Dynamic Changes in Material Properties and Degradation of Poly(ethylene glycol)-Hydrazone Gels as a Function of pH

Covalent adaptable hydrogels (CAHs) dynamically evolve when pushed out of equilibrium by force or change in environmental conditions. Adapting these materials for advanced biological applications, including 3D cell culture and drug delivery platforms, requires in-depth knowledge of the evolution of scaffold microstructure and rheological properties. We use multiple particle tracking microrheology to measure the changes in a poly(ethylene glycol)-hydrazone CAH structure and properties when pushed out of equilibrium by a single change in pH. We determine the CAH degrades rapidly at acidic pH with multiple cycles of almost complete degradation and gelation. At pH 7.1, the scaffold degrades and re-forms cross-links over approximately 1.5 weeks with small oscillations between degradation and gelation. These degradation cycles are well described with first- and second-order reaction kinetics. MPT is sensitive enough to measure the phase transitions in these materials giving new insight into how CAHs evolve and their potential uses in biological applications.