Study of the adhesion of thin plasma fluorocarbon coatings resisting plastic deformation for stent applications

Abstract

Metallic intravascular stents are medical devices (316L stainless steel) used to support the narrowed lumen of atherosclerotic stenosed arteries. Despite the success of bare metal stents, restenosis remains the main complication after 3–6 months of implantation. To reduce the restenosis rate of bare metal stents, stent coating is an interesting alternative. Firstly, it allows the modification of the surface properties, which is in contact with the biological environment. Secondly, the coating could eventually act as a carrier for drug immobilization and release. Moreover, the in vivo stent implantation requires in situ stent expansion. This mandatory step generates local plastic deformation of up to 25% and may cause coating failures such as cracking and delamination. Fluorocarbon films were selected in this study as a potential stent coating, mainly due to their chemical inertness, high hydrophobicity, protein retention capabilities and thromboresistance properties. The aim of this study was to investigate the adhesion properties of fluorocarbon films of three different thicknesses deposited by plasma polymerization in C2F6/H2 on 316L stainless steel substrates. A previously developed small punch test was used to deform the coated samples. According to atomic force microscopy, field emission scanning electron microscopy and x-ray photoelectron spectroscopy characterizations, among the coatings with different thicknesses studied, only those with a thickness of 36 nm exhibited the required cohesion and interfacial adhesion to resist the stent expansion without cracking or delaminating. Otherwise, cracks were detected in the coatings having thicknesses equal or superior to 100 nm, indicating a lack of cohesion.

Electrochemical polishing as a 316L stainless steel surface treatment method: Towards the improvement of biocompatibility

“…Therefore, various surface treatment approaches have been employed to tune the surface texture, energy and chemistry of implants [4][5][6]. In some cases, this has resulted in favourable interactions of the surrounding biological environment with the modified surface [7,8].…”

Methods to Investigate the Adhesion of Soft Nano‐Coatings on Metal Substrates – Application to Polymer‐Coated Stents

“…Previous work demonstrated the influence of the fluorocarbon film thickness on the cohesion properties of these films using the small punch test. [26] The 36 nm thick fluorocarbon films exhibited higher cohesion properties compared to films of 105 nm thickness. However, no metallic compounds (chromium and iron) were detected by XPS, indicating that less than 1% of the substrate was exposed by cracks.…”

“…These ribbon-like structures are formed during the plasma polymerization and are also visible on non-deformed samples. [26,[72][73][74][75][76] The topographical changes at the metal surface, which increased the roughness, proved to influence the adhesion properties and the coating integrity, leading to coating failures. As observed in Figure 3f, a crack aligned with the slip band was observed in the 15 min coated sample, indicating a lack of cohesion.…”

“…[67,68] A 105 nm fluorocarbon film (CF x ) was deposited by a pulsed plasma polymerization of H 2 /C 2 F 6 for 15 min on the pretreated substrates, as previously described. [26] The coated samples were plastically deformed at 25% using a load of 2 200 N and a displacement rate of 0.05 mm Á s À1 , in order to study the effect of a plastic deformation on the coating properties. The 25% deformation represents the maximum localized deformation in a 316L stainless-steel stent after its expansion, as reported by Migliavacca et al [15] Sample Characterization…”

“…In this context, it has to be noted that ultra-thin coatings are attractive as stent coatings due to their higher adhesion and cohesion properties after a plastic deformation, as compared to thin films. [24][25][26] However, the characterization of ultrathin coatings requires the appropriate characterization of the adhesion by sensitive characterization tools. In this paper, the applicability of conventional polymer-metal adhesion tests to ultra-thin and thin polymer-coated stents is discussed.…”