The role of surface oxide thickness and structure on the corrosion and nickel elution behavior of nitinol biomedical implants

Abstract

Biocompatibility is an important factor for metallic medical device implants, and corrosion resistance of implantable alloys is one aspect of biocompatibility. Corrosion behavior of nitinol is strongly dependent upon the nature of the surface oxide, which forms during processing. The surface oxide is comprised of a mixture of titanium and nickel oxides, and subsequent thermal exposure (e.g., during shape setting) and surface removal (e.g., electropolishing, mechanical polishing, etching, etc.) influence its structure. Corrosion behavior is often assessed through testing methods such as cyclic potentiodynamic polarization (e.g., ASTM F2129) and nickel ion release. Studies have suggested that a correlation exists between oxide thickness and nickel ion release, with thicker oxides eluting more nickel. It is hypothesized that the composition of the surface oxide, and not only its thickness, influences the corrosion performance of nitinol. To investigate this, nitinol wire samples were processed to produce surface oxides with different structures both in terms of thickness and composition. These samples were tested per ASTM F2129 and nickel ion release testing.

Surface finishing of Nitinol for implantable medical devices: A review

“…Similar results were observed in several other studies as well. 23,34,46 For instance, Rosenbloom et al 23 reported the E b for EP NiTi samples as ~950 mV whereas the E b for MP NiTi samples was only ~245 mV. Similarly, Thierry et al 34 reported the E b for EP NiTi samples as ~1000 mV with no trace of localized corrosion whereas the E b for MP NiTi samples was ~530 mV (Table 1).…”

“…16,19,20,22 The uniformity of the oxide layer across the entire material surface with respect to thickness and chemical composition is crucial as it determines the efficacy of finishing steps involved. 16,20,23 The stability of the oxide layer is also important since a thick oxide layer can undergo mechanical defects and break whereas a thin oxide layer can be stable and well protective. 20,24,25 There has been a lot of research done to improve the properties of surface oxide with a primary focus on how to prevent the release of nickel ions from NiTi.…”

“…37 The pitting corrosion refers to the breakdown of the passive surface oxide film across a surface to form small cavities (pits) on a device. The crevice corrosion refers to the breakdown of the passive surface oxide film around a crevice (a narrow opening) to form pits in Rosenbloom et al 23 joints, corners, etc. If a material shows an increase in resistance to pitting corrosion, then it will be associated with a higher E b value.…”

“…The results of these studies confirmed that the HT increases the oxide layer thickness of NiTi. 19,23 In Rosenbloom's study, 23…”

Influence of structural size on mechanical properties of nitinol left atrial appendage occluder

“…At present, many scholars are committed to the application of this technology to prevent atrial fibrillation thromboembolism, and great progress has been made [6][7][8]. Nitinol is a kind of shape memory alloy with excellent characteristics of wear resistance, corrosion resistance, high damping and super elasticity, which is widely used in medical devices [9][10][11]. Therefore, LAA occluder is usually made of nitinol.…”