Biological apatites contain many trace elements, such as Mg²⁺, Fe²⁺, CO₃^2-, and F^-. These elements can affect significantly the physicochemical properties of apatites. Furthermore, the substitution of certain trace elements in the apatite crystal influences biological metabolism. Magnesium, for example, seems to promote the proliferation of osteoblasts and thereby the formation of bone.

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Needle-like apatites were successfully formed on titanium plate, bar, and mesh by a hydrothermal-electrochemical method below 200°C in an electrolyte containing calcium and phosphate ions. The deposited needle-like apatites were homogeneously and vertically oriented to the substrate surface. Morphology and crystallinity of the deposited apatite were easily regulated by the electrolytic conditions such as composition and temperature of electrolyte, current density, and substrate. It was confirmed that the electrochemical deposition of apatite had great effect on the promotion of the bioactivity of implant. Furthermore, we applied to produce a novel fabrication method of a composite membrane consisting of oriented needle-like apatite and biodegradable polymer for tissue regeneration in dentistry and medicine.

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Calcium chlorapatite single crystals were converted to hydroxyapatite single crystals by the hydrothermal treatments at 500°C for 12 h in a 6.25 M KOH solution. The mechanism of this topotaxial anion exchange is considered to be dissolution-precipitation of clusters. Calcium chlorapatite and hydroxyapatite single crystals were also converted to fluorapatite single crystals by the hydrothermal treatments at 500°C in a 5M KF solution but that of fluorapatite to hydroxyapatite proceeded by ordinary dissolution and precipitation of ions. The effects of metal ions in chlorapatite and the details of the reaction process were discussed.

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Various studies have been carried out on the biomaterial synthesis using magneto-vector effects, that is one of “vector parameters”. The synthesis of collagen/HAP with seeding osteoblast under 14T of high magnetic fields, namely “magneto-vector effect”, was attempted for high performance scaffolds. In addition, applying the magneto-vector effects, the fabrication of monolithic HAP and HAP with additive in-situ sintered under 10 T was carried out in order to investigate the effect of magneto-vector on the microstructure of HAP related bioceramics. In this paper, the recent results on vector processing for advanced biomaterials are reviewed. Our purpose is to synthesize the vector materials using the high magnetic field and to clarify “magneto-vector effects” on the synthesis of well-microstructured biomaterial.

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The biomaterials used in tissue engineering represent a major area of the current study. When the absorption of the biomaterial is too slow, bone formation is inhibited by itself Their bioabsorption for bone engineering should, therefore, coincide with the rate of endogenous bone formation. Bone consists of two major components, collagen and hydroxyapatite, and is a repository of bone morphogenetic proteins (BMPs). In the field of tissue regeneration, pepsin-digested collagen (so-called atelocollagen) is used for tissue engineering as one of the most useful scaffolds. Its excellent biocompatibility, due to its biological characteristics such as absorbable properties and low antigenecity, has elevated collagen to become a primary resource in medical applications. On the other hand, hydroxyapatite is classified as a non-absorbable material and is not harmonized with bone formation and remodeling. Therefore, there is a need for absorbable bioceramics that allow for bone formation and which gradually biodegrade, to be absorbed by the body and replaced by new bone. The BMPs, osteoinductive molecules discovered from bone, are thought to contribute to signal the local mesenchymal cells to proliferate and differentiate to osteoblasts. In this paper, the characteristics of collagen and ceramics, and animal and human studies using BMPs for bone engineering are reviewed.

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Several kinds of ceramics such as Bioglass ®, glass-ceramic A-W and sintered hydroxyapatite exhibit specific biological affinity, i.e. direct bonding to surrounding bone, when implanted in bony defects. These bone-bonding ceramics are called bioactive ceramics and utilized as important bone substitutes. However, there is limitation on clinical applications, because of their inappropriate mechanical properties such as high Young's modulus and low fracture toughness. Novel materials exhibiting high machinability and flexibility as well as high bioactivity have been desired in medical fields. It is known that bioactive ceramics bond to bone through bone-like apatite layer which is formed on their surfaces by chemical reaction with body fluid. This apatite formation can be well reproduced even in an acellular simulated body fluid (SBF) with inorganic ion concentrations nearly equal to those of human blood plasma. Previous researches have revealed that nucleation of the apatite is triggered by a catalytic effect of silanol (Si-OH) group formed on their surfaces and accelerated by the release of calcium ion (Ca²⁺) from the ceramics into the surrounding solution. These findings bring us an idea that bioactive organic-inorganic hybrids with high flexibility and machinability can be designed by chemical modification of Si-OH group and Ca²⁺ with organic polymers. In the present paper, we review several researches to obtain bioactive organic-inorganic hybrids by such an organic modification of Si-OH and Ca²⁺.

