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Amorphous calcium carbonate-phosphate with a Ca/P ratio of 1.83 was precipitated from Ca2+, Ca2+, PO3–4, CO2-3 − containing solutions at pH 10 and stabilized by ethanol dehydration and followed by heating at 400 °C. The presence of PO3–4 ions in the structure of amorphous calcium carbonate-phosphate provides its increased resistance to transformation into crystalline phases. Aging in a Ca2+, PO3–4 , CO2-3 − containing mother solution at pH 10 for 4 days promotes the transformation of amorphous calcium carbonate-phosphate into amorphous calcium carbonate-phosphate / carbonated hydroxyapatite / calcite. The combined effect of PO3–4 и CO2-3 − ions in the aquatic environment on the crystallization of amorphous calcium carbonate-phosphate into carbonated hydroxyapatite contributes to a maximum stabilization of the amorphous state that provides a high extent of bioactivity.

The calcium phosphate composite octacalcium phosphate / calcite was obtained at pH 5–7 from the CaCO3/Ca(H2PO4)2 aqueous suspension in a galvanostatic mode at a current density of 20 mA/cm2 for 20 min. Drying at 80 °C without the precipitate maturation stage led to a powder formation consisting of brushite, calcite and a small amount of octacalcium phosphate. Prolonged maturation in air for 2 months led to the hydrolytic transformation of brushite into octacalcium phosphate stabilized by calcite. The use of electric current made it possible to increase the amount of octacalcium phosphate in the composite powder with the morphology of lamellar rosettes. Calcination at 800 °C of the composite powders led to the formation of α/β-tricalcium phosphate, β-calcium pyrophosphate, hydroxyapatite, and calcium oxide.

By combining the method of replication of polyurethane foam matrices at 1200 °C and modification in model SBF (Simulated Body Fluid) solutions of various compositions, open-pore calcium phosphate foam ceramics with a porosity of 53-60 % was obtained. The architecture and morphology of the calcium phosphate foam ceramics surface was formed by using polyurethane foam matrices («Granufoam», «STR») with different porosity and quantity of open pores. Modification of the calcium phosphate foam ceramics in SBF solutions of various compositions leads to a slight decrease in porosity to 3 %, which indicates the formation of an ultrathin apatite layer. The calcium phosphate-modified foam ceramics consisted of β-tricalcium phosphate, β-calcium pyrophosphate, α-tricalcium phosphate, and biomimetic apatite. In the standard SBF solution, the formation of apatite on calcium phosphate foam ceramics occurs slowly (14-56 days) and the strength increases by a factor of 2 as compared to the initial one. Soaking of calcium phosphate foam ceramics in SBF without HCO3- leads to the formation of biomimetic apatite with inclusions of calcium chloride dihydrophosphate in spherulites. Modification in a 5-fold concentrated SBF solution for 3-5 days at 37 °C makes it possible to form 6-10 times more biomimetic apatite compared to standard SBF with a 2.5-fold increase in static strength to 0.05 MPa. It has been established that at 800 °C biomimetic apatite crystallizes into β- tricalcium phosphate.

Nanocomposites based on apatitic tricalcium phosphate in an autofibrin matrix were obtained by precipitation at a Ca/P ratio of 1.50, pH 9 and a maturation time from 30 min to 7–14 days. The resorbability of nanocomposites was determined by the composition of calcium phosphates, which, during long-term maturation, formed as the calcium-deficient hydroxyapatite with a Ca/P ratio of 1.66, whereas biopolymer matrix favored the formation of more soluble calcium phosphates with a Ca/P ratio of 1.53–1.59. It was found that the fibrin clot stabilized, along with apatitic tricalcium phosphate, the phase of amorphous calcium phosphate, which after 800 °C was transformed into resorbable α-tricalcium phosphate. Citrated plasma inhibited the conversion of apatitic tricalcium phosphate into stoichiometric hydroxyapatite, which also facilitated the formation of resorbable β-tricalcium phosphate after 800 °C. The combined effect of the maturation time and the biopolymer matrix determined the composition, physicochemical and morphological properties of nanocomposites and the possibililty to control its extent of resorption

Powders of calcium pyrophosphate Ca2P2O7 of γ- and β-modifications have been obtained using the thermal conversion of brushite CaHPO4⋅2H2O synthesized from phosphoric acid H3PO4 and calcium carbonate CaCO3 at a molar ratio P/Ca = 1.1. The resulting powders can be used to create various functional materials, including biocompatible and bioresorbable materials for the treatment of bone defects.

