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

ABSTRACT 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.