  The emerging applications of hydrogels in devices and machines require these
soft materials to maintain robustness under cyclic mechanical loads. Whereas
hydrogels have been made tough to resist fracture under a single cycle of
mechanical load, these toughened gels still suffer from fatigue fracture under
multiple cycles of loads. The reported fatigue threshold (i.e., the minimal
fracture energy at which crack propagation occurs under cyclic loads) for
synthetic hydrogels is on the order of 1-100 J/m2, which is primarily
associated with the energy required to fracture a single layer of polymer
chains per unit area. Here, we demonstrate that the controlled introduction of
crystallinity in hydrogels can significantly enhance their fatigue thresholds,
since the process of fracturing crystalline domains for fatigue-crack
propagation requires much higher energy than fracturing a single layer of
polymer chains. The fatigue threshold of polyvinyl alcohol (PVA) with a
crystallinity of 18.9 wt.% in the swollen state can exceed 1,000 J/m2. We
further develop a strategy to enhance the anti-fatigue-fracture properties of
PVA hydrogels, but still maintain their high water contents and low moduli by
patterning highly-crystalline regions in the hydrogels. The current work not
only reveals an anti-fatigue-fracture mechanism in hydrogels but also provides
a practical method to design anti-fatigue-fracture hydrogels for diverse
applications.
