Determination of the in vivo degradation mechanism of PEGDA hydrogels.

Poly(ethylene glycol) (PEG) hydrogels are one of the most extensively utilized biomaterials systems due to their established biocompatibility and highly tunable properties. It is widely acknowledged that traditional acrylate-derivatized PEG (PEGDA) hydrogels are susceptible to slow degradation in vivo and are therefore unsuitable for long-term implantable applications. However, there is speculation whether the observed degradation is due to hydrolysis of endgroup acrylate esters or oxidation of the ether backbone, both of which are possible in the foreign body response to implanted devices. PEG diacrylamide (PEGDAA) is a polyether-based hydrogel system with similar properties to PEGDA but with amide linkages in place of the acrylate esters. This provides a hydrolytically-stable control that can be used to isolate the relative contributions of hydrolysis and oxidation to the in vivo degradation of PEGDA. Here we show that PEGDAA hydrogels remained stable over 12 weeks of subcutaneous implantation in a rat model while PEGDA hydrogels underwent significant degradation as indicated by both increased swelling ratio and decreased modulus. As PEGDA and PEGDAA have similar susceptibility to oxidation, these results demonstrate for the first time that the primary in vivo degradation mechanism of PEGDA is hydrolysis of the endgroup acrylate esters. Additionally, the maintenance of PEGDAA hydrogel properties in vivo indicates their suitability for long-term implants. These studies serve to elucidate key information about a widely used biomaterial system to allow for better implantable device design and to provide a biostable replacement option for PEGDA in applications that require long-term stability.