Electrochemical deposition of bioactive coatings on Ti and Ti–6Al–4V surfaces

Passivating coatings of brushite (CaHPO4·2H2O) were obtained on Ti and Ti–6Al–4V ELI alloy substrates by cathodic polarization. After soaking in Ringer's solution for 48 h brushite was transformed to hydroxyapatite (Ca10(PO4)6(OH)2) as confirmed by XRD, FT-IR and Raman spectroscopy. Electrochemical cyclic polarization curves of the coated biomaterials obtained in Ringer's solution at pH values of 7.1 and 8.91 as well as in Hank's Balanced Salt Solution (HBSS) at pH value of 7.4 show a nobler behavior than of the uncoated biomaterials. The coated biomaterials had lower corrosion rates than the uncoated biomaterials suggesting a protective character of the hydroxyapatite coating. Electrochemical impedance spectra (EIS) revealed capacitive behavior, owing to the protective, very resistant layer, the thickness of which increased with soaking time. The coated biomaterials presented higher electropositive open circuit potentials compared to the uncoated biomaterials as result of the protective effect of the coating. The morphology of the coatings changed with soaking time as the coatings became denser, smoother and better adhering. Hence such coatings may provide favorable structure for cell adhesion and proliferation.

Research Highlights
► X-ray diffraction confirms formation of primary brushite coating by cathodic electrodeposition.
► After 48 h soaking in Ringer's solution typical reflections of hydroxyapatite were identified.
► Raman spectra of the final coating revealed a single peak assigned to the hydroxyapatite.
► Electrochemical parameters had more favorable values for coated than for uncoated biomaterials.
► Polarization resistances increased with time owing to gradual thickening of the deposited HA coatings.