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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).