Preparation And Characterization Of Dopamine-induced Biomimetic Hydroxyapatite Coatings On The AZ31 Magnesium Alloy

Mg alloys have great potential in biodegradable orthopedic implant applications owing to their rapid degradation in physiological environments and mechanical properties similar to those of bone. However, the high susceptibility to corrosion of Mg alloys in physiological environments and the release of hydrogen gas during corrosion resulted in the loss of mechanical integrity long before the expected service life and a rapid increase in the pH value, which seriously restricted their medical applications.Objective: In this work, a hydroxyapatite(HA) coating was prepared by polydopamine(PDA) induced biomimetic mineralization in a CaP solution to improve the in vitro corrosion resistance and biocompatibility of the AZ31 Mg alloy.Methods: The evolution of the phase structure, chemical composition, and surface morphology of the modified AZ31 Mg alloy substrate was evaluated by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The corrosion resistance of the specimens was evaluated using potentiodynamic polarization(PDP) and H2 evolution testing. Moreover, the cytotoxicity of pristine and modified AZ31 samples was evaluated by examining both the viability and morphology of L-929 fibroblasts.Results: The surface of the PDA/HA coated AZ31 substrate is characterized by flower-like arrays which comprised porous, homogenous flake-like crystals, the thicknesses of the coatings was approximately 20 μm, and the composite coatings were well crystallized, which had no obvious dividing line between the coatings and substrate on the PDA/HA coated AZ31.Potentiodynamic polarization and hydrogen evolution tests demonstrated that the Ecorr value(-1.275 V vs. Ag/AgCl) of AZ31/PDA/HA was the highest and its icorr value(1.82 × 10-6 A/cm2) was the lowest. Compared with the pristine AZ31, Ecorr for AZ31/PDA/HA positively shifted about 200 mV and the Icorr value decreased 25-fold. The accumulated H2 evolution volume of AZ31/PDA/HA decreased to about 5.4 ml, being the least of all magnesium alloy samples.Cytotoxicity testing and cell morphology analysis showed that the L-929 cell viability cultured with the extracts of AZ31, AZ31/PDA, and AZ31/PDA/HA for 1–7 d were all larger than 80%. Cell morphologies in the extracts of the coated samples were normal and healthy, similar to that of the cells with elongated polygonal morphology incubated in the culture media.Conclusion: Uniform HA coatings on biodegradable AZ31 magnesium alloys were successfully achieved through a biomimetic strategy with the assistance of mussel-inspired PDA. The corrosion resistance and biocompatibility of AZ31 had been effectively improved after being coated with PDA or PDA/HA, which providing a basis for clinical applications of biomedical Mg alloys.