Electrochemical Polishing of Nitinol and Other Medical Alloys in Aqueous Electrolytes Using Pulse/Pulse Reverse Electric Fields

Abstract

Nitinol is an important medical alloy, and its unique properties have allowed advancement of minimally invasive procedures for medical and orthodontic applications. Nitinol is a near equiatomic alloy of nickel and titanium whose properties, namely superelasticity, shape memory effect and self-passivity, are particularly important for medical applications. However, surface preparation plays a significant role in the surface properties of Nitinol and consequently influences corrosion behavior in a physiological environment. Surface treatments including mechanical polishing, chemical polishing, electropolishing, anodization and heat treatments have been investigated for corrosion behavior in physiological environments, especially in terms of nickel ion release from the device surface, which is an important consideration for medical applications owing to nickel’s toxicity. Electropolishing is attractive for the preparation of Nitinol surfaces due to improvements in corrosion resistance, biocompatibility and surface quality when compared with other surface preparation methodologies.[i] Traditionally, processing of these materials has been carried out in hazardous chemicals such as hydrofluoric acid, or fluoride salts that convert to hydrofluoric acid,[ii] to electropolish and passivate the device and achieve the surface finish required for insertion into the body. The inclusion of hydrofluoric acid, in addition to concentrated sulfuric-phosphoric acids, makes for an extremely hazardous working environment. Alternatively, non-aqueous electrolytes such as methanol-perchloric acid[iii] or methanol-sulfuric acid[iv] mixtures have been suggested, but these require low-moisture environments and low-temperatures (-10 °C) and industrial implementation is difficult. The current work demonstrates that Nitinol surfaces may be electropolished in simple, aqueous electrolytes using pulse and pulse/reverse electric fields. Electropolishing of Nitinol has been done in dilute sulfuric acid solutions (5-10%) as well as salt electrolytes using pulse/pulse reverse voltage fields under ambient conditions. This work has been demonstrated for Nitinol coupons, wires and shape sets; processing of these surfaces will be discussed. Implementation of a Nitinol wire manufacturing process is an important step in the fabrication of these devices for medical use. Scale up of Nitinol wire electropolishing in terms of apparatus is also currently underway and will be discussed. In the current work, a reel-to-reel apparatus will be utilized to facilitate scale-up activities. A schematic of the reel-to-reel apparatus is shown in the accompanying image. The apparatus design is straight-forward due to limited considerations needed for processing under ambient conditions in simple aqueous electrolytes. Important cell considerations such as cathode configuration and residence time will be discussed along with processing considerations. This method is applicable to other medical alloy wires, devices and implants. [i] R. Steegmueller, T. Fleckenstein, and A. Schuessler, Proceedings of the Materials and Processes for Medical Devices, 163-168 (December 1, 2006). [ii] Dr. James Carlson, Senior Metallurgical Engineer, Cook Medical, private communication, November 2010. [iii] W. Miao, X. Mi, M. Zhu and H. Li, J. Ceramic Processing Research, 7 (4), 367-370 (2006). [iv] K. Fushimi, M. Stratmann and A.W. Hassel, Electrochimica Acta, 52, 1290-1295 (2006). Figure 1