. S10a and b (Supporting information), the crystallization temperature of hydrogels was related to the salt content mainly and decreased dramatically from 0 ℃ in 3 wt% NaCl to −18.65 ℃ in 20 wt% NaCl. Besides, the freezing temperature decreased slightly with the increase of the hydrophobic monomer ratio while the nanoparticles had no obvious influence on the freezing point.

  Moreover, ice forms inevitably on the surface of hydrogels in extremely low-temperature environments, and the most critical problem is how to remove the ice. Therefore, the ice adhesion strength was tested to evaluate the anti-icing ability of CP hydrogels. Fig. S10c (Supporting information) showed that the ice adhesion strength of hydrogels decreased with the reduction of the hydrophobic monomer ratio and increased with repeated icing-deicing tests. Also, the C: P = 1:1 gel had the lowest ice adhesion strength, about 13 kPa. When hydrogels (C: P = 1:1–20 wt% NaCl) with different additives were placed in the anti-icing device for 3, 5, 7 and 24 h, we found that the ice adhesion strength increased gradually as longer freezing time (Fig. 3a). In comparison with glass, the ice adhesion strengths of CP hydrogels were lower by more than one order of magnitude. In addition, although the ice adhesion strength on the hydrogels (C: P = 1:1-Ag-20 wt% NaCl) showed an increasing trend during the 20 cycles of icing-deicing, the gel still maintained a low value (lower than 20 kPa), showing stable and sustainable anti-icing ability (Fig. 3b). Furthermore, anti-frosting ability is also an important parameter to evaluate the anti-icing performance of gels. Compared with other gels, the frost formation of gel (C: P = 1:1–20 wt% NaCl) was slowest which took 58 min (about 20 times that of glass), delaying the frost formation effectively (Fig. S10d in Supporting information).

  Figure 3

  

  Figure 3.  Anti-icing performance of the CP hydrogels. (a) The adhesion strength between ice and various surfaces placed in the anti-icing device for 3, 5, 7, and 24 h. (b) Ice adhesion strength on the surface after 20 cycles. Inset: schematic photograph of the anti-icing test. Electrochemical properties of the CP hydrogels: (c) fitting resistance and conductivity. The inset shows the proposed equivalent circuit. (d) Photographs of LED lights at different tensile ratios. Data are presented as mean ± standard deviation (n = 5).

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  Since ionic conductivity is the basic and important property of the hydrogels used in wearable devices, it was measured by electrochemical impedance spectroscopy (EIS) in the following test. Fig. S11a (Supporting information) shows the Nyquist plots of different CP hydrogels using an electrochemical workstation within a frequency range of 10⁰–10⁵ Hz. The Nyquist plot consisted of the low-frequency region and high frequency region and vertical lines parallel to the imaginary axis at low frequency illustrated the internal charge resistance. Due to the high conductivity of NaCl, CP hydrogels showed good electrochemical properties and the curves of gels at the low frequency were biased towards vertical, indicating a low diffusion resistance and rapid transportation velocity of ions. The R_s (CPE-RP) equivalent fit circuit of the Nyquist curves was presented in the inset of Fig. 3c. Besides, the resistances of gels were obtained by the intercept with the x-axis in Nyquist fitting plots. For instance, the fitting resistance of CP-3 mg/mL AgNPs-20 wt% NaCl was 2.609 ± 0.058 Ω and less than the resistance of the other two types of gels (Fig. 3c). Therefore, the concentration of ions and AgNPs had a great effect on the reduction of the resistance of the gels. The conductivity of CP-3 mg/mL AgNPs-20 wt% NaCl was 19.525 ± 0.767 mS/cm, demonstrating excellent electrical conductivity. Furthermore, the hydrogels at low temperatures (0 and −10 ℃) still maintained good electrical conductivity due to their anti-freezing property while the conductivity decreased at low temperatures (Figs. S11b and c in Supporting information).

