Skin-inspired anti-freezing and durable hydrogel-in-hydrogel sensor with in situ packaging of methacrylated hyaluronic acid for wearable sensing and Morse code transmission.

Hydrogels are promising candidates for flexible and wearable sensors, while the inherent feature of high water content makes them prone to be dehydrated and frozen, leading to the function failure. The existing approaches to improve the lifespan of hydrogel sensors are mainly hindered by complex preparation, limited efficiency, and especially weakened comprehensive performances. In this study, inspired by the separation-synergy mode of the skin, a kind of hydrogel-in-hydrogel sensor was fabricated through a facile in-situ photopolymerization of methacrylated hyaluronic acid. The internal sodium alginate/polyvinyl alcohol hydrogel loaded with Ag nanowires served as the sensor while the external glycerol-water packaging hydrogel played crucial roles in freezing resistance and water retention. Benefiting from the moderate sensing sensitivity and conformal adhesion on human skin, the hydrogel-in-hydrogel sensor was effective in monitoring of human motions and transmission of Morse code. More importantly, this sensor was demonstrated to be reliable in low-temperature environments (-20 °C, > 6 h) and at room temperature (up to 150 days), significantly enhancing its service life. This work provides a new paradigm for developing environment-tolerant and durable hydrogel sensors.