IN-VITRO AND IN-VIVO DEGRADATION OF POLY(PROPYLENE FUMARATE-CO-ETHYLENE GLYCOL) HYDROGELS

The degradation of poly(propylene fumarate-co-ethylene glycol) hydroge ls was examined in vitro in phosphate-buffered saline at pH 7.4 and in vivo in a subcutaneous rat model. These hydrogels have potential appl ication as biodegradable, injectable cardiovascular stents, and, as su ch, their mass loss, dimensional changes, mechanical properties, morph ology, and biocompatiblity over a 12-week time course were evaluated. Three formulations were fabricated: one base formulation consisting of 25% (w/w) PEG, molecular weight 4,600; one high weight percent PEG fo rmulation with 50%; (w/w) PEG; and one high molecular weight PEG formu lation, molecular weight 10,500. All three formulations showed signifi cant weight loss (between 40 and 60%) on the first day due to leaching of the uncrosslinked fraction. Further weight loss was observed only for the low weight percent PEG copolymers in the in vivo case, and a s light increase in volume was observed due to degradative swelling. The mechanical properties of the P(PF-co-EC) hydrogels decreased signific antly in the first 3 weeks, showing the biphasic pattern typical of bu lk degradation. In vitro, the hydrogels showed at least a 20% retentio n of their initial ultimate tensile stress after 3 weeks. The dynamic mechanical properties showed similar retention, with the in vivo mecha nical properties differing from the iri vitro properties only after 6 weeks of degradation. Differences in PEC molecular weight appeared to have little effect, but increasing the weight percent PEG decreased th e rate of degradation both in vitro and in vivo. The morphology of the copolymer films, based on scanning electron microscopy observation, w as not significantly different either among the three formulations or on the time course of the study, suggesting tl,ere were no macroscopic structural changes during this time period. The P(PF-co-EG) hydrogels demonstrated good initial biocompatibility, showing responses charact eristic of biomaterial implants. (C) 1998 John Wiley & Sons, Inc.