Nano- and Micro-Scale Adhesion in Drug-eluting Stents
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1239-VV01-07
Nano- and Macro-Scale Adhesion in Drug-eluting Stents T. Tan1.2, J. Meng
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, N. Rahbar , H. Li , G. Papandreou , CA. Maryanoff and 1,3 W.O. Soboyejo 1 Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ 08544, U.S.A. 2 Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544, U.S.A. 3 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, U.S.A. 4 Department of Civil and Environmental Engineering, University of Massachusetts Dartmouth, North Dartmouth, MA 02747, U.S.A. 5 Cordis Corporation, A Johnson and Johnson Company, Warren, NJ 07059, U.S.A. 6 Cordis Corporation, A Johnson and Johnson Company, Spring House, PA 19446, U.S.A. ABSTRACT This paper presents a combined experimental and theoretical/computational study of adhesion and interfacial fracture between multilayers in a CYPHER® model drug eluting stents (DES). Atomic Force Microscopy (AFM) is used to obtain pull-off forces between coated AFM tips and substrates that simulate the bimaterial surfaces in the DES. Adhesion theories and fracture mechanics concepts are then applied to obtain estimates of the fracture toughness over a range of mode mixities between pure mode I and pure mode II. The trends in the estimates are shown to be in good agreement with experimental measurements of interfacial fracture toughness obtained from Brazil disk specimens over the same range of mode mixities. INTRODUCTION Cardiovascular disease accounts for 30% for the deaths in the United States annually [1]. Drug eluting stents, with coated metallic substrates, have been used to reopen blocked arteries with the help of the expanded balloon. However, there is a need to ensure that the coated drugs and the substrates are well adhered during the service life of the DES structures. In an effort to quantify the adhesion between bimaterial surfaces that are relevant to drug eluting stents, a model DES structure was used to simulate the CYPHER® DES produced by the Cordis Company (Figure 1a). Atomic Force Microscopy (AFM) [2] was used to measure the pull-off forces between the drug-eluting layer and the primer parylene C layer along with the adhesion between the primer layer and the 316L stainless steel layer. The measured pull-off forces were converted to adhesion energies via the Johnson-Kendall-Roberts (JKR) and the Derjaguin-Muller-Toporov (DMT) models. Brazil nut fracture mechanics specimens were also used to measure the interfacial fracture toughness levels for the same interfaces that were studied using AFM techniques. The results show good agreement between the macro-scale fracture mechanics and experiment results. The linkages between the nano- and macro-scale adhesion measurements are also elucidated.
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Figure 1. (a) A schematic of coating layers on the CYPHER® stent; (b) A schematic of Brazil nut disk used in interfacial fracture mechanics test EXPERIMENTAL PROCEDURES (a) Atomic Force Microscopy (AFM) The adhesion bet
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