We have successfully synthesized single-crystal apatite fibers (AF) by a homogeneous precipitation method with urea. The AF has a higher solubility than isotropic HAp powders. Thus, using the AF, we developed apatite-fiber scaffolds (AFSs) that enables three-dimensionally cell culture and induces differentiation into osteoblasts. Recently, we have successfully enhanced mechanical properties of the AFSs using carbon beads (CB) with a diameter of 150 μm and 20 μm, together with uniaxial pressing of the green compacts. In this study, we fabricated the AFSs with enhanced mechanical property (AFS700(50-50)), and then implanted them into tibia of pig to evaluate a biocompatibility to a hard tissue. The porosity of the resulting and compressive strength of typical AFS700(50-50) were 93.5% and 128 kPa, respectively. The AFS700(50-50) had a number of micro pores, macro-pores and interconnected pores. In the histological observation after 13 weeks implantation, newly-formed bone and cells were invaded into pores of the AFS700(50-50), and the scaffolds were incorporated into a cycle of bone remodeling. The AFS700(50-50) was also observed to be replaced with autologous tissue. The present scaffold may be expected as a high performance scaffold for bone regeneration.

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High-density β-tricalcium phosphate (β-TCP) ceramics with a small amount of tri- and pentavalent vanadium ions addition (≤1.0 mol%) were successfully prepared by hot pressing. Tri- and pentavalent vanadium ions were substituted for a Ca ion at the Ca(5) site located in β-TCP crystal structure and for a P ion in the PO₄ group, respectively. The sinterability and mechanical properties of β-TCP doped with vanadium ions (V-β-TCPV) were improved with increasing vanadium ion content. The bending and compressive strengths of V-β-TCPVs were comparable to those of the hard tissue of a living body.

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Hydroxyapatite (HAp) is the principal inorganic ingredient of human teeth and bone, and well-known as an excellent biocompatible material. HAp has been found to be polarized with an external electric field at a high temperature. Electrets are defined as the solids with functions that quasi-permanent surface charges. However, fundamental characteristics of electric fields due to HAp electrets have never been reported. Hence, this purpose is investigation of the properties of the surface electric fields of HAp bioceramic electrets; the surface potentials due to overlapping plural electrets with different electric potentials and signs; and a screening effect on electric fields with electrets due to a conductor and a dielectric. Polarized charges of the HAp electret have generated an electric field around the electret. The principle of superposition has held for the surface potentials of plural electrets. The screening effect by a conductor or a dielectric has been observed.

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Calcium phosphate glasses and glass-ceramics with some limited compositions show bioactivity. The glasses could be prepared by including a small amount of TiO₂ and/or Na₂O: the typical compositions were 60CaO-30P₂O₅-xTiO₂-(10−x)Na₂O in mol%. The glasses include orthophosphate and pyrophosphate groups. Hydroxyapatite (HA) forming ability in a simulated body fluid (SBF) is often examined to know the chemical properties of materials for biomedical applications. On the 60CaO-30P₂O₅-xTiO₂-(10−x)Na₂O glasses, HA formed within 7-10 days. In the case of the glasses, the HA-forming ability is suggested to be influenced by various factors such as the basicity of a gel layer formed on them in SBF and the amount of the functional groups for HA nucleation in the layer. Modification of the glass composition, which MgO was substituted to CaO, improved the glassification tendency of the glasses due to the high field strength of Mg²⁺. The MgO-containing glass included a larger amount of orthophosphate group than the original glass, and showed higher ion-releasing ability. Bivalent ions in the phosphate invert glasses play an important role in their glassification and ion-releasing. The glasses were crystallized and subsequently sintered by heating their powder-compacts. The resulting glass-ceramics consist of β-Ca₃(PO₄)₂ and β-Ca₂P₂O₇ with residual glassy phase. Their HA-forming ability in SBF was enhanced after autoclaving in distilled water. By the autoclaving, sodium, calcium, phosphate ions in the glassy phase around the surface dissolved and anatase crystals formed. The glass-ceramics could be coated strongly on a new β-type titanium alloy using a conventional glazing technique. The glass-ceramic-coated materials were implanted into the femurs of Japanese rabbits. After 1 month, bone tissue contacted directly with the coating. After 5 years of implantation, the coating was clarified to be work as a bioactive material on the titanium alloy.

