Theoretical study of Tris and Bistris effects on simulated body fluids

Simulated body fluids - SBF solutions are useful to evaluate bioactivity as well as to change the surface response of materials not considered fully bioactive. Several simulated body fluids produce similar results during precipitation stage, and an attempt to standardize a specific solution was recently made [T. Kokubo, H. Takadama, Biomaterials. 27, [T. Kokubo, H. Takadama, How useful is SBF in predicting in vivo bone bioactivity?, Biomaterials. 27, (2006) 2907-2915.]]. Moreover, several protocols include buffers as Tris or Bistris to maintain pH near physiological range. Using buffered solutions, two additional reactions may occur for each alkaline earth ion, namely protonation and alkaline earth metal ion complexation. Therefore, the new species Ca(Tris)2+ and Ca(Bistris)2+ modify the free Ca content in fluid and consequently the Ca(free)/P ratio, influencing the stoichiometry of precipitated phosphate. In this paper, we theoretically investigated calcium/buffer interactions for two types of SBF, standard and simplified, in terms of thermodynamic parameters such as activity coefficients, ion activities, and free Gibbs change energy for hydroxyapatite precipitation. Our results show that Tris and Bistris buffers reduce the content of available calcium ion, with a stronger trend towards Bistris. Besides, high pH increases the calcium complexation effects.