A Delayed Cross‐Linking Strategy for Evolvable Hydrogels

Abstract 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.