Compliance for Resistance: The Intrinsic Extraordinary Isotropic Anti-Fatigue Performance of Fish Bladder.

Fatigue failure remains a critical challenge for soft materials used in biomedical implants and soft robotics. Existing strategies, including stiff reinforcement and microstructural alignment, can suppress crack growth but often introduce anisotropy and poor resistance to multidirectional loading. In addition, a persistent orders-of-magnitude disparity remains between fracture toughness and fatigue threshold of synthetic soft materials. Here, the fish bladder of silver carp, characterized by a multilayered hierarchical fibrous architecture, is shown to exhibit extraordinary isotropic fatigue resistance (∼6000 J·m- 2) approaching its fracture toughness. This behavior originates from a "compliance for resistance" mechanism that couples dynamic fiber straightening and rotation, sacrificial bond breakage and reassociation, and layer-specific fiber reorientation with the resulting interfacial sliding to dissipate energy under cyclic loading. Guided by these principles, a biohybrid hydrogel is designed to reproduce this multiscale anti-fatigue mechanism, achieving a high fatigue threshold (>3000 J·m- 2) together with robust antifouling performance. These results uncover fundamental design principles of fatigue-resistant biological soft tissues and provide a general strategy for engineering durable soft materials.