Formation and in Vivo Evaluation of Carbonate Apatite and Carbonate Apatite/CaCO3 Composite Films Using the Thermal Substrate Method in Aqueous Solution

We have studied the formation and carried out an in vivo evaluation of carbonate apatite (CO3-Ap) and CO3-Ap/CaCO3 composite coatings on titanium substrates using the thermal substrate method. The coatings were formed on commercial pure titanium rods (diameter=2 mm, length=5 mm) and plates (thickness=0.3 mm) by the thermal substrate method in an aqueous solution that contained Ca(H2PO4)2, CaCl2, and NaHCO3. The coating experiments were conducted at 40-140°C and pH=8 for periods of 15 or 30 min. The coating temperature and NaHCO3 of the solution had a significant influence on the surface morphology (net-like, plate-like, needle-like, or sphere-like), the phase (single phase of CO3-Ap or binary phase of CO3-Ap and CaCO3), and the carbonate content in the precipitated films. A subsequent autoclave treatment also had an effect on the films. A coated rod was implanted in a 10 weeks old male rat's tibia with a non-coated titanium rod being used as a control. The constructs were retrieved after a period of 14 d postimplantation and examined for new bone formation and for tissue response in the cancellous and cortical bone parts, respectively. Single-phase sphere-like CO3-Ap had high osteoconductivity in the cortical bone region, and this increased with increasing carbonate content in the films. However, the osteoconductivity of the CO3-Ap/CaCO3 composite coatings decreased with increasing total carbonate content.