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The structural disorder in calcium-deficient hydroxyapatite (Ca-def HAp) was investigated by extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure spectroscopy (XANES). Ca-def HAp was prepared by hydrolysis of α-tricalcium phosphate. The Ca/P molar ratio in Ca-def HAp was 1.54. In order to synthesize Ca-def HAp with various Ca/P molar ratios, a hydrothermal treatment in Ca(NO₃)₂ aqueous solution for Ca-def HAp was carried out. The Ca/P molar ratio in the sample obtained by the hydrothermal treatment increased linearly with increasing reaction time. By hydrothermal treatment for 13 days, the Ca/P molar ration in the sample increased up to 1.65 which almost stoichiometric ratio. Ultraviolet-Visible (UV-VIS) absorption at 280nm appeared in Ca-def HAp and the peak intensities decreased with increasing Ca/P molar ratio in Ca-def HAps. The UV-VIS absorption, therefore, could be identified as intrinsic peak due to Ca vacancies in the HAp lattice. The fact also would suggest that Ca ions in the solution were taken into the vacancies in the lattice during the hydrothermal treatments. Intensity of background subtracted and normalized XANES spectra of Ca-def HAp was weaker than that of stoichiometric HAp The reason would be why the distortions are introduced by loosing of structure resulting from deficiency of Ca ion in HAp lattice. From EXAFS analysis of both Ca-def HAp and stoichiometric HAp, it was revealed that the coordination number of Ca ion in Ca-def HAp was lower than stoichiometric HAp. In addition, the Ca-O distance in Ca-def HAp became to be slightly longer than that of stoichiometric HAp Therefore, it could be concluded that the structural disorder resulting from deficiency of Ca ions will caused the extension of Ca-O distance in HAp structure.

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Effects of stress distribution and its change in vivo on density and orientation of biological apatite (BAp) were investigated in rabbit forelimb bones. Bone mineral density (BMD) and BAp alignment were analyzed by a peripheral quantitative computed tomography (pQCT) and a micro-beam X-ray diffraction system, respectively. The intact original rabbit forelimb bone consisting of an ulna and a radius exhibited a one-dimensional preferential alignment of the c-axis of BAp along the longitudinal direction, but the degree of orientation depended strongly on the distance from the neutral point on the bone cross section. This suggests that the BAp alignment is sensitively controlled by the bending stress in addition to the axial stress along the long bone. In the rabbit model with a 10mm defect on the ulna, the BAp alignment as well as BMD in the ulna sensitively changed depending on the loading condition during the bone healing in the defect. The BAp alignment decreased and increased during unloading and reloading, respectively. Therefore, it is concluded that not only BMD but also BAp alignment changes depending on the applied stress on the basis of the functional adaptation in bones.

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New types of proton-conducting phosphate hydrogels were prepared utilizing a chemicovectorial effect. Fine-sized magnesium metaphosphate, calcium metaphosphate, or zinc metaphosphate glass powders reacted with distilled water to hydrate immediately, resulting in formation of the hydrogels with high viscosity. ³¹P MAS-NMR spectra showed that the hydrogels include orthophosphates and long-chain phosphates coordinated to H⁺ or M²⁺ (M=Mg, Ca, Zn) ions; no significant differences in their structures among the hydrogels were shown. Electric double layer capacitors (EDLCs) with electrolytes consisting of the hydrogels showed no oxidation-reduction processes in the range of 0~1V and high rate charge-discharge capabilities. The EDLCs showed excellent self-discharge behaviors; they retained high open circuit potentials of the EDLCs after charging. EDLC consisting of zinc phosphate hydrogel showed much higher (3~6 times higher) specific capacity than those of magnesium phosphate and calcium phosphate hydrogels under the present condition. The possibility as an EDLC electrolyte may be related to polarizability of the hydrogels.

