Sung-Baek Cho

Apatite formation on silica gel in simulated body fluid : Its dependence on structures of silica gels prepared in different media

It has been shown that the prerequisite for glasses and glass-ceramics to bond to living bone is the formation of a layer of biologically active bonelike apatite on their surfaces. The hydrated silica formed on the surfaces of glasses and glass-ceramics plays an important role in nucleating the apatite. In the present study, the structure of the hydrated silica responsible for the apatite nucleation was investigated in an accellular simulated body fluid with ion concentrations nearly equal to those of human blood plasma. Three kinds of porous silica gels were prepared by hydrolysis and polycondensation of tetraethoxysilane in pure water or in aqueous solution containing polyethylene glycol or polyacrylic acid. The silica gels prepared in aqueous solution containing polyethylene glycol or polyacrylic acid had micron-size interconnected pores, whereas the gel prepared in pure water did not. All the gels contained a large volume of nanometer-size pores, almost the same amounts of silanol groups and D2 defect, and showed a high dissolution rate of the silica. Despite this, only the gel prepared in the solution containing polyethylene glycol formed the apatite on its surface in the simulated body fluid. This indicates that only a certain type of structural unit of the silanol group is responsible for the apatite nucleation.

Apatite formation on silica gel in simulated body fluid: effects of structural modification with solvent-exchange.

The prerequisite for glasses and glass-ceramics to bond to living bone is the formation of biologically active bone-like apatite on their surfaces. It has been shown that even a pure silica gel forms the bone-like apatite on its surface in a simulated body fluid. In the present study, pore structure of silica gels prepared by hydrolysis and polycondensation of tetraethoxysilane in an aqueous solution containing polyethylene glycol was modified by 1M HNO3, and 0.1M and 1M NH4OH solution treatments. The three kinds of resultant gels all contained large amounts of silanol groups and trisiloxane rings, but differ greatly in pore structure of nanometre pore size. Irrespective of these differences, all the gels formed the bone-like apatite on their surface in the simulated body fluid. It was speculated that a certain type of structural unit of silanol groups, which is easily formed in the presence of the polyethylene glycol, is effective for the apatite formation.

Apatite-forming ability of silicate ion dissolved from silica gels.

It is known that the prerequisite for glasses and glass-ceramics to bond to living bone is the formation of biologically active bonelike apatite on their surfaces, and a certain type of hydrated silica developed on their surfaces plays an important role in nucleating the apatite. In the present study, the apatite-forming ability of silicate ion dissolved from different silica materials into a simulated body fluid was examined as follows. Polyether sulfone substrates pretreated with O2 plasma were placed in parallel to plates of three kinds of silica gels prepared in different media and silica glass, with a distance of 0.5 mm between them in a simulated body fluid with ion concentrations nearly equal to those of human blood plasma for 4 days, and then soaked in a solution with ion concentrations 1.5 times those of the simulated body fluid for 6 days. After the first soaking, silicon combined with oxygen was detected on the surfaces of the substrates faced to all the silica gels, whereas it was not detected on that faced to the silica glass. After the second soaking, the former formed a bonelike apatite layer on their surfaces, but the latter did not. These results indicate that silicate ion which is dissolved from the silica gels and adsorbed on the substrates has an apatite-forming ability, irrespective of the microstructure of the original silica gels.

Induction of bioactivity of a non-bioactive glass-ceramic by a chemical treatment

Glass-ceramic A-W(Al), which was prepared by heat treatment of a MgO-CaO-SiO2-P2O5-Al2O3 glass to precipitate crystalline apatite and wollastonite, shows a higher mechanical strength than glass-ceramic A-W, which was prepared by heat treatment of a MgO-CaO-SiO2-P2O5 glass to precipitate the same types of crystalline phases. The former, however, does not show bone-bonding ability, i.e. bioactivity, whereas the latter shows it. In the present study, in order to induce bioactivity of glass-ceramic A-W(Al), it was treated with HCl or NaOH solutions with different concentrations, and its bioactivity was evaluated by examining the apatite formation on its surface in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma. When the glass-ceramic A-W(Al) was pretreated with HCl aqueous solutions with concentrations over 0.1 M, it formed the bone-like apatite on its surface in SBF. This was attributed to the formation of a hydrated silica on its surface by the HCl treatment.

Dependence of Bone-like Apatite Formation on Structure of Silica Gel

ABSTRACT It has been shown that the prerequisite for glasses and glass-ceramics to bond to living bone is the formation of biologically active bone-like apatite on their surfaces, and that a hydrated silica on their surfaces plays an important role in nucleating the apatite. In the present study, structure of the hydrated silica responsible for the apatite nucleation was investigated. Silica gels were prepared by hydrolysis and polycondensation of tetraethoxysilane(TEOS) in aqueous solution containing no polymer, polyacrylic acid or polyethylene glycol. All these gels contained large volume of nanometer-size pore, silanol groups and D2 defect and showed high dissolution rate of the silica. Despite it, only the gel prepared in the solution containing polyethylene glycol formed the apatite on its surface in the simulated body fluid. The same gel did not form it when the gel lost its nanometer-size pores, silanol groups and high dissolution rate of the silica by heat treatment at 1000 °C. The heat-treated gel recovered its apatite-forming ability by a H 2 O gas plasma treatment. This indicates that only a certain type of structural unit of the silanol group is responsible for the apatite nucleation.

