Chemical stimuli-induced reversible bond cleavage in covalently crosslinked hydrogels

Stimuli-responsive hydrogels are known as a new category of smart materials with responsivity toward various stimulants. Controllable reversible bond dissociation and spontaneous self-healing without adding any external force in stimuli-responsive hydrogels containing reversible covalent bonds is an interesting subject of study because of its potential applications in cell culturing, tissue engineering, drug delivery, wound healing, shape memory and self-healing systems, etc. Physical and chemical properties of hydrogels could precisely be controlled by applying different stimuli. Therefore, swelling properties, crosslinking density, gelation and viscosity, viscoelastic behavior, and rheological properties of hydrogels are tunable by applying desired triggers. In this review, conventional functional groups used for the preparation of stimuli-cleavable and self-healable hydrogels containing reversible covalent bonds, preparation methods, bond dissociation and formation mechanisms, advantages and disadvantages of each method, and the potential applications are investigated. Chemical stimuli-responsive hydrogels containing pH- and redox-responsive species with reversible bond dissociation as well as dual- and multi-responsive hydrogels prepared from pH- and redox-cleavable bonds are discussed in detail.