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Glass-ceramics of the phosphorus containing Na₅RSi₄O₁₂-type (R=rare earth; Sm) Na⁺-superionic conductors (Narpsio-V) were prepared by crystallization of glasses with the composition Na_3.9Sm_0.6P_0.3Si_2.7O₉. The effects of He⁺ ion implantation are discussed with respect to the conduction properties. Large decrease of ionic conductivity has been observed in the glass-ceramics by ion implantation of He⁺ ions at 200 keV with fluences of 10¹⁴ and 10¹⁵ ions/cm². Upon this implantation the ionic conductivity of the glass-ceramics at 300°C is decreased from 0.534x10^-1 S/cm (before ion implantation) to 0.021×10^-1S/cm (10¹⁴ ions/cm²) and to 0.039×10^-1 S/cm (10¹⁵ ions/cm²).

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We recently developed α-tricalcium phosphate (TCP) ceramic with continuous pores in the range 10-50 μm by a conventional sintering processing. It is important to control the degradation rate of porous biodegradable ceramics for an application as scaffold for bone regeneration. β-TCP is well known as biodegradable ceramic showing lower solubility than α-TCP. To develop porous calcium phosphate ceramics with controlled degradation rate, this study focused on fabrication of biphasic TCP porous body consisting of a mixture of α-and β-TCP. Their chemical durability was evaluated by exposure to aqueous solutions with pH=6. Biphasic TCP porous body with the continuous macro-pores was successfully fabricated through the same process of the α-TCP porous body by addition of Mg. The biphasic TCP ceramic formed octacalcium phosphate (OCP) after soaking in the buffered solution at pH=6 for I day, while delayed formation of OCP was identified from α-TCP porous body after soaking for 7 days. The addition of Mg to TCP composition easily brings about a production of biphasic TCP bodies consisting of α- and β-phase with continuous macro-pores, and allows control of degradation rate of the porous calcium phosphate ceramics.

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Diethylenetriamine- and pentanediamine-intercalated α-zirconium phosphates (α-ZrP) were synthesized and their adsorption properties for fragrant gases with aldehyde group such as citral, benzaldehyde and trans-2-hexenal were examined. Diethylenetriamine-intercalated α-ZrP adsorbed these gases more than pentanediamine-intercalated α-ZrP. These gases were adsorbed through the interaction of aldehyde group with amino or imino group of intercalated amines. The arrangement of these amines in the interlayer region changed after the adsorption of aldehyde gases.

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Adsorption mechanisms of Ponceau R (PR) by hydroxyapatite (HAP) in an aqueous phase were discussed. PR was adsorbed mainly through isomorpous substitution of phosphate ion on the surface of HAP with terminal sulfonate group of PR and through electrostatic attractive force between calcium ion on the surface and the sulfonate group of PR. Added NaCl interfered the adsorption of PR when the concentration of PR was low. This was explained in terms of ionic strength effect, that is, electrostatic attractive force between PR and HAP was weakened by the NaCl added. On the other hand, added NaCl accelerated the adsorption when a concentration of PR was high. This was due to the fact that the added salt dehydrated the PR ions which are adsorbed and/or located close to the surface of HAP, resulting in acceleration of hydrogen bonding between-OH groups and of the hydrophobic interaction between dimethyl benzene groups of PR ions on the surface of HAP, that is, lateral interaction of PR on the surface became significant.

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A number of researchers have used a simulated body fluid (SBF) to get an in vitro preliminary assessment of bone-bonding ability of an artificial material by examining apatite-forming ability in the SBF, before animal experiments. However, the common method for preparing the SBF and for detecting an apatite formed on a surface of a material in the SBF has not been established yet. The ion concentrations of the conventional SBF are not exactly equal to those of human blood plasma. In the present study, the detailed preparing method, and a newly improved SBF were proposed. In vitro assessments of apatite-forming ability in the conventional and proposed SBFs were performed.

