Degradable hydrogels: Design mechanisms and versatile applications

Recent environmental issues raised by the durability of polymers call for designing degradable products. Hydrogels with widespread applications in superabsorbent and hygiene products are traditionally discarded as household wastes. In other hi-tech applications such as drug delivery and tissue engineering, hydrogels are required to be removed after fulfilling the target service. Therefore, the design and control of degradation would significantly promote their sustainable application. Degradation can be achieved by employing dynamic physical bonds that set a lifetime for the structure integrity. However, target-specific and spatiotemporal control over degradation can be obtained along with structural integrity by employing reversible chemical cross links. Besides the reaction chemistry, many physical parameters like crosslinking density and network topology further affect the degradation profile. As such, the custom design of a degradable hydrogel for a specific application is not trivial and requires multifacet knowledge of physical chemistry of degradation. In this review, we provide a roadmap by first overviewing the library of available degradation mechanisms and then exploiting recent application fields, with an emphasis on how degradability unlocks new opportunities for the design and utility of hydrogels.