Preparation of SBF with different HCO3- content and its influence on the composition of biomimetic apatites.

The bioactivity of bone and dental implant materials is usually tested in vitro using simulated body fluid (SBF). The composition of common SBF differs from that of blood plasma in that it has a higher Cl- and a lower HCO3- concentration, which affects the composition of in vitro formed bone-like apatite. Five different SBFs with a composition of 142 Na+, 5 K+, 2.5 Ca2+, 1 Mg2+, 1SO4(2-), 1HPO4(2-), and 136 (Cl-+HCO3-) mmol/l were prepared with HCO3- concentrations ranging from 5 to 27 mmol/l. The SBF solutions were prepared by mixing stable concentrated solutions, which increase the reproducibility of in vitro tests due to negligible changes of pH during preparation. The high stability of thus prepared SBF enables the evaluation of hydroxyapatite formation on the surface of bioactive materials without the negative effect of spontaneous precipitation. Furthermore, the use of concentrated solutions offers a facile way to prepare SBF with different ionic contents and thus modify the composition of Ca-P layers precipitated on the surface of the bioactive materials exposed to the SBF solutions. The SBF solutions were shown to be supersaturated with respect to slightly carbonated apatite. The Fourier transform infrared (FT-IR), Raman and X-ray analyses of the precipitated layers indicate that the HCO3- content in SBF influences the composition and structure of the calcium phosphates obtained. It can be supposed that as long as the HCO3- concentration in the testing solutions is lower than 20 mmol/l, only B-type HCA precipitates. At higher HCO3- concentrations, it can be assumed that A-type HCA forms as well considering FT-IR, Raman and X-ray measurements.