Comparative study of the degradation of polylactide and polytartrate implants in vitro

Poly(D/L-lactide (PLA) and 2',3'-(1',4'-diethyl-L-tartryl)-poly-(2,3-O-isopropylidene)L-tartrate (PTA) were investigated with respect to their hydrolytic degradation. Implants were made of compressed polymer and melted polymer. They were immersed in a buffer solution (pH 7.4) and stored in a water bath at 30 degrees C. Change of surface morphology, content of water, glass transition temperature, existence of crystalline degradation products and decrease of average molecular weight were used to assess the extent of hydrolytic degradation. Glass transition temperature of PTA devices, which is influenced by content of absorbed water, decreased more rapidly in comparison to the implants made of PLA. The importance of non-freezing water accumulated at the ester linkages for the hydrolytic degradation could be shown. Decrease in average molecular weight was determined by measuring intrinsic viscosities and using the Staudinger equation. The average molecular weight of PLA implants decreased for compressed and melted devices from 17400 to 5000 and 8500, respectively, during 7 weeks of immersing. Implants made of PTA (average molecular weight of about 23000) provided an average molecular weight of approximately 1 000 after 3 weeks storage in the buffer solution.