The degradation, swelling and erosion properties of biodegradable implants prepared by extrusion or compression moulding of poly(lactide-co-glycolide) and ABA triblock copolymers.

In the design of parenteral delivery systems the modulation of the biodegradation of a polymer matrix represents a promising strategy to control drug release. We have investigated the degradation of ABA triblock copolymers, consisting of poly(lactide-co-glycolide) A-blocks and poly(oxyethylene) B-blocks, and PLG, poly(lactide-co-glycolide), with respect to swelling behaviour, molecular weight loss and polymer erosion. Implants were prepared by either compression moulding or extrusion using a laboratory ram extruder. Insertion of an elastoplastic B-block did not lower the processing temperature, but the entanglement of the polymer chains was significantly reduced as can be seen from the diameters of the extruded rods. The swelling of the rods showed a volume extension of 130% for an ABA containing 50% PEO and 20% for an ABA containing 20% PEO. Using 1H-NMR it was found that protons in the B-blocks of the swollen ABA copolymers were mobile, while the A-blocks remained rigid during incubation. The analysis of the pH inside ABA rods using electron paramagnetic resonance, EPR, gave a pH of 5.2 after incubation with a subsequent increase to pH 6.0 during the first day, approaching the pH of the medium after nearly 33 d. Acidic degradation products did not accumulate inside the ABA rods. Degradation and erosion started immediately upon incubation. By contrast, PLG rods showed the typical profile of degradation and erosion. In this case, the influence of the geometry of the device was insignificant. Consequently, ABA triblock copolymers may widen the spectrum of parenteral drug delivery with regard to release of pH-sensitive drugs as well as erosion-controlled release kinetics.