Synthesis and characterization of sol–gel hydroxyapatite coatings deposited on porous NiTi alloys

A hydroxyapatite (HA) coating was deposited onto a porous NiTi alloy via dip-coating using a sol–gel procedure with triethyl phosphite and calcium nitrate as phosphorus and calcium precursors, respectively. Adjusting the concentration and viscosity of the sol as well as changing the spin-coating rotational velocity or dip-coating times, enabled uniform coatings with controllable thickness at the sub-micron scale to be successfully deposited on the external surface and within the pores of the porous NiTi alloy. Cross-sectional SEM analysis and EDS characterization of the HA films show that the coating on the inner surface of the pores is thicker than that on the outer surface. The results of an immersion test in a Tris solution show that the HA coating possesses excellent stability, and the rates of Ni ion release through the HA coatings on the porous NiTi alloys of different porosity ratios in a simulated body fluid decrease markedly compared with the uncoated alloys. There is also a remarkable increase in the apatite forming ability of the HA coated porous NiTi alloy in a calcium containing solution.

Research highlights
► A uniform HA coating layer with a thickness at a sub-micron scale has been deposited not only on the surface of the porous NiTi alloy but also inside the pores without blocking the pores via the sol–gel route.
► The sol–gel derived HA coating on porous NiTi alloy shows good stability in Tris solution and much reduced rate of Ni ion release in SBF.
► There is a remarkable increase in the apatite forming ability of the HA coated porous NiTi alloy in FCS and the inner surface of porous NiTi alloy has an increased apatite deposition rate due to the rough surface.
► The sol–gel method provides an effective way to produce the HA coating on porous NiTi alloys with the enhanced apatite forming ability beneficial for biomedical applications.