Hydroxyapatite Coatings Incorporating Silicon Ion Releasing System on Titanium Prepared Using Water Glass and Vaterite

A new type of bioactive coating with the ability to release soluble silica species was prepared on a titanium substrate surface using water glass and vaterite (CaCO3) particles and precipitating hydroxycarbonate apatite (HCA) from solution. Silica was incorporated because soluble silica and calcium ions have been reported to stimulate osteogenic cells to produce new bone matrix. Water glass was coated on the substrate surface and then heated at 300°C for 2 h and then soaked in 0.1M HCl aqueous solution at 50°C for 120 min, resulting in the formation of silica layer on the surface. One of vaterite, calcite, aragonite, or CaCl2 was coated on the silica layer to enhance HCA formation on the surface in modified simulated body fluid (1.5 SBF). Vaterite was found to encourage the most rapid HCA formation. The titanium substrates coated with silica and HCA layers, continuously released about 53 mM of silicon species a day. The HCA layer contained a trace amount of silicon, which was expected to be released from the silica layer and trapped in the layer during 1.5 SBF-soaking. Osteoblast-like cells (MC3T3-E1) proliferated better on the substrate releasing soluble silica than on an HCA layer on titanium containing no silica.