The roles of poly(lactic acid) chain conformation and configuration on the enthalpy relaxation kinetics of amorphous poly(lactic acid) were examined. Enthalpic relaxation data, which were scaled to the same supercooling from the initial fictive temperature, were taken for three types of the polymer containing various d-lactyl monomers (5.7%, 13.0%, 50%) to assess the effects of configurational defects. The kinetics data were very similar from sample to sample. The effects of configurational defects were assessed using the generalized Kohlrausch−Williams−Watts (KWW) equation solved by the Tool−Narayanaswamy−Moynihan (TNM) equation. The major effect of increasing the d-lactyl contents was to lower the PLA glass transition temperature, thereby accelerating the kinetics of enthalpic relaxation. Configurational defects showed no significant effect on the other KWW/TNM fit parameters (x, Δh, ln A). A slightly larger KWW β stretched exponential parameter is observed for greater (50% d) than for lower (5.7% d) amount of d-lactyl monomer, although these differences are just within the experimental error. Raman spectroscopy showed that conformation does not change appreciably during physical aging.

Drug‐eluting stents (DES) are a preferred treatment modality for occlusive coronary artery disease. First‐generation DES have demonstrated high levels of efficacy. However, concerns have been raised over late thrombotic events. XIENCE V™ everolimus‐eluting coronary stent is a second‐generation DES designed to be more deliverable and safe, while maintaining efficacy in a broad patient population compared with first‐generation DES. 1 − 3 As a drug/device combination product, the overall performance of a DES is determined by its components and how well they are integrated. XIENCE V utilizes the MULTI‐LINK VISION® stent, the antiproliferative drug everolimus, a fluorinated polymer drug carrier, poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP), and a stent‐specific delivery system. A DES coating must fulfill the multiple goals of biocompatibility, controlled drug release and maintenance of the coating durability through stent crimping, and expansion in vivo. The XIENCE V coating utilizes a two‐layer coating system composed of an acrylate primer and a fluorinated copolymer drug reservoir. Fluorinated polymers have a long history of use in permanent vascular implant applications. The XIENCE V fluorinated copolymer offers in vivo biocompatibility combined with excellent chemical stability and high purity. Described in this article are the design rationale and polymer selection criteria. The hemocompatibility and biocompatibility of the fluorinated polymer coating are discussed. Characterization results on drug release control, possible drug release mechanism, coating integrity, coating uniformity, and fatigue resistance are also presented.

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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.

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