Characterization of Surface-Modified Halloysite Nanotubes by Thermal Treatment Under Reducing Atmosphere.

In recent years, the halloysite (Al2Si2O5(OH)4 · 2H2O) has been highlighted owing to its naturally occurring one-dimensionalmicrostructure that enables versatile applications. Due to the demand for enhancing surface interaction, several types of research such as acid/base treatments have been conducted on the halloysite nanotubes. The objective of this study is to investigate the structural and surface properties of thermally treated halloysites under reducing atmosphere. The heat treatment is carried out in a gas-flow furnace at 400-800 °C under various atmosphere, e.g., ambient air, 4% H2-balanced Ar, and 99.99% H2. The thermal treatment of halloysites under reducing atmosphere show a similar phase transition around 500 °C as the heating under air. However, the halloysite reduced in pure hydrogen shows a significant increase of the zeta-potential, -36.7 mV for a 600 °C-treated sample, compared to the other samples. The mechanisms of the zeta-potential increase for the halloysite was also explored.

Separation of Halloysite Nano Tubes (HNTs) by Homogenization of Quartz Contaminated Kaolins.

Natural halloysite kaolin contains a lot of impurities such as quartz phases and varies in morphology and size during their formation in the earth. So to utilize as a new type of natural nano material, removing quartz impurities from kaolin clays without scathe the tube morphologies are necessary. So to remove quartz impurities from kaolin by forming a well deflocculated aqueous slip without fracturing the morphology of tubes, the slip of homogenized halloysite clay was recovered by adding polyvalent metallic cations and anionic polyelectrolyte flocculants simultaneously to selectively flocculate the mixture of quartz and halloysite, whereby the halloysite particles form floes and the tubular halloysite remains in suspensions. Then, the uniform size and tubular shape of halloysite was obtained which could be suitably used as a container or a carrier to encapsulate nanomaterials.

Polylactic Acid/Nanostructured Si-Substituted ß-Tricalcium Phosphate Composites for Biodegradable Fixation Medical Devices.

Organic/inorganic biocomposite materials for biodegradable fixation medical devices require osteoconductivity, biocompatibility, and adequate mechanical properties with biodegradation behavior. The objective of this study was to investigate the effect of Si ions substituted in ß-tricalcium phosphate (ß-TCP) on the mechanical properties of organic/inorganic biocomposites. Biodegradable composite materials were prepared with polylactic acid (PLA) as the matrix and nano Si-substituted ß-TCP as the osteoconductive filler by solvent mixing and conventional molding. The nanostructured Si-substituted ß-TCP powders were synthesized by co-precipitation, controlling the quantity of Si ions. The amount of nanostructured Si-substituted ß-TCP powders in composites was varied in the 0-40 wt% range and the material properties were compared with those of pure ß-TCP/PLA composites. The influence of Si ions on the mechanical properties of the composites was evaluated by tensile and flexural tests.

Interfacial Property of Dental Cobalt­Chromium Alloys and Their Bonding Strength with Porcelains.

Porcelain-fused-to-metal crown is one of the widely-used prostheses among the dental porcelain restorations. Nonprecious metals like Ni­Cr and Co­Cr have extensively been used for metal-ceramic restorations due to advantages such as inexpensive price, hardness, durability, resistance to deformation, thin thickness of metal of porcelain area, and other mechanical and physical properties. However, the immediate advantage of the Co­Cr alloy is comparable performance to other base metal alloys, but without an allergenic nickel component. To achieve clinical longevity of porcelain-fused-to-metal (PFM) crowns, it is essential to have adequate bond strength between the metal substrate and porcelain. Any type of metal-ceramic fracture failure can become a costly and timeconsuming problem, both in the clinic and laboratory. Therefore, the suitability of the Co­Cr alloy for dental applications is critically associated with its ceramic bonding capacity. In this study, Co­Cr metal alloys modified by acid-etching and sandblasting, oxide layer was formed for subsequent bonding to porcelain ceramics. By both acid-etching and sandblasting oxide layer was formed and showed higher bonding strength at a proper condition, but debonding was occurred at porcelain layer so that they showed highest bonding strength by combined these two kind of surface treatment. Because the oxide film was formed more densely in a vacuum at the portions where more sophisticated concavo-convex were formed on the surface of a metal.

