V. I. Putlyaev

Bioresorbable carbonated hydroxyapatite Ca10−xNax(PO4)6−x(CO3)x(OH)2 powders for bioactive materials preparation

The purpose of this work was to find and investigate a correlation between the carbonate ion content in crystalline lattice and defect structure, and solubility of the materials; finally, to prepare the materials under study for in vitro tests. Various techniques, such as XRD, FTIR, TEM, FESEM/EDX, TG/DTA, AES (ICP), wet chemical analysis, Ca-ionometry, microvolumetric analysis of evolved CO2, BET adsorption, were applied to determine the efficiency of carbonate substitution, and to quantify the elemental composition, as well as to characterize the structure of the carbonated hydroxyapatite and the site(s) of carbonate substitution,. It was shown that AB-type substitution prevails over other types with the carbonate content increase. According to in vitro tests, the bioactivity of the samples is correlated with the carbonate content in carbonate-doped hydroxyapatite due to accumulation of defects in carbonated hydroxyapatite nanocrystals.

Physicochemical bases of differences between the sedimentometric and laser-diffraction techniques of soil particle-size analysis

Comparison of the particle-size distributions in different soils showed that the sedimentation method (Kachinskii pipette method) gives higher (by 1.5–5 times) values of the clay content than the laser diffraction method. This is related to the significant variation in density of soil solids, which is taken to be constant in the sedimentation method. Therefore, particles of significantly larger size and lower density fall into this fraction. Using optical, electron, and confocal microscopy, it was shown that the low density of soil particles of silt size falling into the sedimentometric clay fraction is related to the organomineral shell (film) around the soil microparticles. This shell contributes to the linking of microparticles into aggregates at the lower average density. As a result, these aggregates have significantly larger size and lower density and settle with the same velocity as the small particles with the average density of the solid phase during the sedimentation particle-size analysis.

Calcium phosphate powders synthesized from solutions with [Ca2+]/[PO43−]=1 for bioresorbable ceramics

Calcium phosphate powders for manufacturing bioceramics were synthesized via precipitation from stock solutions of (NH4)2HPO4 and Ca(NO3)2, or CaCl2 or Ca(CH3COO)2 with [Ca2+]/[PO43−] = 1, without pH regulation. Properties of powdered samples, including density and microstructure of ceramics sintered at 900, 1000, 1100°C, were studied. The following pairs of precursors such as Ca(NO3)2/(NH4)2HPO4, CaCl2/(NH4)2HPO4, Ca(CH3COO)2/(NH4)2HPO4 gave both insoluble calcium phosphates and the corresponding by-products of synthesis — NH4NO3, NH4Cl, NH4CH3COO. These by-products were released from the calcium phosphate precipitates in the course of heating to the temperature of sintering. Owing to specific buffer properties of the solutions being formed during synthesis, the pH value varied in a wide range during the precipitation process leading to different final values of pH and, thus, to different target phase(s) after annealing at 900–1100°C. After sintering, the samples based on the powders synthesized from Ca(NO3)2/(NH4)2HPO4 consisted of β-Ca2P2O7, whereas the samples based on the powders derived from CaCl2/(NH4)2HPO4 were composed of β-Ca2P2O7 and β-Ca3(PO4)2, and the samples based on the powders synthesized from Ca(CH3COO)2/(NH4)2HPO4 contained only β-Ca3(PO4)2. All the powders can be considered as the precursors for fabrication of bioceramics with enhanced resorption.

Hydroxyapatite-based ceramic materials prepared using solutions of different concentrations

Hydroxyapatite, Ca10(PO4)6(OH)2, powders with enhanced sinterability have been synthesized through precipitation from calcium nitrate and ammonium hydrogen phosphate solutions at pH 9, t= 60°C, and a Ca/P atomic ratio of 1.67, and their properties have been studied: phase composition, particle size distribution, loose density, and green density. The initial solution concentration is shown to influence the properties of the powders and the ceramics fabricated from them. Comparison of the particle size distributions in disaggregated powders and the grain size distributions in the ceramics indicates that the ceramics inherit the structure of the corresponding powders. Optimizing the synthesis conditions in order to enhance the sinterability of the powders, we obtained green compacts with the highest shrinkage rate in the range 850–950°C and shrinkage onset at 600°C, which is 100–150°C lower in comparison with powders synthesized in earlier studies from calcium nitrate and ammonium hydrogen phosphate.

Phase equilibria in the tricalcium phosphate-mixed calcium sodium (potassium) phosphate systems

Phase equilibria in the quasi-binary sections Ca3(PO4)2-CaMPO4 (M = Na, K) are distinguished by high-temperature isomorphism of glaserite-like phases α’-Ca3(PO4)2 and α-CaMPO4. The main differences of the Ca3(PO4)2-CaKPO4 system from the Ca3(PO4)2-CaNaPO4 system are a shift of invariant equilibria toward higher temperatures, deceleration of phase transformations, and the emergence of polymorphism of an intermediate phase of an ordered solid solution based on α-CaKPO4. The low-temperature modification of this phase of a composition near Ca8K2(PO4)6 has the apatite structure with unoccupied hexagonal channels.

