Passive heating into designable temperatures with thermal responsive hydrogel for smart thermal managements

• • The passive thermostat maintains a temperature variance of 0.287 °C over 3 h in cold winter, enabling stable thermal management without active control—critical for reducing energy consumption in building HVAC systems. • • The LCST of the PNIPAM hydrogel can be precisely tuned within 30–38 °C, allowing designable set-point temperatures that match specific application requirements, such as personal comfort or battery thermal safety. • • The system achieves reversible switching between solar heating and cooling via the hydrogel's transparent-to-opaque phase transition, eliminating the need for mechanical or electrical components and thus lowering maintenance costs. • • On extremely hot summer days, the thermostat maintains a stable, relatively low temperature by autonomously disabling solar heating, demonstrating adaptive cooling capacity that prevents overheating in photovoltaic or electronic devices.