Molecular Chain Rearrangement-Induced\nIn Situ Formation\nof Nanofibers for Improving the Strength and Toughness of Hydrogels

Although poly(vinyl alcohol) (PVA) hydrogel has high\nelasticity\nand is suitable for cartilage tissue engineering, it is difficult\nto have both high strength and toughness. In this study, a simple\nand universal strategy is proposed to prepare strong and tough PVA\nhydrogels by in situ forming nanofibers on the original network structure\ninduced by a molecular chain rearrangement. Quenching–tempering\nalteratively in ethanol and water several times is carried out to\nstrengthen PVA hydrogels (PVA-Et<i><sub>n</sub></i> hydrogels)\ndue to the advantages of noncovalent bonds in adjustability and reversibility.\nThe results show that, after three quenching–tempering cycles,\nPVA-Et<sub>3</sub> hydrogel with water content up to 79 wt % shows\ncomprehensive improved mechanical properties. The compression modulus,\ntensile modulus, fracture strength, tensile strain, and tear energy\nof the PVA-Et<sub>3</sub> hydrogel are 270, 250, 260, 130, and 180%\nof the initial PVA hydrogel, respectively. The improved mechanical\nproperties of the PVA-Et<sub>3</sub> hydrogel are attributed to the\nstrong cross-linked PVA chains and hydrogen bond-reinforced nanofibers.\nThis study not only provides a simple and efficient solution for the\npreparation of strong and tough polymer scaffolds in tissue engineering\nbut also provides new insights for understanding the mechanism of\nimproving the mechanical properties of polymer hydrogels by adjusting\nthe molecular structure.