Electrochemical Deposition of Apatite/Collagen Composite Coating on NiTi Shape Memory Alloy and Coating Properties
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Electrochemical Deposition of Apatite/Collagen Composite Coating on NiTi Shape Memory Alloy and Coating Properties Tao Sun, Min Wang * Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong * Corresponding author: [email protected] ABSTRACT In this investigation, an apatite/collagen composite coating was formed at 37°C on a NiTi shape memory alloy (SMA) through electrochemical deposition using double-strength simulated body fluid (2SBF) which contained dissolved collagen. Surface characteristics, wettability and stability of the composite coating were subsequently studied. Scanning electron microscope (SEM) examination of the surface of composite coatings revealed that many collagen fibers were embedded in apatite with flake-like structure and apatite nanocrystals nucleated and grew on collagen fibrils. Energy dispersive X-ray (EDX) spectroscopy analysis showed that the Ca : P ratio of the composite coating was about 1.35, which is close to that of octocalcium phosphate. Transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) analysis were also conducted for the composite coating. Compared to bare NiTi SMA samples, the potentiodynamic polarization curves of NiTi SMA samples with the composite coating displayed lower corrosion current density, more positive corrosion and breakdown potential, suggesting that the composite coating was chemically stable and provided corrosion resistance for NiTi SMA. INTRODUCTION NiTi shape memory alloys (SMAs) are promising metallic biomaterials for orthopaedic and dental implants [1]. However, in spite of their excellent potential, the safety and reliability of NiTi SMAs in clinical applications are still in controversy, not only because of their bioinertness but also because of the toxic Ni ion release [2, 3]. Therefore, to improve the biocompatibility and bioactivity of NiTi SMAs, many investigations have been conducted on the surface modification of these metals for their intended biomedical applications. Choi et al. coated NiTi SMA with calcium phosphate by dipping it in an oversaturated calcium phosphate solution [4]. Sun and Wang fabricated apatite/TiO2 composite coating on NiTi SMA through the H2O2-oxidation treatment and subsequent biomimetic deposition [5]. Michiardi et al. investigated a novel oxidation treatment of NiTi SMA to obtain Ni-free surface to improve the biocompatibility [6]. Compared to other surface modification techniques, electrochemical deposition, which can form a suitable coating on metal implant surface, is increasingly gaining attention owing to the ease of process control, ease of varying the coating composition, possibility of protein delivery and suitability for complex implant geometry. Synthetic hydroxyapatite (HA) is similar to the mineral component of living bones, while collagen is the major organic component of bone matrix. Compared to single component coatings of HA or other bioactive apatite, apatite/collagen composite appeared to have better interactions with ost
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