A multi-model, large range and anti-freezing sensor based on a multi-crosslinked poly(vinyl alcohol) hydrogel for human-motion monitoring

Conductive hydrogels are capturing intensive attention for versatile applications in flexible wearable devices on account of their unique combination of softness, stretchability, conductivity and biocompatibility. However, most of the hydrogel sensors can only serve as single-type sensors to detect strain or pressure, accompanied by a limited detection range. Moreover, the poor anti-freezing performance is also a serious problem to be addressed for their practical applications. Herein, a multi-model, large range and anti-freezing hydrogel sensor was constructed from a high-mechanical and ionic conductive multi-crosslinked poly(vinyl alcohol) (M-PVA) hydrogel, which was prepared via incorporating chain entanglement interaction and complexation between Fe3+ ions and hydroxyl groups into the microcrystalline network through immersion treatment in Fe2(SO4)3 solution. The three reversible and reconstructable crosslinks within the M-PVA hydrogel worked in tandem to achieve ultra-stretchability (1120%), supercompressibility (98%), high toughness, fast self-recoverability and excellent fatigue resistance. Meanwhile, the introduction of Fe3+ and SO42- ions endowed the M-PVA hydrogel with good ionic conductivity and remarkable anti-freezing properties (-50 °C), which benefited the M-PVA hydrogel to act as a freezing-tolerant dressing. The assembled multi-model hydrogel sensor can sensitively and stably detect large range elongation (∼900%), compression (∼70%), bend and pressure (up to 4.60 MPa) concurrently, as well as various human activities including speaking, finger bending and treading behavior. Notably, the hydrogel sensor was capable of maintaining excellent mechanical flexibility and sensitive sensing capacity at low temperature. The M-PVA hydrogel is a promising flexible sensing material for versatile applications in ionic skin, motion recognition and intelligent wearable devices.