Mechanochemical synthesis of strontium- and magnesium-substituted and cosubstituted hydroxyapatite powders for a variety of biomedical applications

Abstract

Hydroxyapatite (HA) is a biocompatible material widely used in various biomedical applications. Stoichiometric HA has low bioactivity and does not possess antibacterial properties. One way to modify HA properties is to substitute some ions in stoichiometric HA, i.e., to dope HA, for attaining desired properties, e.g., improved bioactivity or an antibacterial effect. This work shows that a soft mechanochemical method of HA synthesis allows to obtain bioactive HA containing strontium ions, magnesium ions, or both. Substituted HAs with substitution degree x(Mg) = 0.5 mol or x(Sr) = 1.5 mol were synthesized, as was cosubstituted HAs contained both magnesium and strontium ions with x(Mg) = 0.5 mol or x(Sr) = 1.5 mol. The resulting materials were investigated by simultaneous thermal analysis, scanning electron microscopy, granulometry, powder X-ray diffraction, Fourier transform infrared, Raman spectroscopy, and X-ray photoelectron spectroscopy. Thermal stability of the obtained materials was studied too. The partial calcium substitution with strontium and/or magnesium changed both HA crystal lattice parameters and positions of absorption bands of phosphate groups in Fourier transform infrared and Raman spectra. X-ray photoelectron spectra of the synthesized materials contain peaks of all elements with binding energies corresponding to substituted HA. The strontium substitution, unlike the magnesium substitution, did not reduce HA thermal stability. The crystal lattice of Sr-substituted HA is stable up to 1300 °C, just as stoichiometric HA. Magnesium cations left HA at 800 °C, forming Mg-substituted β-Ca3(PO4)2. At the cosubstitution, thermal stability of HA became slightly lower. Partial decomposition of the material began already at 700 °C. Accordingly, during manufacturing of medical devices from the proposed materials, their low thermal stability should be taken into account. To preserve the structure of Mg-Sr-cosubstituted HA, heat treatment conditions should not exceed 700 °C.