Mechanisms for high fatigue threshold of prestretched PVA hydrogel with aligned nanofibrils. Schematic illustration of nanofibril morphology in (A) notched prestretched PVA hydrogel where crack is perpendicular to the longitudinal direction of nanofibrils, (C) notched prestretched PVA hydrogel where crack is parallel to the longitudinal direction of nanofibrils, and (E) freeze-thawed PVA hydrogel. Corresponding confocal images of notched samples under different stretches for (B) prestretched PVA hydrogel where crack is perpendicular to the longitudinal direction of nanofibrils, (D) prestretched PVA hydrogel where crack is parallel to the longitudinal direction of nanofibrils, and (F) freeze-thawed PVA hydrogel. The yellow arrows in confocal images indicate the direction of aligned nanofibrils around crack tip. (Scale bars: B, 250 µm; D, 100 µm; F, 250 µm.)