Fatigue Behaviors\nof Anisotropic Hydrogels with a\nMacroscopic Lamellar Bilayer Structure and Swelling Effects

The application of soft materials for long-term use requires\na\nprofound understanding of their fatigue mechanisms and structural\nevolution under cyclic loading conditions. In this work, we studied\nthe fatigue resistance behaviors of an anisotropic hydrogel composite\nconsisting of periodically stacked, polymerized bilayers embedded\nin an elastic hydrogel matrix. The hydrogel composite exhibits high\ntoughness and self-resilience under monotonic loading due to efficient\nenergy dissipation from the lamellar bilayers, which act as reversible\nsacrificial bonds. We found that at a loading rate similar to the\nmonotonic loading test, bilayers only modestly enhance the fatigue\nthreshold itself but significantly suppress the fatigue crack extension\nrate above the fatigue threshold. Specifically, the fatigue crack\nextension length per cycle is only 1/10,000 that of the pristine elastic\nhydrogel. This enhancement in fatigue fracture resistance is only\nmodestly reduced in the fully swollen sample.