Seigo Okawa

Bio: Seigo Okawa is an academic researcher from Niigata University. The author has contributed to research in topics: Titanium & Casting. The author has an hindex of 12, co-authored 30 publications receiving 453 citations.

Layered structure of cast titanium surface.

Abstract: The present study concerns the surface layered structure of the cast Ti. A commercial Ti was cast into a mold which was made of a phosphate-bonded Al2O3/SiO2 investment. Elemental analyses of the interfacial zone of the casting were made under an electron probe micro-analyzer.The interfacial zone was composed of four layers: the outermost reaction or casting burn layer, the second layer of an O-and Al-stabilized α case, the third layer in which Si, P, O, and C were inhomogeneously concentrated, and the fourth layer which consisted of acicular or plate-like crystals. It was observed that the larger the cast volume and the higher the mold temperature, the thicker became each layer and the coarser became the acicular grains. Probably, the layered structure was formed through decomposition of reducible species in the burnout investment and diffusion of the resulting elements into the casting.

Morphological and chemical analysis of different precipitates on mineral trioxide aggregate immersed in different fluids.

Abstract: The purpose of this study was to analyze the ultrastructures and chemical compositions of precipitates formed on mineral trioxide aggregate (MTA; White ProRoot MTA) immersed in distilled water (DW) or phosphate buffered saline (PBS), based on the attribution that MTA's bioactivity and sealing ability are influenced by its interaction with the external fluid environment. After 1 and 14 days of immersion, precipitates formed on MTA disks were analyzed using wavelength-dispersive X-ray spectroscopy electron probe microanalyzer with image observation function (SEM-EPMA; EPMA1601, Shimadzu, Kyoto, Japan), and Fourier transform-infrared (FT-IR) spectroscopy. On DW specimens, cubic-like crystals containing Ca, O, and C (17, 66, and 17 at% respectively) were produced. State analysis of calcium k(β)spectrum also revealed calcium hydroxide. On PBS specimens, acicular-spherical and lath-like crystals with Ca/P molar ratios of 1.42 and 1.58 respectively were produced. In conclusion, the precipitates formed on DW specimens were identified as calcium carbonate and calcium hydroxide primarily, whereas the precipitates on PBS specimens were inferred to be amorphous calcium phosphate.

Surface contamination of titanium by abrading treatment.

Abstract: This study investigated the contamination of abraded Ti surfaces. Using a polishing machine, specimens were abraded with waterproof SiC grit papers under water cooling. The abraded surfaces were examined using element analysis, X-ray diffraction, and hardness tests. Contaminant deposits with dimensions reaching about 30 microns were observed throughout the surface. In these deposits, Ti was apparently reduced by about 10% and replaced by Si and O. The chemical bond state of the Si was similar to that of SiC or a titanium silicide. The O was solute in Ti, which increased the surface hardness. The contaminant deposits were amorphous or very thin. The contamination of Ti, the extent of which was related to hardness, resulted from a reaction with abrasives.

Chemical mechanical polishing of titanium with colloidal silica containing hydrogen peroxide--mirror polishing and surface properties.

Abstract: Chemical mechanical polishing (CMP) of cpTi (Ti) was carried out using two types of slurries, acidic and basic colloidal silica containing H2O2 up to 3 wt%, to obtain flat and mirror-polished surfaces without any contaminated and reacted layers. Polishing behavior and surface properties were investigated using AFM, EPMA, and XPS. Weight loss of Ti polished by CMP using the basic slurry was larger than that using the acidic one, and surface roughness was less than 2 nm RMS when basic slurry containing 3 wt% H2O2 was used. Moreover, three kinds of chemical species, OH-, O2-, and H2O, were detected on the Ti surfaces polished by CMP using these slurries. Results of this study showed that CMP using colloidal silica containing H2O2 successfully created a mirror-polished surface without contaminated and reacted layers.

Uptake of calcium and silicon released from calcium silicate-based endodontic materials into root canal dentine

Abstract: Han L, Okiji T. Uptake of calcium and silicon released from calcium silicate–based endodontic materials into root canal dentine. International Endodontic Journal. Aim To compare Biodentine and White ProRoot mineral trioxide aggregate (MTA) with regard to Ca and Si uptake by adjacent root canal dentine in the presence of phosphate-buffered saline (PBS). Methodology Root canals of bovine incisor root segments were instrumented, filled with either Biodentine or MTA (n = 20 each) and then immersed in Ca-and Mg-free PBS for 1, 7, 30 or 90 days (n =5 each). Unfilled, unimmersed dentine specimens (n =5 ) served as controls. The specimens were sectioned longitudinally, and the ultrastructure of the dentine‐ material interface and the elemental composition/ distribution in the material‐adjacent dentine were analysed using a wavelength-dispersive X-ray spectroscopy electron probe microanalyser with image observation function. Data were statistically analyzed using one-way anova and Tukey’s honestly significant difference test or the Mann‐Whitney U-test. Results Along the material‐dentine interface, both materials formed a tag-like structure that was composed of either Ca- and P-rich crystalline deposits or the material itself. The width of a Ca- and Si-rich layer detected along the dentine layer of the material‐ dentine interface showed increases over time. The Caand Si-rich layer width was significantly larger (P < 0.05) in Biodentine than MTA at 30 and 90 days. Conclusions Both Biodentine and MTA caused the uptake of Ca and Si in the adjacent root canal dentine in the presence of PBS. The dentine element uptake was more prominent for Biodentine than MTA.

Investigation of the hydration and bioactivity of radiopacified tricalcium silicate cement, Biodentine and

Abstract: a b s t r a c t Objective. Novel root-end filling materials are composed of tricalcium silicate (TCS) and radiopacifier as opposed to the traditional mineral trioxide aggregate (MTA) which is made up of clinker derived from Portland cement and bismuth oxide. The aim of this research was to characterize and investigate the hydration of a tricalcium silicate-based proprietary brand cement (BiodentineTM) and a laboratory manufactured cement made with a mixture of tricalcium silicate and zirconium oxide (TCS-20-Z) and compare their properties to MTA AngelusTM. Methods. The materials investigated included a cement containing 80% of TCS and 20% zirconium oxide (TCS-20-Z), BiodentineTM and MTA AngelusTM. The specific surface area and the particle size distribution of the un-hydrated cements and zirconium oxide were investigated using a gas adsorption method and scanning electron microscopy. Un-hydrated cements and set materials were tested for mineralogy and microstructure, assessment of bioactivity and hydration. Scanning electron microscopy, X-ray energy dispersive analysis, X-ray fluorescence spectroscopy, X-ray diffraction, Rietveld refined X-ray diffraction and calorimetry were employed. The radiopacity of the materials was investigated using ISO 6876 methods. Results. The un-hydrated cements were composed of tricalcium silicate and a radiopacifier phase; zirconium oxide for both Biodentine TM and TCS-20-Z whereas bismuth oxide for MTA AngelusTM. In addition BiodentineTM contained calcium carbonate particles and