Powders of calcium pyrophosphate Ca2P2O7 in the form of γ- и β-modifications have been produced as a result of thermal conversion of brushite CaHPO4∙2H2O synthesized from phosphoric acid H3PO4 and calcium carbonate CaCO3 at the molar ratio P / Ca = 1.1. The resulting powders can be used for production of various functional materials including biocompatible and bioresorbable ones for the treatment of bone defects.

Hardening composite materials based on calcium phosphates (hydroxyapatite, monocalcium phosphate monohydrate) and polymer binders (polyvinyl alcohol, polyvinylpyrrolidone) were obtained. It was found that the amount of brushite and tricalcium phosphate grows with increasing calcium acid phosphate content in the composition. The introduction of polymeric binders into the composition of hardening composite materials promotes growth of their mechanical strength and inhibition of structural-phase transformations of calcium phosphates at 800°C. Compositions based on 5% solutions of polymers and powders of hydroxyapatite or its mixtures with weight content 10 – 40% monocalcium phosphate monohydrate can be used as a consumable for 3D printing of calcium phosphate biomaterials and ceramic.

Ceramic in the system Ca2P2O7–Ca(PO3)2 was obtained by firing cement stone fabricated from a powder mixture which included β-tricalcium phosphate β-Ca3(PO4)2 and monocalcium phosphate monohydrate Ca(H2PO4)2 ∙ H2O. The molar ratio Ca/P of the initial powder mixtures was set as Ca/P ≤ 1. X-ray phase analysis shows all ceramic samples after firing at 1000°C to include β-calcium pyrophosphate β-Ca2P2O7, and a sample of the ceramic made from a powder mixture with the maximum content of monocalcium phosphate monohydrate Ca(H2PO4)2 ∙ H2O also contained β-calcium polyphosphate β-Ca(PO3)2. The obtained calcium phosphate ceramic with reduced firing temperature can be used to develop a resorbable matrix for tissue engineering structures.

Hydroxyapatite (HA or HAp) is one of the most preferred biomaterials, specifically for bone tissue engineering. HAp is available naturally and is also chemically synthesized. The properties, shape, size and crystalline structure and applications of HAp vary widely depending on the source and extraction methods used. In addition to conventional chemical approaches such as precipitation or sol-gel techniques, newer methods such as microwave synthesis and atomic-layer deposition provide an opportunity to generate HAp with desirable structure and properties. Various methods used for the synthesis of HAp have their own pros and cons. Hence, it is essential to understand the role of specific methods and conditions on the properties and structure of HAps in order to obtain HAp with properties suitable for specific applications. In addition to pure HAp, substantial efforts have been made to dope HAp with various minerals or bioentities to enhance their suitability for medical, environmental remediation and other approaches. In this review, we provide an overview of the various chemical methods used to produce HAp, properties of the HAp produced and its potential applications. Particular focus of this paper is on the co-relation between properties and processes used to synthesis HAp. This review will enable readers to quickly understand the importance of synthesis methods and conditions on the properties of HAp and choose appropriate means to generate HAp with desired properties for specific applications.

Thermoplastic polymer composites have been obtained by stretching a polylactide (PLA) melt containing 1–10% amorphized hydroxyapatite (HA). The physicochemical properties of the obtained composites have been studied by XRD, thermal analysis, and microscopy. It is shown that PLA/HA composites are well extruded through nozzles with a diameter of 0.4 mm, retain their shape during 3D printing by the FDM method, and can be used to create three-dimensional porous biomaterials.

The introduction of a 15 – 85 wt.% powder of monocalcium phosphate monohydrate Ca(H2PO4)2 · H2O into the hydroxyapatite suspension after annealing at 1200°C leads to the formation of resorbable calcium phosphate phases (tricalcium phosphate, calcium pyrophosphate), which makes it possible to control the phase composition of foam ceramics.

