Nanostructured Hydroxyapatite Coatings for Improved Adhesion and Corrosion Resistance for Medical Implants
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Nanostructured Hydroxyapatite Coatings for Improved Adhesion and Corrosion Resistance for Medical Implants Zongtao Zhang, Matthew F. Dunn, T. D. Xiao Inframat Corporation, 74 Batterson Park Road, CT 06032 Antoni P. Tomsia and E. Saiz MS 62-203, Lawrence Berkeley Lab, Berkeley, CA 94720 Abstract: Hydroxyapetite (HA) coating on medical implant has been used in commercial application for several decades. The coating, commercially made by thermal spray method, functions as a intermediate layer between human tissues and the metal implant. The coating can speed up early stage healing after operation but the life span is much limited by low interfacial bond strength, which comes from the dissolution of amorphous HA in human body fluid during its service. This amorphous phase is formed in coating process under high temperature. To overcome these problems, we have developed a novel room temperature electrophoretic deposition process to fabricate nanostructured HA coating. This nanostructured HA coating significantly improved coating’s bond strength up to 50-60 MPa, 2-3 times better than the thermal sprayed HA coating. The nanostructured HA coating also has corrosion resistance 50100 times higher than the conventional HA coating. X-ray diffraction shows that all the HA coating is fully crystalline phase. It is expected that the implants with the nanostructured HA coating will have much longer service life. Other benefits derived from this process include room temperature deposition, the ability to control the coating microstructure and phases, and low cost for production. INTRODUCTION The field of hydroxyapatite (“HA”) coatings is growing very rapidly. Since the 1980’s, titanium implants typically have been coated with HAP using thermal spray techniques. There are two major shortcomings associated with plasma-sprayed HAP coatings. First, due to the high temperature plasma (≥15,000oC), a large fraction of crystalline HAP turns amorphous while in transit. This phase is soluble in body fluids and results in subsequent dissolution of the material during operation. Second, while thermal spray coatings in general are characterized by strong mechanical bonds, on a relative scale, this type of bond is weak in comparison to metallurgical or chemical bonds. In the 1990’s, a chemical precipitation method was used for coating porous implants. This process operated at a temperature lower than 100oC, overcoming the HA dissolution problem, but the bonding between the HA coating and substrate interface is very week. In fact, the poor adhesion of HA coatings to titanium substrates is the single most troublesome aspect of today’s technologies in relation to the viability of commercial implantable products [1]. Free-standing crystalline nanostructured HA (“n-HA”) has been synthesized by Ying et al. [2] using a wet chemical synthesis technique. Note that this material has not been fabricated as a coating. Powder agglomerates were consolidated into bulk form for testing of mechanical properties such as fracture toughness, and bend and compressiv
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