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Biological apatite, in bones and teeth as hard tissue in human body, contains a few carbonate ions. Therefore, it isn't completely equal to stoichiometric hydroxyapatite (HA). Chemical solubility for an HA ceramic is especially affected to the carbonate ions in lattice. The purpose of this work has been the investigation of the electrical polarization effect on B-type carbonated hydroxyapatite (CHA) ceramics. B-type CHA was synthesized by wet chemical method, and the CHA was sintered at 800°C in carbon dioxide atmosphere. From result of FTIR analyses were shown the carbonate absorption bands belong to substitution for phosphate site (B-type) in the CHA ceramics. The CHA ceramics were polarized with voltage in various high temperatures, and analyzed to estimate the electrical stored charged in the polarized CHA ceramics by thermally stimulated depolarization current (TSDC) measurement. The TSDC spectra of the CHA ceramics showed the maxim current density around at 450-550°C, respectively. Bioactivities of the polarized CHA ceramics were evaluated by deposition of bone-like apatites crystals in simulated body fluid (SBF). The SBF immersion test demonstrated that positively charged surface of the CHA ceramic accelerated deposition of the bone-like apatites compared with that of non-polarized CHA ceramic.

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Chemical treatment of a Ti substrate with 6 mass% H₂O₂ and subsequent thermal treatment yielded surface titania gel with enhanced ability of spontaneous in vitro deposition of apatite or bioactivity. We examined the effects of the chemical treatment time and the concentration and pH of the H₂O₂, solutions on both the formation of titania gel and in vitro bioactivity. The optimum treatments consisted of 6 mass% H₂O₂ treatment at 60°C for 3-6h and subsequent thermal treatment at 400°C for 1h. The formation of the titania gel layer proceeded in a pH range of 4 to 4.6, while the corrosion of the titania gel layer was enhanced at a pH below 4.3. The reactions between the titanium substrates and H₂O₂ involved the oxidation of titanium, titania gel formation and Ti (IV) dissolution. The optimized chemical and thermal treatments(CHT method) were applicable for providing a Ti mesh screen with high in vitro apatite-forming ability as well as Ti plates with or without grooves.

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Zeta potential of hydroxyapatite (HAP, Ca₁₀(PO₄)₆(OH)₂) particle in an aqueous phase was measured in the presence of various concentrations of NaCl, sodium dodecylsulfate (SDS), and polyethylene glycol mono-p-isooctyl- phenyl ether (TX-100). It was found that the intact particle was negatively charged. Zeta potential of the particle became more negative with a concentration of SDS because of its adsorption. The potential decreased furthermore with a concentration of TX-100 at a given concentration of SDS when NaCl was not added to the system. On the other hand, the potential became less negative with a concentration of TX-100 at a constant concentration of SDS when 500 mmol/L NaCl was added. The mechanisms to explain these facts and the effect of NaCl in particular were discussed, taking into consideration the mixed micelle formation, hydration, adsorption of SDS and TX-100, and shifting of the slipping plane into an aqueous phase.

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Porous and dense hydroxyapatites were prepared at 900 to 1300°C in air atmosphere by the pressureless sintering technique. In the present paper, the usage of these synthesized hydroxyapatites ceramics and porous hydroxyapatite from the cattle bone as an industrial waste was attempted with a view to the removal of Cd²⁺ion from the polluted water. The pore characteristics of these hydroxyapatites sintered ceramics and cattle bone were evaluated by the mercury porosimeter. Especially, the ability of Cd²⁺ ion into these hydroxyapatites ceramics with various types of microstructures was investigated by an ICP equipment. Cd removability of HAp in 100 ppm Cd solution at pH 6.5 was twice higher than that of the active carbon. Especially, porous HAp and massive cattle bone possessed higher ability, compared to dense HAp, because of their high surface area.

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Hydrolysis of calcite was investigated in 0.1M KH₂PO₄ solution containing potassium fluoride (0 and 1005 ppm as F) at 37°C by means of solution analyses (pH, Ca, P and F concentrations), X-ray diffraction, ³¹P magic angle spinning nuclear magnetic spectroscopy (MAS NMR), Fourier-transformed infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). With the non-fluoride solution, octacalcium phosphate (OCP) was a main product of the hydrolysis for 10 days though some amount of calcite still remained. With increasing in fluoride concentration in the solution, OCP formed less and apatite phase formed instead. Remaining calcite was also decreased with fluoride concentration. Fluoride in the solution was completely absorbed in the product after hydrolysis for 10 days except for the highest fluoride concentration of 1000 ppm. FT-IR analysis showed that the apatite phase formed was fluoridated B-type carbonated apatite, in which CO₃^2- ion substituted for PO₄^3- ion. Fluoride ion was found to promote fluoridated carbonated apatite formation directly and/or through OCP transformation.

