Preparation of hydroxyapatite (HAP) was investigated by the spray-pyrolysis technique from methanol-water-nitric acid solution of calcium nitrate and phosphoric acid between 800° and 1400°C. The powders obtained consisted of crystalline HAP and a small amount of α-tricalcium phosphate under the experimental conditions used. The content of HAP phase in products increased as the pyrolysis temperature and/or the water content in the solution increased. One phase HAP was obtained by heating the as-prepared powders at 800°C for 1h. Spherical HAP was prepared by this method. The amount of good spherical particles in the powder products increased with increasing Pyrolysis temperature and/or with decreasing water content in the solution. Preparation of HAP with inorganic constituents similar to human bone was also examined. HAP containing Mg, Na, K, Cl and F ions was obtained and it showed good crystallinity.

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Erosion process of the zinc polycarboxylate cement in various acid solutions was investigated by means of determination of Zn and Mg leached from the cement and SEM observation of the surface structure. The amount of species leached from the cement increased with increasing dissociation constant of the acid, that is, with decreasing pH of the solution. Partial neutralization of the acid decreased the amounts of Zn and Mg. When the degree of neutralization (=equivalent of NaOH added/equivalent of the acid originally present) reached 1, the amount leached was almost negligible. However, in the acids which formed stable complexes with Zn and Mg, certain amounts of Zn and Mg were released from the cement. Especially with EDTA which formed much stable complexes, the amount leached was independent of the degree of neutralization and showed high value. Erosion of the cement in the acid solutions consisted of two competitive processes, that is, dissolution of the unreacted ZnO particles and erosion of the matrix. The preference of these two processes depends on the dissociation constants of the acids and the formation constants of the complexes between the acids and Zn or Mg. The former occurs preferentially at a lower degree of neutralization. The latter also proceeds in strong acids such as HCl or in acids forming stable complexes such as citric acid and EDTA. With increasing degree of neutralization, erosion of the matrix occurs exclusively with ZnO particles taken off the cement in the latter two acids.

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Amorphous calcium phosphate, after thermally dehydrated, was used as a starting material for preparation of sintered β-Ca₃(PO₄)₂. Specific surface area and pore size distribution of the dehydrated powders were examined from the adsorption isotherms in order to clarify the relation between the powder properties of dehydrated samples and the sintering process. Sintered β-Ca₃(PO₄)₂ with more than 90% of theoretical density was prepared from crystalline α-tricalcium phosphate which was obtained at 700°C, while dense bodies were formed by hot pressing at 24MPa and 1100°C for 30min from the starting powders calcined at low temperature. The hindrance of densification by mesopores (20-200Å) in the dehydrated powder particles might be eliminated by the thermal crystallization and the hot pressing.

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Ultrafine powder of pure hydroxyapatite was synthesized by the reaction of Ca(CH₃COO)₂⋅H₂O and (NH₄)₂HPO₄ at 40°C for 3h. The powder particles had a specific surface area of 177m₂/g and were platy. The dependence of composition and sinterability on the Ca/P molar ratio in starting solutions was studied. When the Ca/P molar ratio was 1. 68, stoichiometric powder with fairly good sinterability was obtained. A highly-densified and translucent hydroxyapatite body was obtained when the powder was sintered at 1000°C for 1h after calcination at 700°C for 3h and ultrasonic treatment.

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In order to make calcium hydroxyapatite (Ca-HAP) compacts with satisfactory size and required shape as an artificial tooth root implant, a centrifugal settling method using a centrifugal tube with desired shape has been developed, which gives good shaping yield. The good shaping yield results from: (1) the selection of the Ca/P ratio, pH and reaction temperature of the solutions so as to keep the specific surface area over 70m₂/g and the agglomerate size as large as possible; (2) use of centrifugal tubes having low affinity with the Ca-HAP particles; (3) application of an adequate revolution rate and centrifugal force to attain the uniform packing of particles; and (4) slow drying rate to avoid rapid and irregular shrinkage of segregated Ca-HAP. The relative density of a sintered body after heat treatment at 1100°C was more than 98%.

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Two rectangular bars of glass-ceramic A-W, its parent glass G, Bioglass Bg or hydroxyapatite ceramic HAp were respectively bound and soaked in an acellular simulated body fluid for various periods up to 6 months. Tight chemical bond was formed between the two bars after a certain period of time for all these bioactive materials. The ability of forming such bond increased in the order HAp<G=A-W<Bg. The bond was achieved by an apatite phase newly formed at the interfaces of the specimens in the simulated body fluid. In the present arrangement of the specimens, bonding took place in the peripheries within the interface, indicating that wider area for bonding and accordingly stronger bonding can be expected if the arrangement is designed so that the fluid might circulate into the central part of the interface. The present results indicate a possibility that when artificial bone made of those bioactive materials is accidentally broken in a body, the separate pieces can be recombined like the natural bone.

