A Simple Method to Functionalize the Surface of Plasma Electrolytic Oxidation Produced TiO₂ Coatings for Growing Hydroxyapatite

Highlights: • TiO₂ coatings with porous surfaces were produced by plasma electrolytic oxidation. • Simple pre-immersion in K₂HPO₄ could functionalize the surfaces of the TiO₂. • Such pre-immersion enhanced substantially the growth of hydroxyapatite in SBF. • Growth mechanisms of hydroxyapatite via the pre-immersion have been proposed. • MTT assay shows great osteoblast-like cell activity on the obtained hydroxyapatite. - Abstract: Conventionally, hydrothermal treatment was often used to modify the TiO₂ surface prior to the growth of hydroxyapatite (HA) that is one of the most important implant biomaterials. In this work, a simple pre-immersion of the obtained TiO₂ in a weak base, instead of the conventionally high pressure-temperature hydrothermal pre-treatment, was conducted prior to the growth of HA. Firstly, anatase TiO₂ coatings with porous surfaces were produced by plasma electrolytic oxidation with optimized processing parameters. X-ray diffraction patterns and field-emission microscopy reveal that the anatase TiO₂ films with porous surfaces were produced by plasma electrolytic oxidation. Subsequently, the films were pre-immersed in 0.1–2 M K₂HPO₄ solutions for only 10 min. Fourier transform infrared spectroscopy shows that the −OH functional groups were generated after such pre-immersion, which could enhance significantly the growth of a single phase of HA in simulated body fluid (SBF). Growth mechanisms of HA via the pre-immersion treatment and soaking in SBF have been proposed. Moreover, the proliferation rate and attachment of the MG-63 osteoblast cells were greatly enhanced on the obtained HA compared to that without the immersion pre-treatment from the MTT assay and morphology analyses. This simple immersion pre-treatment evidently provides an easy route for the growth of HA and has great potential for biomedical applications.