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HYDROXYAPATITE (Ca10(PO4)6(OH)2; hereafter, HA) ceramic and other related calcium phosphate ceramics are mainly used in the field of bioceramics, such as for artificial bones and dental implant surgery, due to their identical chemical composition and high biocompatibility with natural bones. Hence, these materials have received considerable attention over the past three decades as implant materials.[1,2] HA is also frequently used as a drug carrier, ocular implant, scaffold matrix for tissue engineering, and coating agent on biomedical implants.[3,4] However, applications of HA

HUI-TING CHEN, Associate Professor, and MOO-CHIN WANG, Professor, are with the Department of Fragrance and Cosmetic Science, Kaohsiung Medical University, Kaohsiung 80782, Taiwan, R.O.C. Contact e-mail: [email protected] KUO-MING CHANG and SZU-HAO WANG, Professors, are with the Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80782, Taiwan, R.O.C. WEI-JEN SHIH, Project Manager, is with the Metal Industries Research and Development Center, Kaohsiung 81160, Taiwan, R.O.C. WONG-LONG LI, Professor, is with the Department of Materials Science and Engineering and with the Institute of Nanotechnology and Microsystems Engineering, National Cheng Kung University, Tainan 70101, Taiwan, R.O.C. Manuscript submitted March 2, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A

are currently limited to powders, coatings, porous bodies, and nonbearing implants due to processing difficulties and the poor mechanical properties of conventional HA.[5] HA ceramic was coated on the implant substrate alloys to have high strength, good process ability, suitable specific density, and excellent corrosion resistance in the living body. Recently, many methods such as electrochemical methods,[6–12] electrophoretic methods,[13,14] and plasma spraying[15–17] have been underdeveloped. Although the most widely applied HAcoating procedure is the plasma spray process,[15–17] the major problem of the phase transition and decomposition of HA still exists in spray-coating process. To avoid these problems, the application of HA coating using electrochemical deposition methods has attracted considerable attention.[6–12] The coating structure and composition controls are possible because of the relatively low processing temperature. Highly irregular objects can be coated relatively quickly. However, electrochemical deposition can result in a pH increase at the interface, which is attributed to electron incorporation to form OH ions and H2 through water reduction. H2 gas evolution at the interface leads to a heterogeneous coating.[15] Zhitomirsky[14] pointed out that more homogeneous coating is obtained using organic solutions to avoid the negative effects of H2

gas. Moreover, Wang et al.[7] also reported that when calcium phosphate coating is deposited on a Ti-6Al-4V substrate using an electrolytic process under 1.07 9 102 MPa, in this pressure environment the bubbles quickly lift up from the cathode surface, making the deposit regular