Fabrication of Wollastonite Coatings on Zirconia by Room Temperature Spray Process.

Wollastonite-based bioceramic powders were synthesized by solid-state reaction between calcite and silica powder. To control the phase and bioactivity of wollastonite, we also prepared four kinds of wollastonite-based bioceramics by adding an enstatite (MgSiO3) powder. After solid state reaction at 1000-1300 degrees C, micron-sized powders were obtained by milling and screening. By using these powders, wollastonite coatings were fabricated on zirconia substrates by a room temperature spray process, and their microstructure and composition under different coating condition were investigated. Wollastonite coatings on zirconia substrates were formed with the homogeneous microstructure and nanoscaled grain size. The phase composition of the resultant wollastonite coatings was similar to that of the starting powders, however, the grain size of the wollastonite particles was reduced to about 100 nm due to their formation by particle impaction and fracture. The wollastonite coating layer exhibited bioactivity in a stimulated body fluid and forming ability of 25 nm sized-hydroxyapatite precipitates of during in vitro test in SBF solution.

Microscale tribological behavior and in vitro biocompatibility of graphene nanoplatelet reinforced alumina.

Graphene nanoplatelets were added as reinforcement to alumina ceramics in order to enhance microscale tribological behavior, which would be beneficial for ceramic-on-ceramic hip implant applications. The reduction in microscale wear is critical to hip implant applications where small amounts of wear debris can be detrimental to patients and to implant performance. The addition of the GNPs lead to improvements in fracture toughness and wear (scratch) resistance of 21% and 39%, respectively. The improved wear resistance was attributed to GNP-induced toughening, which generates fine (~100nm) microcracks on the scratch surface. In addition, active participation of GNPs was observed in the scratch subsurface of GNP-reinforced samples through focused ion beam sectioning. Friction coefficients are not significantly influenced by the addition of GNPs, and hence GNPs do not act as solid state lubricants. In vitro biocompatibility with human osteoblasts was assessed to evaluate any possible cytotoxic effects induced by GNPs. Osteoblast cells were observed to survive and proliferate robustly in the GNP-reinforced samples, particularly those with high (10-15vol%) GNP content.

Influence of Starting Powders on Hydroxyapatite Coatings Fabricated by Room Temperature Spraying Method.

Three types of raw materials were used for the fabrication of hydroxyapatite coatings by using the room temperature spraying method and their influence on the microstructure and in vitro characteristics were investigated. Starting hydroxyapatite powders for coatings on titanium substrate were prepared by a heat treatment at 1100 °C for 2 h of bovine bone, bone ash, and commercial hydroxyapatite powders. The phase compositions and Ca/P ratios of the three hydroxyapatite coatings were similar to those of the raw materials without decomposition or formation of a new phase. All hydroxyapatite coatings showed a honeycomb structure, but their surface microstructures revealed different features in regards to surface morphology and roughness, based on the staring materials. All coatings consisted of nano-sized grains and had dense microstructure. Inferred from in vitro experiments in pure water, all coatings have a good dissolution-resistance and biostability in water.

Long and short range order structural analysis of In-situ formed biphasic calcium phosphates.

BACKGROUND: Biphasic calcium phosphates (BCP) have attracted considerable attention as a bone graft substitute. In this study, BCP were prepared by aqueous co-precipitation and calcination method. The crystal phases of in-situ formed BCP consisting of hydroxyapatite (HAp) and ß-tricalcium phosphate (ß-TCP) were controlled by the degree of calcium deficiency of precursors. The long and short range order structures of biphasic mixtures was investigated using Rietveld refinement technique and high resolution Raman spectroscopy. The refined structural parameters of in-situ formed BCP confirmed that all the investigated structures have crystallized in the corresponding hexagonal (space group P63/m) and rhombohedral (space group R3c) structures. RESULTS: The crystal phases, Ca/P molar ratio, and lattice parameters of in-situ formed BCP consisting of HAp and ß-TCP were controlled by the degree of calcium deficiency of calcium phosphate precursors. The significant short range order structural change of BCP was determined by Raman analysis. CONCLUSIONS: The long and short range order structural changes of in-situ formed BCP might be due to the coexistence of ß-TCP and HAp crystal phases.

