Microstructural and electrochemical characterization of hydroxyapatite-coated Ti6Al4V alloy for medical implants

Abstract

The in vitro behaviors of the etched, electrochemically anodized, and hydroxyapatite (HA)-coated Ti6Al4V alloys were investigated through microstructural analysis, electrochemical measurements, and immersion tests in the Hank’s solution. A nanometer-scale, bonelike porous structure with a layer of TiO2on top was formed during the anodization process. The surface of the coated substrate was composed of a thin TiO2layer adjacent to the substrate, a thick monolithic HA on the outside, and a composite layer of TiO2and HA in the middle. The anodization significantly improved the stability of the Ti6Al4V alloy in Hank’s solution due to a layer of TiO2formed on the surface. The precoated HA further improved the stability of the Ti6Al4V alloy due to a composite layer of TiO2and HA. The barrier layer of the composite of TiO2and HA was suggested by the capacitive behavior of the HA-coated substrate in the electrochemical impedance spectroscopy. The electrochemical measurements implied a high tendency for the new formation of HA on the precoated HA and the anodized substrates, which was confirmed through the immersion tests. The newly formed HA on the anodized substrate was scattered over the entire surface. The newly formed HA on the HA-precoated surface mingled with the precoated HA, and gradually a new layer of HA was formed on top. These proved the favorable condition of the anodized surface as a prerequisite step for coating HA and the conductive promotion of new HA formation on the precoated surface. The new formation of HA during the immersion might suggest that artificial joints pretreated through anodization and HA coating could induce strong bonding to the bone due to the easy growth of new HA.