Surface modification of NiTi alloy with tantalum to improve its biocompatibility and radiopacity

Abstract

Nickel–titanium alloy, with the features of shape memory effect, superelasticity and biocompatibility offers many unique advantages for the biomedical application. These applications have included everything from surgical tools to permanent implants [1–3]. However, many researchers highlighted the selective dissolution of Ni ion from the NiTi alloy during the corrosion process, which could lead to potential danger [4–6]. Different surface treatments have been investigated to improve the corrosion resistance of NiTi implants [7–10]. In modern medicines, to analyze the motion of implants, it is necessary to estimate the 3D position and orientation of each component using radiography [11]. When small stents, guidewires and catheter are quite thin and space farther apart, the detection of the implantable devices or tools become very difficult [12, 13]. The methods to enhance the visibility under X-ray include adding a mark of noble metals and coating with gold, etc. Although gold plating of nitinol components to improve the radiopacity has been achieved by means of electroplating technique, problems such as risks of galvanic corrosion, biocompatibility and hydrogen embrittlement still exist [14]. Tantalum metal has successfully been used for implants for half a century [15]. Since the galvanic potentials of tantalum and nitinol are very similar, the galvanic corrosion effect is almost immeasurable and complete failure of the implants is impossible [16]. Tantalum and tantalum oxide particles are often used to improve radiopacity of other materials [17, 18]. No problems have been reported concerning its biocompatibility [19–22]. It has been found that tantalum coatings are 100% pinhole-free and show great potential within both industrial and medical applications [23]. Tantalum coating with a very ductile nature is suitable where a product can be improved by combining material characteristics of the substrate with an exceptional corrosion-resistant surface [24, 25]. As a result, tantalum is an excellent candidate for usage as coatings for NiTi alloys to improve its anti-corrosion property and radiopacity. For coating applications, a good bonding strength between the coating and the substrate is required in order to ensure the lifetime service in the implanting environment [26]. The multi arc ion-plating technique, with the feature of high packing density and good adhesion, can satisfy this requirement [27]. Accordingly, the purpose of the present study is to develop a novel biocompatible and radiopaque Ta coating on NiTi alloy by multi-arc ion plating technique, the surface characterization, corrosion behavior and hemocompatibility are investigated. The chemical composition of the experimental alloy was Ti-50.6 at%. All samples were mechanically polished to 1 lm and then ultrasonically cleaned in acetone, alcohol and distilled water successively, then dried and loaded into the deposition chamber. The tantalum coatings were deposited by multi arc ion-plating technique [27]. Prior to deposition, the surfaces of the NiTi alloy substrates were further cleaned by Ar ion bombardment with energy of 1100 eV for 10 min. And the temperature of the NiTi alloy substrates was held to be 300 C. The substrate bias voltages were controlled to be )300 V with an arc current be 75 A, deposition pressure Y. Cheng AE Y. F. Zheng (&) Department of Materials and Nanotechnology, College of Engineering, Peking University, Beijing 100871, China e-mail: yfzheng@pku.edu.cn