In vitro real-time aging and characterization of poly (L/D-lactic acid)

Bioabsorbable polymers are being extensively studied for possible application as suture materials, bone plates, pins, screws, reconstructive bone substitutes, sheets for adhesion prevention, and drug delivery agents. For any application, the rate and nature of the degradation process of the polymers must be understood and documented. In vitro aging studies may be used to characterize polymer degradation and mechanical property retention over time. Studies were planned to be conducted at various temperatures and under several loading conditions to develop a master curve for degradation which could be correlated with the degradation rates of an in vivo study. This paper covers the first six months of the planned two-year in vitro studies conducted at 37°C in a phosphate-buffered saline solution. Among the most attractive candidate materials for low-load bearing bioabsorbable devices are the members of the poly (α-hydroxy acids) family. In particular, a high molecular weight grade of poly (L/D,L-lactic acid) (90/10) was selected for the device construction material because of its inherently high mechanical strengths and moderate degradation time. Extruded rods were produced for the in vitro immersions and in vivo implants. The analyses included mechanical, thermal, and chemical testing. Mechanical testing during the first six months was performed in both wet (37°C) and dry (RT) conditions. In general, the mechanical properties measured wet (37°C) were lower than those measured dry. Overall, there was not much net change in the mechanical properties over the six month time period. However, there is an initial decrease, then an increase and finally a slight drop again. The degradation of the rods involved loss in molecular weight as the water molecules attacked the labile ester bonds. The water molecules may also serve as a plasticizer as evidenced by the initially lowered glass transition temperature and mechanical properties. Subsequent increase in these properties indicates a possible increase in crystallinity as the shortened, plasticized polymer chains are able to realign more easily. Also, presumably, the amorphous D/L-lactic acid regions are degraded first, diffusing out of the polymer, allowing the more crystalline L-lactic acid portions of the polymer to realign at the elevated temperature