(531h) Microrheology As a Tool to Measure Cell-Material Interactions and Degradation of Covalently Adaptable Hydrogel Scaffolds

Microrheology measures material properties both spatially and temporally. This technique is an ideal measurement technique to understand the dynamic changes in a complex scaffold during degradation and how cells interact, remodel and degrade a 3D matrix. In this work we measure scaffold degradation of a covalently adaptable hydrogel, a poly(ethylene glycol) (PEG)-hydrazone scaffold, and cell-mediated remodeling of a PEG-peptide scaffold. We use multiple particle tracking microrheology (MPT) to characterize the spatial and temporally changes in both materials. In MPT, one micrometer fluorescently labeled probe particles are embedded in the matrix and the movement of these particles is tracked and related to rheological properties using the Generalized Stokes-Einstein Relation. Degradation of the covalently adaptable hydrogel is initiated by pushing the scaffold out of equilibrium by changing the pH of the incubating buffer. The scaffold is measured through time and oscillations in breakage and reformation of bonds is measured throughout the degradation reaction. The critical relaxation exponent and critical degradation time are identified with time-cure superposition. It is found that this scaffold undergoes breakage and reformation once during degradation at an acidic pH and repeatedly at physiological pH. Additionally, MPT is used to measure PEG-peptide scaffold degradation due to remodeling by human mesenchymal stem cells (hMSCs) encapsulated in 3D. In this work, MPT identifies that scaffold degradation is greatest furthest from the cell with the amount of cross-links inversely related to the distance from the cell. The sensitivity and ability to measure material properties both spatially and temporally with MPT has enabled the discovery that hMSCs will not degrade the scaffold directly under it to enable spreading and attachment prior to motility.