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Surface characterization of poly(lactic acid)/everolimus and poly(ethylene vinyl alcohol)/everolimus stents

Two model drug eluting stents of poly(lactic acid) (PLA)/everolimus and poly(ethylene vinyl alcohol) copolymer (EVAL)/everolimus have been investigated using complementary surface analysis techniques including AFM, XPS, and ATR-IR to assess their structure and its relation to drug release. Different surface morphologies were observed for these stents, with phase separation evident on the PLA coating and a homogeneous system for the EVAL-based coating. This indicates a potentially different drug distribution for the different stents, although both showed a surface enrichment of the drug compared to the bulk. Dissolution studies for PLA/everolimus stents showed an immediate loss of drug from the surface as well as a longer term polymer matrix erosion. The EVAL/everolimus stent also displayed a loss of drug from its surface, but an intact surface after 28 days in dissolution media. These data are discussed in relation to the different release mechanisms occurring in the stents.