(701g) Characterization of Enzymatic Degradation in a Thiol-Ene Hydrogel Using Multiple Particle Tracking Microrheology

The design of hydrogel matrices for cell encapsulation and tissue regeneration has become increasingly complex.  Often times, researchers seek to recapitulate specific biophysical and biochemical cues critical for the resident cell population, but the cellular microenvironment is dynamic, as cells migrate, remodel, and modify their local extracellular niche.  Therefore, an in depth understanding of changes in the local microstructure and rheological properties of the synthetic matrix during degradation would be extremely beneficial.  Multiple particle tracking microrheology (MPT) enables simultaneous characterization of rheological properties and visualization of the microstructure in an evolving cell-laden hydrogel scaffold.  MPT is a passive microrheological technique that measures the Brownian motion of fluorescently labeled probe particles embedded in the material, which is directly related to rheological properties using the Generalized Stokes-Einstein Relation (GSER). The hydrogel scaffold consists of a multi-arm poly(ethylene glycol) (PEG) end functionalized with norbornene that is cross-linked with both a nondegradable PEG-dithiol and a matrix metalloproteinase (MMP) degradable peptide (KCGPQG*IWGQCK). The ability to tailor the amount of degradation in the material enables fundamental studies of the material properties with degradable cross-links close to the critical fraction of cross-links required to form a gel, p_c,A, calculated using Flory-Stockmayer theory. Sensitivity of MPT measurements near this critical point quantifies the material properties and heterogeneity while visualizing the material microstructure at the gel-sol transition. Results from these investigations show that degradation of the material occurs at a slightly higher value of degradable cross-links than Flory-Stockmayer theory predicts. This is due to nonidealities in the hydrogel matrix, which are not accounted for in the theory. Finally, the spatial information gained by MPT measurements quantify the rheological properties that result from cellular remodeling and degradation enabling the identification of the matrix conditions that encourage migration of encapsulated cells. Understanding the microstructural and rheological properties of a material around the gel-sol transition will enable us to improve our understanding of how cells remodel their microenvironment when encapsulated in gels, and more precisely design and manipulate this parameter to improve tissue regeneration.