Masaru Akao

Dense polycrystalline β-tricalcium phosphate for prosthetic applications

A method is described for preparing dense, polycrystallineβ-tricalcium phosphate. The compressive and flexural strengths of the polycrystalline bodies sintered at a temperature of 1150° C for 1 h were found to be 459 and 138 MPa, respectively. Observation of the fracture surfaces by scanning electron microscopy indicates a predomination of transgranular failures. The polycrystalline tricalcium phosphate is non-toxic in a cell culture system.

PREPARATION AND MECHANICAL PROPERTIES OF CALCIUM PHOSPHATE/COPOLY-L-LACTIDE COMPOSITES

New artificial bone materials were prepared using calcium phosphates, hydroxyapatite and β-tricalicum phosphate, and copoly-L-lactide, CPLA. Calcium phosphate powder and CPLA were mixed at 453 K for 10 min with various mixing ratios. Scanning electron microscope observations indicated that the composites of β-tricalicum phosphate and CPLA were homogeneously dispersed and highly adhesive. Young’s modulus of the composites was the same as bone, and bending strength was over half that of bone. The improvement of Young’s modulus compared to the original two materials was due to a composite effect. The composites are expected to be usable as artificial bone materials.

Hydrothermal growth of carbonate-containing hydroxyapatite single crystals

Single crystals of carbonate-containing hydroxyapatite have been grown hydrothermally by gradually heating with a temperature gradient applied to the vessel, using CaHPO4, H2O and carbon dioxide as dry ice. The optimum conditions to obtain large single crystals are: CaHPO4, 10 g/l; final temperature gradient, 33.5°C/cm; heating rate, 0.005°C/min; and carbon dioxide, 55 g/l. The largest crystal obtained was 12 mm in length and 200 μm in width. The CaP molar ratio and the CO2 content of the single crystals were ranging from 1.60 to 1.67 and 0.09 to 0.65 wt%, respectively. Carbonate groups substituted only for OH groups.

Thermal expansion of hydroxyapatite-β-tricalcium phosphate ceramics

Abstract The thermal lattice expansion of calcined hydroxyapatite and β-tricalcium phosphate single-phase powder was investigated by high-temperature X-ray diffractometry. The thermal expansion of densely sintered hydroxyapatite-β-tricalcium phosphate composite ceramics was examined by thermomechanical analysis. Hydroxyapatite-β-tricalcium phosphate mixtures with a Ca/P atomic ratio varying from 1.50 to 1.67 were precipitated from Ca(OH) 2 and H 3 PO 4 solution at room temperature. The powder, calcined in air at 800°C for 3 h, was pressed in a mould at 98 MPa. The pressed powder pellets were sintered in air at 1150°C for 1 h. High-temperature X-ray powder diffraction data were measured in the temperature range from 25 °C to 1150 °C and calibrated by a platinum internal standard technique. Thermo- mechanical analysis was carried out with a 0.08 kPa compressive load in the range from 25 °C to 1250° C with an α-Al 2 O 3 polycrystalline standard. The hydroxyapatite and β-tricalcium phosphate are characterised by non-linear thermal expansion between 25 ° C and 600° C because of HPO 4 2− condensation and dehydration. In the upper temperature region, the hydroxyapatite c -axial dimension expanded sharply from 1000° C to 1150° C owing to the formation of oxyapatite. The c -axial dimension of β-tricalcium phosphate shrank from 850 to 950° C which was ascribed to the formation of an intermediate phase between the β- and α-tricalcium phosphate phases. The mean (25–1000 °C) thermal expansion coefficient of the densely sintered hydroxyapatite-β-tricalcium phosphate ceramics increased almost linearly with an increase in β-tricalcium phosphate content.

Sintered hydroxyapatite for a percutaneous device and its clinical application

The sintered hydroxyapatite was designed to be utilized as a percutaneous device. The device was percutaneously fixed through the back skin of mongrel dogs. The tissue reaction around the implants was examined histologically. At 1 month the sintered hydroxyapatite was closely contacted with the skin tissue and downgrowth of epidermis was not observed. The long-term implantation ranging from 3 to 17 months showed that the depth of epidermis downgrowth was limited to only 1 mm. The amino acid composition of fibrous capsule formed around the hydroxyapatite showed close resemblance to vertebrate periosteum. It was confirmed that the sintered hydroxyapatite had good compatibility and long-term stability with skin tissue.

