Stretchable, Self-Healing, and Bioactive Hydrogel with High-Functionality N,N'-bis(acryloyl)cystamine Dynamically Bonded Ag@polydopamine Crosslinkers for Wearable Sensors

Abstract

Hydrogels present attractive opportunities as flexible sensors due to their soft nature and tunable physicochemical properties. Despite significant advances, practical application of hydrogel-based sensor is limited by the lack of general routes to fabricate materials with combination of mechanical, conductive, and biological properties. Here, a multi-functional hydrogel sensor is reported by in situ polymerizing of acrylamide (AM) with N,N'-bis(acryloyl)cystamine (BA) dynamic crosslinked silver-modified polydopamine (PDA) nanoparticles, namely PAM/BA-Ag@PDA. Compared with traditional polyacrylamide (PAM) hydrogel, the BA-Ag@PDA nanoparticles provide both high-functionality crosslinks and multiple interactions within PAM networks, thereby endowing the optimized PAM/BA-Ag@PDA hydrogel with significantly enhanced tensile/compressive strength (349.80 kPa at 383.57% tensile strain, 263.08 kPa at 90% compressive strain), lower hysteresis (5.2%), improved conductivity (2.51 S m^-1) and excellent near-infrared (NIR) light-triggered self-healing ability. As a strain sensor, the PAM/BA-Ag@PDA hydrogel shows a good sensitivity (gauge factor of 1.86), rapid response time (138 ms), and high stability. Owing to abundant reactive groups in PDA, the PAM/BA-Ag@PDA hydrogel exhibits inherent tissue adhesiveness and antioxidant, along with a synergistic antibacterial effect by PDA and Ag. Toward practical applications, the PAM/BA-Ag@PDA hydrogel can conformally adhere to skin and monitor subtle activities and large-scale movements with excellent reliability, demonstrating its promising applications as wearable sensors for healthcare.

Keywords: health monitoring; hydrogel; self‐healing; sensor.

Scheme 1

Schematic illustration of the PAM/BA‐Ag@PDA nanocomposite hydrogel for wearable sensors.

Figure 1

Characterization of the BA‐Ag@PDA nanoparticles. a) TEM image of the BA‐Ag@PDA nanoparticles. b) SEM and EDS mapping images of the BA‐Ag@PDA nanoparticles. c) XRD patterns of the BA, PDA, Ag@PDA, and BA‐Ag@PDA. d) XPS analysis of the PDA, Ag@PDA, and BA‐Ag@PDA nanoparticles. e) UV–vis–NIR absorption spectra of the BA, PDA, Ag@PDA, and BA‐Ag@PDA. f) FTIR spectra of the BA, dopamine, PDA, Ag@PDA, and BA‐Ag@PDA. g) SEM and EDS mapping images of the PAM/BA‐Ag@PDA network in the dry state. h) Tensile stress–strain curves of the PAM/BA‐Ag@PDA hydrogel with different contents of AM and Ag@PDA. Insert: Image of the 15AM/1.4BA‐1.0Ag@PDA group under twist and stretch condition. i) Compressive stress–strain curves and j) lap shear stress–strain curves of the PAM/BA‐Ag@PDA hydrogel with different contents ofAg@PDA.

Figure 2

Strain sensing properties of the PAM/BA‐Ag@PDA hydrogel‐based resistive strain sensors. a) Photographs of the PAM/BA‐Ag@PDA hydrogel sensor to strains during the uniaxial stretching test. b) Relative resistance changes (ΔR/R₀ signal) of the PAM/BA‐Ag@PDA hydrogel sensor at different stretching speeds (50% min⁻¹, 100% min⁻¹, 150% min⁻¹) for one cycle. c) Response and recovery time of the PAM/BA‐Ag@PDA hydrogel sensor under small tensile strain. d) Dynamic response of the PAM/BA‐Ag@PDA hydrogel sensor under a series of unloading step‐down strains from 100% to the initial state. e,f) Relative resistance changes of the PAM/BA‐Ag@PDA hydrogel sensor under increasing strain increments of 5% and 40%. g) Relative resistance changes of the PAM/BA‐Ag@PDA hydrogel sensor during cyclic tensile test at 10%, 20%, 30%, and 40% strain (strain frequency: 0.04 Hz). h) Relative resistance changes of the PAM/BA‐Ag@PDA hydrogel sensor at 50%, 100%, and 200% strain for 100 cycles (stretching–releasing speed: 120 mm min⁻¹).

Figure 3

a) Self‐healing behavior of the PAM/BA‐Ag@PDA hydrogel after NIR irradiation. b) The LED electrical performance of the self‐healed PAM/BA‐Ag@PDA hydrogel. c) Relative resistance changes of the self‐healed PAM/BA‐Ag@PDA hydrogel at 50% and 100% strain for 200 consecutive cycles (stretching–releasing speed: 120 mm min⁻¹). Relative resistance changes of the d) original and e) self‐healed PAM/BA‐Ag@PDA hydrogel at 30th, 60th, 90th, 120th, 150th, and 180th cycles in the strain range of 0–100%.

Figure 4

a) UV–vis absorption spectra of the DPPH free radicals incubated with different concentrations of PAM/BA‐Ag@PDA hydrogel. b) DPPH radical scavenging percentage of the PAM/BA‐Ag@PDA hydrogel at different concentrations. c) Photothermal efficiency of the PAM/BA‐Ag@PDA hydrogel at different NIR power densities. Insert image: Photostability of the PAM/BA‐Ag@PDA hydrogel after four on/off NIR irradiation (1.5 W cm⁻²) cycles. d) Antibacterial activity of the PAM/BA‐Ag@PDA hydrogels with or without NIR irradiation (808 nm, 1.5 W cm⁻², 3 min). e) Biocompatibility of the PAM/BA‐Ag@PDA hydrogel determined by CCK‐8 and Live/Dead assay.

Figure 5

Detecting strain changes in human movement for healthcare monitor and human–machine interface. Relative resistance responses of the PAM/BA‐Ag@PDA based resistive strain sensor to: a) neck twisting, b) subtle throat movements, c) finger bending with different amplitudes, d) biceps contracting, e) real‐time sensing of breathing during standing (normal), running, and recovery by attaching the sensor to the abdomen. f) Photograph of the robot hand integrated with five hydrogel‐based resistive strain sensors on the back of fingers. g) Electrical signals of the robot hand for different gestures. h) Photograph of the application of the PAM/BA‐Ag@PDA hydrogel sensor to monitor knee‐joint activity during the squats. i) Real‐time relative resistance changes of the PAM/BA‐Ag@PDA hydrogel sensor used for large deformation of the knee‐joint.