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The formation of strong bond between material and bone tissue, studied by Hench, Kokubo, Yamamuro, or others in 1980's is revisited to obtain some insight into apatite deposition on meso- or microporous silica gel, derived by phase separation in the system tetraethoxysilane, polyethylene glycol, and water. Testing the apatite-depositing ability on the silica gels indicated that the increase in calcium, phosphate, and hydroxyl ions in Kokubo's simulated body fluid differently affected the apatite nucleation rate, and suggested necessity of modifying the original simplistic model of apatite nucleation: calcium ion adsorption triggered nucleation. Apatite formation was reviewed on anatase layer prepared on Ti due to chemical (H₂O₂) and heating treatments (CHT procedure), and balanced clustering of calcium and phosphate ions is significant for the apatite nucleation. CHT layer is proposed as a reference material for bone-bonding testing.

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Adsorption behavior of bovine serum albumin (BSA) and osteopontin (OPN) on osteoconductive hydroxyapatite (HA) and non-osteoconductive alpha-type alumina (α-Al₂O₃) was studied as function of protein concentration and pH, using Bradford dye binding assay. BSA showed a much larger binding capacity on α-Al₂O₃ than that on HA irrespective of BSA solution concentration and pH. Furthermore, OPN is likely to show the same adsorption characteristic as BSA. Definite correlation was not observed between the albumin or OPN adsorption capacity and the osteoconductivity of materials, suggesting that other factors (e.g., orientation, arrangement, etc. of albumin and/or OPN) likely govern expression of the osteoconductivity.

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Spherical β-tricalcium phosphate (β-TCP) granules composed of rod-shaped particles are expected to be useful as bioresorbable bone substitutes. The β-TCP granules can be prepared by the heat treatment of Ca-deficient hydroxyapatite (HA) granules that are synthesized by hydrothermal treatment of α-TCP granules. However, when the Ca/P molar ratio of Ca-deficient HA becomes higher than 1.50, depending on the hydrothermal conditions, not only the β-TCP but also the HA phase appears following heat treatment. To resolve this problem, phosphate ions were incorporated into the Ca-deficient HA granules or monetite was added to decrease the Ca/P molar ratio to less than 1.50 in the starting composition. We revealed that these processes were effective in obtaining the pure β-TCP granules composed of rod-shaped particles and thus preventing the formation of HA.

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Octacalcium phosphate (OCP) has known to be a precursor phase of biological apatite in bones and teeth. The aim of this study was to evaluate OCP-mediated cement as a root canal filling material for primary teeth. According to the characterization with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), OCP-mediated cement has transformed into dentin-like carbonated hydroxyapatite 6weeks after immersion in medium with 30% fetal bovine serum. The canal sealing ability was estimated by dye penetration test. The penetration depth of India ink into the OCP-mediated cement decreased significantly with time. The sealing ability of OCP-mediated cement improves along with the apatite formation.

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Fe(III)-treated hydroxyapatites, HAP-300-Fe and HAP-400-Fe, were prepared from hydroxyapatites, HAP-300 (stoichiometric type) and HAP-400 (Ca-deficient type), respectively, and applied to photo-Fenton reaction of dyes as heterogeneous catalysts in comparison with other Fe catalysts such as Fe₂O₃, FePO₄·2H₂O, and FeOOH. Among the Fe catalysts, HAP-400-Fe revealed the highest consumption and mineralization in photo-Fenton reaction of alizarin red S (ARS), and also indicated good catalytic activity to other dyes, alizarin, neutral red, methyl red, methyl orange (MO), Congo red, and indigo carmine. Generation of hydroxyl radical •OH as an oxidation species from H₂O₂ was observed in the all Fe catalysts by an indirect method using N, N-dimethylamino-4-nitrosoaniline, which is exclusively sensitive to •OH. In order to elucidate catalysis mechanism of Fe catalysts, kinetic investigation based on Michaelis-Menten theory was conducted in the reaction of ARS and MO. As a result, HAP-400-Fe indicated high ability of bearing many dye molecules but low degradative catalysis ability to H₂O₂, compared with Fe₂O₃. However, judging from high mineralization ability as well as repeating availability (5 times), HAP-400-Fe is considered as an excellent heterogeneous catalyst for photo-Fenton reaction.

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The present study was designed to investigate whether an octacalcium phosphate/gelatin (OCP/Gel) composite can repair a defect created in rabbit tibia within 4 weeks. Histological and μCT studies showed that the implantation of OCP/Gel composite (40 wt% OCP) enhances cortical bone deposition over the defect accompanied by the formation of cartilage and bone marrow with cancellous bone within 2 weeks. The defects were repaired even in the control group (defect only) within 4 weeks; however, the repaired cortical bone was thinner than that in the OCP/Gel-implanted group. The results suggest that an OCP/Gel composite could be effectively used as a bone substitute material for autografts.

