Electrochemical properties and bioactivity of hydroxyapatite coatings prepared by MEA/EDTA double-regulated hydrothermal synthesis

In this study, we develop hydroxyapatite coatings that consist of micrometric hexagonal crystals grown on the Ti/TiO2 substrate. The coatings are synthesized by a hydrothermal method. The advance of our hydrothermal approach lies in the concomitant use of two reagents: ethylenediamine tetraacetic acid and monoethanolamine. The phase identification, surface morphology, and elemental composition of structures are examined by x-ray diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersive x-ray spectroscopy. We also assess the in vitro bioactivity of hydroxyapatite structure and uncoated materials after incubation in the SBF solution. Simultaneously, we conduct a series of experiments to investigate electrochemical properties of titanium with and without hydroxyapatite by the means of electrochemical impedance and potentiodynamic polarization experiments in the Ringer's solution. Our results show that monoethanolamine-assisted hydrothermal method is an efficient and promising approach to obtain hydroxyapatite coatings with excellent crystal quality and Ca/P ratio close to the stoichiometric value of the Ca10(PO4)6(OH)2 phase. The SBF immersion tests indicate high bioactivity of hydroxyapatite coating after 7 days of incubation. However, the hydroxyapatite coating does not improve the corrosion behavior of the metallic titanium substrate. Electrochemical studies of all structures show the highest corrosion resistance for the Ti/TiO2 surface as compared to other samples. The lowest value of corrosion current density (jcor = 0.65 ± 0.19 nA cm−2) is found for the Ti/TiO2 surface. This result could be attributed to the porous morphology of the hydroxyapatite layer which could favor direct flow of electrolyte between the corrosive medium and the titanium substrate or to the adverse effect of hydrothermal synthesis on the barrier properties of the TiO2 intermediate layer.