A Delayed Cross-Linking Strategy for Evolvable Hydrogels

Biological tissues grow or evolve through a series of complicated processes of matter and energy internalization, which are highly challenging to mimic in synthetic materials. Herein, a delayed cross-linking strategy is developed to program the reactivity of cross-linking sites and make hydrogels evolvable. The polymer networks are constructed by combining polyvinyl alcohol (PVA) with a polyzwitterion comprising both cationic quaternary ammonium and anionic phenylboronic acid groups (PQBA). Shielding of phenylboronic acid groups in ion pairs and polyzwitterion microdomains delays the cross-linking between PVA and PQBA. Mechanical stimulations unlock the phenylboronic acid groups and dramatically accelerate the cross-linking reaction. A simple stretching treatment makes the hydrogels stronger. Training the hydrogels with five cycles of 200% stretching results in up to ≈13.0- and ≈22.8-fold of enhancements in tensile strength and maximum Young's modulus, respectively. The hydrogels can also self-evolve in a damage-healing process, the fracture strength and maximum Young's modulus of the hydrogels increase by at most ≈7.5- and ≈27.2-fold after five times of repeated tensile rupture and self-healing. The study demonstrates the possibility of designing “living polymeric materials” by programming the cross-linking kinetics of polymer networks.