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Hydrolysis of calcite was investigated in 0.1 mol/L KH₂PO₄/KB₂PO₄ solutions with various pHs between 4.5 and 8.5 at 60°C by means of solution analyses (pH, Ca and P concentrations), X-ray diffraction, Fourier-transformed infrared spectroscopy and scanning electron microscopy. Calcite was mostly hydrolyzed within several days and the final product was octacalcium phosphate (OCP) or apatite phase depending on their initial pHs. OCP was initially formed in the solution with lower pH and it was transformed into apatite phase with increasing in pH. Calcite was directly transformed into apatite in 0.1 mol/L K₂HPO₄ solution with the highest initial pH. These phenomena were probably related to solubility of apatite, OCP and calcite at pH during hydrolysis. FT-IR analysis showed that the apatite phase formed was B-type carbonated apatite, in which CO₃^2- ion substituted for PO₄^3- ion.

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Details of the reaction of β-tricalcium phosphate (β-TCP) with NaOH solution at 90°C were clarified in a NaOH concentration wider range than that previously reported. At a 0.8 mass% NaOH solution, no reaction occurred even after 6 d. At 2-23 mass% the simple reaction β-TCP→hydroxyapatite (HAp) occurred and its completion reaction time reduced from over 24h at 2 mass% to within 1.0h at 23 mass%. At 24-33 mass% the consecutive reaction β-TCP→Ca(OH)₂→HAp occurred. where the first step was comparatively fast and the second was slow. The second step proceed completely at 24-25 mass%, whereas at 30-33 mass% reached an equilibrium mixture of 85%HAp and 15%Ca(OH)₂. At 34-50 mass% the simple reaction β-TCP→Ca(OH)₂ was completed within 0.5h, and no subsequent change occurred. The solubility singular points in the system β-TCP-Ca(OH)₂-HAp were estimated to be 23 mass% (7.1 mol/dm³) for the β-TCP-Ca(OH)₂ system, and 32 mass% (10.8 mol/dm³) for the Ca(OH)₂-HAp system.

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Ca-type phillipsite (PHI) synthesized from natural zeolite was covered with the needle-like scaly nano-apatite crystals by a hydrothermal treatment at 80°C for 8h under autogenous pressure. The crystal structure of PHI was not destroyed but the surface morphology was only changed by the formation of hydroxyapatite (HA) with 100×30 nm in size. The HA nano-crystals were grown in a reaction between Ca ions discharged from the nano-channels and cavities of PHI and phosphate ions in (NH₄)₃PO₄ solution as a reaction solution. The Ca ions in Ca-type PHI were involved in the formation of HA on the surface of NH₄-type PHI. The novel nanocomposite obtained here is an ideal material for adsorbing and fixing radioactive elements.

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Hydrothermal methods are useful for preparation of hydroxyapatite (HAp) whiskers. In particular, the method utilizing tripolyphosphate compound as a starting material has an advantage in that the whiskers can be prepared with high production efficiency at relatively low hydrothermal temperature. In the present work, large-sized HAp whiskers of 50~85μm in length were prepared by a hydrothermal treatment of calcium tripolyphosphate using a nitric acid to adjust the pH value before the reaction at 140°C for 24h. The whisker length was controlled by content of the nitric acid. Formation of by-products could be inhibited by addition of a trace amount of titanium dioxide.

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We have developed the novel and simple fabrication of porous hydroxyapatite by electrophoretic deposition (EPD) method in H₂O/ethanol system. In this study, the porous hydroxyapatite bulks through the controlled use of H₂ gas generation in the aqueous EPD process could be successfully prepared. Especially, it was found that this aqueous EPD process in combination with a sintering process could advantageously result in unique porous bulks containing unidirectionally aligned continuous pores with some hundreds μm in diameter. These unique microstructured porous hydroxyapatite prepared by EPD process will lead to the usage of porous fillers of bone etc.

