Electrophoretic deposition of hydroxyaptite on homogenized magnesium based alloy for biomedical applications

Abstract

Comparing with cobalt alloys, titanium alloys and stainless steels, magnesium (Mg) alloys are promising biodegradable implant materials. However, Mg alloys suffer fast degradation rate and lose the structural integrity before the healing process ends. Among the phase composition of Mg alloy microstructure is beta (P) or intermetallic phase, which is nobler than the alpha phase and it can worsen the degradation conduct of Mg alloys due to its galvanic effects. The aim of this study was to evaluate the effects of P phase on the degradation rate which hinders the clinical application of Mg AZ91 alloy. The influence of P phase and the hydroxyapatite powders were employed to slow down the initial degradation rate of Mg AZ91 alloy. Sample pieces were cut from Mg grade AZ91 alloy ingot and milled into 10mm x 10mm x 3mm. The samples were prepared and divided into two groups. The first part was classified as as-received samples (sample A) while the remaining ones were processed for homogenization heat treatment to reduce the P phase. Homogenization heat treatment was performed on samples at different temperatures and times of 350oC/10h, 370oC/10h, 390oC/5h, 390oC/10h, 410oC/5h, 410oC/10h, 430oC/5h and 430oC/10h, and named as sample B. Both samples A and B were ground using 400 to 4000 grit silicon carbide paper and polished using alumina until mirror like surface was achieved. They were subsequently etched in 2% nital for 10s. Then, these samples were characterized using optical microscope, X-ray diffractometer (XRD) and field emission scanning electron microscope equipped with energy dispersive X-ray spectroscope (FESEM-EDX). Microhardness test was carried out on AZ91 samples with the aid of Vickers’ microhardness tester. Thereafter, the hydroxyapatite powders were synthesized using a simple wet chemical precipitation technique and characterized by using FESEMEDX, XRD, transmission electron microscope and Fourier transform infrared spectroscopy. The stability of hydroxyapatite colloidal suspensions was analyzed by using Zeta potential measurement. The hydroxyapatite powders were deposited on sample B via electrophoretic deposition technique at different voltages and times of 10, 20, 30, 40, 50 and 60V, and 180, 300, 450, 600, 750 and 900s, respectively. The uncoated and coated AZ91samples were characterized by using FESEM-EDX and XRD. The immersion evaluation of coated samples was investigated in stimulated body fluid at a room temperature. Electrochemical potentiodynamic polarization tests were performed to measure the degradation rate of uncoated, homogenized and coated samples. The results showed that the higher the homogenization heat temperature or the longer the dwelling time, the better the homogenous distribution of the large beta particles over the microstructure. Also, the Vickers’ microhardness results revealed improvement in mechanical strength from sample A (75.35HV), sample B (92.18HV) to coated samples (140.18HV). Significant drops in corrosion current density and degradation rate were recorded from sample A, sample B and coated samples of 64.7, 37.9, 0.015 |iA/cm2 and 1.421, 8.35 x 10-1, 3.73 x 10-4 mm per year, respectively; which implied a good level of corrosion protective behavior. In addition, in-vitro evaluation revealed a high bioreactivity within the stimulated body fluid and the formation of flower-like layers, which could slow down the degradation rate of Mg AZ91 alloy. Thus, a substantial improvement in mechanical strength, degradation rate and success in in-vitro bioactivity of coated samples were achieved.