Carbonated hydroxyapatite of predominant B-type of substitution has been synthesized at various temperatures, carbonate concentrations and time of immersion in mother solution. Carbonated hydroxyapatite synthesized at 80 °C contains the largest amount (up to 9 wt.%) of calcite impurity and has a low specific surface area (40 m2/g). Lowering the synthesis temperature to 20 °C leads to a slight decrease in the content of calcite impurity (up to 5–7 wt.%) and an increase in the specific surface up to 125 m2/g. The introduction of the maturation stage for carbonated hydroxyapatite precipitated at 20 °C for 4 days suppresses the calcite impurity formation and leads to the increase in the specific surface up to 155 m2/g.

The structural and physicochemical mechanisms of interaction of bone tissue with titanium implants under conditions of tuberculous osteitis are studied using scanning electron microscopy, X-ray diffraction analysis, IR spectroscopy, and thermal analysis. Physiological regeneration of bone tissue during treatment of tuberculous osteitis is accompanied by an increase in the organic component, embrittlement of the mineral phase of the bone matrix, and its decomposition to fine particles. The presence of titanium implants under physiological conditions contributes to the activation of the synthesis of the apatite phase in the regenerating bone. The primary mechanism of implantation osteogenesis in tuberculous osteitis is the formation of an inorganic phase of hydroxyapatite mainly in the regions of destruction of the titanium alloy.

The introduction of 10 – 50% (by weight) brushite powder, preheated at 50° and then fired at 1200°C, into a permeating suspension based on hydroxyapatite powder heat-treated at 800°C results in partial decomposition of hydroxyapatite to the α- and β-modifications of tricalcium phosphate. The presence of a tricalcium phosphate phase in calcium phosphate foam ceramic promotes an increase of the porosity from 40 to 60% and the resorption with significant static strength reduction from 3 – 7 to 0.3 MPa.

The physicochemical properties and structure of the bone matrix are studied in a case of simulated tuberculous osteitis without treatment and after a full course of specific antibacterial therapy. Using X-ray diffraction, infrared spectroscopy, thermal analysis, and scanning electron microscopy, we revealed that tuberculous osteitis causes fine disintegration of the bone matrix due to an increase in nonstoichiometric hydroxyapatite, formation of amorphous calcium phosphates, and a decrease of the organic phase, which is accompanied by embrittlement of the bone matrix. Excessive growth of carbonate-hydroxyapatite crystals of a mixed AB substitution type leads to excessive osteogenesis, accompanied by the uncontrolled growth of bone trabeculae. The presence of “liming” regions in the crystal lattice of hydroxyapatite increases the “binding” properties of the bone matrix in which mycobacteria are immobilized and removed in the composition of detritus.

The introduction of a 10−50 mass.% of brushite powder or tricalcium phosphate heated at 50−70°C in a hydroxyapatite slip leads after annealing at 1200°C to formation of resorbable calcium phosphate phases which make it possible to regulate the degree of bioactivity of the foam ceramics.

Powder mixtures prepared by mechanical activation from synthetic hydrated acidic calcium phosphates Ca(H2PO4)2 · H2O and CaHPO4 · 2H2O were used to obtain resorbable ceramic in the system Ca (PO3)2–Ca2P2O7. The phase composition of the ceramic after firing in the interval 700 – 1000°C was represented by biocompatible and bioresorbable phases: calcium polyphosphate Ca(PO3)2, tromelite Ca4P6O19, and calcium pyrophosphate Ca2P2O7. The obtained materials can be used to fabricate resorbable implants for regenerative treatment of defects of bone tissue.

Mesoporous calcium phosphate granules ranging in mesopore size from 11 to 19 nm have been prepared using cryogenic processing (–18°C) of gels and extrusion of calcium phosphate paste. The granules based on acid calcium phosphates have a relatively small specific surface area (45 m2/g) in comparison with the neutral and basic calcium phosphates (84–155 m2/g). Preclinical in vivo trials on rats show that the presence of calcium phosphate granules in a bone defect considerably accelerates reparative osteogenesis in comparison with a control group of animals.

We have examined the effect of synthesis and heat-treatment conditions on the phase composition of composites based on hydroxyapatite and zirconia. The zirconia present in the composite based on a mechanical powder mixture crystallizes predominantly in tetragonal symmetry at heat-treatment temperatures in the range 400–600°C and in monoclinic symmetry at 800°C. The zirconia present in the composites prepared via the mixing of hydroxyapatite gel with hydrated zirconia sol or coprecipitation crystallizes only in monoclinic symmetry. Annealing at 1300°C leads to the formation of a solid solution with the composition Ca0.15Zr0.85O1.85 in all of the composites as a consequence of complete or partial hydroxyapatite decomposition and the thermal diffusion of Ca2+ ions into the ZrO2 lattice. In the composites prepared by liquid-phase processes, heat treatment in the range 800–1300°C produces the smallest changes in the hydroxyapatite structure, which ensures high biological activity of the material.

