The degradation, swelling and erosion properties of biodegradable implantsprepared 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 biodegra dation of a polymer matrix represents a promising strategy to control drug release. We have investigated the degradation of ABA triblock copolymers, c onsisting of poly(lactide-coglycolide) A-blocks and poly(oxyethylene) B-blo cks, and PLG, poly(lactide-co-glycolide), with respect to swelling behaviou r, molecular weight loss and polymer erosion. Implants were prepared by eit her compression moulding or extrusion using a laboratory ram extruder. Inse rtion of an elastoplastic B-block did not lower the processing temperature, but the entanglement of the polymer chains was significantly reduced as ca n 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% fo r an ABA containing 20% PEG. Using H-1-NMR it was found that protons in the B-blocks of the swollen ABA copolymers were mobile, while the A-blocks rem ained rigid during incubation. The analysis of the pH inside ABA rods using electron paramagnetic resonance, EPR, gave a pH of 5.2 after incubation wi th 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 accum ulate inside the ABA rods. Degradation and erosion started immediately upon incubation. By contrast, PLG rods showed the typical profile of degradatio n and erosion. In this case, the influence of the geometry of the device wa s insignificant. Consequently, ABA triblock copolymers may widen the spectr um of parenteral drug delivery with regard to release of pH-sensitive drugs as well as erosion-controlled release kinetics. (C) 2000 Elsevier Science Ltd. All rights reserved.