Polymers for biodegradable medical devices: VII. Hydroxybutyrate-hydroxyvalerate copolymers: degradation of copolymers and their blends with polysaccharides under in vitro physiological conditions

Abstract

The hydrolytic degradation of hydroxybutyrate-hydroxyvalerate copolymers was monitored in vitro at 37°C and pH 7.4. Direct use of bulk properties such as weight loss and tensile strength did not reveal substantial changes in the polymer matrix over degradation periods of several months. Despite this, the polymers were demonstrated to undergo significant modification during this period, in ways that markedly influence their subsequent behaviour. Combined use of goniophotometry and surface energy measurements revealed that surface modification begins at an early stage and is accompanied by diffusion of water into the matrix and a progressive increase in polymer porosity. Relatively little change in the molecular weight and some increase in the crystallinity of the matrix occurred during these early months. As a result, the tensile strength of the polymer varies little in this period. As the porosity of the matrix increases, hydrolytic chain scission within the matrix and diffusion out of degradation products proceeds more effectively. Decrease in matrix molecular weight, increase in matrix erosion, weight loss and loss of tensile strength began at a much more dramatic rate. The apparent resistance of the polymer to degradation in the early months is followed by an accelerated degradation phase around and beyond 1yr. The use of fillers that can dissolve or hydrolytically degrade more rapidly than the hydroxybutyrate matrix accelerates the development of porosity within the matrix and thus enhances the decomposition process.