Processing and Development of Nano-Scale HA coatings for Biomedical Application
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Processing and Development of Nano-Scale HA coatings for Biomedical Application Afsaneh Rabiei* and Brent Thomas** Department of Mechanical and Aerospace Engineering, **Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27695-7910, U.S.A. ABSTRACT Functionally graded Hydroxyapatite coating with graded Crystallinity across the thickness of the film has been processed and tested as a more effective orthopedic/ dental implant coating. The present study aims to increase the service-life of an orthopedic/ dental implant by creating materials that form a strong, long lasting, bond with the Ti substrate as well as juxtaposed bone. The health relatedness of the new material is to increase bonding between an implant and juxtaposed bone so that a patient who has received joint or dental replacement surgery may quickly return to a normal active lifestyle. Cross-sectional transmission electron microscopy analysis displayed that the films have a graded crystal structure with the crystalline layer near the substrate and the amorphous layer at the top surface. Compositional analysis was performed using SEM-EDX at the top surface as well as STEM-EDX at the cross section of the film. The average calcium to phosphorous ratio at the surface is 1.46, obtained by SEM-EDX. The Ca/P ratios in the crystalline and amorphous layers of the film are 1.6 to 1.7, close to the ratio of 1.67 for HA. INTRODUCTION In order to improve the bioactivity of metal implants, Hydroxyapatite (HA: Ca10(PO4) (OH) 6 2) or is generally applied as a coating to the metal substrate [1]. This will cause a direct bonding and fast stabilization of implant because of HA’s similarity with the inorganic components of human bone [2-4]. The bond strength of a coating layer with the metal substrate is an important factor to be considered seriously. Particulate debris at the bone prostheses interface with HA coated implants has been found to cause a foreign body response that is destructive to the surrounding tissues [5]. Dissolution rate of the implant must also be compatible with the rate of bone growth or otherwise the coating is not effective. The dissolution rate of crystalline HA has been observed to be very low, while that of the amorphous phase HA is considerably high [1]. Various techniques have been used to deposit calcium phosphate on metal substrates such as sputtering, electron beam deposition, laser deposition, and plasma spraying [6- 19]. So far, plasma spraying is the only technique being commercialized and most widely used because of its simplicity and cost effectiveness. Regardless of the processing technique, the amorphous HA generally has a high dissolution rate in aqueous solutions. Therefore, the HA coatings need to be subsequently heat-treated in order to convert the as deposited amorphous phase into a crystalline phase [16- 19 and 20]. However, the heat treatment increases the processing time and cost and can even cause residual stresses in the HA coating due to a thermal expansion mismatch between
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