  In addition, the bifilar resistance of CP hydrogels increased with the increase of the length, proving indirectly uniform distribution of AgNPs and consistent electrochemical properties (Fig. S11d in Supporting information). To further verify the good conductivity of the CP hydrogels visually we made a device using batteries, wires, and lamp beads (Fig. 3d). At first, the hydrogel did not touch the wiring of the lamp bead and as the gel stretched and contacted with the wiring, the red lamp bead on the left lit up. With further stretching of the gel, it gradually contacted with the wiring of other colored lamp beads and the corresponding lamp beads lit up and remained stable. Similarly, the tensile conductivity of CP gel was performed by a closed circuit attached to the lamp beads (Fig. S12 in Supporting information). We observed that the lamp bead darkened significantly with the stretching of the gel, which demonstrated that the resistance increased obviously as the gel stretched. Consequently, CP hydrogels have good electrochemical conductivity at both high and low concentrations of NaCl solution.

  Based on the high conductivity, the CP hydrogels have potential application in a resistivity-type strain sensor for perceiving mechanical deformation. As depicted in Fig. 4a, the relative resistance changes of the CP hydrogels in 3 wt% NaCl during repeated stretching at strain (20%–120%) were recorded, showing a good perception of various strains and sensing capabilities. Furthermore, the hydrogel sensor was stretched continuously at 100% strain for 100 cycles and not broken after 100 cycles on account of good mechanical strength. The amplitude and waveform of electrical signals were stable, continuous, and consistent (Fig. 4b), indicating the outstanding stability and repeatability of the hydrogels in long-term application. Notably, the CP hydrogels showed prominent sensing sensitivity and relatively high gauge factors (GF) by applying strain (Fig. 4c). GF was an important characterization parameter of sensitivity for gel strain sensors. In 3 wt% NaCl, the linear electromechanical response of the hydrogels to strain was divided into two stages which the GF was 0.26 and 2.69 in the strain range of 20%–80% and 80%–250%. However, the GF of gel in 20 wt% NaCl was 0.93 within a strain range of 20%–250% because of higher conductivity and smaller relative resistance variations overall.

  Figure 4

  

  Figure 4.  The electrochemical sensing performance of the hydrogel (C: P = 1:1-Ag-3 wt% NaCl). (a) The relative resistance changes of the hydrogel during repeated stretching under strain (10%–100%). (b) The cycling stability test of the hydrogel to a maximum strain of 100% for 100 cycles. (c) The relative resistance changes for the various hydrogels under different tensile strains (0%–250%). The relative resistance change curves of the hydrogel as a strain sensor to detect various human motions: (d) walking and running, (e) squatting and standing, (f) normal breathing and deep breathing, (g) finger bending, (h) arm bending, (i) speaking.

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  Since the CP hydrogels possess multi-functional properties, containing high mechanical strength, strong adhesion, satisfactory antibacterial activity, fair anti-icing ability, high electrical conductivity, and strain sensitivity, etc., the hydrogels were suitable as wearable sensors perfectly that could be mounted on human skin to monitor human motions. As shown in Figs. 4d–i, the hydrogels were adhered to different parts of the body, such as knee joints, knuckles, and elbows, to detect the relative resistance variations of gel when the joint bent repeatedly. The hydrogel sensors were also capable of monitoring movements, including running, walking, standing, and squatting. Squatting would stretch the hydrogels increasing resistance. On the contrary, the resistance decreased with hydrogels' shrinkage when standing. In particular, subtle motions were able to be detected accurately by the hydrogel sensors, for instance, attached to the abdomen to detect the respiration states. Fig. 4f shows the recording of sensor signals of normal breathing and deep breathing. In addition, when adhering to the throat the hydrogel sensors could produce effective resistance feedback on simple pronunciation, such as saying "Yes" (Fig. 4i). As a result, these experimental results proved CP hydrogels had great application potential in wearable devices.

