A bilayer hydrogel actuator based on a microgel island-bridge structure for integrated sensor arrays and intelligent rehabilitation systems.

With the aging global population and rising chronic diseases, intelligent rehabilitation systems gain attention. Poly(N-isopropylacrylamide) (PNIPAM) hydrogels show a thermo-responsive phase change and flexibility but suffer from poor mechanical properties and single functionality. Herein, a dual-network PNIPAM-polyacrylamide-microgel hydrogel (P-M) with a microgel island-bridge structure is designed, which achieves a tensile strength of 31.6 kPa and an elongation at break of 729.6% via strong hydrogen bonding and topological entanglement. Subsequently, a bilayer P-M/polyacrylamide-Cu1.8S (P-M/P-C) hydrogel is fabricated by introducing Cu1.8S microspheres, endowing the hydrogel with excellent electrical conductivity, photothermal conversion ability and strain-sensing performance. The P-M/P-C hydrogel sensor features a high gauge factor (a GF of up to 5.44), fast response and recovery times (100 ms and 100 ms, respectively), and outstanding cycling stability (no obvious signal degradation after 500 stretching cycles at 0%-100% strain). Benefiting from the island-bridge structure, this work achieves higher strength and larger elongation than those of most reported flexible hydrogel sensors and soft electronic devices. The prepared hydrogel sensor can be adopted to accurately detect human motion signals, and a 4 × 4 hydrogel sensing array is constructed to realize object mass differentiation and spatial localization. Meanwhile, fixed-point thermostatic control and real-time monitoring of the transport process are realized. The proposed bilayer multifunctional hydrogel provides new ideas for further development of smart rehabilitation platforms and intelligent wearable devices.