Papers by Author: Chikara Ohtsuki

Abstract: We synthesized TiO₂ nanoparticles dispersed hydroxyapatite (TiO₂-modifed HAp) by solvothermal treatment of α-tricalcium phosphate (α-TCP) using water-isopropanol solution including tetraisopropyl orthotitanate (TTIP). Rod-shaped HAp with TiO₂ nanoparticles were formed by solvothermal treatment. HAp formation decreased with increasing the isopropanol fraction in the solution with TTIP. With increasing the treatment period, the rate of HAp formation in sample increased. The aspect ratio of HAp decreased with increasing the isopropanol fraction in the solution. Synthesized TiO₂-modifed HAp samples shows the photocatalytic decoloration of methylene blue under UV irradiation.

Abstract: Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) can be obtained from calcium carbonates through dissolution-precipitation reaction in a phosphate solution under a hydrothermal condition, with keeping its external shape. In this study, we assumed preparation of hydroxyapatite honeycombs from a calcite (CaCO₃) honeycomb. Calcite honeycomb was hydrothermally treated in a phosphate solution. After hydrothermal processing for 24 h, calcite transformed partially to hydroxyapatite phase and its external shape was kept. Moreover, specific surface areas of the specimens were increased after the hydrothermal processing. Consequently, this processing is useful to prepare honeycomb structure of hydroxyapatite from calcium carbonates.

Abstract: α-tricalcium phosphate (α-TCP), which shows higher solubility than β-TCP, is bioabsorbable and receives special attention for its ability of turning into hydroxyapatite (HAp) in a physiological condition. Problems occasionally occur as α-TCP porous body is too brittle to be handled. Compressive strengths of β-TCP and HAp dense sintered blocks are reported to be comparable to that of human cortical bone. However, α-TCP dense body has rarely been reported. For fabrication of dense sintered body, two-step sintering (TSS) was applied in this study. The TSS generally has firing processes at the lower temperatures, following that at the higher temperatures. TSS is known as one of most effective processing to prevent grain growth comparing with conventional sintering (CS). Dense body of α-TCP was fabricated by both CS and TSS. TSS processing involves heating specimens to T₁ temperature, followed by holding at a relatively lower T₂ temperature for 12 h. Microstructures of the sintered bodies were characterized and mechanical properties were also evaluated. The specimen prepared by TSS processing with T₁ 1400 °C, T₂ 1300 °C showed the lowest porosity (2.7%) and highest compressive strength (714 MPa) among the prepared specimens. TSS processing might be applicable on densification of calcium phosphate powders to fabricate dense body.

Abstract: We reported the apatite-forming ability of CaO-SiO₂ spherical particles obtained through a sol-gel processing. In this study, we synthesized the CaO-SiO₂ containing silver (Ag) ions using silver standard solution to induce antibacterial property and evaluated the effects of Ag on their apatite forming ability in a simulated body fluid (SBF). The particles with 30CaO·70SiO₂ starting composition in Ag standard solution with range 0 to 500 ppm Ag content were synthesized through the sol-gel processing. The prepared 30CaO·70SiO₂ could be obtained as spherical particles with regardless of Ag contents. The prepared Ag-contained 30CaO·70SiO₂ particles formed apatite on their surfaces after soaking in SBF regardless of Ag contents. Consequently, Ag-contained 30CaO·70SiO₂ spherical particle could be obtained without decrease of its apatite forming ability.

Abstract: Osteoconductive bone-repairing materials with mechanical properties analogous to those of human bone can be prepared through the combination of an osteoconductive ceramic filler with an organic polymer. Osteoconduction is archived from apatite formation on substrates. Previously, we reported that novel osteoconductive spherical particles in a binary CaO-SiO₂ system were produced through a sol-gel process as ceramic filler for the fabrication of composites. In this study, we fabricated the composites consisting of polyetheretherketone (PEEK) and 30CaO·70SiO₂ (CS) spherical particles and evaluated the effects of heat treatment in the range of 320-360 °C on apatite formation of the composites in a simulated body fluid. The prepared composites of PEEK and CS particles form hydroxyapatite on their surfaces in the simulated body fluid. The induction periods of hydroxyapatite on the composites decreased with increasing the amount of CS particles and decreasing the temperature for heat treatment. The apatite formation was affected by exposure of ceramic filler on the polymer matrix.

Abstract: Calcium phosphatepolymer composites have been produced for bone-repairing. We have focused development of composites by a crystal growth technique in a hydrogel matrix, that is regarded as gel-mediated processing. Under the gel-mediated condition, reaction temperature is one of the major parameter to determine microstructure of the precipitated crystals. In the present study, we investigated effects of the reaction temperature on formation of calcium phosphates through gel-mediated processing where double diffusion technique was applied. Crystalline phases of calcium phosphate formed in the hydrogel were varied from octacalcium phosphate (OCP) to hydroxyapatite (HAp) with increasing the reaction temperature. OCP crystals formed at 4 and 40 °C had granular or spherical shape, while HAp crystals formed at 80 °C had rod shape. The HAp crystals were composed of a large number of fibrous crystals. The rod-shaped HAp crystals were oriented in the direction of ion diffusion. Formation of oriented HAp crystals was generated by increase in diffusion rate of ions in the hydrogel matrix after increasing reaction temperature in gel-mediated processing.

