English
Biological tissues grow or evolve through a series of complex processes of material and energy internalization, which is highly challenging to imitate in synthetic materials. Here, a delayed cross-linking strategy was developed to program the reactivity of cross-linking sites and make the hydrogel evolutionary. This polymer network is constructed by combining polyvinyl alcohol (PVA) with multi zwitterionic groups containing cationic quaternary ammonium and anionic phenylboronic acid groups (PQBA). The shielding of phenylboronic acid groups in ion pairs and multi zwitterionic microdomains delayed the crosslinking between PVA and PQBA. Mechanical stimulation releases phenylboronic acid groups and significantly accelerates crosslinking reactions. Simple stretching treatment can make the hydrogel stronger. The tensile strength and maximum Young's modulus of hydrogel were increased by 13.0 and 22.8 times respectively after five 200% tensile cycle training. The hydrogel can also self evolve in the process of damage repair. After five repeated tensile fractures and self repair, the breaking strength and maximum Young's modulus of the hydrogel increase by about 7.5 times and 27.2 times at most. This study demonstrates the possibility of designing "active polymer materials" by programming the crosslinking kinetics of polymer networks.
