Nanoscale mechanical measurement determination of the glass transition temperature of poly(lactic acid)/everolimus coated stents in air and dissolution media

Localized atomic force microscopy (AFM) force analysis on poly(lactic acid) (PLA) and poly(lactic acid)/everolimus coated stents has been performed under ambient conditions. Similar Young's modulus were derived from both PLA and PLA/everolimus stent surface, namely 2.25 ± 0.46 and 2.04 ± 0.39 GPa, respectively, indicating that the drug, everolimus does not significantly effect the mechanical properties of PLA up to a 1:1 (w/w) drug loading. Temperature controlled force measurements on PLA only coated stents in air and in a 1% Triton surfactant solution allowed the glass transition temperature (Tg) of the polymer to be determined. A significant drop of the Young's modulus in solution was observed at 36 °C, suggests that in vivo the Tg of the polymer is below body temperature. The possible consequences on drug release and the mechanisms by which this may occur are considered.