Abstract 241: In Vivo Response to Dynamic Hyaluronic Acid Hydrogels

Tissue-specific elasticity arises in part from developmental changes in extracellular matrix over time, e.g. ~ 10-fold myocardial stiffening in the chicken embryo. When this time-dependent stiffening is mimicked in vitro with a thiolated hyaluronic acid (HA-SH)/poly(ethylene glycol) diacrylate (PEGDA) hydrogel, improved cardiomyocyte maturation has been observed. However, host interactions, matrix polymerization, and stiffening kinetics remain uncertain in vivo, and each plays a critical role in therapeutic applications using HA-SH. In order to assess in vivo feasibility and biocompatibility of HA-SH/PEGDA hydrogels, subcutaneous injections were first performed. Hematological and histological analysis of subcutaneously injected HA-SH/PEGDA hydrogels showed minimal systemic immune response and host cell infiltration. Most importantly, subcutaneously injected HA-SH/PEGDA hydrogels exhibited time dependent porosity and stiffness changes at a rate similar to hydrogels polymerized in vitro, as measured by atomic force microscopy. When injected intramyocardially, host cells begin to actively degrade HA-SH/PEGDA hydrogels within 1-week post-injection, continuing this process while producing matrix to nearly replace the hydrogel within 1 month post-injection. While non-thiolated HA did not degrade after injection into the myocardium, it also did not elicit an immune response, unlike HA-SH/PEGDA, HA-SH/low concentration PEGDA, or high concentration HA-SH only hydrogels, where visible granulomas and macrophage infiltration were present at 1 month post-injection, as indicated by CD45 (lymphocyte marker) and CD68 (macrophage marker) staining, likely due to reactive thiol groups. Altogether, these data suggest that the HA-SH/PEGDA hydrogel responds appropriately in a less vascularized niche and stiffens as had been demonstrated in vitro, but in more vascularized tissues, in vivo applicability appears limited.