A thermally responsive phase-change hydrogel for skin-mountable multifunctional sensors

Hydrogels have been entensively explored for their intrinsic biocompatible and adhesive properties. However, limitations such as low viscosity, easy dehydration, and difficulty in peeling off the skin. Herein, we develop a hydrogel with a dual crosslinked network of gelatin and polyvinyl alcohol, which shows a unique thermally responsive reversible phase-change property between the flowing fluid state and the viscoelastic gel state. The enhanced hydrogel-based device can be steadily attached to the skin for reliable monitoring or quickly removed by controlling the temperature. Besides, the developed hydrogel exhibits ultra-thin, anti-drying, and self-healing properties. By introducing PEDOT: PSS and graphene dispersion, the hydrogel can be used as skin-mountable electrodes for high-fidelity electrocardiogram signal capture in the long term. By incorporating cobalt nanoporous carbon, a bilayer structure composed of triboelectric and conductive hydrogels is fabricated for non-contact sensing based on hydrogel-triboelectric nanogenerators. This work provides an idea for preparing viscosity-adjustable gels and a promising strategy for developing skin-mountable multifunctional sensors.