In vitro degradation of porous poly(L-lactic acid) foams

This study investigated the in vitro degradation of porous poly(L-lactic acid) (PLLA) foams during a 46-week period in pH 7.4 phosphate-buffered saline at 37 degrees C. Four types of PLLA foams were fabricated using a solvent-casting, particulate-leaching technique. The three types had initial salt weight fraction of 70, 80, and 90%, and a salt particle size of 106-150 mu m, while the fourth type had 90% initial weight fraction of salt in the size range 0-53 mu m. The porosities of the resulting foams were 0.67, 0.79, 0.91, and 0.84, respectively. The corresponding median pore diameters were 33, 52, 91, and 34 mu m. The macroscopic degradation of PLLA foams was independent of pore morphology with insignificant variation in foam weight, thickness, pore distribution, compressive creep behavior, and morphology during degradation. However, decrease in melting temperature and slight increase in crystallinity were observed at the end of degradation. The foam half-lives based on the weight average molecular weight were 11.6 +/- 0.7 (70%; 106-150 mu m), 15.8 +/- 1.2 (80%, 106-150 mu m), 21.5 +/- 1.5 (90%, 106-150 mu m), and 43.0 +/- 2.7 (90%, 0-53 mu m) weeks. The thicker pore walls of foams prepared with 70 or 80% salt weight fraction as compared to those with 90% salt weight fraction contributed to an autocatalytic effect resulting in faster foam degradation. Also, the increased pore surface/volume ratio of foams prepared with salt in the range 0-53 mu m enhanced the release of degradation products thus diminishing the autocatalytic effect and resulting in slower foam degradation compared to those with salt in the range 106-150 mu m. Formation and release of crystalline PLLA particulates occurred for foams fabricated with 90% salt weight fraction at early stages of degradation. These results suggest that the degradation rate of porous foams can be engineered by varying the pore wall thickness and pore surface/volume ratio. (C) 2000 Elsevier Science Ltd. All rights reserved.