Increased corrosion resistance of stent materials by converting current surface film of polycrystalline oxide into amorphous oxide

Abstract

Current efforts of new stent technology have been aimed largely at the improvement of intravascular stent biocompatibility. Among the chemical characteristics of metallic stents, surface oxide corrosion properties are paramount. Using our unique technique, the currently marketed 316 L stainless steel and nitinol stent wires covered with polycrystalline oxide were chemically etched and then passivated to form amorphous oxide. Excellent metallic-stent corrosion resistance with an amorphous oxide surface was demonstrated in our previous in vitro study. For in vivo validation, we compared the corrosion behavior of different oxide surfaces on various forms of test wires in the abdominal aorta of mongrel dogs using open-circuit potential and cyclic anodic polarization measurements. After conduction, the retrieved test wires were observed under scanning electron microscope. No passivity breakdown was found for wires covered with amorphous oxide, while wires with polycrystalline oxide showed breakdown at potentials between +0.2 to + 0.6 V. It has been proven that severe pitting or crevice corrosion occurred on the surface of polycrystalline oxide, while the surface of amorphous oxide was free of degradations in our experiment. We have demonstrated that this amorphous oxide coating on metallic material provides better corrosion resistance, not only in vitro but also in vivo, and it is superior not only in strength safety but also in medical device biocompatibility.

Trivalent and Hexavalent Chromium Issues in Medical Implants

“…Lin reports that the work of Parker et al indicate that activated macrophage cells can actually decrease the corrosion rate of 316L stainless steel [5,13,21]. The decreased corrosion was attributed to an enhancement of the alloy's oxide film by the released oxidizing compounds.…”

“…Both layers are n-type semiconductors. In studies of stainless steels having an amorphous oxide film, the films show minimal interaction between the blood and amorphous oxide film [4,5]. Su and Shih report, through a proprietary process, the normally polycrystalline oxide layer can be removed and replaced with an amorphous n-type semiconductor layer with a higher Cr and O concentration.…”

Coronary Arterial Drug-Eluting Stent: From Structure to Clinical

“…In addition, it is suspected of producing mild inflammation when contacting monocytes 38 . In order to solve these problems, the surface of Ni-Ti is passivated to increase the Ti oxide concentration and decrease the Ni concentration 39,40 , and it is coated by some materials such as polyurethane 41 , Ti nitride 42 and polycrystalline oxides 43 to improve the corrosion resistance. Another problem is that the Ni-Ti alloy does www.intechopen.com not have adequate visibility under fluoroscopy 44 though it can be visualized by MRI 45 .…”

Electrochemical characterization of vascular bare-metal stents. A novel approach modifying the mini-cell system

“…Applications of in vitro corrosion analyses include the characterization of surface modifications [9,11,13,14], coatings [5,8,10,12,15,16] and new materials [6,17,18]. In vivo OCP measurements have been performed in dogs [19] and humans [20]. The majority of the studies found that the surface features crucially influence corrosion resistance.…”

“…In this regard, the mini-cell system (MCS) is a promising approach as it enables a variety of electrochemical analysis techniques on micro surfaces [21]. The aim of the present study was to investigate the electrochemical properties of different stents, modifying the MCS to match the arising requirements: first of all, the study intended to characterize the surface´s electrochemical properties as its corrosion resistance has been described as having a major impact on biocompatibility [19]. Furthermore, the aim was to investigate stents in a final deployed form and finished condition as all steps in manufacturing potentially influence corrosion properties [22].…”