One-step approach for nano-crystalline hydroxyapatite coating on titanium via micro-arc oxidation

Nano-crystalline hydroxyapatite (HAp) films were formed at the surface of Ti by a single-step micro-arc oxidation (MAO) using Ca²⁺ and P⁵⁺ ion-containing electrolytes. The HAp films were 10-25 μm thick, showing strong crystallinity dependence on the CaCl₂ concentration in the electrolytes. Also, the formation of an amorphous CaTiO₃ interlayer was identified to exist between the HAp and Ti substrates. In contrast to the previous researches using K₂HPO₄ for the electrolytes, KH₂PO₄ was used in this study, and this could allow the formation of the crystalline HAp layer. It is suggested as the most probable mechanism for the HAp formation that the high-density hydroxyl groups of TiO(OH)₂, formed by the reactions between the amorphous CaTiO₃ interlayer and the H⁺ ions from the dissolution of the KH₂PO₄, can play a key role in the nucleation and crystal growth of HAp by attracting Ca²⁺ and P⁵⁺ ions in the electrolytes.