T. V. Safronova

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.

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.

Resorbable calcium phosphates based ceramics

Resorbable monophase ceramics and ceramic composites based on calcium pyrophosphate (CPP, Ca2P2O7), tricalcium phosphate (TCP, Ca3(PO4)2), and hydroxyapatite (HAp, Ca10(PO4)6(OH)2) were prepared via thermal treatment of a HAp (Ca/P = 1.67) and CPP (Ca/P=1) mixture. High-temperature solid-state reaction between HAp and CPP leading to TCP formation was studied within the range 700–800 °C. The metastable α-TCP phase was observed in this range as a product of the solid-state reaction. The reaction had been completed before sintering of the powders started. The microstructure of CPP/TCP composites was found to be duplex-like, consisting of large CPP grains with smaller TCP grains among CPP ones. In the case of the monophase ceramics with starting HAp/CPP ratio corresponding to TCP, grain size was less than 300 nm.

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.

Powder systems for calcium phosphate ceramics

The present review describes approaches to the preparation of powder formulations for production of calcium phosphate ceramic materials. In order to mold semifinished powder items by means of various methods powder formulations, comprising synthetic powders and fabrication binders, are used.

Fabrication of osteoconductive Ca3–xM2x(PO4)2 (M = Na, K) calcium phosphate bioceramics by stereolithographic 3D printing

Osteoconductive ceramic implants based on Ca3–xM2x(PO4)2 (M = Na, K) double phosphates and having the Kelvin structure, tailored macropore size (in the range 50–750 μm), and a total porosity of 70–80% have been produced by stereolithographic 3D printing. We demonstrate that, to maintain the initial geometry of a model and reach sufficiently high strength characteristics of macroporous ceramics (compressive strength up to 9 MPa and fracture toughness up to 0.7 MPa m1/2), the polymer component should be removed under specially tailored heat treatment conditions. Based on our results on polymer matrix destruction kinetics, we have found heat treatment conditions that ensure a polymer removal rate no higher than 0.1 wt%/min and allow one to avoid implant cracking during the firing process.

Powders Mixtures Based on Ammonium Pyrophosphate and Calcium Carbonate for Preparation of Biocompatible Porous Ceramic in the CaO–P2O5 System

Powder mixtures are considered containing ammonium hydrophosphate and calcium carbonate intended for preparing biocompatible porous ceramic in the CaO–P2O5 system. Pore formation on heating in workpieces based on the powder mixtures in question occurs due to occurrence of gas liberation in the presence of melt in the NH3–H2O–CO2–CaO–P2O5 or CO2–CaO–P2O5 systems. Gas phase formation is due to liberation of gaseous water and ammonia, thermal hydrolysis of calcium polyphosphate, decomposition of calcium carbonate, and oxidation of carbonized organic compounds remaining within a powder mixture in the form of associated calcium carbonate synthesis reaction product.

Sintering of HAp precipitated from solutions containing ammonium nitrate and PVA

Bioceramics based on hydroxyapatite (HAp) is known to be a prospective material for biomedical applications. However, sintering of HAp is still understudied in sense of reasonable selection of controlling parameters. In particular, the role of impurities and co-products of powder fabrication is still questionable. The data concerning the role of ammonium nitrate coming to precipitated HAp from the mother liquor, its effect on powder compaction and subsequent sintering, are not available. Nanosized powders of HAp were fabricated via conventional wet-precipitation technique by dropwise adding of Ca(NO 3 ) 2 solution (0.25 -1.67 M) to the stock solution of (NH 4 ) 2 HPO 4 (0.15-1.00 M) with pre-adjusted pH at 60°C in presence of polyvinyl alcohol (PVA). PVA was added to the stock solution in order (i) to block crystal growth during synthesis , (ii) to improve stability of Hap suspension to sedimentation, (iii) to regulate an aggregation of HA nanoparticles during synthesis and in the stage of drying. NH 4 NO 3 – a by-product of the precipitation reaction, presented in as-precipitated powder in amount of 30%, was evaluated as an additive affecting a compaction of the powder and the initial stage of a sintering. The powder samples were tested by XRD, FTIR, light-scattering , TEM and SEM/EDX to get particle sizes, morphology and chemical composition, dilatometry. Ceramics were sintered at 700-1250°C and evaluated with SEM/EDX and density measurements. Addition of PVA to the stock solution in the course of HAp precipitation is a promising technique to control an aggregation of HAp nanoparticles in the stages of drying and sintering. PVA acting as a surfactant in the solution and as a binder in dry powder can keep highly reactive small HAp particles within large agglomerates providing better molding of the powder and controllable densification of ceramics. The effect of PVA on microstructure of the HAp powder and their sintering behaviour is discussed in terms of self-organisation concept and synergetics.

Ceramics Based on Brushite Powder Synthesized from Calcium Nitrate and Disodium and Dipotassium Hydrogen Phosphates

Brushite (CaHPO4 · 2H2O) powder has been synthesized in aqueous 1.0 M solutions of calcium nitrate dipotassium hydrogen phosphate, and disodium hydrogen phosphate at a Ca/P ratio of unity, without adjusting the pH of the reaction. After synthesis and drying, the fraction of a reaction by-product (NaNO3, KNO3, or their mixture) in the powder was about 20 wt %. After firing at temperatures from 800 to 1000°C, the ceramics prepared using the powder synthesized from Ca(NO3)2 and Na2HPO4 consisted of β-Ca2P2O7 and β-NaCaPO4. After firing at temperatures from 900 to 1100°C, the ceramics prepared using the powder synthesized from Ca(NO3)2 and K2HPO4 consisted of Са10К(РО4)7 and СаК2Р2О7. The ceramic composites produced in this study can be recommended as materials for resorbable bone implants.

Octacalcium phosphate as a precursor for the fabrication of composite bioceramics

We have studied the formation of octacalcium phosphate (OCP) in various buffer solutions. Brushite hydrolysis in acetate and succinate buffer solutions at 60°C and pH 5.75 ensures rapid synthesis of pure OCP and substituted OCP (sOCP), which allows a rather large amount of this phosphate (at least 10 g in a single synthesis run) to be obtained in 50–60 min. The observed differences in phase composition between the OCP and sOCP thermolysis products make it possible to obtain biphasic ceramic composites of various kinds: β-TCP/β-CPP (Ca/P = 1.33) in the case of OCP and β-TCP/HA (Ca/P = 1.54) in the case of sOCP. Ceramics with a density of 80% of theoretical density and higher produced using the OCP precursors synthesized in this study have a uniform microstructure, possess the desired microporosity, and are potentially attractive for further advances in the field of bioresorbable osteoplastic materials.