Kinetic Analysis of Degradation in Thioester Cross-linked Hydrogels as a Function of Thiol Concentration, p<i>K</i><sub>a</sub>, and Presentation
Benjamin J. Carberry, Juan J. Hernandez, Adam Dobson, Christopher N. Bowman, Kristi S. Anseth
Abstract
Stimuli-responsive degradation of hydrogels has established and emerging utilities ranging from controlled release of biological products to sacrificial molding. Although covalent adaptable networks are particularly amenable for these applications, their degradation kinetics have yet to be clearly elucidated in these swollen material systems. In this work, the thiol–thioester exchange reaction in cross-linked poly(ethylene glycol) (PEG) hydrogels is characterized to determine the relative effects of thiol concentration, pKa, and pH on network mass loss and mechanical properties. From these measurements, we identify that network thiols liberated from exchanged thioester cross-links play a significant role in hydrogel degradation, resulting in predictable changes in the storage modulus but nonideal mass loss profiles. These findings highlight how the mechanism of thioester exchange degradation can give rise to useful deviations from ideal degradation behavior, such as amplified degradation rates and linear mass loss profiles. Furthermore, this work empowers the design of on-demand degradable hydrogels with uniquely tunable degradation and mass loss.