On the surface of silicon nanowires (SiNWs) synthesized by gold (Au)-catalyzed chemical vapor deposition (CVD), Au particles 5-20 nm in diameter are formed if the growth conditions are within a specific range. We studied the mechanism of Au particle formation by growing SiNWs under different conditions, specifically by dynamically changing the growth parameters during the growth process. We show that insufficient supply of Si source to the Au-Si eutectic on top of the SiNWs enhances the migration of Au atoms on the surface of SiNWs in the form of Au-Si eutectic, which is precipitated on the surface as Au particles during cooling. We also show that using Au-Si eutectic on the surface of SiNWs as a catalyst enables one-step growth of branched SiNWs.

The highly oxygen-permeable material, poly-dimethylsiloxane (PDMS), has the potential to be applied to cell culture microdevices, but cell detachment from PDMS has been a major problem. In this study, we demonstrate that a combination of collagen covalently immobilized PDMS and an adequate oxygen supply enables the establishment of a stable, attached spheroid (hemispheroid) culture of rat hepatocytes. The bottom PDMS surfaces were first treated with oxygen plasma, then coupled with aminosilane followed by a photoreactive crosslinker, and they were finally reacted with a collagen solution. X-ray photoelectron spectroscopy (XPS) and contact angle measurements showed that the covalent immobilization of collagen on the surface occurred only where the crosslinker had been introduced. On the collagen-conjugated PDMS surface, rat hepatocytes organized themselves into hemispheroids and maintained the viability and a remarkably high albumin production at least for 2 weeks of culture. In contrast, hepatocytes on the other types of PDMS surfaces formed suspended spheroids that had low albumin production. In addition, we showed that blocking the oxygen supply through the bottom PDMS surface inhibited the formation of hemispheroids and the augmentation of hepatocellular function. These results show that appropriate surface modification of PDMS is a promising approach towards the development of liver tissue microdevices.

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The concentration of electrically active impurities in in situ boron (B)-doped silicon (Si) nanowires (SiNWs) synthesized by gold (Au)-catalyzed chemical vapor deposition (CVD) is studied by Raman spectroscopy. B-doped SiNWs exhibit an asymmetric Raman spectrum due to Fano resonance between discrete phonon Raman scattering and continuous electric Raman scattering caused by the excitation of holes in the valence band. To quantitatively evaluate the concentration of active B atoms from the asymmetric spectral shape, the spectra are fitted by a Fano resonance formula and asymmetry parameters are extracted. From the comparison of the asymmetry parameter with those obtained for reference samples, the concentration of active B atoms in SiNWs is estimated. The effects of thermal annealing in nitrogen and oxygen gases on the active B concentration are also studied. The annealing in nitrogen ambience for a short period significantly increases the concentration of active B atoms especially when the doping level is high, while longer period annealing decreases the active B concentration. The reduction of active B concentration is more significant when SiNWs are annealed in oxygen gas ambience. By combining Raman results with high-resolution transmission electron microscope observations, the growth mechanism of B-doped SiNWs is discussed. We show that the number of B atoms doped into SiNWs via Au catalysts is very limited and high B concentration layers are grown by conformal growth on the sidewall of SiNWs.

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The paper reports the first successful fabrication of MgB 2 superconducting tape using a flexible metallic substrate as well as its strong pinning force, which was verified by direct measurement of transport critical current density. The tape was prepared by depositing MgB 2 film on a Hastelloy tape buffered with an YSZ layer. The J c of the tape exceeds 10 5 A/cm 2 at 4.2K and 10T, which is considered as a common benchmark for magnet application. The J c dependence on magnetic field remains surprisingly very small up to 10T, suggesting that the tape has much better magnetic field characteristic than conventional Nb-Ti wires in liquid helium.The newly discovered MgB 2 superconductor 1 is expected to be useful for various electric power applications as well as electronic device applications because its transition temperature is much higher than those of conventional metallic superconductors such as Nb-Ti and Nb 3 Sn. In order to evaluate the potentiality for p ower applications, the development of wire processing techniques is essential. The first attempt at wire fabrication was the magnesium vapor diffusion to boron fibers presented by Canfield et al 2 . Recent efforts at wire fabrication have centered on developing the powder-in-tube (PIT) process due to its greater easiness of scaled up production [3][4][5][6][7] . The transport critical current density J c at 4.2K and in self-field of the PIT processed tapes has already exceeded the practical level of 10 5 A/cm 2 . However, the J c rapidly decreases in an applied magnetic field due to its weak pinning force. The J c values at 4.2K and 10T reported so far for PIT processed wires and tapes remain as low as the order of 10 3 A/cm 2 , although the values are being steadily improved.In contrast to the relatively poor in-field J c values of the tapes and bulks, there are several papers on MgB 2 thin films, which reported very much higher Hc 2 values and J c values in applied magnetic fields than those of tape and bulk samples 8,9 . Kim et al 9 reported a large transport J c value of 10 5 A/cm 2 at 5T and 15K for the c-axis oriented MgB 2 film prepared on an Al 2 O 3 substrate. Those results suggest to us that the MgB 2 phase formed by vapor deposition techniques has extremely strong pinning force. However, all experiments on the thin films have been performed using ceramic substrates, which are not suitable for long length production of flexible conductor. This paper reports a successful new approach to fabricate MgB 2 tape that is the combination of the high critical current density achieved by vapor deposition and the use of flexible metallic substrate tape. The technique is similar to the so-called coated conductor techniques developed for YBa 2 Cu 3 O y (Y-123), where Y-123 thin films are deposited on various buffer layers on a metallic substrate 10 -12 . The MgB 2 tape obtained in this work has an excellent transport J c of 1.1x10 5 A/cm 2 at 4.2.K and 10T. It is also surprising that the J c dependence on the magnetic field remains extremely small up to 10T, ...

Spherical three-dimensional (3D) cellular aggregates are valuable for various applications such as regenerative medicine or cell-based assays due to their stable and high functionality. However, previous methods to form aggregates have shown drawbacks, being labor-intensive, showing low productivity per unit area or volume and difficulty to form homogeneous aggregates. We proposed a novel strategy based on oxygen-permeable polydimethylsiloxane (PDMS) honeycomb microwell sheets, which can theoretically supply about 80 times as much oxygen as conventional polystyrene culture dishes, to produce recoverable aggregates in controllable sizes using mouse insulinoma cells (MIN6-m9). In 48 hours of culture, the PDMS sheets produced aggregates whose diameters were strictly controlled (≃32, 60, 90, 150 and 280 mm) even at an inoculum density eight times higher (8.0×105 cells/cm(2) ) than that of normal confluent monolayers (1.0×105 cells/cm(2) ). Measurement of the oxygen tension near the cell layer and glucose/lactate analysis clearly showed that cells exhibit aerobic respiration on the PDMS-based culture system. Glucose-responsive insulin secretion of the recovered aggregates showed that the aggregates around 90 mm in diameter secreted the largest amounts of insulin. This confirmed the advantages of 3D cellular organization and the existence of a suitable aggregate size, above which excess organization leads to a decreased metabolic response. These results demonstrated that this microwell-based PDMS culture system provides a promising method to form size-regulated and better functioning 3D cellular aggregates of various kinds of cells with a high yield per surface area.

We have applied phthalocyaninatosilicon (SiPc) covalently linked to one or two tetramethyl-1-piperidinyloxyl (TEMPO) radicals as fluorescence probes for detecting ascorbic acid in biological systems.