Rheological Analysis of the Gelation Kinetics of an Enzyme Cross-linked PEG Hydrogel

The diverse requirements of hydrogels for tissue engineering motivate the development of cross-linking reactions to fabricate hydrogel networks with specific features, particularly those amenable to the activity of biological materials (e.g., cells, proteins) that do not require exposure to UV light. We describe gelation kinetics for a library of thiolated poly(ethylene glycol) sulfhydryl hydrogels undergoing enzymatic cross-linking via horseradish peroxidase, a catalyst-driven reaction activated by hydrogen peroxide. We report the use of small-amplitude oscillatory shear (SAOS) to quantify gelation kinetics as a function of reaction conditions (hydrogen peroxide and polymer concentrations). We employ a novel approach to monitor the change of viscoelastic properties of hydrogels over the course of gelation (Δ tgel) via the time derivative of the storage modulus (d G'/d t). This approach, fundamentally distinct from traditional methods for defining a gel point, quantifies the time interval over which gelation events occur. We report that gelation depends on peroxide and polymer concentrations as well as system temperature, where the effects of hydrogen peroxide tend to saturate over a critical concentration. Further, this cross-linking reaction can be reversed using l-cysteine for rapid cell isolation, and the rate of hydrogel dissolution can be monitored using SAOS.