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Strontium/carbonate co-substituted two-phase system of hydroxyapatite and octacalcium phosphate derived from cuttlefish bone

Hydroxyapatite (HAp) is the most widely used calcium phosphate (CaP) for hard tissue applications due to its compositional similarity to biological apatite, biocompatibility and bioactivity. Along with HAp, octacalcium phosphate (OCP) and tricalcium phosphates (TCP), are gaining increasing attention. As bone apatite is a multi-substituted carbonated HAp containing trace elements, one of the used approaches to improve the biological and physicochemical properties of CaPs is ionic substitution. In this study Sr-substituted CaPs, with varying strontium content (0, 1, 5 and 10 mol%), have been prepared by using cuttlefish bone as precursor of calcium and various trace ions. The effect of the doping on phase composition, crystal structure, morphology and thermal stability of synthesized materials was studied. Mineralogical composition of the as-prepared and the heat-treated powders were determined using X-ray diffraction analysis. The lattice parameters, the cell volume and phase composition of prepared powders were determined by Rietveld refinement using software DIFFRAC.SUITE TOPAS V.5.0. Ions release test was studied during 7 days of incubation in simulated body fluid (SBF) at 37 °C. The chemical composition of as-prepared unsubstituted and Sr-substituted CaP powders, and concentration of released ions were determined by inductively coupled plasma mass spectrometry. The precipitated powders were composed of HAp, OCP and amorphous CaP. After heat treatment, crystalline phase of β-TCP was detected. Along with strontium, unsubstituted CaP is doped with carbonate, sodium and magnesium ions, as they are present in cuttlefish bone. Sr-substituted CaP systems have similar trace element content (Na+ and Mg2+) as unsubstituted CaP system. CaP systems are CO3-substituted by B-type substitution as in natural bone mineral. Obtained results revealed an increase of cell volume (Å3), lattice a-axis (Å), lattice b-axis (Å) and c-axis (Å) for all obtained CaP phases (OCP, HAp and β-TCP), due to larger ionic radius of Sr2+ (1.12 Å) in comparison to substituted Ca2+ (0.99 Å) ion. Strontium ions are located at Ca1 site in hydroxyapatite, Ca3, Ca4, Ca7 and Ca8 sites in OCP and Ca1, Ca2, Ca3 and Ca4 sites in β-TCP structure. Obtained results have shown that two-phase system of OCP and HAp can be synthesised from biogenic source at mild conditions. As result of using biogenic source, prepared CaP system is co-substituted with magnesium, sodium and carbonate ions, along with strontium as target substitution ion. Determination of ion position and substitution mechanism is of great importance to understand in vitro ion release.

hydroxyapatite, octacalcium phosphate, strontium, Rietveld refinement, co-substitution

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