Assessment of phase stability and in vitro biological properties of hydroxyapatite coatings composed of hexagonal rods

Abstract We describe fabrication of hydroxyapatite coatings consisting of micrometric hexagonal crystals on titanium substrate. The aim was to evaluate their phase stability and in vitro performance. We employed a facile hydrothermal method assisted with a chelating agent and monoethanolamine as regulators of the nucleation and crystal growth. After the hydrothermal reaction, the hydroxyapatite coatings were sintered at different temperatures up to 950 °C. We performed in-depth investigations into the coating surface morphology, topography, wettability, and chemical composition using scanning electron microscopy, profilometry, sessile drop technique, x-ray diffraction and Raman spectroscopy. We assessed the bioactivity of hydroxyapatite structure sintered at different temperatures after incubation in the SBF solution. Simultaneously, we conducted a series of in vitro experiments based on MTT assay using MC3T3 mouse preosteoblast cell line to study the cytocompatibility of synthesized specimens. The results show that the thermal processing of hydroxyapatite coatings leads to their thermal decomposition to β-TCP phase already at 700 °C. As the heat treatment progresses, the edges of hexagonal crystals are melting and smoothing. We show that while as-prepared hydroxyapatite coating exhibits remarkable bioactivity after just 3 days, the sintered samples are not bioactive at all. Biological examination with MC3T3 cells indicates good compatibility of all coatings, with cell proliferation being enhanced on samples heated at higher temperatures. In conclusion, the heat treated HAp coatings with different morphologies and different HAp/β-TCP phase ratios influence the in vitro bioactivity and response of preosteoblast cells.