Magnesium incorporated hydroxyapatite: Synthesis and structural properties characterization

Synthetic hydroxyapatites are widely used in bone tissue engineering because of their similar composition with the inorganic phase of hard tissues. Biological apatites, however, are calcium-deficient apatites with many di- and tri-valent ion substitutions. In this study, stoichiometric hydroxyapatite (HA) powders were prepared by wet-chemical precipitation method, and the effect of Mg incorporation on the resulting solid solution was investigated. X-ray diffraction (XRD) analysis confirmed that the substitution of Mg for Ca in apatite lattice resulted in a slight increase in a lattice and more emphasized decrease in c lattice parameters: 0.0966% and 0.2964%, respectively. The results indicated an increase in the d-spacing of Mg-doped HA (MHA). Scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDX) analysis showed that the Mg, C, and O elements were evenly distributed. Transmission electron microscopy (TEM) analysis revealed that incorporation of Mg did not significantly alter the size of the precipitated crystals. Although XRD patterns suggested smaller crystallite size, such a result was still consistent with TEM results, wherein change in size was not significant in Mg-doped HA (MHA) in comparison to HA. Moreover, the incorporation of impurity ions into the HA lattice did not alter the high-temperature phase stability often required for processing. The comparison of HA and MHA samples before and after heat treatment showed that the apatite structure did not decompose or undergo any phase transformation at high temperatures. A proportion of the Mg added did not substitute into the HA lattice. The Mg(OH)2 phase, as observed in the XRD pattern of the Mg-added sample, was a second phase that was easily washed by a citrate solution. In both HA and MHA samples, the calcium phosphate phase remained as a single-phase hexagonal calcium hydroxyapatite before and after heat treatment.