Surface Corrosion of Nanoscaled Hydroxyapatite During an In Vivo Experiment.

Hydroxyapatite (HA) is widely used as a bioactive ceramics as it forms a chemical bond with bone. However, the drawback to using this material is its inferior mechanical properties. In this research, surface corrosion and disintegration of nanoscaled HA in a dog were studied, and the mechanism by which phase-pure HA dissolved in vivo was investigated. Biological properties of HA in vivo are affected by the grain-boundary dissolution followed by a surface corrosion and microstructural disintegration. This kind of dissolution process, apparently evidenced at the grain boundary, causes particle generation, which indicates that both long-term bone in-growth and mechanical properties can dramatically deteriorate. Implant dissolution by osteoclasts in vivo is also observed on the surface of hydroxyapatite. Implant surface showed an aggressive corrosion by an osteoclast resorption. Severe and deeper dissolution underwent close to osteoclast resulting in formation of smaller and more round particle shape.

Morphologies of nano-sized apatite formed on titanium substrate by biomimetic process.

The apatite was formed on the titanium plates with NaOH and heat treatments by biomimetic process. The influence of titanium surface microstructure on the apatite formation onto titanium substrate in SBF solution was investigated. After biomimetic process, nano-sized apatite layers were found on the Ti plates with NaOH and heat treatments. However, the morphologies of formed apatite on substrate had different shapes such as coated, load-like, and linked. The morphology of apatite formed by biomimetic process would be affected by alkaline treatment, and substrate morphology and phase.

Improvement of the stability of hydroxyapatite through glass ceramic reinforcement.

Hydroxyapatite has achieved significant application in orthopedic and dental implants due to its excellent biocompatibility. Sintered hydroxyapatites showed significant dissolution, however, after their immersion in water or simulated body fluid (SBF). This grain boundary dissolution, even in pure hydroxyapatites, resulted in grain separation at the surfaces, and finally, in fracture. In this study, hydroxyapatite ceramics containing apatite-wollastonite (AW) or calcium silicate (SG) glass ceramics as additives were prepared to prevent the dissolution. AW and SG glass ceramics were added at 0-7 wt% and powder-compacted uniaxially followed by firing at moisture conditions. The glass phase was incorporated into the hydroxyapatite to act as a sintering aid, followed by crystallization, to improve the mechanical properties without reducing the biocompatibility. As seen in the results of the dissolution test, a significant amount of damage was reduced even after more than 14 days. TEM and SEM showed no decomposition of HA to the secondary phase, and the fracture toughness increased, becoming even higher than that of the commercial hydroxyapatite.

Nanostructured hydroxyapatite by microwave sintering.

In this work, nanostructured HA ceramics with dense microstructure were prepared by microwave sintering process and their microstructures were compared with the case of conventional sintering. Commercially obtained HA powder with Ca/P molar ratio of 1.67 was used as a starting material. The powder of granular type consists of nanocrystalline particles of 20-30 nm in size. The as-received HA powder or the powder calcined at 800 degrees C, followed by ball-milling was used for the preparation of HA disks. Microwave sintering was conducted at 1200 degrees C for 5 min with a heating rate of 50 degrees C/min. HA ceramics with the sintered densities of approximately 96-97% of the theoretical were obtained. XRD analysis showed that all detectable peaks are identical to pure hydroxyapatite. The HA sintered body made of calcined and ball-milled powder showed uniform microstructure with grain size of 300-400 nm and with finer sub-grains of 30-40 nm.