Functional inhomogeneous salt-hydrogel PVA/PAM/lithium nitrate trihydrate with reversible adhesion, mechanical and humidity sensitivity

Hydrogels, as a class of functional material, have been used in various fields such as flexible sensors, actuators, and energy devices. Nevertheless, their practical application is often hampered by the poor mechanical properties and susceptibility to dehydration. In this work, using hygroscopic salt lithium nitrate trihydrate (LNH) as the phase-change material, PVA/PAM/LNH salt hydrogels were prepared in a mixture of water and glycerol. The mechanical performance of the hydrogels reached to 326 kPa from 31.5 kPa after the crystallization triggered by crystal seed at room temperature. Since phase transition triggered by crystal seed primarily occurs at the hydrogel surface, the surface contains a higher concentration of LNH crystals, forming an inhomogeneous structure. The composite hydrogel exhibits reversible adhesion, higher water retention ability enabling it to maintain long-term flexibility at low relative humidity. Moreover, the abundant Li + ions present in the gel enable high conductivity, humidity sensitivity and contribute to the piezoelectric and piezoresistive capabilities.