Pre-Freezing-Assisted Wet Annealing Yields Mechanically Robust Hydrogels with Tunable Water Content

Simultaneously enhancing hydrogels' strength, stretchability, toughness, fatigue resistance, and tunable water content remains a challenge. Here, a universal "pre-freezing-assisted wet-annealing" strategy addresses these issues in poly(vinyl alcohol) (PVA) hydrogels. This dual-phase process enables controlled phase separation during prefreezing (promoting gelation from low-concentration polymer solutions for a high water content) and thermally driven polymer conformational rearrangement during wet-annealing (facilitating network densification, chain entanglement, and crystallite formation). The resulting hydrogels exhibit exceptional tensile strength (11.9 MPa), fracture strain (1275.4%), toughness (80.2 MJ m^-3), fracture energy (21.8 kJ m^-2), and fatigue threshold (902.8 J m^-2), outperforming conventional single-step hydrogels. Multiscale characterizations reveal enhanced crystallinity and intensified hydrogen bonding as key contributors. Notably, the water content is tunable (55-88%). These hydrogels exhibit superior fatigue and swelling resistance, along with low friction coefficients (0.03-0.15) under physiological conditions, promising for underwater load-bearing applications. This work establishes a versatile processing paradigm for engineering high-performance hydrogels via synergistic phase transition and macromolecular reorganization.