P. V. Evdokimov

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.

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.

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.

Reinforcement of Brushite Cement Based on α-TCP by Rigid Polylactide Framework

Brushite cement obtained from α-Ca3(PO4)2 and orthophosphoric acid in the presence of Mg(H2PO4)2 with compressive strength of 18 MPa was armored with a framework with the Kelvin architecture made of polylactide by thermoextrusion 3D printing. It is shown that armoring of cement with polylactide framework allows one not only to increase the compressive strength of such a structure (up to 35 MPa) but also to significantly improve its deformability (up to 15%).

Resorption of Ca 3 – x M 2 x (PO 4 ) 2 (M = Na, K) Calcium Phosphate Bioceramics in Model Solutions

The resorbability of bioceramics in the Ca3(PO4)2–CaNaPO4–CaKPO4 system is evaluated in an approach involving thermodynamic assessment of solubility and investigation of the dissolution kinetics in model media, in particular in citric acid solutions. Thermodynamic calculation indicates high solubility of the Ca5Na2(PO4)4, α-CaМPO4, β-CaKPO4, and β-СаK0.6Na0.4PO4 phases. Investigation of the dissolution kinetics of ceramics has made it possible to identify two distinct types of behavior of resorbable materials in weakly acidic solutions: with fast resorption kinetics in the case of the phases based on nagelschmidtite solid solutions and α-CaМPO4 disordered high-temperature solid solutions, and with a nearly constant, relatively low dissolution rate and a high solubility limit in the case of β-СaK1 – xNax-based phases.

Preparation of β-Ca 3 (PO 4 ) 2 /Poly(D,L-lactide) and β-Ca 3 (PO 4 ) 2 /Poly(ε-caprolactone) Biocomposite Implants for Bone Substitution

Highly permeable macroporous implants of various architectures for bone grafting have been fabricated by thermal extrusion 3D printing using highly filled β-Ca3(PO4)2/poly(D,L-lactide) (degree of filling up to 70 wt %) and β-Ca3(PO4)2/poly(ε-caprolactone) (degree of filling up to 70 wt %) composite filaments. To modify the surface of the composite macroporous implants with the aim of improving their wettability by saline solutions, we have proposed exposing them to a cathode discharge plasma (2.5 W, air as plasma gas) in combination with subsequent etching in a 0.5 M citric acid solution. It has been shown that the main contribution to changes in the wettability (contact angle) of the composites is made by the changes produced in their surface morphology by etching in a low-temperature plasma and citric acid. An alternative approach to surface modification of the composites is to produce a carbonate hydroxyapatite layer via precipitation from a simulated body fluid solution a factor of 5 supersaturated relative to its natural analog (5xSBF).

Synthesis of calcium phosphate powder from calcium lactate and ammonium hydrogen phosphate for the fabrication of bioceramics

A calcium phosphate powder has been synthesized from aqueous 0.25, 0.5, and 1.0 M calcium lactate and ammonium hydrogen phosphate solutions atat a Ca/P = 1, without pH adjusting. According to X-ray diffraction data, the as-synthesized powder consisted of brushite (CaHPO4 · 2H2O) and octacalcium phosphate (Ca8(HPO4)2(PO4)4 · 5H2O). After heat treatment in the range 500–700°C, the powders were gray in color because of the destruction of the reaction by-product. The powders heat-treated in the range 500–700°C consisted largely of γ-Ca2P2O7. The ceramics prepared from the synthesized powders by firing at 1100°C consisted of β-Ca2P2O7 and β-Ca3(PO4)2.