Phase Crosslinking Strategy to Construct Impact‐Resistant and Buffer‐Enhanced Bilayer Hydrogels with High Interfacial Adhesion

Inspired by the rigid exoskeleton and elastic inner tissues of crustaceans, a bilayer gel integrating high-strength rigidity and soft cushioning with high interfacial adhesion (1060 ± 40 J m^- ²) is developed via a stepwise solid-liquid phase crosslinking strategy. Herein, a prefrozen high-concentration polyvinyl alcohol (PVA) solution forms a solid-state structural framework, while a subsequently cast low-concentration PVA solution generates a flexible layer. Partial thawing of the frozen gel during casting triggers molecular chain interpenetration at the interface, synergistically enhanced by controlled molecular penetration, freeze-thaw cycles, and salt-induced crystallization. This method prevents gravity-driven molecular diffusion, enabling programmable fabrication of gels with unidirectional (positive/inverse) or multidimensional concentration gradients. The rigid layer, reinforced by salt- induced crystallization via 1 M Na₂SO₄ solution, exhibits a compressive modulus of 1493 ± 94 kPa at 50% strain, while the soft layer provides energy-dissipating cushioning. As a proof of concept, a helmet-like bilayer structure demonstrates simultaneous impact resistance and shock absorption. This work establishes a universal platform for constructing complex soft-hard composites through interfacial molecular entanglement and crystallization, addressing challenges in gradient-material fabrication for diverse biomechanical applications. The strategy with gravity-independent gradient control and dual-phase integration mechanism offers new avenues for designing multifunctional biomimetic materials.