Beyond Swelling and Shrinking: Achieving a Quasi‐Isovolumetric Phase Transition in Water‐Driven Thermo‐Responsive Hydrogels via Enthalpy–Entropy Compensation

Thermo-responsive hydrogels hold promise in various fields for their reversible phase transition behavior, but often at the expense of high energy consumption from external thermal inputs and volumetric swelling/shrinkage from phase transition. Herein, we present a water-driven phase transition strategy that circumvents thermal triggers while retaining upper critical solution temperature (UCST)-type thermo-responsiveness via enthalpy-entropy compensation. The UCST phase transition arises from entropy loss due to hydrophobic interactions within the hydrogel networks. By modulating the enthalpy/entropy balance, we achieve hydrogels with desired responsiveness, exemplified by a rapid (130 s) and quasi-isovolumetric (volume change of 1.2) phase transition under mild conditions (water, 25°C). This strategy leverages water as a stimulus, enabling phase transitions that align with the compatibility requirements of biogenic materials, since the risks related to thermal triggers can be avoided. Our strategy thus offers a pathway to thermo-responsive hydrogels without thermal energy input, while mitigating volumetric instability challenges in practical applications, such as body temperature triggered information encryption and human brain mimic dynamic memory-forgetting.