Development of Nanolayer Hydroxyapatite (HA) on Titanium Alloy via Superplastic Deformation Method
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TRODUCTION
TITANIUM alloys are commonplace materials for load-bearing orthopedic implants because of their superior biocompatibility,[1] corrosion resistance, and interesting specific mass compared with other high-strength alloys such as steels.[2] Similar to other metallic implants, titanium is bioinert. Hence, the potential uses of titanium in biomedical applications are limited because of ineffective osteointegration with the host tissue.[3–5] Hydroxyapatite (HA) bioceramic is a bone-bonding material that has been employed in numerous prosthetic applications because of its osteoconductive properties and ability to bond chemically to living bone.[1,3,6] Although beneficial, HA possesses disadvantages such that its bending strength and fracture toughness are inferior to that of human bones. Hence, HA can be used only for applications in which an insignificant magnitude of stress needs to be borne.[6,7] Attempts to overcome these disadvantages have been made by using HA as a surface coating on bioinert metallic substrates such as titanium, Ti-6Al-4V, and stainless steel 3I6L.[8] An alternative solution is to develop a composite composed of Ti powder and HA. The coating has been applied extensively for metallic prostheses with the aim of improving bone apposition[9] and implant fixation, well as reducing healing time.[10] Numerous methods of surface coating techniques have been established to deposit HA-based coatings. These methods range from the conventional press-andsinter method to more elegant approaches such as sputtering,[11] electrophoretic deposition,[9] and plasma HIDAYAH MOHD KHALID, PhD Candidate, ISWADI JAUHARI and ADIBAH HANEEM MOHAMAD DOM, Lecturers, are with the Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, 50603 Malaysia. Contact e-mail: imanhidayah@ gmail.com Manuscript submitted October 20, 2011. Article published online May 16, 2012 3776—VOLUME 43A, OCTOBER 2012
spraying.[12] Regardless of the coating methodology, the separation or loss of HA coatings is progressive with time in vivo, which may cause cell-mediated osteolysis and subsequent implant loosening.[13] In our previous study on the implantation of HA into superplastic Ti-6Al-4V, we were successful in producing a micrometer-thickness HA layer on the titanium substrate. The process was conducted through continuous pressing of the superplastic substrate onto HA powder under an initial pressure of 60.3 MPa at a specific range of temperature and time. The wear test results of the implanted layer showed that the reduction in thickness of the implanted layer is more evident in thicker implanted layers. It was described that the thicker layer would experience a higher bending moment along its thickness compared with the thinner layer one, during the wear tests under a fixed compression pressure. Unless the layer can resist the moment of force acting along it, the thicker layer would easily be worn out.[14] The observation was supported by previous research, which indicates that thin HA coatings (~2 lm) have a significantly gr
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