Yusuke Tsutsumi

Evaluation of corrosion resistance of implant-use Ti-Zr binary alloys with a range of compositions.

Although titanium-zirconium (Ti-Zr) alloy has been adopted for clinical applications, the ideal proportion of Zr in the alloy has not been identified. In this study, we investigated the biocompatibility of Ti-Zr alloy by evaluating its corrosion resistance to better understand whether there is an optimal range or value of Zr proportion in the alloy. We prepared pure Ti, Ti-30Zr, Ti-50Zr, Ti-70Zr, and pure Zr (mol% of Zr) samples and subjected them to anodic polarization and immersion tests in a lactic acid + sodium chloride (NaCl) solution and artificial saliva. We observed pitting corrosion in the Ti-70Zr and Zr after exposure to both solutions. After the immersion test, we found that pure Ti exhibited the greatest degree of dissolution in the lactic acid + NaCl solution, with the addition of Zr dramatically reducing Ti ion dissolution, with the reduction ultimately exceeding 90% in the case of the Ti-30Zr. Hence, although the localized corrosion resistance under severe conditions was compromised when the Zr content was more than 70%, metal ion release reduced owing to Zr addition and the corresponding formation of a stable passive layer. The results suggest that Ti-30Zr or a Zr proportion of less than 50% would offer an ideal level of corrosion resistance for clinical applications.

Magnetic susceptibility, artifact volume in MRI, and tensile properties of swaged Zr–Ag composites for biomedical applications

Zr-Ag composites were fabricated to decrease the magnetic susceptibility by compensating for the magnetic susceptibility of their components. The Zr-Ag composites with a different Zr-Ag ratio were swaged, and their magnetic susceptibility, artifact volume, and mechanical properties were evaluated by magnetic balance, three-dimensional (3-D) artifact rendering, and a tensile test, respectively. These properties were correlated with the volume fraction of Ag using the linear rule of mixture. We successfully obtained the swaged Zr-Ag composites up to the reduction ratio of 96% for Zr-4, 16, 36, 64Ag and 86% for Zr-81Ag. However, the volume fraction of Ag after swaging tended to be lower than that before swaging, especially for Ag-rich Zr-Ag composites. The magnetic susceptibility of the composites linearly decreased with the increasing volume fraction of Ag. No artifact could be estimated with the Ag volume fraction in the range from 93.7% to 95.4% in three conditions. Youngs modulus, ultimate tensile strength (UTS), and 0.2% yield strength of Zr-Ag composites showed slightly lower values compared to the estimated values using a linear rule of mixture. The decrease in magnetic susceptibility of Zr and Ag by alloying or combining would contribute to the decrease of the Ag fraction, leading to the improvement of mechanical properties.

Response of preosteoblasts to titanium with periodic micro/nanometer scale grooves produced by femtosecond laser irradiation

To investigate the cellular response to designed topography in vitro, we studied the adhesion, proliferation, osteogenic differentiation, and calcification of mouse preosteoblasts (MC3T3-E1) cultured on titanium (Ti) surfaces with periodic micrometer scale grooves containing nanometer scale ripples in the vertical direction fabricated by single-shot, femtosecond laser irradiation (fsTi). The surface composition and chemical state of fsTi were almost the same as those of mirror-polished Ti without femtosecond laser irradiation (mTi). Cells cultured on fsTi were highly aligned, whereas the cell proliferation rate on fsTi was less than that on mTi. Higher gene expressions of Spp1 and Bglap1 were detected in cells cultured on fsTi than those on mTi, indicating that the periodic micro/nanometer scale grooves topography promoted osteogenic differentiation and calcification. This initial activation of osteoinduction on fsTi generated calcified deposits that were thicker and larger than those on mTi and hence, osteoconductivity was promoted on fsTi. Our findings indicate that femtosecond laser irradiation is a technique with potential for controlling biomaterial-cell interfaces and, in particular, the promotion of osseointegration of Ti.

