An investigation of adhesion in drug‐eluting stent layers

An atomic force microscopy (AFM) method was developed to quantify the adhesion forces between and cohesive forces within the layers of a drug‐eluting stent (DES). Surface pairs representing both the individual components and the complete chemistry of each layer within the DES were prepared. As a model, the CYPHER® Sirolimus‐eluting Coronary Stent was studied. This DES consists of a stainless steel stent substrate, a parylene C primer layer, and a drug‐eluting layer that contains poly(ethylene‐ co ‐vinyl acetate), poly( n ‐butyl methacrylate), and sirolimus (rapamycin). Coated AFM tips and two‐dimensional substrates or coupons, which act as surrogates to the CYPHER® Stent, were prepared and characterized. The force–displacement measurements were conducted to evaluate the adhesion between the middle parylene C layer and the 316L stainless steel substrate, the adhesion between the parylene C layer and the outer drug‐eluting layer, and the cohesion between the three constituents of the drug‐eluting layer. The average adhesion forces between the parylene C to drug layer varied from 88 to 167 nN, and the drug layer‐to‐drug layer interactions were between 194 and 486 nN within the model CYPHER® Stent coating. All the adhesion forces measured were larger than those observed for gold–gold interactions, which yielded a pull of force of 19 nN (Zong et al., J Appl Phys 2006;100:104313–104323). © 2008 Wiley Periodicals, Inc. J Biomed Mater Res, 2008