Self-healing Hydrogels Based on Dynamic Chemistry and Their Biomedical Applications

Self-healing materials are able to repair themselves automatically after external normal damages. This special property of self-healing materials could enhance the service life and ensure more safety while using such materials especially mechanically. Therefore, it has become a new emerging smart material which has the specialty to manage damages instead of traditional materials to avoid it. Self-healing hydrogels based on constitutional dynamic chemistry have received a lot of attentions recently with their intrinsic dynamic structures namely cross-linked polymer networks by dynamic bonds. Dynamic chemistry bonds contain both non-covalent bonds and reversible covalent bonds. Non-covalent bonds (weak effects) include hydrogen bonds, van der Waals forces, coordinative bonds, hydrophobic effects, etc. Reversible covalent bonds usually include those chemistry bonds that could be reversible in mild conditions as imine bonds, disulfide bonds, acylhydrazone bonds and so on. Those hydrogels with dynamic cross-linked networks could not only manage external damages and repair themselves as self-healing materials but also gain multi-responsive properties to environmental stimuli. The latter specialty comes from the adptive property of dynamic bonds to react to changes of reaction environments like pH, temperature and chemical reactants, thus building the foundation to develop self-healing hydrogels further into a multi-functional self-adaptive smart soft matter, which is of great significance for research to enrich multi-functional materials. Besides, hydrogel as a soft matter have long been vastly used in biomedical applications due to their superior biocompatibility and resemblance to biological tissues as mainly components (usually more than 70%) of hydrogel are water. They are playing a more and more important role in biomedical applications such as drug delivery systems, cell culture, tissue engineering and manmade biomimetic materials. Developing multi-functional smart soft matter with self-healing property as self-healing hydrogels would be quite helpful to this emerging field with unexpected more biomedical materials. This paper reviews recent works about self-healing hydrogels based on dynamic chemistry and their future biomedical applications. Systems based on multiple-hydrogen bonds, coordination effects, hydrophobic effects, acylhydrazone bonds and imine bonds are specifically discussed.