Synthesis methods for nanosized hydroxyapatite with diverse structures.

Boron carbide is a strategic material, finding applications in nuclear industry, armour for personnel and vehicle safety, rocket propellant, etc. Its high hardness makes it suitable for grinding and cutting tools, ceramic bearing, wire drawing dies, etc. Boron carbide is commercially produced either by carbothermic reduction of boric acid in electric furnaces or by magnesiothermy in presence of carbon. Since many specialty applications of boron carbide require dense bodies, its densification is of great importance. Hot pressing and hot isostatic pressing are the main processes employed for densification. In the recent past, various researchers have made attempts to improve the existing methods and also invent new processes for synthesis and consolidation of boron carbide. All the techniques on synthesis and consolidation of boron carbide are discussed in detail and critically reviewed.

Synthesis and consolidation of boron carbide: a review

The first part of this review looks at the fundamental properties of hydroxyapatite (HAP), the basic mineral constituent of mammalian hard tissues, including the physicochemical features that govern its formation by precipitation. A special emphasis is placed on the analysis of qualities of different methods of synthesis and of the phase transformations intrinsic to the formation of HAP following precipitation from aqueous solutions. This serves as an introduction to the second part and the main subject of this review, which relates to the discourse regarding the prospects of fabrication of ultrafine, nanosized particles based on calcium phosphate carriers with various therapeutic and/or diagnostic agents coated on and/or encapsulated within the particles. It is said that the particles could be either surface-functionalized with amphiphiles, peptides, proteins, or nucleic acids or injected with therapeutic agents, magnetic ions, or fluorescent molecules. Depending on the additive, they could be subsequently used for a variety of applications, including the controlled delivery and release of therapeutic agents (extracellularly or intracellularly), magnetic resonance imaging and hyperthermia therapy, cell separation, blood detoxification, peptide or oligonucleotide chromatography and ultrasensitive detection of biomolecules, and in vivo and in vitro gene transfection. Calcium phosphate nanoparticles as carriers of therapeutic agents that would enable a controlled drug release to treat a given bone infection and at the same be resorbed in the body so as to regenerate hard tissue lost to disease are emphasized hereby as one of the potentially attractive smart materials for the modern medicine.

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Nanosized hydroxyapatite and other calcium phosphates: chemistry of formation and application as drug and gene delivery agents.

Due to large grain sizes, the biological properties of the conventional calcium phosphate (Ca–P) bioceramics are limited to a great extent. Progresses in nanotechnological approaches now allow the fabrication of nanocrystalline Ca–P bioceramics. In this article we first review current methodologies of the Ca–P nanocrystal syntheses and nanoceramic processes. In particular, we emphasize in this article the fabrication of porous Ca–P nanoceramics using a modified co-precipitation synthesis and its microwave sintering. Subsequently, the biological properties of the three-dimensional porous Ca–P nanoceramics, involving protein adsorption, cell adhesion, bone repair, osteoconductivity and osteoinductivity, are introduced in detail on the basis of the in vitro protein adsorption and cell adhesion, and in vivo intramuscular and bone implant experiments. Because of high specific surface area, nano-level surface topography, high surface defects and interconnecting macropores with abundant micropores, the Ca–P nanoceramics can well initiate and regulate a cascade of gene activities of cells, thereby resulting in higher in vivo osteoconductivity and osteoinductivity than the conventional ones. Finally, the degradability, potential risk, and anticancer activity of the nanoceramics are discussed. In summary, because of the chemical and macro-/nanoscale structural similarities with bone, the Ca–P nanoceramics are hopeful of becoming a new generation of biomaterials for hard tissue repair.

Fabrication, biological effects, and medical applications of calcium phosphate nanoceramics

Calcium phosphate materials have been used increasingly in the past 40 years as bone graft substitutes in the dental and orthopedic fields. Accordingly, numerous fabrication methods have been proposed and used. However, the controlled production of spherical calcium phosphate particles remains a challenge. Since such particles are essential for the synthesis of pastes and cements delivered into the host bone by minimally-invasive approaches, the aim of the present document is to review their synthesis and applications. For that purpose, production methods were classified according to the used reagents (solutions, slurries, pastes, powders), dispersion media (gas, liquid, solid), dispersion tools (nozzle, propeller, sieve, mold), particle diameters of the end product (from 10 nm to 10 mm), and calcium phosphate phases. Low-temperature calcium phosphates such as monetite, brushite or octacalcium phosphate, as well as high-temperature calcium phosphates, such as hydroxyapatite, β-tricalcium phosphate or tetracalcium phosphate, were considered. More than a dozen production methods and over hundred scientific publications were discussed.

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Synthesis of spherical calcium phosphate particles for dental and orthopedic applications

The influence of precursor concentration, pressure, temperature and time of hydrothermal synthesis on the development of calcium hydroxyapatite structure has been analyzed. The obtained results show that it is possible to adjust the conditions of hydrothermal synthesis from solutions of relatively high concentrations to obtain calcium hydroxyapatite nanopowders of well-defined structure. The relationship between the synthesis and the lattice parameters, as well as the crystallite size and the microstructure of synthesized hydroxyapatite has been established. The synthesized powders are preferentially carbonated hydroxyapatite of the B type in the form of agglomerates that accommodate two-modal size pores of 1.5-10 and 50-200 nm. The structure of calcium hydroxyapatite particles consists of crystallites 8-22 nm in size, bound within prime particles, which size is between 10 and 63 nm, that in turn form bigger agglomerates 200 nm in size, which further cluster building up agglomerates 5-20 microm in size.

Hydrothermal synthesis and nanostructure of carbonated calcium hydroxyapatite.

Thermodynamics of gas phase carbothermic reduction of boron-anhydride

Wet spray pyrolysis of fine, well-dispersed a SiO2 sol was used for the deposition of thin films of silicon dioxide. The sol was obtained by hydrothermal precipitation of silicon acid from a solution at pH = 10. The morphology, roughness, phase composition, chemical homogeneity and the mechanism of the films were investigated by SEM, EDS and IR spectroscopy. The obtained results show a complete covering of the titanium substrate with SiO2 after 3 h of deposition. It was observed that the film thickness increased from 3 to 19 microm, the roughness of the film decreased from 12 to 3 microm, while the morphology of the deposit changed considerably. A hydroxyapatite film was prepared on the so-obtained SiO2 thin film by spray pyrolysis deposition and its morphology and phase composition were investigated.

Thin films of SiO2 and hydroxyapatite on titanium deposited by spray pyrolysis

Synthesis of nanostructured carbonated calcium hydroxyapatite by ultrasonic spray pyrolysis

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Papers

Hydrothermal synthesis and nanostructure of carbonated calcium hydroxyapatite.

Thermodynamics of gas phase carbothermic reduction of boron-anhydride

Thin films of SiO2 and hydroxyapatite on titanium deposited by spray pyrolysis

Synthesis of nanostructured carbonated calcium hydroxyapatite by ultrasonic spray pyrolysis