Hydrothermal synthesis of hydroxyapatite coatings with oriented nanorod arrays
Hydroxyapatite coatings (HACs) with oriented nanorod arrays have been fabricated according to the following stages: (i) sol–gel preparation of bioglass coatings (BGCs) on Ti6Al4V substrates; and (ii) transformation of the BGCs to the HACs in a simulated body fluid (SBF) under hydrothermal conditions. After soaking the BGCs in SBF for 12 h under hydrothermal conditions, the elongated HA rods deposit on the surfaces via a dissolution–precipitation reaction. The TEM image and corresponding ED pattern indicate that the HA rods in the HACs are single crystals with a preferential (002) orientation. With increasing the reaction time to 2 days, most of the HA rods are converted to HA particles due to Ostwald ripening. If SBF is replaced by deionized water, the obtained coatings are rod-like HA with the poor crystal orientation. Beside the reaction time and reaction medium, the reaction conditions play an important role in the morphologies of the HACs. Although a HA layer deposits also on the BGCs after soaking in SBF at 37 °C, the HA crystals exhibit plate-like structure. SBF immersion tests and biocompatibility tests by using human bone marrow stromal cells (hBMSCs) as cell models indicate that the HACs with oriented nanorod arrays exhibit great in vitro bioactivity and biocompatibility. The oriented HA rods on the coatings can not only promote the formation of bone-like apatite, but also improve the adhesion, spreading and proliferation of hBMSCs. The excellent in vitro bioactivity and biocompatibility suggest that the HACs with oriented nanorod arrays have great potential for bone implants.