Bone-bonding behavior of three heat-treated silica gels implanted in mature rabbit bone

Silica gel has been reported to induce apatite nucleation on its surface in vitro and it can act as a stimulant that induces formation of chemical apatite (Ca-P) layers on the surfaces of bioactive glass-ceramics. In this study, apatite formation in response to and the bone-bonding behavior of solica gels implanted in the tibiae of mature rabbits were studied. Implants were made from three silica gels treated at 400, 800, and 1000°C, and the effects of such heat treatment on the above parameters were investigated. The silica gel was made by hydrolysis and polycondensation of tetraethoxysilane in aqueous solution containing polyethylene glycol. Rectangular implants (15 mm × 10 mm × 2 mm) of each heat-treated silica gel were implanted into both tibial bones of mature male rabbits, which were killed 4 or 8 weeks after implantation, and the tibiae containing the implants were dissected out. The bone-implant interfaces were investigated using Giemsa surface staining, contact microradiography, scanning electron microscopy-electron probe microanalysis, and X-ray diffraction. Histologically, no bonding of bone to any of the silica gels was observed at any time postimplantation. Soft tissue was observed at the bone-silica gel interface, but there were no giant foreign body or inflammatory cells. A Ca-P-rich layer was observed only on small areas of the surfaces of the silica gels treated at 400 and 800°C 4 and 8 weeks after implantation. X-Ray diffraction analysis confirmed the presence of hydroxyapatite in these Ca-P-rich layers. At no time after implantation was a Ca-P-rich layer observed on the surface of silica gel treated at 1000°C. It is thought that a special type of silanol group, which forms on silica gel treated below 800°C, is responsible for the apatite nucleation. A Ca-P layer does not always form reliably on the surfaces of silica gels.

The Compressive Strength Development of Briquette Ash by Alkali Activated Reaction

Non-sintering cement was manufactured with briquette ash. Alkali activator for compression bodies used a NaOH solution. In order to apply alkali-activated briquette ash and the non-sintering cement to concrete, several experimental studies were performed. It was necessary to study the binder obtained by means of a substitute for the cement. This study concentrated on strength development according to the concentration of NaOH solution, the curing temperature, and the curing time. The highest compressive strength of compression bodies appeared as 353 kgf/cm cured at 80C for 28 days. This result indicates that a higher curing temperature is needed to get a higher strength body. Also, geopolymerization was examined by SEM and XRD analysis after the curing of compression bodies. According to SEM and XRD, the main reaction product in the alkali activated briquette ash is aluminosilicate crystal.

Effects of Li 2 O Addition and Heat-Treatment on Formability of FeS 2 Powder for Cathode of Thermal Battery

has been widely used for cathode materials in thermal battery because of its high stability and current capability at high operation temperature. Salts such as a LiCl-KCl were added as a binder for improving electrical performance and formability of cathode powder. In this study, the effects of the addition of in LiCl-KCl binder on the formability of powder compact were investigated. With the increasing amount of addition to LiCl-KCl binder salts, the strength of the pressed compacts increased considerably when the powder mixture were pre-heat-treated above . The heat-treatment resulted in promoting the coating coverage of particles by the salts as was added. The observed coating as addition might be attributed to the enhanced wettability of the salt rather than its reduced melting temperature. The high strength of compacts by the addition and pre-heat-treatment could improve the formability of raw materials.

Characteristics of Ceramic Separator Impregnated by Molten Salt for Thermal Batteries

Thermal batteries are primary power sources for military applications requiring high reliability, robustness and long storage life. Conventional electrodes for thermal batteries are prepared by compacting powder mixtures into pellets. Separator is composed of halide mixture, such as LiCl-KCl eutectic salt, blended with MgO to immobilize the molten salt. In order to increase the power density and energy density, the resistance of electrolyte should be reduced because the resistance of electrolyte is predominant in thermal batteries. In this study, wetting behaviors and impregnation weight of molten salts as well as the micro structures of ceramic felt were investigated to be applicable to thin electrolyte. Discharge performances of single cell with the ceramic separator impregnated by molten salt were evaluated also. Zirconia felt with high porosity and large pore outperformed alumina felt in wetting characteristics and molten salt impregnation as well as discharge performances. Based on the results of this study, ceramic felt separator impregnated with molten salt have revealed as an alternative of conventional thick MgO based separator with no conspicuous sign of thermal runaway by short circuit.

Compressive Strength Properties of Geopolymer Using Power Plant Bottom Ash and NaOH Activator

When a new bonding agent using coal ash is utilized as a substitute for cement, it has the advantages of offering a reduction in the generation of carbon dioxide and securing the initial mechanical strength such that the agent has attracted strong interest from recycling and eco-friendly construction industries. This study aims to establish the production conditions of new hardening materials using clean bottom ash and an alkali activation process to evaluate the characteristics of newly manufactured hardening materials. The alkali activator for the compression process uses a NaOH solution. This study concentrated on strength development according to the concentration of the NaOH solution, the curing temperature, and the curing time. The highest compressive strength of a compressed body appeared at 61.24MPa after curing at for 28 days. This result indicates that a higher curing temperature is required to obtain a higher strength body. Also, the degree of geopolymerization was examined using a scanning electron microscope, revealing a micro-structure consisting of a glass-like matrix and crystalized grains. The microstructures generated from the activation reaction of sodium hydroxide were widely distributed in terms of the factors that exercise an effect on the compressive strength of the geopolymer hardening bodies. The Si/Al ratio of the geopolymer having the maximum strength was about 2.41.