  In summary, a multifunctional hydrogel combining satisfactory mechanical, adhesiveness, antibacterial, anti-icing, and sensing properties has been successfully developed with cation-π interactions as physical crosslinks. Based on the multiple non-covalent interactions, the hydrogels exhibited good mechanical properties. On account of the synergistic effect of cationic-π and electrostatic interactions, the hydrogels manifested a robust adhesion to various substrates. Meanwhile, benefitted from the addition of AgNPs, the hydrogels demonstrated satisfactory antibacterial activity and long-term antifouling stability. Besides, the hydrogels possessed good anti-freezing, anti-icing and anti-frosting ability in the presence of NaCl additives. The hydrogels also manifested high conductivity, cyclic stability and high sensing sensitivity, indicating that the hydrogel sensors had a great application potential in wearable devices to monitor human motions and physiological signals, such as joint bending, movements, breathing and speaking. This work may provide a simple and effective design strategy for the innovation of multi-functional hydrogels to meet broader application scenarios.

  Ethical approval

  The sensing experiments about monitoring human motions were assessed and approved by the Ethics Committee and the informed consent of the participants was obtained. The participant was the author Z. Wang.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Yuanmao Fu: Writing – review & editing, Writing – original draft, Investigation. Ziang Wang: Methodology, Investigation, Conceptualization. Kefan Wu: Formal analysis, Data curation. Feiyang Li: Methodology, Investigation. Xian Zhang: Investigation. Hongyuan Cui: Methodology. Xiaolin Wang: Funding acquisition, Conceptualization. Hui Guo: Writing – review & editing, Supervision, Funding acquisition. Yuezhong Meng: Conceptualization.

  Acknowledgments

  Y. Fu and Z. Wang contribute equally to this work. The authors acknowledge financial support from the Guangdong Basic and Applied Basic Research Foundation (No. 2023A1515012218), Macau Science and Technology Development Fund (Nos. FDCT 0009/2020/AMJ, 0027/2023/RIB1), National Natural Science Foundation of China (No. 32301104), and Fundamental Research Funds for the Central Universities, Sun Yat-sen University (No. 23ptpy165).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110479.

  
  
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• 

  Scheme 1  Schematic representation for the fabrication, gelation mechanism, and applications of the bio-inspired multifunctional CP hydrogels.

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  Figure 1  (a) Water content and NaCl solution content of various CP hydrogels in 3 wt% NaCl solution. (b) Water contact angles of CP hydrogels. (c) Energy dispersive spectrometer (EDS) images of the hydrogel (C: P = 1:1–3 mg/mL Ag-20 wt% NaCl). The influence of chemical composition on the mechanical performance of CP hydrogels: uniaxial stress-strain curves of hydrogels about different (d) molar feed ratios, (e) additive amounts of Ag and (f) concentrations of NaCl solution (Ag/Cu). Data are presented as mean ± standard deviation (n = 5).

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  Figure 2  Adhesive performance and antibacterial activity of the CP hydrogels. (a) Digital photos of hydrogel (C: P = 1:1-Ag-20 wt% NaCl) adhered to different objects. (b) The adhesion strength of the various hydrogels to glass. (c) The antibacterial ratio and (d) images of hydrogels loaded with AgNPs incubated with E. coli and S. aureus for 12 h. (e) Photos of inhibition zones of hydrogels against E. coli and S. aureus for 24 h. Scale bar: 1 cm. Data are presented as mean ± standard deviation (n = 5).

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  Figure 3  Anti-icing performance of the CP hydrogels. (a) The adhesion strength between ice and various surfaces placed in the anti-icing device for 3, 5, 7, and 24 h. (b) Ice adhesion strength on the surface after 20 cycles. Inset: schematic photograph of the anti-icing test. Electrochemical properties of the CP hydrogels: (c) fitting resistance and conductivity. The inset shows the proposed equivalent circuit. (d) Photographs of LED lights at different tensile ratios. Data are presented as mean ± standard deviation (n = 5).

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  Figure 4  The electrochemical sensing performance of the hydrogel (C: P = 1:1-Ag-3 wt% NaCl). (a) The relative resistance changes of the hydrogel during repeated stretching under strain (10%–100%). (b) The cycling stability test of the hydrogel to a maximum strain of 100% for 100 cycles. (c) The relative resistance changes for the various hydrogels under different tensile strains (0%–250%). The relative resistance change curves of the hydrogel as a strain sensor to detect various human motions: (d) walking and running, (e) squatting and standing, (f) normal breathing and deep breathing, (g) finger bending, (h) arm bending, (i) speaking.

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