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.

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.

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+/-0.39GPa, 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 (T(g)) of the polymer to be determined. A significant drop of the Young's modulus in solution was observed at 36 degrees C, suggests that in vivo the T(g) of the polymer is below body temperature. The possible consequences on drug release and the mechanisms by which this may occur are considered.

The comonomer effect on the structures of poly(vinylidene fluoride–hexafluoropropylene) P(VDF–HFP) copolymers was analyzed by Raman spectroscopy. The HFP content of these copolymers varies from 5% to 15%. Because of steric interactions involving the bulky HFP comonomers, the predominant chain conformation has extensively more gauche conformers in comparison to the neat PVDF. Based on both experimental and simulation studies, specific spectroscopic features in the 400–900cm−1 region have been identified that are characteristic of irregular chain conformations elucidating the perturbing effect of HFP on the equilibrium chain statistics of PVDF in the amorphous phase. In addition, these spectroscopic features were revealed to be extremely sensitive to the relative placements of the CF3 units with respect to other fluorine atoms along the chain.

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, boolean OR triangle h, In A). A slightly larger KWW 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.

High-quality polarized Raman spectra have been obtained for various poly(vinylidene fluoride) (PVDF) structured, crystalline and amorphous. The results encouraged us to revisit the Raman band assignment, especially within the conformational sensitive region (400-1100 cm(-1)) and to use the new understanding to characterize the amorphous region of PVDF. Vibrational bands have been assigned on the basis of polarization characteristics observed and the potential energy distribution (PED) calculated. The simulated results agree well with the experimental polarized Raman study. On the basis of the calculated PED, combined with the simulation of different conformational sequences (tttt, tttg tgtg' tggg gggg), spectroscopic features (band intensity at 648 cm-1 and the frequency change of the 856 cm-1 band) were associated with the distribution of rotational isomeric states. Two rotational isomeric state (RIS) models were analyzed and compared in the simulation of the amorphous state. On the basis of the spectroscopic features of experimental and simulated Raman spectra, it was concluded that the model which predicts higher gauche population more accurately describes the amorphous state. This analysis provides an opportunity to describe the amorphous state in a quantitative manner.