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A vascular access tube made of sintered hydroxyapatite tubing was developed by animal experimentation. Anticoagulability of a tube, 3.8mm in internal diameter and 2.0cm long and made of sintered hydroxyapatite, was tested by insertion in the jugular vein of nonheparinized dogs under anesthesia. If patency lasted over 2h, the test was ended by venous occlusion. The patent time was longer for the tubes (106±35min) pretreated by 24h incubation in autologous heparinized plasma at 37°C and for those (102±36min) implanted subcutaneously one week earlier in the abdominal wall of the recipient dog than for those (46±30min) treated by dipping in autologous heparinized blood for 5min. The mean shear rate and activated coagulating time were not significantly different among the three groups. No rupture or pseudoaneurysm was observed in 15 dogs in which 26 tubes of 2.6-2.8mm internal diameter were implanted in the carotid arteries and jugular veins when they were examined at an average of 2 months after implantation. In one dog, the tube 2.5mm in internal diameter in a carotid artery was revealed angiographically to be patent on the 85th day after implantation. It is concluded that appropriate pretreatment can make sintered hydroxyapatite more biologically compatible and the use of its tubes an excellent blood means of access.

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Apatite-biologically active glass composite (ABC) was synthesized and its microstructure, bioactivity and bending strength were studied. The existence of fluorapatite (FAp), Na₂Ca₂Si₃O₉ and the remainder glass was confirmed by XRD and SEM. From in vitro experiment, rapid formation of stable FAp film in 7 days was observed by IRRS, SEM and EPMA. Bending strength of ABC increased slightly with treatment time in trisaminomethane buffer solution to 30 days while under similar condition the strength of sintered hydroxyapatite (HAp) decreased gently. The increase of bending strength of ABC was closely related with the surface ion migration to form FAp. Radical crack tip blunting due to ion migration and/or stress releasing around tip was considered to be the main reason. From in vivo experiment, the bone bonding strength of ABC was 27MPa, higher than that of sintered HAp. The bone bonding strength, in this case, is equivalent to the fracture shear strength between bone and material after implantation in rabbit femur during 8 weeks. Furthermore, just the same as the result of in vitro experiment, FAp film formation on the surface of ABC was also observed in vivo, which suggested the possibility of slight increase of bending strength in living body. For clinical application as bone replacement or tooth implant, possible advantage of ABC regarding bending strength is expected to be (1) strength increase and (2) strength recovery after introduction of a new crack, during implantation in living body.

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Mixtures of hydroxyapatite and zirconia containing 3mol% Y₂O₃ (TZ-3 Y) or 2mol% Y₂O₃ (TZ-2 Y) were hot-pressed under the conditions of 1300°-1400°C, 10min and 20MPa, and the bending strength and fracture toughness of the hot-pressed bodies were measured. The reactivity between hydroxyapatite and zirconia was also examined. The zirconia content was up to 50vol%. The bending strength and fracture toughness increased with an increase in zirconia content. Properties were improved with zirconia (TZ-2 Y) than with zirconia (TZ-3 Y). Compacts containing 50vol% zirconia (TZ-2 Y) fired at 1400°C showed the highest mechanical properties: bending strength; about 400MPa and fracture toughnees; about 3.0MPa·m^1/2. These values were two or three times as large as those of hydroxyapatite single compacts. The following two thermal transformations were observed in the present system: (i) decomposition of hydroxyapatite to α-tricalcium phosphate and (ii) transition of tetragonal zirconia to cubic phase.

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Surface modification of an FRS carbon dental implant (FRS implant) with a fine Rahmen structure surface, was investigated in order to give wettability to the FRS implant surface. The wettability was given by oxidation of FRS implant surface only. The FRS implant was prepared by the deposition and infiltration of low temperature pyrolytic carbon (LTPC) from cis-1, 2-dichloroetylene. Surface oxidation onto the LTPC plate was carried out by several methods. The LTPC plate surface was not oxidized by reagent oxidation with hot concentrated nitric acid at 95°C. Then the LTPC plate surface was treated by electrolytic oxidation with phosphoric acid, which gave hydrophilic property. But micro-Vickers hardness of the LTPC plate surface decreased from 500 to 290kgf/mm², while the LTPC plate surface was found to be strongly etched by scanning electron microscopy. So surface oxidation by low temperature plasma of oxygen (13.56MHz, 25W, 10min) was carried out to the LTPC plate. This treatment modified the surface of LTPC plate to highly hydrophilic, and the hardness showed the same value before plasma treatment. Surface condition of the LTPC plate was not changed by this treatment. Accordingly the low temperature plasma treatment is regarded as the best modifying method for the FRS carbon dental implant.

