Effect of Magnesium Substitution on Structural Features and Properties of Hydroxyapatite.

Hydroxyapatite (HAP) is the main mineral component of bones and teeth. It is widely used in medicine as a bone filler and coating for implants to promote new bone growth. Ion substitutions into the HAP structure highly affect its properties. One of the most important substituents is magnesium. This paper presents new results obtained using high-precision hybrid density functional theory calculations for Mg/Ca substitutions in HAP in a wide magnesium concentration range within a 2 × 2 × 2 supercell model. Experimental data on the mechanochemical synthesis of HAP-Mg samples with different Mg concentrations are also presented. A comparison between the experiment and the theory showed good agreement: the HAP-Mg unit cell parameters and volume decreased with increasing degree of Mg/Ca substitution. The changes in the distances between the Ca and O, Ca and H, and Mg and O ions upon Mg/Ca substitution in different calcium positions was analyzed. The resulting asymmetry and distortion of the cell parameters were evaluated. It was shown that bulk modulus, energy levels, and band gap depend on the degree of Mg substitutions in the Ca1 and Ca2 positions. The formation energies of Mg/Ca substitutions showed non-monotonic behavior that was different for Ca1 and Ca2 positions. The Ca2 position had a slightly higher probability (~5 meV/f.u.) of substitution than Ca1 position at a Mg concentration x = 0.5. At x = 1, substitution in both positions can coexist. The simulated IR spectra for different Mg/Ca substitutions showed that Mg in the Ca2 position changes the IR spectrum more significantly than Mg in the Ca1 position. Similar changes were recorded in the IR spectra of the synthesized samples. The electronic structure is shown to be sensitive to the number and position of substitutions, which may be used to tweak the optical properties of the HAP-Mg material.

Selective Laser Melting of Hydroxyapatite: Perspectives for 3D Printing of Bioresorbable Ceramic Implants.

Hydroxyapatite, being the major mineral component of tooth enamel and natural bones, is a good candidate for bone tissue engineering applications. One of the promising approaches for manufacturing of three-dimensional objects is selective laser sintering/melting which enables the creation of a dense structure directly during 3D printing by adding material layer-by-layer. The effect of laser irradiation with a wavelength of 10.6 µm on the behavior of mechanochemically synthesized hydroxyapatite under different treatment conditions was studied for the first time in this work. It was shown that, in contrast to laser treatment, the congruent melting is impossible under conditions of a relatively slow rate of heating in a furnace. Depending on the mode of laser treatment, hydroxyapatite can be sintered or melted, or partially decomposed into the more resorbable calcium phosphates. It was found that the congruent selective laser melting of hydroxyapatite can be achieved by treating the dense powder layer with a 0.2 mm laser spot at a power of 4 W and at a scanning speed of 700 mm/s. Melting was shown to be accompanied by the crystallization of a dense monolayer of oxyhydroxyapatite while preserving the initial apatite crystal lattice. The thickness of the melted layer, the presence of micron-sized pores, and the phase composition can be controlled by varying the scanning speed and laser power. This set of parameters permits the use of selective laser melting technology for the production of oxyhydroxyapatite biodegradable implants with acceptable properties by 3D printing.

Detonation Spraying of Hydroxyapatite on a Titanium Alloy Implant.

Hydroxyapatite (HA), the major mineral component of tooth enamel and natural bones, is a good candidate for bone tissue engineering. Synthetic HA is used for making coatings on metallic implants intended for medical applications. A HA coating renders the implant biocompatible and osteoinductive. In addition, it improves fixation and the overall performance of the implanted object. In the present work, HA coatings were deposited on a medical titanium alloy implant with mesh geometry and a developed surface by detonation spraying. The feedstock powder was HA obtained by the dry mechanochemical method. Single-phase HA coatings were obtained. The coatings were formed not only on the surfaces normal to the particle flow direction, but also on the sides of the mesh elements. Despite partial melting of the powder, no decomposition of HA occurred. This work demonstrates the prospects of detonation spraying for the production of HA coatings on metallic implants with complex geometries.

Lanthanum-silicate-substituted apatite synthesized by fast mechanochemical method: Characterization of powders and biocoatings produced by micro-arc oxidation.

Lanthanum-silicate substituted apatite with equal concentrations of the substituents in the range of 0.2-6.0 mol were produced by a fast method - mechanochemical synthesis. This method makes it possible to synthesize a nanosized single-phase product by activating reaction mixtures containing CaHPO4, CaO, La(OH)3 and SiO2·H2O for 25-30 min in AGO-2 and AGO-3 planetary mills. The structure of the apatites was investigated by the FTIR and XRD methods. It was found that the synthesized samples with substituent concentrations up to 2 mol are substituted oxy-hydroxyapatites, at higher concentrations, they are substituted oxyapatites. The mechanochemically synthesized apatite with a substituent concentration of 0.5 mol was used for depositing biocoatings on titanium substrates by the micro-arc oxidation method. The structure of the coatings is mainly amorphous. In vitro biological tests demonstrated high biocompatibility of the coatings and the absence of cytotoxic action on mesenchymal stem cells.