Injectable polymeric hydrogels for the delivery of therapeutic agents: A review

Since drug delivery systems have become one of the most promising areas of human health related research, the applications of biomaterials such as hydrogels have been widely investigated. Possessing unique hydrophilic, biocompatible network structures and the ability to form solid-like gel states once administered, injectable hydrogels facilitate the encapsulation and release of therapeutic agents, including drugs, proteins, genes and cells, in a controllable manner. A wide and diverse range of techniques have been used to generate hydrogels, from chemical cross-linking, such as photo-polymerization, click chemistry, enzyme-catalyzed reactions, Schiff's base reactions, and thiol-based Michael reactions, to physical cross-linking induced by temperature, pH, ionic interaction, guest-host inclusion, stereo-complexation or complementary binding. This review covers the utilization of various injectable hydrogel systems for the delivery of therapeutic agents from the viewpoint of cross-linking methods.