Passive heating into designable temperatures with thermal responsive hydrogel for smart thermal managements
Passive thermal management systems lack adaptive capacity and precise temperature control, limiting their deployment in dynamic environments. This study introduces a passive thermostat comprising a thermal-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel upper layer and a photothermal conversion bottom layer. The hydrogel undergoes a reversible phase transition between transparent and opaque states, modulating solar heating and cooling to maintain a designable, stable temperature. By tuning the lower critical solution temperature (LCST) of the hydrogel between 30 and 38 °C, the thermostat achieves a set-point equal to the LCST under open-air conditions. Experimental validation over 3 h in cold winter demonstrated a temperature variance of only 0.287 °C, while on extremely hot summer days the system maintained a stable, relatively low temperature by switching off solar heating. These results establish a viable pathway for energy-free, precise thermal regulation using renewable solar resources, with potential applications in building energy efficiency, personal thermal management, and electronic device cooling.