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In order to enhance the handling ability and mechanical properties of chelate-setting β-tricalcium phosphate (β-TCP) cement, various polysaccharides, such as sodium alginate, sodium dextran sulfate, sodium chondroitin sulfate, and chitosan (Chito GL and W10), were added to mixing solutions for cement preparation. When mixing solutions containing the above additives were used, the handling ability of all the cement pastes was improved compared with the case of pure water without polysaccharide. Among the examined cement specimens, the cement with the highest compressive strength (17.4 MPa) was obtained using Chito W10. Although the viability of cells co-cultured with cements fabricated using additives was less than that of the control and water on the first day, the number of cells increased until the fifth day. The addition of chitosan (Chito W10) into the mixing solution is promising in the fabrication of chelate-setting β-TCP cement with enhanced handling ability and mechanical properties.

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Bio-absorbable granules originated from animal bone were easily prepared by using high velocity rotation- crushing apparatuses. Frozen bovine bone, porcine bone and human femoral bone were crushed together with saline ice for 1 min under 12,000 rpm-rotation number of a stainless-blade in a stainless vessel or a ZrO₂-blade in a ZrO₂ vessel. The crushed granules exhibited granular sizes of 0.5-2 mm, which were efficient for regeneration of alveolar bone. They were dissolved at 293K under 500 rpm for 20 min in 2.0% HNO₃ aqueous solutions to obtain completely demineralized bone matrix (DBM) granules. The DBM granules were implanted into the subcutaneous tissue of the back region in nude mice. At 4-6 weeks after the implantation, osteoinduction or cartilage-induction as well as bio-absorption were recognized on the surface layers of both DBM granules. The DBM granules may be applied as new biomimetic materials for bone-regeneration therapy and physiological activation substances for biochemistry.

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Octacalcium phosphate (OCP) has a biological affinity for living bone. It is worthwhile investigating the behavior of crystal growth, transformation and dissolution of OCP crystals under physiological conditions, with a similar concentration in inorganic composition to that of human blood plasma, to understand the affinity for living bone. We examined the behavior of OCP crystals after exposure to a simulated body fluid (SBF). No crystalline phase changes were exhibited by samples soaked in SBF, but formation of calcium phosphate was assumed to occur because of decreases in concentration of both calcium and phosphate ions in SBF. The molar ratio of the reduction in calcium and phosphate ions concentrations (ΔCa/ΔP) at 1 day was 1.42, eventually converging to a value of 1.33. These results indicated that OCP was formed on OCP crystals through an intermediate amorphous phase and OCP soaked in SBF was transformed into hydroxyapatite to only a negligible extent in the solution.

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Yttria-stabilized zirconia is currently used as an orthopedic and dental material, because of its excellent mechanical properties. In this study, we have improved the bioactivity of yttria-stabilized zirconia by a combination of electrical polarization and chemical treatment. The phase transformation from tetragonal to monoclinic ZrO₂ after alkaline treatment was inhibited on positively charged yttria-stabilized zirconia surfaces compared with negatively charged and conventional surfaces. During polarization, some oxide ions move from the positively charged surface to the negatively charged surface, leading to an increase in oxygen vacancies on the positive surface and hence greater formation of Zr-OH when this surface was exposed to alkaline solution. The bioactivity was assessed by immersing the samples in simulated body fluid and evaluating the growth of apatite on the surfaces. The combination of polarization and alkaline treatment increased the bioactivity in vitro.

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Hydroxyapatite (HAp) nano-particles were synthesized by a hydrothermal process under different initial pH conditions (pH 9.00∼10.50). All resulting particles were identified to be single-phase HAp. With increasing of initial pH values, Ca/P molar ratios increased closely to that of the stoichiometric HAp value of 1.67. The morphologies and ultrastructures of these particles were observed using high-resolution transmission electron microscopy. These results indicated that the resulting particles were fiber-shaped single crystals with preferred orientation to the a-plane at low pH values. However, at high pH values, the resulting particles were plate-shaped polycrystalline crystals with preferred orientation to the c-plane.

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Manganese (II) ions were substituted for Ca²⁺ ions at the Ca(4) and Ca(5) sites located in the A column of the β-tricalcium phosphate (β-TCP) crystal structure, and effects of additions of Mn²⁺ ions on proliferation and differentiation of osteoblasts were examined using mouse MC3T3-E1 osteoblast-like cells. The X-ray diffraction patterns of β-TCP doped with manganese (II) ions (Mn-β-TCP) indicated the formation of β-TCP solid solution to 13.64 mol%. MC3T3-E1 cells attached on Mn-β-TCP well proliferated compare to those on β-TCP. In particular, Mn-β-TCP doped with 3 mol% showed the most excellent cell proliferation and the highest ALP activity level. We consider that the new bone formation may be promoted by dissolution of calcium ions and manganese ions from Mn-β-TCP. The present Mn-β-TCP is expected as an implant biomaterial with enhanced bioactivity.

