Hierarchical crack-resistant, tissue-mimetic hydrogels enabled by progressive nanocrystallization of anisotropic polymer networks

Addressing the persistent challenge of reconciling extreme mechanical robustness with tissue-mimetic functionality in hydrogels, we present a phase-transition-guided hierarchical engineering strategy that progressively architectures anisotropic polyvinyl alcohol networks through sequential mechanical training, wet-annealing, and salting-out. This triphasic processing induces programmable structural evolution: (1) mechanical training aligns polymer chains, (2) wet-annealing relaxes the stress while stabilizes oriented crystallites through solvent-plasticized rearrangement, and (3) salting-out densifies the network via chain aggregation and hydrogen-bond proliferation. The resultant hierarchical architecture achieves high fatigue resistance (threshold: 2083 J·m^-2) through multi-scale energy dissipation: sacrificial hydrogen bonds consume energy, while aligned crystalline domains pin the crack and deflect crack propagation via anisotropic stress redistribution. Demonstrating tissue-surpassing mechanics (tensile strength: 61 ± 3 MPa, toughness: 106 ± 27 MJ·m^-3, fracture energy: 85 ± 9 kJ m^-2) coupled with biological functionality, the hydrogel directs cell alignment through contact guidance while resisting swelling-induced dimensional instability (<1.2% volume change in physiological saline). This biomimetic engineering strategy establishes a universal route to design synthetic extracellular matrices that concurrently emulate the anisotropic mechanics of tendons and crack-blunting resilience of cartilage, critical for load-bearing tissue regeneration.