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The scaffolds composed of monolithic β-TCP and β-TCP/α-TCP composites were fabricated by a low-loaded environmental processing. The pore introduction was attempted using starch, instead of polymer beads. The aqueous slurry of the mixture of β-TCP and starch was prepared for an aqueous slip-casting. Problem on the heterogeneity distribution of the starch in aqueous slip-casting was solved by the addition of calcium phosphate gel into the slurry of β-TCP/starch. Sintering of green bodies prepared by slip-casting was done by a pressureless sintering technique. The pore characteristics of sintered β-TCP/starch samples were evaluated. Their microstructures of sintered bodies were observed by SEM. Effect of starch and calcium phosphate gel on the microstructures of sintered β-TCP bodies was investigated..

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Organic polymer-hydroxyapatite (HAp) composites are much attractive due to the advantages of low Young's modulus and ability of bone-bonding, i.e. bioactivity. Cellulose (CEL) is one of abundant natural polymers and exhibits biological compatibility and high chemical durability. In the present study, CEL-HAp composites were prepared through mechanochemical reaction and the subsequent aging. The prepared composites contained needle-like low-crystalline HAp crystals with carbonate ions. The composites formed apatite on their surfaces after soaking in simulated body fluid (SBF) for 7 d. The composites are therefore expected to be useful bone-repairing materials with low Young's modulus and bioactivity.

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A novel self-curing paste, which is composed of calcium phosphates powder and acetyl cellulose dissolved in ethyl lactate, has been developed as a bone-repairing material that does not need any mixing operation before its use and sets in the body. This paper focused on investigations of the mechanical properties and bioactivity of the paste after exposure to physiological solutions. Compressive strength of the set paste increased with increment of the polymer concentration and of calcium phosphates powder/polymer solution (P/L) ratio, and strain at the yield point increased with increasing the polymer concentration and with decreasing P/L. The mechanical properties of the set paste including calcium phosphates and acetyl cellulose were analogous to those of human cancellous bone. The paste was able to form bone-like hydroxyapatite on its surface in a simulated body fluid (SBF). This indicates that the paste has a potential to show bone-bonding ability, i.e. bioactivity. From these results, this paste can achieve both the mechanical properties corresponding to human cancellous bone and show bioactivity as well as easy handling.

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A continuous series of solid solutions in the system of strontium and calcium hydroxyapatite, Ca_10-xSr_xHAP (x= 0-10), was successfully synthesized by both high-temperature mixing method (HTMM) and low-temperature mixing method (LTMM) at 200°C for 12 hours under hydrothermal conditions. The samples were characterized by chemical analysis, electron microscopic observation and X-ray powder-pattern fitting. The site of the metal ions in the solid solutions was analyzed by the Rietveld method. The lattice constants of the solid solutions prepared by both of the methods varied linearly with Sr contents. It was found that Sr²⁺ ions in the solid solutions prepared by these two methods preferentially occupied the Ca (II) site in the apatite structure.

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Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: HA) is widely used as bioceramics. In general, sintering process is indispensable to give shape to the ceramics. However, it is impossible to control the crystal face of materials surface. Using hydrothermal reactions for HA preparation, it is possible to control the crystal face and the chemical composition. The authors reported various kinds of HA materials prepared by the unique hydrothermal methods.
In the present investigation, porous HA sheet with tailored crystal surface were prepared by the hydrothermal method. The HA sheet prepared at 120 °C under the saturated vapor pressure of water for 3h was composed of rod-shaped HA crystals. These crystals were about 30 μm in length with aspect ratio of about 10 and they had non-stoichiometric HA composition of Ca/P molar ratio less than 1.67. This material must be suitable as scaffold for cultured bone, for bone graft material and for drug delivery system (DDS).

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The present work was investigated about hydrothermal synthesis of Ce³⁺ and Mn²⁺ activated calcium hydrogenphosphate anhydrate (CaHPO₄:Ce³⁺,Mn²⁺) phosphors. CaHPO₄:Ce³⁺,Mn²⁺ was a novel unprecedented phosphor, which was synthesized by hydrothermal treatment of dipping amorphous calcium phosphate (ACP) in formic acid aqueous solution. The emission intensity of the CaHPO₄:Ce³⁺,Mn²⁺ phosphor was affected remarkably by synthetic conditions such as Ce/Ca and Mn/Ca atomic ratio, concentration of formic acid, temperature and aging time. The optimum conditions for the synthesis of CaHPO₄:Ce³⁺,Mn²⁺ phosphor were follows; Ce/Ca atomic ratio, 0.01; Mn/Ca atomic ratio, 0.02; concentration of formic acid, 1.0 mol·dm^-3, hydrothermal temperature, 170°C; retention time, 0 h.

