A multilayer material consisting of hydroxyapatite (HAp) and calcium titanate (CaTiO
3) was hydrothermally synthesized on a titanium substrate. The titanium substrate was treated with a mixed solution of CaCl
2 and KOH at 150°C to form a CaTiO
3 layer. The thickness of the CaTiO
3 layer increased with increasing KOH concentration, but the adhesion between the coating layer and the substrate diminished. In an alkaline solution such as KOH, titanium substrates dissolved and reacted with calcium ions, and a CaTiO
3 layer precipitated on the substrate. A layer of HAp formed on the CaTiO
3 coating layer from the mixed solution containing CaCl
2, K
2HPO
4 and KOH, when the molar ratio of Ca/P was less than 1.67, which was the ratio of stoichiometric HAp. Calcium was supplied from both the reacting solution and the CaTiO
3 layer, and was precipitated as HAp. Decomposition of a part of CaTiO
3, and the consequent supply of Ca from the coating layer were necessary to form the HAp layer. The multilayer formed without peeling, when HAp was precipitated on the CaTiO
3 layer at 220°C for 24h with a pH of 8-10. The adhesion force between the substrate and the CaTiO
3 layer did not decrease after the formation of HAp, though the force decreased with increasing KOH concentration during synthesis of the CaTiO
3 layer. After soaking the HAp/CaTiO
3 multilayer in simulated body fluid (SBF) for 35 d at 37°C, the multilayer was covered with very fine needlelike particles of HAp, though neither the surface of the CaTiO
3 coating layer nor the titanium substrate were altered after the same treatment.
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