Key Engineering Materials Vols. 309-311

Abstract: The properties of sintered hydroxyapatite (HA), obtained from bovine femoral shafts via calcination method, were investigated utilizing scanning electron microscopy (SEM) and X-ray diffraction analysis together with measurements of microhardness, density, and compression strength. The production of HA from natural sources is preferred due to money and time saving reasons. The results indicate the new HA materials as promissing in biomedicine, since similar mechanical behaviour was obtained with previous studies.

Abstract: In this study, hydroxyapatite (HA) material, obtained from calcinated bovine bone (BHA), was mixed with 0.25, 0.50, 1.00, and 2.00 wt% Li2CO3. The pressed pellets were sintered at various sintering temperatures between 900°C and 1300°C. Measurements of compression strength, microhardness, and density, along with SEM observation and X-ray diffraction analysis were performed. The experimental results showed that the samples with 0.25 and 0.50% Li2CO3 reached a maximum of densification and the highest values of compression strength and microhardness were achieved after sintering at 1300°C. The wetting effect of a Li2O-associated glassy phase was observed even from 900°C.

Abstract: A high magnetic field is a useful tool to control the crystal alignment of non-magnetic materials such as ceramics and polymers. In the case of Hydroxyapatite crystal, the a,b-axis is aligned parallel to the direction of an imposed magnetic field. This fact implies that the alignment of the c-axis is not controllable only using a high static magnetic field due to the freedom of the c-axis in a plane perpendicular to a magnetic field direction. In this study, a high static magnetic field and mold rotation was simultaneously so applied during a slip casting process as to align the c-axis of HAp poly crystals.

Abstract: The hydrothermal hot-pressing (HHP) technique was used to prepare hydroxyapatite (HAp) ceramics from a HAp powder with low crystallinity, and the effects of processing factors such as temperature, loading pressure and reaction time on densification were investigated. The crystallinity of HAp was increased by the HHP treatments. With the increase in reaction temperature, loading pressure and reaction time, the density and Vickers hardness of the compacts increased. The HAp compact with high relative density of 83% and high Vickers hardness of 2.9 GPa was successfully prepared by the HHP treatment at 200°C for 3 h under loading pressure of 60 MPa.

Abstract: This paper reports neutron diffraction data and its analysis that characterise a biocompatible hydroxyapatite composite material. The neutron data has elucidated the crystal structure, and enabled the positions of the hydrogen atoms to be determined. The data also shows the improvement of crystallinity during the heat treatment process. An extension of the work involved looking at a hydroxyapatite – carbon nanotube composite material, and neutron diffraction has shown that the retention of the carbon nanotubes in the composite material has been successful. The nanotubes have had no affect on the hydroxyapatite structure.

Abstract: Hollow hydroxyapatite microspheres (H-HAMs) with controlled characteristic mesoporous structure on the shell were successfully fabricated by using core template technology and sol-gel process. The fabrication of H-HAMs mainly included three distinct steps: the preparation of core template spheres of chitin by emulsifying chitin solution in oil, the formation of core-shell composite spheres of chitin-HA/chitin after a layer of chitin/HA solution coated on the surface of chitin core templates by means of a gelling process of chitin solution with the help of water in the cores, and the harvest of H-HAMs through a special sintering procedure to remove chitin. The size and shape of H-HAMs were chiefly determined by the size and morphology of core templates. The thickness of shells was easily controlled by altering water content of the starting template particles, and the characteristic mesoporous structure on the shell was related to the proportion of chitin in the chitin/HA composite solution and the sintering temperature. H-HAMs with characteristic mesoporous architecture on the shell have many potential applications such as used as a carrier for sustained release of drugs in the therapy of hard tissue system.

Abstract: Solid-state Nuclear Magnetic Resonance (NMR) has been used for a detailed structural analysis of different amorphous calcium phosphate (ACP) and hydroxyapatite (HAp) samples. For ACP the NMR data reveal the presence of hydrogenphosphate units in much larger quantities as known previously also showing differences between the samples. Furthermore, phosphate units close to water and a third group (yet unknown) are found in the spectra. A novel proposal has been established to explain the Ca-deficiency in nanocrystalline HAp. It consists of a crystalline core of stoichiometric composition which contains only 50% of the total phosphate amount. The residual phosphate is contained in a disordered surface layer with about 1 nm thickness. Most importantly, this surface layer contains hydrogenphosphate groups to a large extend and carbonate units. A Ca/P ratio of 1.52 is obtained from the NMR data which is very close to the value of 1.51 of the chemical analysis. Thus, NMR allows the investigation of the disordered mineral-protein interface and first examples concerning bone and nacre are presented and discussed.

Abstract: Studying on the interaction between proteins and calcium phosphate implants is one of the basic subjects in biomaterials science and engineering. In this work, zeta potentials and contact angles of hydroxyapatite (HA) and biphasic calcium phosphate (BCP) ceramics were measured, and the adsorption behaviors of bovine serum albumin (BSA) on the surfaces were investigated. The adsorption isotherms of BSA on both ceramics follow the Langmuir type, however, BCP shows higher BSA adsorption ability. The differences of contact angles and zeta potentials in HA and BCP are the key factors to determine their ability to bind BSA molecules.

Abstract: Mesoporous HAp aggregates were composed of needle-like crystals that were aligned to the c-axes direction perpendicular to the flat surface of plate-like aggregates. The pore size distributions of the HAp aggregates increased with increasing heat treatment temperature e.g. 3-20 and 26-52 nm, respectively, before and after heat-treated at 600 °C. The adsorption proteins on the mesoporous HAp aggregates were investigated by liquid chromatography using the HAp aggregates as an adsorbent. Elution molarity ratios of acidic proteins on the mesoporous HAp aggregates before and after heat-treatment at 600 °C increased with increasing molecular weight in the order of ferritin > fibrinogen > catalase > albumin, whereas the elution molarity ratios of basic proteins were considerably lower than those of acidic proteins. These results suggested that penetration of the large size acidic proteins into the pore of the as-prepared mesoporous HAp aggregate was slightly inhibited but were the proteins could easily penetrate into the pore of the heat-treated mesoporous HAp aggregates and then were selectively adsorbed on the mesoporous wall consisting of the a-surface of the HAp aggregate. Conversely, the basic proteins could be adsorbed on the wide outer surface of the plate-like HAp aggregates. Therefore, the elution molarity ratios of basic proteins decreased with decreasing the specific surface area by the heat treatment, independent of the mesoporous structure.

Abstract: Hydroxyapatite (HAp) also shows the high absorption ability for proteins with low degradations. In this study, to modify the absorption ability and to obtain high surface area, metal ions (Zn, Mg, Fe) were introduced in preparing the HAp nanocrystals, and porous microparticles were subsequent fabricated by a spray drying method. HAp microparticles with Zn, Mg, Fe ions had a single phase of low crystalline HAp. The specific surface area of the HAp, Mg-HAp, Fe-HAp and Zn-HAp were 87, 150, 152, 167 m2/g, respectively. In the adsorption experiment of a lysozyme as a basic proteine, Zn-HAp showed the highest adsorption ability. On the other hand, the HAp microparticles witn metal ions showed higher adsorption ability for BSA as a acitic protein than pure HAp microparticles.