Comparative study of calcium phosphate and calcium carbonate precipitation in model systems of increased complexity

Processes of calcium phosphates (CaP) and calcium carbonates (CaCO3) formation and transformation, due to their biological relevance, are of special interest in the biomimetic preparation of hard tissue implant materials. Both these classes of minerals appear in a number of different solid phases, amorphous or crystalline, stable or metastable, which opens a possibility of fine tuning chemical and structural composition of a biomaterial by a strict control of formation conditions. Although much is known about these systems, the mechanisms of their formation are not yet fully elucidated and their comparison is scarce. The aim of this study is to contribute to the general understanding of mechanisms underlying formation of biologically relevant minerals by comparing properties of solid phases formed in CaP and CaCO3 systems at identical initial experimental conditions (supersaturation, constituent ions ratio, ionic strength and presence of relevant inorganic additives). Three model systems of increased complexity, which contain: (a) only constituent ions, (b) constituent ions and increased ionic strength and (c) constituent ions and physiologically relevant co-ions, were investigated for each class of biominerals. Experiments were initiated by mixing the reactant solutions at strictly controlled hydrodynamic and thermodynamic conditions, while the progress of reaction was continuously followed potentiometrically. Precipitates obtained after 1 hour reaction time were filtrated and analysed by IC, PXRD, FT-IR, TEM, EPR and TG. The results gave insight into the precipitation boundaries and supersaturation domains for formation of different CaP and CaCO3 solid phases. Recent studies have shown that the biominerals in vivo can form via amorphous precursor and its subsequent transformation. Therefore, in each system special attention was given to the formation of amorphous phases. It was found that their transformation into more stable modifications is prolonged by increasing complexity of precipitation system.