Breakdown of biodegradable polymers the breakdown of poly(D,L lactideco-glycolide polymers in agitated and stagnant environments

Poly( d,l lactide-co-glycolide) copolymers are used extensively in biomedical implants due to their outstanding biocompatibility, bioresorbability and the fact that wide ranges of physical, thermal, mechanical and biological properties are covered by varying chemical and configurational chain structures. Present trends in invasive surgical devices, are focusing on merging scaffolding-type implants with controlled drug release systems. This can enhance the surgical success of such procedures by delivering the appropriate amounts of drug (e.g. antibiotic) to an exact site thereby minimizing side effects. Examples of such applications are stents, and catheters, which are tubular-type implant devices. To improve the accuracy of these bifunctional systems, a more complete understanding of the physical and chemical breakdown mechanisms of poly(d,l lactide-co-glycolide) copolymers is necessary. Although much research has been carried out on these types of breakdown processes, one area that has not been fully explored is the eflect of a flowing medium around a degrading implant device. In tubular-type devices where bodily fluids arc present, the physical effect of agitation on tbe breakdown process can be significant. Three types of poly( d,llactide-co-glycolide) copolymers were used in the experimental analysis. These copolymers varied in molecular weight and the compositional ratios of lactide to glycolide. Their breakdown in Ringer's solution at 3JOC under agitated and stagnant medium conditions, was analysed by pi I changes of the medium, weight loss and water absorption of the copolymer matrix, Macroscopy, Ellipsometry, Atomic Force Microscopy (AFM,) [)ilferential Scanning Calorimetry ([)SC), Gel Permeation Chromatography (GPC), and Fourier Transform Infi"a-Red Spectroscopy (FTJR). It was confirmed that the copolymers of higher molecular weight degraded more slowly than the lower molecular weight ones. Higher glycolide ratios also decreased degradation time, as expected. The uncapped carboxylic end groups present in copolymer RG5031 I significantly increased its degradation rate, as in all other factors it was the same as RG503. Experimental data showed a clear heterogeneous degradation process observable in the copolymer of highest molecular weight which had a ratio of 50:50 laetide to glyeolidc, (RG755). This was observed as an outer shell formation causing an inner autocatalytic effect in the copolymer matrix. This type of heterogeneous degradation was not as definitive in the other two lower molecular weight polymers (RG503, RG503H), which also had a higher glycol ide ratio (25:75 lactide: glycolide). With respect to stagnant versus agitated, experimental data point to a faster degradation rate in stagnant conditions for polymer RG755. Agitation did not conclusively affect the breakdown rate of copolymers RG503 and RG503I-I. The increased degradation rate in RG755 under stagnant versus agitation, is seen as the effect of the decrease in pH concentration on the outer surface of the agitated samples, compared to a high pH concentration surrounding stagnant specimens which helps to autocatalyse the hydrolysis breakdown mechanism. No significant difference was noted between agitated and stagnant samples of RG503H probably due to their lower molecular weights and the glycolide segments hydrophilicity having more influence over degradation rates than the autocatalytic effect of pH concentration.