Hydrolytic degradation of the coral/poly(DL-lactic acid) bioresorbable material.

A bioresorbable composite material composed of a high molecular weight poly(DL-lactic acid) and granules of natural coral (40/60 w/w) was made by blending in acetone with further evaporation of the solvent. The blend and the unfilled polymer matrix were compression moulded and machined to yield 12 x 12 mm plates of different thicknesses (1.4 mm for the blend vs 2.1 mm for the polymer). The resulting plates were allowed to age at 37 degrees C in a pH 7.4 isoosmolar phosphate buffer solution taken as a model of body fluids. Hydrolytic degradation of the blend was investigated comparatively with the unfilled polymer matrix. The fate of the specimens was monitored by weighing, enzymatic assay, size-exclusion chromatolography and differential scanning calorimetry. Data on water absorption, weight loss, release of L-lactic acid, molecular weight and morphology changes conclusively showed that the presence of coral dramatically modified the degradation characteristics of poly(DL-lactic acid). In particular, the faster internal degradation observed for the unfilled polymer matrix was not detected for the blend. This finding was assigned to the buffering effect of calcium carbonate which neutralized the carboxyl end groups of the degradation products and to the presence of coral/polymer interfaces which facilitated ion exchanges with the external solution, both contributing to eliminate autocatalysis.