(347e) Macrophage Interrogation of PEG-Based Hydrogels Used in Tissue Engineering Applications

Photopolymerized poly(ethylene glycol) (PEG) based hydrogels hold promise as in situ forming cell carriers for a wide range of tissue engineering applications. Because PEG is considered bioinert, crosslinked PEG is attractive as a base chemistry to control the hydrogel structure and to which biological functionality can be incorporated in a systematic manner. The success of PEG-based hydrogels in vivo, however, will partly depend on how the host responds to the implanted material. The goals for this study were to explore the in vitro and in vivo host response to PEG-based hydrogels. The hydrogels were prepared from 20% w/w PEG diacrylate to produce PEG-only hydrogels and PEG hydrogels immobilized with a commonly employed cell adhesion moiety, RGD. Medical grade silicone served as the positive control for the classic foreign body response. In vitro, primary murine macrophage attachment was similar on all three surfaces (PEG-only, PEG+RGD, and silicone). Morphologically, macrophages were generally rounded on the PEG hydrogels while cell spreading was evident on the PEG+RGD hydrogels and silicone after 4 days of culture. Macrophage activation was assessed by gene expression for pro-inflammatory cytokines. Specifically, interleukin-1β and tumor-necrosis factor-α expressions were significantly up-regulated on the PEG-only gels by 290- and 340-fold when compared to silicone (p<0.05) after 2 days of culture. PEG+RGD was not statistically different from silicone. A similar response was observed in vivo when the hydrogels were implanted in subcutaneous pockets of immunecompetent mice (C57/bl6). After 4 weeks, a fibrous collagenous capsule had formed around the silicone and PEG+RGD hydrogels characteristic of a typical foreign body reaction. However, PEG-only gels resulted in a robust inflammatory reaction characterized by a thick layer of macrophages at the material interface with observable signs of gel degradation. Both PEG-only and PEG+RGD hydrogels were shown to be susceptible to oxidative biodegradation, although minimal signs of degradation were observed in vivo for the PEG+RGD hydrogels. In conclusion, PEG hydrogels appear to be pro-inflammatory while the presence of RGD attenuates this negative reaction leading to a moderate foreign body reaction, although the exact mechanisms are still under investigation. Overall, our findings suggest that the incorporation of biological moieties into PEG hydrogels, such as RGD, may be critical to the success of using PEG hydrogels in vivo for tissue engineering applications.