Rred to a study with tri(ethylene glycol) diacrylate (TEGDA). [21] PEGDA hydrogel-based biomaterial systems are generally fabricated from pre-polymers of molecular weight between 2,000 and 10,000 Da. [16, 22] These higher molecular weight pre-polymers have a much lower crosslink density than low molecular weight TEGDA and higher ratios of hydrolysable esters relative to backbone groups. Thus, the degradation profile of TEGDA is not an ideal comparison. In an effort to develop a PEG-based gel with increased biostability and to identify PEGDA degradation mechanism, we synthesized PEG diacrylamide (PEGDAA) hydrogels with a hydrolytically stable amide group in place of the ester of PEGDA, Figure 1. [14] We previously reported on the properties of these gels and the in vitro hydrolytic stability relative to PEGDA hydrogels. These studies demonstrated that PEGDA gels undergo accelerated hydrolysis under alkaline conditions and that degradation rates can be increased by decreasing crosslink density (i.e. increasing molecular weight and/or decreasing concentration). In contrast, no measureable change of the PEGDAA hydrogels occurred under similar accelerated hydrolysis conditions. Although PEG hydrogels are typically characterized as bioinert, PEG-based devices can promote a degree of non-specific protein adsorption and/or complement activation in vivo, which can result in macrophage recruitment, attachment and activation at the implantation site. [235] Macrophage adhesion and fusion to form foreign body giant cells (FBGCs) during frustrated phagocytosis of an implanted biomaterial generates a privileged microenvironment between the cell membrane and the material surface. Upon activation, macrophages and FBGCs secrete degradative agents including acids, reactive oxygen intermediates (ROIs), and enzymes into this microenvironment. [26, 27] The close cellmaterial interaction prevents buffering of these agents and results in a high concentration at the material surface to promote degradation. Acids present in the microenvironment can reduce the pH to as low as 4 to cause hydrolysis of the endgroup esters of PEG. [28] This is widely considered to be the source of in vivo degradation of PEGDA hydrogels; however, the ether backbone of PEG is also susceptible to oxidation that can be mediated by ROIsJ Biomed Mater Res A.Nusinersen Author manuscript; available in PMC 2015 December 01.Afoxolaner NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptBrowning et al.PMID:24428212 Pagereleased from adherent macrophages and FBGCs. [29] Thus, the observed degradation of PEGDA hydrogels could be a result of ester group cleavage via hydrolysis, ether cleavage via oxidation, or some combination of the two. [20, 24] Reports in current literature lack a suitable control for determination of the relative contribution of hydrolysis and oxidation to the overall PEGDA degradation in vivo. PEGDAA hydrogels do not provide increased resistance to oxidative degradation compared to PEGDA hydrogels due to their ether backbone; however, they can serve as a hydrolytically-stable control for in vivo degradation studies. In this work, PEGDA and PEGDAA hydrogels were fabricated with similar initial compressive moduli and swelling ratios. First, in vitro degradation profiles under accelerated hydrolytic and oxidative conditions were characterized at 37 by measuring changes in swelling ratio and modulus over time. Samples were then implanted subcutaneously using a standard rat cage model for up.