THE MECHANICAL PROPERTIES AND SOLUBILITY OF STRONTIUM-SUBSTITUTED HYDROXYAPATITE

The mechanical properties and solubility of sintered Sr-substituted hydroxyapatites were examined in order to investigate the influences of Sr incorporation into hydroxyapatite in bones and teeth on them. Hydroxyapatite (HAp), Sr-substituted hydroxyapatite (Ca.SrAp), and strontium-hydroxyapatite (SrAp) were synthesized and sintered. The bending strength and Youngs modulus were measured. Also, solubility in isotonic NaCl solution was examined. The Youngs modulus of Sr-substituted hydroxyapatite (Sr wt% = 1.0) and hydroxyapatite surface treated with Sr ion were greater than that of hydroxyapatite. The bending strengths of Sr-substituted hydroxyapatites (Sr/Ca = 4/6 and less) were reduced slightly compared to that of hydroxyapatite. The bending strength of hydroxyapatite surface treated with Sr ion was increased compared to that of hydroxyapatite. The solubility in isotonic NaCl solution at 37 degrees C was increased with an increase in Sr content. The effects of Sr on the mechanical properties and solubility of bones and/or teeth are discussed.

Application of hydroxyapatite-sol as drug carrier

The application of hydroxyapatite-sol as a drug carrier is being developed. Hydroxyapatite-sol which is a suspension consisting of hydroxyapatite nano-crystals, was synthesized using an ultrasonic homogenizer. The size of the crystals was 40 x 15 x 10 mm3 on average and their specific surface area was 100 m2/g. An amount of a glycoside antibiotics adsorbed onto hydroxyapatite nano-crystals was measured. The drug adsorbed 0.2 mg per 1 mg of hydroxyapatite. The affect of the drug adsorbed onto the hydroxyapatite was investigated using cancer cells. The drug, adsorbed onto the hydroxyapatite nano-crystals, inhibited cancer cell growth.

Surface properties of hydroxyapatite ceramic as new percutaneous material in skin tissue

Hydroxyapatite ceramic (HAC) was studied for utilization as a percutaneous device to prevent exit-site and tunnel infection along peritoneal dialysis catheters. As a result, it was found that HAC had good compatibility with skin tissue compared with silicone rubber and glassy carbon. In the present study, the surface properties of HAC after long-term implantation in skin of dogs was evaluated by scanning electron microscopic observation, solubility and bending tests. At 12 weeks after implantation, the surface of HAC was eroded severely by body fluid or cells, and the grain boundaries were clearly relieved. Ingrowth and adhesion of collagen into the etched ditch of HAC were observed. At 12 months, the eroded surface had been tightly covered and adhered to by a collagenous layer in spite of ripping off the surrounding fibrous capsule. In solubility tests, HAC dissolved at the rate of 0.23 mg cm−2 HAC surface area per year. On the other hand, the bending strength decreased by about 10% at 4 weeks and scarcely decreased after that time. From these results, it was confirmed that the solubility of HAC produced tight contact and strong adhesion with skin tissue to prevent bacterial infection, and HAC was practical for a permanent percutaneous device.

Preparation and characterization of double layered coating composed of hydroxyapatite and perovskite by thermal decomposition

A new modified thermal decomposition method is described for preparing a double layered coating on titanium plates which includes an initial perovskite (CaTiO3) layer followed by a hydroxyapatite (HA) layer on top. The characterization of the coating was studied by X-ray diffractometry and infrared spectroscopy and indicated that the double layer consisted of carbonate HA and CaTiO3 and the thickness of the layer was 4 microns. The coating was performed on the inner surfaces of 50-200 microns sized pores and was also consistent in the smallest of the pores even those of 50 microns. Bone formation was examined in canines at 2-32 week intervals and was dominant on coated plates and in large-sized pores before 16 weeks. However, after 16 weeks bone ingrowth was similar in non-coated and coated plates and in all pore sizes. The results indicated that HA could only influence early bone ingrowth, though good bone ingrowth into small pores indicated that HA exhibited enhanced osteocompatibility. Our methodology ensured the stability of the HA layer consequently minimizing the problems associated with HA loss.

Effect of Hydroxyapatite Microcrystals on Macrophage Activity

Hydroxyapatite (HAp) microcrystals were synthesized by a neutralization reaction of Ca(OH)2 suspension and H3PO4 solution using an ultrasonic homogenizer. The in vitro interaction of HAp microcrystals with rat peritoneal macrophages was investigated by measuring the viability, acid phosphatase (ACP) activity, lactate dehydrogenase (LDH) activity and intracellular calcium content. HAp calcined at 800 degrees C and alpha-alumina particles (alumina) were used as comparative materials. Macrophages actively phagocytosed HAp microcrystals by dissolving them. However, no damage in macrophages exposed to HAp microcrystals was observed by transmission electron microscopy. Macrophages in the presence of HAp microcrystals showed less ACP and LDH activity and higher intracellular calcium content than those in the presence of calcined HAp and alumina. HAp microcrystals had excellent biocompatibility to macrophages as well as sintered HAp.