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Porous and dense tricalcium phosphate (TCP) ceramics with 10-40% porosity were prepared by sintering β-TCP and α-TCP powders at 1200°-1500°C. Then the TCP ceramics were treated with water or succinate aqueous solution. Hydroxyapatite (HAp) by the water-treatment and an HAp-like compound (octacalcium phosphate; OCP) by the succinate-treatment were formed in porous TCP (dispersion type) and on dense TCP (coating type), respectively. These composites had higher strengths than untreated TCP ceramics. For instance, diametral tensile strength increased from 15 (porosity=22%) to 21MPa (20%) by the water-treatment, and from 8 (33%) to 18MPa (16%) by the succinate-treatment. The increases in strength were considered to be due to two fashions, i.e., the entanglement of HAp (complexing effect) in the water-treatment and the decrease in porosity (packing effect) in the succinate-treatment.

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Tricalcium phosphate-zirconia composite ceramics were prepared. Mixtures of β-tricalcium phosphate and zirconia with 3 or 6mol% yttria were sintered in air at 1400°C. Phases of the β-tricalcium phosphate and zirconia almost remain unchanged after heating at 1400°C. The best material, composed of β-tricalcium phosphate and 67wt% zirconia with 6mol% yttria, has an average bending strength of 199MPa and fracture toughness value of 2.15MPa·m^1/2.

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In order to obtain high strength β-tricalcium phosphate (β-TCP) ceramics, effect of alkali earth metals addition was investigated. Addition of magnesium increased the bending strength of the sintered β-TCP to 2010kgf/cm². Formation of a solid solution by replacement of magnesium inhibited the phase transformation of β-TCP into α-TCP, and contracted the crystal lattice. The strengthening of β-TCP ceramics was assumed to be due to the generation of partial stresses in the sintered body by magnesium addition.

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Hydroxyapatite powders were prepared by calcining freeze-dried mixtures of calcium acetate and tri-ethyl phosphate, in which no calcium phosphate was formed. Results of X-ray powder diffraction were compared with those of commercially available hydroxyapatite powders. The optimum operating conditions were determined by varying factors such as calcination temperature, Ca/P ratio of the solution, and retention time.

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Thermal decomposition process of Al-iso-propoxide as precursor material for alumina was analyzed with a TG-DTA apparatus. Alumina powder was prepared by calcination of the raw material in air and under reduced pressure at several temperatures. The calcined materials were identified by an X-ray powder diffraction method to be γ-alumina below 900°C and α-alumina above 1100°C, of which crystal structures were independent of the atmospheric environment. The powder briquettes were made by a CIP (cold isostatic pressing) method, which were sintered in the temperature range of 1600° to 1800°C. The sinterabilities of alumina were discussed relative to the effects of atmosphere, crystal system, calcination temperature, and firing temperature.

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Hydroxyapatite (HAp) ceramics are attractive materials for bone and tooth implant, since they closely resemble bone and tooth mineral and have been reported to show superior biocompatibility and ability to bind directly to bone. Recently, several workers have attempted to develop HAp coated subperiosteal and endosseous subperiosteal implant materials, because it is practicaly difficult to make these implant materials with complicated shapes (for example, saddle type and frame-work structure) by sintered HAp ceramics alone, and furthermore, such implant materials are too weak to function after implantation even if it were possible. The purpose of this study is to estimate the effects of Li₃PO₄ on the sintering of HAp, especially lowering of sintering temperature of HAp, because the reduction of sintering temperature is considered to be effective to control the reaction between HAp and other materials. Li₃PO₄ was very effective; Addition of 1.1wt% Li₃PO₄ gave dense sintered HAp's by 920°C-100min and 940°C-30min firings. Under these conditions, the bulk density of sintered HAp ceramics was 3.10g/cm³. SEM photographs of etched surfaces of sintered bodies indicated that addition of Li₃PO₄ accelerates grain growth of HAp.

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