Metal‐ion‐mediated healing of gels

Abstract

Self-healing is common in biological systems. A cut or a wound heals with time by cell migration from the adjacent tissues followed by cell proliferation and remodeling, leaving behind only a scar or weld line. Healing is also seen in synthetic linear polymers. Two pieces of polymers are welded by the reptation of chains across the interface at temperatures above the glass-transition or melting temperature. Unlike linear polymers, crosslinked polymeric networks cannot heal naturally because of the lack of reptation ability of the network chains across an interface. However, several smart ways for healing crosslinked polymers have been reported in the recent past. One of the techniques makes use of repair chemicals that are embedded in hollow fibers or microcapsules and that initiate self-repair when released during crack propagation. Another technique uses a reversible Diels– Alder chemistry, in which a furan and a maleimide react around 80 8C to reform a cracked network. The aforementioned strategies cannot be used to heal lightly crosslinked and water-swollen polymer networks because of their soft nature and the presence of water. In this work, we meet the challenge of demonstrating a healing phenomenon in hydrophilic gels. We show that certain hydrogels that have flexible hydrophobic side chains with a terminal carboxyl group undergo healing at the ambient temperature mediated by transition-metal ions. Healing occurs by the formation of coordination complexes that are much stronger than other noncovalent interactions such as chain entanglements and hydrogen bonding. The weld so formed then grows in strength with time, eventually leading to the process of metal-ion-mediated healing of the gels.