Biomimetic calcium phosphate coatings on Ti6Al4V: a crystal growth study of octacalcium phosphate and inhibition by Mg2+ and HCO3−

The biomimetic approach for coating metal implants allows the deposition of new calcium phosphate (Ca-P) phases. Films elaborated at physiological conditions might have structures closer to bone mineral than hydroxyl-apatite (HA) plasma-sprayed coatings. In this study, different Ca-P coatings have been deposited through a two-step procedure. After cleaning and etching, Ti6Al4V plates were pretreated by soaking in a simulated body fluid (SBF), i.e., a solution containing inorganic components in concentration more or less similar to body fluids: a thin amorphous carbonated Ca-P layer precipitated on the metal substrate. Second, by soaking these thinly coated metal substrates in another SBF, with different concentrations, the thin amorphous carbonated Ca-P layer led to the fast precipitation of a second and thick Ca-P layer. Different SBF solutions were used in order to investigate the influence of magnesium and carbonate ions. From SBF containing only Ca2+ and HPO42− ions, an octacalcium phosphate layer grew epitaxially on the substrate. When Mg2+ was added into this SBF, the coating was composed of Ca-deficient apatite crystals, while the addition of HCO3− in SBF led to the formation of a B-carbonated apatite layer. Magnesium and carbonate acted as inhibitors of crystal growth. The three phases obtained by our biomimetic process are closer to bone mineral structure than plasma-sprayed HA. Therefore, the obtained results may be particularly relevant for the development of biomimetic Ca-P coatings with optimal bioactivity.