Reversible Phase Change‐Induced Hardening and Softening for Conditions‐Adaptive and Mechanics‐Reconfigurable Applications

The mechanical soft–hard transition of hydrogels is desired in conditions‐adaptive deformation and mechanics reconfiguration applications. However, highly efficient, stimuli‐responsive, and reversible transition strategies are hard to achieve. Inspired by the supercooling of salt‐aqueous solutions, solid and supersaturated hydrogels are prepared based on a hydrophilic polymer network and salt‐aqueous solution. The inner crystallization‐ or melting‐induced reversible phase‐change realizes the switch between the soft hydrogel (modulus: 0.1 MPa) and rigid composite (modulus: 24.0 MPa). The soft and supersaturated hydrogels easily deform to achieve diverse new 3D models and the unfamiliar soft–hard transition makes temporary shapes be efficiently fixed (hardening). Interestingly, the initial hydrogel's shapes can be regenerated relying on the resilience of the polyacrylamide network when the crystal is melted (softening). Shape memory, complex surface morphology replication, rapid mold application, and self‐supporting laminated glass are accomplished by this unique crystallization‐melting introduced soft–hard transition. This phase change soft‐hard switching strategy will broaden the functionalities of hydrogels.