Biocompatible calcium phosphates were synthesized by mixing calcium- and phosphate-containing solutions in medium with various pH value, which contained up to 6 vol.% of platelet-poor plasma (PPP). Depending on the pH of precipitation medium, the influence of PPP components favored the formation of calcium-deficient hydroxyapatites with varying composition. In neutral medium, dicalcium phosphate dihydrate (DCPD) was the predominant phase but the buffering properties of PPP favored formation up to 24 wt.% of apatitic tricalcium phosphate. In alkaline medium, negatively charged surface of the PPP component macromolecules inhibited Ca2+ diffusion, which in turn lowered the Ca/P molar ratio of the formed hydroxyapatite from 1.67 to 1.62. After soaking in simulated body fluid (SBF), DCPD underwent hydrolysis yielding 29 wt.% of octacalcium phosphate and 19 wt.% of apatitic tricalcium phosphate and hydrolyzed DCPD/PPP sample contained 9 wt.% and 42 wt.% of octacalcium phosphate and apatitic tricalcium phosphate. Calcium-deficient hydroxyapatite in HA/PPP sample consumed the ions of SBF with an increase of its Ca/P molar ratio from 1.62 up to 1.66. In both cases, the Ca/P molar ratio of the calcium phosphates increased, indicating the apatite forming ability of the obtained calcium phosphates. Obtained results seem to be promising in development of new generation of hybrid composites based on calcium phosphates and blood components.

Photocatalytic activity of gypsum cements that are based on the building plaster blend and synthetic calcium sulfate and contain 2–10 wt % titanium dioxide of anatase modification was studied. The decomposition rate of the model contaminant (Rhodamine C) on the surface of a plaster blend containing 2 wt %TiO2 becomes seven times higher under exposure to a full spectrum of a UV lamp, and 29 times higher under UV-A irradiation at λ > 320 nm. It was found that there are two competing processes (photocatalysis and direct photolysis), which affect the decomposition rate of Rhodamine C on the surface of the cements, depending on the UV irradiation spectrum.

The influence exerted by conditions of the secondary maturation of hydroxyapatite gel on the composition of a xerogel was determined. It was found that the prolonged stage of the secondary maturation within pH 7.6–7.1, the low pH value of distilled water or a solution of the acid (HCl, H3PO4) and the long time of contact of the hydoxyapatite gel with the acid solution assist the formation of α-tricalcium phosphate impurity. A singlephase hydroxyapatite is formed in the presence of distilled water with pH 5.3–5.6, as the time of the secondary stage of maturation of hydroxyapatite gel within pH 7.6–7.1 is decreased and the total washing time is decreased up to 34–40 days.

The influence of various dehydration procedures on the physicochemical properties of hydroxylapatite gel was examined. The best dehydration procedure is freezing, leading to the formation of a nanocrystalline hydroxylapatite powder without intermediate formation of a monolithic xerogel. Examination by the layer self-arrangement method, scanning electron microscopy, and atomic-force microscopy showed that the powder particles are spherical aggregates from 0.4 to 2.0 μm in size, consisting of nanoparticles 16–30 nm in diameter.

Calcium hydroxyapatite (HAP) powder was synthesized from a water solution of potassium hydrophosphate and a suspension of calcium hydroxide in a water solution containing calcium and potassium ions, hydroxide ions, and acetate ions. The Ca(OH)2 amount in the calcium containing component was 50, 75, and 100%. The HAP powder synthesized was used to produce ceramic. After kilning at 700°C the density of the ceramic obtained from the HAP powder containing a by-product of the reaction was 2.9 – 2.6 g/cm3 and bending strength 35 – 45 MPa with grain size 200 nm. The phase composition was represented by HAP and calciumpotassium double orthophosphate. After kilning at 1200°C the density of the HAP-based ceramic made from powder with no by-product of the reaction was 2.2 – 2.3 g/cm3 and the bending strength was 45 – 55 MPa with grain size 500 – 1000 nm.