Structure design, preparation and performance of a novel composite coating on medical magnesium-zinc alloy

Magnesium alloy is a promising implant material, but its inherent defects limit its clinical application. In this article, the CaP pre-coating and polycaprolactone (PCL) composite coatings doped with different hydroxyapatite (HA) and zinc oxide (ZnO) nanoparticles were successively prepared on the surface of Mg6Zn alloy using hydrothermal method and dip coating technique. Results show that ZnO compounds was formed in the CaP pre-coating on the solution treated Mg6Zn alloy, which can enhance the bonding strength of CaP compounds with alloy substrate. Comparing with the PCL coating on bare alloy, the bond strength of the PCL coating on CaP pre-coating was increased by about 5 times. Therefore, the preparation of CaP pre-coating is necessary because it can help to improve the bonding strength of the PCL coating on the substrate, and effectively avoid the peeling, blistering and loss of the barrier effect of the PCL coating. The nanoparticles in the composite coating play important roles for fabricating multifunctional coating. Firstly, doping HA and ZnO nanoparticles help to increase surface roughness of the composite coating, which is beneficial to cell attachment and proliferation. Secondly, the formation of porous structure in the composite coating enable it the ability to control the degradation rate of the matrix by adjusting the porosity. Thirdly, the composite coating doped with ZnO nanoparticles brings better antibacterial properties for magnesium alloy implants. The alloy coated with CaP pre-coating and (2wt.%nHA + 2wt.%nZnO)/PCL composite coating has antibacterial rate of over 90 % and cell proliferation rate of about 90 %, demonstrating it can be used as a potential multifunctional medical implant material.