Structural Characterization and Biological Fluid Interaction of Sol-Gel-Derived Mg-Substituted Biphasic Calcium Phosphate Ceramics

Sol-Gel chemistry has been used to prepare undoped and Mg-substituted biphasic calcium phosphate (BCP) ceramics composed of hydroxyapatie (HAp) and whitlockite (beta-TCP) phases. Differences series of samples have been synthesized with different Mg-doping levels (from 0 to 5 atomic % of CA atoms substituted) and different temperatures of calcination (from 500 to 1100 degrees C). All of the powdered samples were systamatically treated by Rietveld refinement to extract the quantitative phase analysis and the structural and microstructural parameters, to locate the Mg crystallographic sites, and to refine the composition of the Mg-substituted phases. The temperature dependence of the weight amount ratio between HAp and beta-TCP is not monotonic because of the formation of minor phases as Ca2P2O7. CaO MgO and CaCO3 and certainly an amorphous phase. On the other hand the Mg stabilizing feature on the beta-TCP phase has been evidenced and explained. The mechanism of stabilization by small Mg2+ is different from that by large Sr2+. Nevertheless in both cases the beta-TCP stabilization is realized by an improvement of the environment of Ca4 site unusally face-coordinated to a PO4 tetrahedron. The substitution of a Mg atom in the Ca5 site allows considerable improvement of the bond valence sum of the unusual Ca4 polyhedron. The temperatures of calcination combined with the amount of Mg atoms introduced allow monitoring of the phase composition of the BCP ceramics as well as their microstructural properties. The bioactivity properties of the BCP samples are improved by the presence of Mg atoms in the structure of the beta-TCP phase. The mechanism of improvement is mainly attributed to an accelerated kinetic of precipitation of a calcium phosphate layer at the surface comprising HAp and/or beta-TCP phases.