Stimuli-Responsive Covalent Adaptable Hydrogels Based on Homolytic Bond Dissociation and Chain Transfer Reactions

Abstract

Covalent adaptable hydrogels containing dynamic covalent bonds are gaining significant interest based on their ability of stimuli-controlled reversible bond dissociation, structural reorganization, color change, and self-healing through covalent bond exchange or dissociation. Potential applications of such hydrogels have been explored in coatings, sealants, tissue adhesives, soft robotics, tissue engineering, and stimuli-responsive lithography. Stimuli-induced homolytic bond cleavage leads to the formation of radicals with the ability of recombination or transfer to induce bond exchange and color variation. The incorporation of such stimuli-responsive homolytically cleavable bonds in hydrogels can lead to stimuli-controlled plasticity, stress relaxation, self-healing, structural reorganization, mechanochromism, and mechanoluminescence. The resultant smart materials are interesting for different applications, ranging from patterning and shape-shifting, cell encapsulation and culturing, and protein binding to strain sensing and damage reporting. The recent progress in the preparation of covalently adaptable hydrogels based on stimuli-responsive homolytical bond dissociation and recombination or chain transfer will be discussed in this review. More specifically, the different types of chemistry that can be used for development of covalent adaptable hydrogels based on light-induced, temperature-induced, and mechanically induced homolytic bond dissociation will be discussed. Moreover, the applications of the resultant covalent adaptable hydrogels will be highlighted, focusing on stress sensing and damage reporting, tissue engineering, and self-healable hydrogels, as well as stimuli-controlled patterning and shape-shifting.