Flaw-sensitivity of polyvinyl alcohol hydrogel under monotonic and cyclic loads

Polyvinyl alcohol (PVA) hydrogels have been widely used for various biomedical applications, such as artificial tissues and surgical patches. Extensive researches have focused on improving the toughness and fatigue thresholds of PVA hydrogels. However, the flaw-sensitivity of PVA hydrogels has rarely been reported. Here, we investigate the flaw-sensitivity for both PVA hydrogel and a composite hydrogel of PVA reinforced by a knitted fabric. The composite hydrogel exhibits larger flaw-sensitivity length than PVA hydrogel under both monotonic and cyclic loading. The endurance flaw-sensitivity length of the composite hydrogel is ∼0.30 mm, more than three times that of PVA hydrogel (∼0.08 mm). Both the digital image correlation (DIC) measurement and the finite element simulation reveal that the larger flaw-sensitivity length of the composite hydrogel is attributed to the stress deconcentration mechanism of bridging fibers. We further show that the improved flaw-sensitivity of composite hydrogels endows better suture retention capacity, which is vital for biomedical scenarios. Under cyclic loading of a suture, the PVA hydrogel ruptures after 5000 cycles, whereas the composite hydrogel remains intact after 500,000 cycles. It is hoped that this work may provide insights for the design of flaw-insensitive hydrogels.