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Hydroxyapatite was sintered by either the pulse electric current sintering (PECS) process or pressureless sintering (PLS) process in this paper. Sintering by PECS process at 700°C with a heating rate of 100°C/min under applied pressure of 30MPa led to the dense hydroxyapatite bodies without other calcium phosphate phases. Their samples of hydroxyapatite prepared at 700°C by PECS showed very fine microstructure with the average grain size of approximately 150 nm and very narrow grain size distribution, whereas samples sintered by PECS over 800°C revealed the tremendous grain growth in the matrix. Nevertheless, it was found that the significant decrease of sintering temperature for dense hydroxyapatites could be achieved by PECS, compared to the PLS process with a heating rate of 10°C/min. The effect of the sintering process on the microstructure of hydroxyapatite bodies was discussed.

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Calcium hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂: HA, is the inorganic principle component of natural bones and teeth, The present study deals with the preparation of HA ceramics with a different OH amount by spark plasma sintering (SPS) from HA fine crystals synthesized hydrothermally. The dense hydroxyapatite ceramics with controlled OH were prepared at the temperatures from 800°C to 1000 °C for 10 min. Transparent hydroxyapatite ceramics were prepared by SPS at 900 °C and 1000 °C. The amount of OH ion in the structure of hydroxyapatite was decreased with increasing temperature of sintering. The hydrothermally treated HA fine crystals have the high sinterability.

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Crystallization of amorphous calcium phosphate (Ca-P) coating on Ti substrate has been investigated by Fourier transform infrared absorption spectroscopy and transmission electron microscopy (TEM). High resolution TEM was also used to elucidate crystal structure of the crystallized grains. The Ca-P coating on Ti substrate was performed by radio frequency magnetron sputtering of a sintered hydroxyapatite (HAp) target. As-sputtered Ca-P coating having 1.95 of Ca/P ratio was amorphous. Annealing the as-sputtered amorphous coating at 600°C resulted in crystallization of the coating. Crystallized coating mainly consisted of tetracalcium phosphate (TTCP) and HAp grains. Grain sizes of TTCP and HAp were about 10-100 nm in diameter. CaO grains were also observed only near surface of annealed coating.

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Hydroxyapatite coating (HA-C) material has been applied as orthopedic and dental implant, and we have investigated several properties of hydroxyapatite coating (HA-C) on titanium (Ti) substrate. Recently, in order to shorten the period until osteointegration and to increase the bone contact rate, surface modification of Ti and its alloys have been reported. However, Ti was used as a substrate in this study. Ti surface was modified by concentrated NaOH solution and subsequent heat treatment. The surface modified substrate was soaked into calcium and phosphate solutions to obtain apatite coating layer. The obtained layer was verified to hydroxyapatite (HA) by fourier-transform infrared spectroscopy (FT-IR) and thin-film X-ray diffractometry (TF-XRD). The bioactivity of this coating layer has been confirmed by the simulated body fluid (SBF) immersion.

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Single-crystal apatite fibres were synthesized from aqueous solutions in the Ca(NO₃)₂-(NH₄)₂HPO₄-HNO₃systems by a homogeneous precipitation method using urea. The resulting fibres with long-axis sizes of 60-100μm were composed of carbonate-containing apatite with preferred orientation along the c-axis. We have developed porous scaffold for tissue engineering of bone using the single-crystal apatite fibres. The resulting apatite-fibre scaffolds have large pores with diameters of 110-250μm and high porosities of 98-99%. The scaffolds were biologically evaluated using two kinds of cells, osteoblastic cells (MC3T3-E1) and rat bone marrow cells. In both cases, the cells cultured in the scaffolds showed excellent cellular response, such as good cell proliferation and enhanced differentiation into osteoblasts. We conclude that such scaffolds with high porosity and large pore size may be effective as the matrix of tissue engineered structures for promoting regeneration of bone.

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