Abstract: Morphological control on hydroxyapatite (HAp) crystals is one of attractive researches to produce novel bioactive materials for repairing bone defects. Hydrothermal processing has been applied to fabricate well-grown crystals of HAp, through a reaction from appropriate calcium and phosphate sources. We recently found that oriented structure was observed when single crystal of calcium carbonate was used as a starting material for the hydrothermal processing. However, the detailed process on the orientation of HAp crystals has been not clarified yet. In the present study, we investigated the effects of the crystalline planes of calcite on HAp formation through the hydrothermal condition. Calcite single crystals with either of {100} or (111) plane was used as a starting material to examine the difference of the reactivity in a phosphate solution. After the hydrothermal treatment at 160 °C for 24 h, the surfaces and cross-sections of starting materials were characterized. Rod-shaped HAp crystals were oriented on the surface of the starting material with {100} plane, whereas small HAp crystals formed on the starting material with (111) plane followed by growth to same direction that were observed on the sample with the {100} plane. The difference in the morphology of the formed HAp was caused by the reactivity of each crystal plane.

Abstract: Chemical modification of titanium substrate provides ability of hydroxyapatite (HAp) formation that is important property for bone-bonding capability after implantation in bony defects. Potential of the HAp-formation is occasionally reduced. In the present study, we investigated potential of the HAp-formation on titanium oxide (titania) with doped silicon or aluminum in simulated body fluid (SBF). Sol-gel processing was applied to prepare titania with doped silicon (TSx) or aluminum (TAx), in its nominal composition ranging from 0 to 10 mol%. Specific surface area of the prepared samples was gradually increased with increasing the amounts of silicon or aluminum. Zeta potential of TAx was definite changed from negative charge to positive charge with increasing aluminum amounts, but TSx slightly changed to be positive with increasing silicon amounts. The pure titania sample free from doping of silicon or aluminum showed formation of calcium phosphate precipitates, that is HAp-formation, after soaking in SBF for 14 d. In contrast, all the titania samples with doped silicon or aluminum hardly showed evidence of precipitates of calcium phosphates, although absorption of calcium and phosphate ions were detected. Especially, TAx showed remarkable adsorption of phosphate ions. Aluminum-doping in titania enhances the adsorption of phosphate ion on the surface, but reduce nucleation rate of calcium phosphates in body environment.

Abstract: For the purpose of bioinert coating on electronic devices, we developed the non-hydrolytic sol-gel derived organic-inorganic hybrid materials by addition of epoxy groups which can adhere strongly to the surface of electronic silicon device. The adhesion and chemical properties of hybrids were investigated as a function of epoxy group contents. The hybrids were prepared from 3-metacrloxypropyltrimethoxysilane (MPTS) and 3-glycidoxypropyltrimethoxysilane (GPTS) and diphenylsilanediol. The transparent hybrids were obtained after curing by UV irradiation. The adhesion properties of the hybrids were estimated by the maximum load to resist in a scratch test. The adhesion property of the hybrids increased with addition of GPTS and the highest adhesion was obtained from the hybrid with 5-10 mol% of GPTS. From the element analysis, Si concentrations of all the solutions were less than 2 mM after soaking for 7 d. The Si concentrations were not changed with increasing soaking period. The addition of epoxy groups is effective on improvement of adhesion property of the silica-based hybrid without loosening its chemical stability.

Abstract: The sensing of protein adsorption by silver nanoparticles/hydroxyapatite composites was investigated using a phosphate buffer solution containing bovine serum albumin (BSA) or lysozyme (LSZ). The adsorption of BSA and LSZ on the composites prepared without using trisodium citrate was similar to plain hydroxyapatite, whereas composites prepared with trisodium citrate showed lower BSA adsorption and higher LSZ adsorption than plain hydroxyapatite powder. Because the ability of the adsorption is mainly governed by surface charges of the powders, the usage of trisodium citrate during the preparation is assumed to produce high negative charges on the surface. The protein adsorption resulted in peak shifts in localized surface plasmon resonance (LSPR) spectra. The peak shifts clearly corresponded to the concentration of the surrounding proteins up to the point of saturation of adsorption on the hydroxyapatite. The silver nanoparticles/hydroxyapatite composites are promising candidate materials for detection of protein adsorption by measurement of LSPR peak shifts, that may be attributed to changes in the dielectric properties of the matrix fluid surrounding the silver nanoparticles.