Adhesion and differentiation behaviors of mesenchymal stem cells on titanium with micrometer and nanometer-scale grid patterns produced by femtosecond laser irradiation: ADHESION AND DIFFERENTIATION OF hMSCs ON MICRON AND NANO GRIDS

To clarify the effects of grid topographies with different scales on cell morphology and functionalization, we investigated the adhesion and differentiation of human mesenchymal stem cells (hMSCs) to titanium surfaces with micron, nano, and micron/nano (hybrid) grid topographies created by femtosecond laser irradiation. The results showed that cellular adhesion and differentiation strongly depended on the scales of the grid topography. hMSCs cultured on micron and hybrid grid topographies showed regulation of cellular adhesion plaques following the surface topography and were vinculin-positive, whereas filamentous vinculin was evident at the filopodia of hMSCs cultured on nanogrids. The findings indicate that the micron grid topography was beneficial for cell colonization by anchoring the cells to the substrate surface, whereas the nanogrid topography was beneficial for cell locomotion. With the superposition effect of the micron and nanogrids, micro/nanohybrid grid topography strongly promoted cell adhesion. This differential adhesion induced differences cell differentiation. Nanogrids promoted differentiation of hMSCs, particularly osteogenic differentiation. These findings provide a basis for the design of novel biomaterial surfaces that can regulate specific cellular functions.

Biosafety, stability, and osteogenic activity of novel implants made of Zr 70 Ni 16 Cu 6 Al 8 bulk metallic glass for biomedical application

Superior mechanical and chemical properties of Zr70Ni16Cu6Al8 bulk metallic glass (BMG) demonstrate its promise as a novel biomaterial for fabrication of implants. The aim of the present study was to validate mechanical, chemical, and biological properties of Zr70Ni16Cu6Al8 BMG through comparison with titanium (Ti). Our data indicated higher tensile strength, lower Youngs modulus, and reduced metal ion release of Zr70Ni16Cu6Al8 BMG compared with Ti. Biosafety of bone marrow mesenchymal cells on Zr70Ni16Cu6Al8 BMG was comparable to that of Ti. Next, screw-type implant prototypes made of Zr70Ni16Cu6Al8 BMG were fabricated and inserted into rat long bones. Zr70Ni16Cu6Al8 BMG implants indicated a higher removal-torque value and lower Periotest value compared with Ti implants. In addition, higher amounts of new bone formation and osseointegration were observed around Zr70Ni16Cu6Al8 BMG implants compared with Ti implants. Moreover, gene expression analysis displayed higher expression of osteoblast- and osteoclast-associated genes in the Zr70Ni16Cu6Al8 BMG group compared with the Ti group. Importantly, loading to implants upregulated bone formation, as well as osteoblast- and osteoclast-associated gene expression in the peri-implant area. No significant difference in concentrations of Ni, Al, Cu, and Zr in various organs was shown between in the Zr70Ni16Cu6Al8 BMG and Ti groups. Collectively, these findings suggest that Zr70Ni16Cu6Al8 BMG is suitable for fabricating novel implants with superior mechanical properties, biocompatibility, stability, and biosafety compared with Ti. STATEMENT OF SIGNIFICANCE Titanium is widely used to fabricate orthopedic and dental implants. However, Titanium has disadvantages for biomedical applications in regard to strength, elasticity, and biosafety. Recently, we developed a novel hypoeutectic Zr70Ni16Cu6Al8 BMG, which has superior mechanical and chemical properties. However, the validity of Zr70Ni16Cu6Al8 BMG for biomedical application has not been cleared. The aim of the present study was to validate the mechanical, chemical, and biological properties of Zr70Ni16Cu6Al8 BMG for biomedical applications through comparison with Titanium. The present study clarifies that Zr70Ni16Cu6Al8 BMG has good mechanical properties, corrosion resistance, and osteogenic activity, which are necessary features for biomedical applications. The present study provides for the first time the superiority of Zr70Ni16Cu6Al8 BMG implants to Titanium implants for biomedical applications.