Methods to Investigate the Adhesion of Soft Nano-Coatings on Metal Substrates – Application to Polymer-Coated Stents Franc ¸ois Lewis, Diego Mantovani * Introduction Thin films are widely used to modify the surface properties of materials, thus providing new insights into different fields, such as the electronic, aerospace and medical industries. In most cases, the adhesion between the coating and the substrate needs to be as high as possible in order to fabricate durable devices. A coating failure could affect the reliability and the performance of the coated device. The adhesion evaluation of thin polymer films deposited on metal substrates has become a major challenge for medical devices, in particular for coated and drug-eluting stents. Stents are thin wire-mesh tubes inserted in partially blocked arteries to prevent the obstruction of blood flow and to act as internal scaffolding. [1] Typically, the length and diameter of a stent vary in the ranges 8 to 32 mm and 2.5 to 4.0 mm, respectively, and the strut size is about 100 mm. [2–4] About one million of these devices, mainly made of 316L stainless steel, are implanted worldwide each year. [5] To date, three coated/drug-eluting stents have been approved by the Food and Drug Administration (FDA) and have shown a decrease in the restenosis rate – that is, reoccurrence of the blockage – as compared to bare metal stents. [6] However, serious clinical problems remain such as in-stent restenosis, thrombosis, stent fractures and coating failures. [7–14] Topographic changes of the metal surface occurring during the stent expansion are a major cause of the adhesion failures. In fact, the local plastic deformation could reach up to 25% in high stress area (nodes) of a 316L stainless steel stent, as evaluated by numerical simulation. [15] Furthermore, the stent design, the nature of the material and the expanded diameter also influence the localized deformation. [15–21] The FDA has provided guidance to the stent industry to evaluate the safety and effectiveness of coated and drug- Feature Article F. Lewis, D. Mantovani Laboratory for Biomaterials and Bioengineering, Department of Materials Engineering & University Hospital Research Center, Laval University, Quebec City, QC, G1K 7P4, Canada D. Mantovani Laboratory for Biomaterials and Bioengineering, Department of Materials Engineering, Pavillon Adrien-Pouliot, 1745-E, Laval University, Quebec City, QC, G1K 7P4, Canada Fax: þ1 (418) 656-5343; E-mail: diego.mantovani@gmn.ulaval.ca Soft coatings are widely used to tailor the surface chemistry of materials without altering their bulk properties. However, the strength of adhesion between the coating and the substrate must be high enough for long-term applications. This has become a major challenge in the medical field, especially for polymer-coated stents, mainly due to several coating failures reported after mechanical expansion during clinical implantation. In this work, the applicability of current polymer- metal adhesion tests to polymer-coated stents is dis- cussed. The small punch test was proposed as an adhesion test that allows fundamental studies on the adhesion and coating properties. This adhesion test was applied to thin fluorocarbon coatings deposited by plasma on 316L stainless steel. Macromol. Mater. Eng. 2009, 294, 11–19 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mame.200800229 11