Hydroxyapatite of platelet morphology synthesized by ultrasonic precipitation from solution

Hydroxyapatite (HA) synthesis by precipitation with urea from aqueous solutions of calcium nitrate and ammonium hydrogenphosphate is studied. Ultrasonication during the synthesis decreases the size of platelet HA crystals from several micrometers to 200–300 nm. At low calcium concentrations in solution, the crystallizing phase is carbonate-hydroxyapatite, whereas at high calcium concentrations, octacalcium phosphate (OCP) precedes hydroxyapatite crystallization.

Synthesis of Nanocrystalline Calcium Hydroxyapatite from Calcium Saccharates and Ammonium Hydrogen Phosphate

The synthetic hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 (HAP) is a mineral component of the bone tissue of mammals. HAP is biocompatible with the human body, capable of biointegration with the bone tissue, has no negative influence on the immune system, is nontoxic, and shows osteoconductive behavior. Therefore, HAP is used to design ceramic materials for bone implants [1]. The activity of powders toward sintering in the production of ceramic materials increases with a decrease in the particle size of the powder [2, 3]. For bioceramics, an important feature is the absence of acute cytotoxicity, which is determined by the chemical purity of the initial powder. To prepare HAP containing no toxic or associated products difficult to remove, the reaction of calcium hydroxide with ammonium phosphate is used. In this case, one component, Ca(OH) 2 , is taken as a suspension [4]. An obvious drawback of the synthesis of calcium phosphates from a poorly soluble compound is that dissolution of Ca(OH) 2 is the rate-limiting step, and this increases the duration of synthesis. The formation of calcium phosphates may also occur on the surface of Ca(OH) 2 particles, which shifts the reaction to a region of slower diffusion through a solid calcium phosphate bed and gives rise to an inhomogeneous target product. The use of soluble calcium compounds for preparing highly dispersed powders is preferred, because this implements the known bottom-up approach to the creation of a dispersion system (from small to large) [5]. In the presence of saccharose, the solubility of calcium hydroxide increases due to formation of soluble calcium saccharates [6]. Organic compounds being adsorbed on the surface of the inorganic crystals formed prevent their growth and thus ensure the formation of a finely crystalline powder [7]. The purpose of the present work was to prepare nanocrystalline HAP containing a biocompatible associated product, saccharose, from ammonium hydrogen phosphate and calcium saccharates. The study was carried out with solutions of calcium saccharates Ca n C 12 H 22 – 2 n O 11 at different n ( n is the Ca : C 12 H 22 O 11 molar ratio in the saccharate synthesis) and HAP samples synthesized from different calcium saccharates and ammonium hydrogen phosphate.

Properties of amorphous calcium pyrophosphate powder synthesized via ion exchange for the preparation of bioceramics

A novel approach has been developed for the synthesis of amorphous hydrous calcium pyrophosphate powder consisting of nearly isometric particles: treatment of an aqueous suspension of platelike crystalline γ-calcium pyrophosphate particles with an ion exchange resin in H+-form. After firing at 1000°C, the relative density of calcium pyrophosphate-based ceramics produced from the powder is 80%. The relatively low firing temperature for sintering the ceramics is ensured by the improved sinterability of the powder due to the specific features of its micromorphology and phase composition.

Additive Technologies for Making Highly Permeable Inorganic Materials with Tailored Morphological Architectonics for Medicine

The review discusses methods to manufacture osteoconductive inorganic materials based mainly on calcium phosphates with given macroscopic porosity using rapid prototyping. Requirements for composition and morphological architectonic of these materials are considered, and three-dimensional printing techniques that are most often used to achieve this are described.

BIPHASE CaO-P2O5 CERAMIC BASED ON POWDER SYNTHESIZED FROM CALCIUM ACETATE AND AMMONIUM HYDROPHOSPHATE

Biphase ceramic containing hydroxyapatite Ca10(PO4)6(OH)2 and β-tricalcium phosphate β-Ca3(PO4)2 has been obtained from a uniform nanocrystalline calcium phosphate powder with the structure of apatite, synthesized from water solutions of calcium apatite and ammonium hydrophosphate with molar ratio Ca/P = 1.5 at room temperature without pH regulation. After firing at 1100°C the material contained 75% β-Ca3(PO4)2 and the grain size did not exceed 600 nm. It is suggested that the carbon formed in the interval 200 – 500°C, as a result of the carbonization of the organic components of synthesis adsorbed by particles of powder, be used as a physical barrier impeding intense mass transfer up to 800°C, which makes it possible to obtain an ultradisperse ceramic with grains smaller than 1 μm.