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Morphological control of hydroxyapatite (HAp) is very difficult because this compound has very low solubility. However, CaCO₃, a calcium salt with the same insolubility, dissolves easily in solutions containing CO₂. Therefore, HAp was easily dissolved by CO₂ blowing into the HAp suspension. The dissolution amount was 200 times that found without CO₂ blowing. Thus, a high-concentration HAp solution could be prepared through the simple process of CO₂ blowing. Moreover, flower-like HAp particles were deposited by outgassing the dissolved CO₂ in the HAp solution under heating.

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II-CaSO₄-80SiO₂-20P₂O₅ glass composites were successfully prepared as bulk or film by sol-gel method, and characterized by XRD, FT-IR, SEM, and EDX. In this composite system, SEM and EDX analyses suggested that CaSO₄ was not dissolved into glass or decomposed during the preparation process. Apatite-forming ability of the prepared film was evaluated by SBF immersion experiment and apatite-formation by alternate soaking process. This composite was partially dissolved in SBF, that is, partial bioresorbability was exhibited. By ASP, Ca-deficient hydroxyapatite was found to be formed onto the composite film surface, easier than glass without II-CaSO₄ surface.

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The aim of this work is to develop a highly-functional tissue engineering scaffold from silicon-containing apatite fibres (Si-AF). Firstly, Si-AFs were synthesized by a homogeneous precipitation method. Starting solution with a Ca/(P+Si) ratio of 1.67 was prepared by mixing Ca(NO₃)₂·4H₂O, (NH₄)₂HPO₄, Si(OC₂H₅)₄ (TEOS), (NH₂)₂CO and HNO₃. The concentrations of TEOS in the starting solution were 0 (AF), 0.8 (0.8Si-AF) and 1.6 (1.6Si-AF) mass%. Next, Si-AF scaffolds (Si-AFS) were fabricated by firing the compacts consisting of Si-AF and carbon beads of 150 µm diameter. The resulting Si-AFSs were characterized using X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Results showed that the material properties of the 0.8Si-AFS were similar to those of the conventional AFS without silicon. Consequently, we can conclude that the 0.8Si-AFS offers as a potential novel scaffold material, creating a three-dimensional cell culture environment.

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We have developed calcium-phosphate cement (CPC) that can be formed into any desired shapes during an operation. The novel hydroxyapatite (HAp) and tricalcium phosphate (TCP) cements set on the basis of chelate-bonding ability of inositol phosphate (IP6). In the present study, we evaluated dissolution rate of various CPCs under conditions of acetic acid buffer solution at pH5.5. The concentration of calcium ions released from the CPCs with HAp, β-TCP and α-TCP phases in the acetic acid buffer solution was continuously measured using a calcium ion meter. The dissolution rate of samples decreased in order of α-TCP > β-TCP > HAp derived from wet process (wet-HAp) > commercially-available HAp.

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Two types of β-tricalcium phosphate/poly(lactic-co-glycolic acid) scaffolds were prepared by an electrospinning method. Mechanical tensile properties and cell proliferation behavior of the conventional nonwoven fibermat (Sample-A) were compared with those of another type of fibermat including flat areas around cross-linked portions between skeletal fibers (Sample-B). Sample-A showed larger elongation in the mechanical-tensile test than Sample-B. Murine osteoblast-like cells adhered between the fibers in Sample-A, while they adhered and spread on the flat faces in Sample-B. Remarkable proliferation of the cells was observed on the Sample-B at the early stage of the culture.

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Tricalcium phosphate doped with a small amount of both V(III) and V(V) ions (V-β-TCPV) was successfully prepared using a solid-state reaction, and its in vitro cell cytotoxicity was investigated using V79 cells. We found that the V(III) ion was substituted for a Ca ion at the Ca(5) site located in the β-Ca₃(PO₄)₂ (β-TCP) crystal structure and that the V(V) ion was substituted for a P ion in the PO₄ group of the β-TCP crystal structure. The cell cytotoxicity of the V-β-TCPVs increased gradually with increasing amounts of vanadium ion doping. The β-TCP doped with 1.0 mol% vanadium ions showed low toxicity. V-β-TCPVs doped with a small amount of vanadium ions are expected to be significant new implant biomaterials.

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The heading title of the article [Phosphorus Research Bulletin 26, 39-52 (2012)] was incorrect because of the editor's mistake on the volume number. The corrected volume number of the heading title is "Vol.26".

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