Bioinspired 2D Isotropically Fatigue-Resistant Hydrogels.

Engineering conventional hydrogels with muscle-like anisotropic structures can efficiently increase the fatigue threshold over 1000 J m-2 along the alignment direction; however, the fatigue threshold perpendicular to the alignment is still as low as ≈100-300 J m-2 , making them nonsuitable for those scenarios where isotropic properties are desired. Here, inspired by the distinct structure-properties relationship of heart valves, a simple yet general strategy to engineer conventional hydrogels with unprecedented yet isotropic fatigue resistance, with a record-high fatigue threshold over 1,500 J m-2 along two arbitrary in-plane directions is reported. The two-step process involves the formation of preferentially aligned lamellar micro/nanostructures through a bidirectional freeze-casting process, followed by compression annealing, synergistically contributing to extraordinary resistance to fatigue crack propagation. The study provides a viable means of fabricating soft materials with isotropically extreme properties, thereby unlocking paths to apply these advanced soft materials toward applications including soft robotics, flexible electronics, e-skins, and tissue patches.