Slow-water strategy for fracture-resistant hydrogels.

Water in hydrogels has often been overlooked or deemed detrimental to the mechanical properties of hydrogels due to the plasticizing effect of water, which weakens interchain interactions and accelerates crack propagation. Here, we propose a slow-water strategy that imparts enhanced fracture resistance to hydrogels. By converting free water into slowly migrating intermediate water, we suppress excess chain mobility by reducing the free volume around polymer chains, thereby enhancing interchain interactions and substantially extending the load-transfer length. Notably, even with only 13% polymer content and without additional network design, the hydrogels achieve orders-of-magnitude improvements in fracture toughness and fatigue threshold solely through medium design, and these enhancements are demonstrated to be dominated by the intermediate water. The strategy demonstrates versatility, providing insights into the role of intermediate water in biological systems and enabling innovative approaches to reinforcing biomimetic materials under complex stress conditions.