Electrochemical Hydroxyapatite Coatings on Nitinol Stents for the Reduction of Metal Ions Elution
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Electrochemical Hydroxyapatite Coatings on Nitinol Stents for the Reduction of Metal Ions Elution Daisuke Kondo1, Tomohiko Yoshioka1, Toshiyuki Ikoma1, Kensuke Takamatsu2, Kunihiro Ohta2 and Junzo Tanaka1 1 Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan 2 Tamachi Industries Co. Ltd., 4-10-2 Minami-ooi, Shinagawa-ku, Tokyo 140-0013, Japan
ABSTRACT Nitinol was coated with biocompatible calcium phosphate materials by pulsed electrolytic deposition (ELD) to reduce toxic metal-ions elution. The pulse ELD for the stents was carried out with changing the current off-periods (toff) of the pulse wave. The pulse ELD suppressed the generation of H2 gas due to the electrolysis of water on a calcium phosphate layer and improved the adhesiveness of the coating layer on nitinol compared with a conventional DC-ELD. The coating layers were identified to be octacalcium phosphate (OCP) at lower toff, while they were transformed to dicalcium phosphate anhydraous (DCPA) with an increase of toff. The layers of OCP or DCPA on the nitinol surface were subjected to a NaOH treatment at 60ºC for 3days to transform them into hydroxyapatite (HAp). From results of a metal-ions elution test, the deposited calcium phosphates suppressed nickel ions elution at one quarter compared with the bare nitinol stent. These results indicate that the pulse ELD of biocompatible calcium phosphate materials on the nitinol stent was one of the best techniques to create firmly attached coating on it and reduce toxic nickel ions elution. INTRODUCTION Nitinol (nickel and titanium alloy) is one of the best possible candidates for shape memory related applications, such as coronary, biliary and esophageal stents with self-expanding property [1]. However, the nickel ions elution from the stent causes allergic reaction [2]. Permanent polymers such as polyethylene-co-vinyl acetate and poly n-butyl methacrylate have been utilized for a surface coating to prevent the nickel ions elution; however the permanent polymers could cause inflammation and restenosis [3-5]. Therefore, it is necessary to improve biocompatibility of coatings. In this study, hydroxyapatite (Ca10(PO4)6OH2 :HAp) was coated onto the nitinol surface by electrolytic deposition (ELD). HAp is well known as a typically biocompatible ceramic material and ELD is an attractive method to create calcium phosphate coatings onto complex shaped substrates. Reactions of ELD of calcium phosphate are indicated as follows; (1) electrolysis of water, (2-3) decomposition of phosphoric acid and (4-6) precipitation of calcium phosphates [6-8]. 2H2O + 2e- → H2 + 2OHOH- + H2PO4-→HPO42- + H2O OH- + HPO42-→PO43- + H2O Ca2+ + HPO42-→CaHPO4 8Ca2+ + 2HPO42- + 4PO43- + 5H2O →Ca8(HPO42-)2(PO43-)4 ∙ 5H2O 10Ca2+ + 6PO43- + 2OH- →Ca10 (PO4)6(OH)2
(1) (2) (3) (4) (5) (6)
Although Ban et al. has already described calcium phosphates depositiosn on various metal substrates by ELD and effects of substrates on the formation of calcium phosphates, there are few
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