Biomimetic apatite coatings—Carbonate substitution and preferred growth orientation

Biomimetic apatite coatings were obtained by soaking chemically treated titanium in SBF with different HCO₃⁻ concentration. XRD, FTIR and Raman analyses were used to characterize phase composition and degree of carbonate substitution. The microstructure, elemental composition and preferred alignment of biomimetically precipitated crystallites were characterized by cross-sectional TEM analyses. According to XRD, the phase composition of precipitated coatings on chemically pre-treated titanium after exposure to SBF was identified as hydroxy carbonated apatite (HCA). A preferred c-axis orientation of the deposited crystals can be supposed due to the high relative peak intensities of the (0 0 2) diffraction line at 2θ = 26° compared to the 100% intensity peak of the (2 1 1) plane at 2θ = 32°. The crystallite size in direction of the c-axis of HCA decreased from 26 nm in SBF5 with a HCO₃⁻ concentration of 5 mmol/l to 19 nm in SBF27 with a HCO₃⁻ concentration of 27 mmol/l. Cross-sectional TEM analyses revealed that all distances correspond exactly to the hexagonal structure of hydroxyapatite. The HCO₃⁻ content in SBF also influences the composition of precipitated calcium phosphates. Biomimetic apatites were shown to have a general formula of Ca_10−x−yMg_y(HPO₄)_x−z(CO₃)_z(PO₄)_6−x(OH)_2−x−w(CO₃)_w/2. According to FTIR and Raman analyses, it can be supposed that as long as the HCO₃⁻ concentration in the testing solutions is below 20 mmol/l, only B-type HCA (0 < z < 1; $w=0$) precipitates. At higher HCO₃⁻ concentration, it can be assumed that AB-type HCA (z = 1;$0<w<1$) is formed.