Hydrolytic degradation of devices based on poly(DL-lactic acid) size-dependence.

The hydrolytic degradation of aliphatic polyesters derived from lactic and glycolic acids (PLA/GA polymers) has been previously shown to proceed heterogeneously in the case of large size devices, the rate of degradation being greater inside than at the surface. A qualitative model based on diffusion-reaction phenomena was proposed which accounts for the formation of the more stable outer layer. However, this model also suggested that devices with dimensions smaller than the thickness of the outer layer should degrade less rapidly than larger ones. In an attempt to check this hypothesis, 15 x 10 x 2 mm compression moulded plates, millimetric beads and submillimetric microspheres and cast films, derived from the same batch of poly (DL-lactic acid) polymer were allowed to age comparatively in isoosmolar 0.13 M phosphate buffer, pH 7.4, at 37 degrees C. Ageing of the various devices was monitored by measuring water absorption, weight loss, L-lactic acid formation, pH and molar mass changes. As expected, large size plates and millimetric beads degraded heterogeneously and much faster than homogeneously degraded submillimetric films and particles.