New approaches to mapping through-thickness variations in the degradation in poly (lactide-co-glycolide)

Biodegradable poly (lactide-co-glycolide) (PLGA) copolymers have been used for many years for biomedical applications such as soluble sutures, orthopaedic implants and more recently as potential tissue scaffold materials. The rate at which the copolymers degrade can be manipulated from a period of days to months by changing the lactide/glycolic acid ratio. Degradation of PLGA copolymers occurs by hydrolysis of the ester bonds in the polymer backbone. The hydrolysis reaction is autocatalytic and is accelerated by the build up of degradation products in the bulk of the material. As a consequence, material degradation is expected to be non-uniform through the specimen thickness with the material at the centre degrading at a faster rate than at the surface. Despite many studies of PLGA degradation, information on this local variance is sparse as the techniques used to track the process are usually bulk measures. In this study, two new approaches for monitoring degradation have been developed that enable local measurements of degradation to be made throughout the specimen over an extended period of time. Chemical and mechanical variations in the structure of the polymer have been mapped using attenuated total reflectance infrared spectroscopy (ATR-FTIR) and nanoindentation. These have produced comparable results and show that the degradation rate at the centre of the specimens is almost an order of magnitude higher than at the surface.