Eui Kwan Koh

A 60-Hz sinusoidal magnetic field induces apoptosis of prostate cancer cells through reactive oxygen species

Purpose: To explore the effects of power frequency magnetic fields (MF) on cell growth in prostate cancer, DU145, PC3, and LNCaP cells were examined in vitro. Materials and methods: The cells were exposed to various intensities and durations of 60-Hz sinusoidal MF in combination with various serum concentrations in the media. To analyze MF effects on cell growth, cell counting, trypan blue exclusion assay, Western blot analysis, flow cytometry, enzyme-linked immunosorbent assay (ELISA), semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), fluorescence microscopy, and spectrofluorometry were used. Results: MF exposure induced significant cell growth inhibition and apoptosis in an intensity- and time-dependent manner, in which cell cycle arrest, cleaved Caspase-3, and reactive oxygen species (ROS) increased. Pretreatment with a Caspase-3 inhibitor or antioxidant, N-acetyl-L-cysteine (NAC), significantly attenuated MF-induced cell growth inhibition and cell death. Media replacement experiments failed to show any notable change in the MF effects. Conclusions: These results demonstrate 60-Hz sinusoidal MF-activated cell growth inhibition of prostate cancer in vitro. Apoptosis together with cell cycle arrest were the dominant causes of the MF-elicited cell growth inhibition, mediated by MF-induced ROS. These results suggest that a possibility of using 60-Hz MF in radiation therapy of prostate cancer could usefully be investigated.

Free Carrier Concentration Gradient along the

The charge carrier concentration variance in a freestanding Si-doped GaN single crystal grown by hydride vapor phase epitaxy (HVPE) has been investigated. A transverse micro-Raman scattering measurement along the [0001] c-axis on the cross section of the n-type GaN was performed to obtain the profile of the free carrier concentration. The analysis of the coupled modes of plasmons and longitudinal optical (LO) phonons revealed the presence of a free carrier concentration gradient along the c-axis, which gradually varies from 2.3×1017 (N face) to 9.3×1017 cm-3 (Ga face). The possible physical origins of the charge carrier concentration gradient in the GaN crystal were discussed on the basis of current theories and experimental results of the growth mechanism of an n-doped GaN single crystal.

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In an effort to improve the initial nucleation and crystal quality of the GaN epilayer, N-ion implantation on the sapphire (0001) substrate has been attempted. As a result of implantation with 55 keV nitrogen ions (N+), to a dose ranging from 1×1015 to 1×1017 cm-2, prior to GaN epilayer growth, the formation of a thin and disordered AlN phase was observed. The presence of the N-ion-implanted surface decreased internal free energies in the growth of the GaN epilayer, and the misfit strain was relieved to allow the formation of the AlN phase on the N-ion-implanted sapphire substrate. Through the estimation of the lattice strain value for GaN epilayers, it was confirmed that the reduction of strain was achieved with the N-ion-implanted surface with the ion dose of 1×1016 cm-2. The present results show that N-ion implantation of the sapphire (0001) surface can improve the properties of GaN epilayers grown by metalorganic chemical vapor deposition (MOCVD).

-Axis of a Freestanding Si-doped GaN Single Crystal

GaN growth was carried out on Al/sub 2/O/sub 3/(0001) which was ion beam pre-treated to reduce the stain and dislocation density in the film. The ion dose was fixed to 1/spl times/10/sup 16/ ions/cm/sup 2/ and the ion energy used was 800eV and 5.5keV. The 800eV ion beam was reactive N/sub 2//sup +/ ions and the 5.5keV ion beam was from a normal ion implanter. Both ion beam pretreatments resulted in the formation of a disordered amorphous phase on the substrate surface. The GaN buffer and main growth was carried out by MOCVD,the strain was monitored using Raman spectra and dislocations were observed by TEM. The Raman shift clearly showed that the strain in the film was decreased by ion beam pre-treatment of sapphire prior to GaN deposition. The defect density in the film was reduced by up to 55% with ion beam pretreatment.

Implantation of N ions on sapphire substrate for improvement of GaN epilayer

AbstractThe authors have developed and evaluated a new method that uses transit dose in the treatment room in verification of dose delivery to patients. Five intensity modulated radiotherapy (IMRT) plans were selected from actual cancer patients, and transit dose was measured using MapCHECK 2 and an anthropomorphic phantom. The criteria used as a verification tool for the dose delivery to the patient were gamma-index-based dose comparison between the computed dose and measured dose. When the doses were delivered to an anthropomorphic phantom normally, the average passing rate was 95.2% based on a gamma index analysis. This feasibility study suggested that transit dose-based quality assurance can provide information about the accuracy of an inhomogeneity correction algorithm and patient positioning during treatment, allowing its use as a verification tool for actual dose delivery to patients in the treatment room.

Improved crystalline quality of GaN by substrate ion beam pre-treatment

The structural and optical properties of GaN epilayers grown on intentionally strained sapphire (0001) substrates were investigated. In general, sapphire substrate experiences the tensile stress whereas GaN epilayer experience the compressive stress in the case of GaN/sapphire system. To examine the effects of intentionally strained sapphire substrate on the GaN epilayers, it was obliquely implanted with 2.4 MeV energy Cl/sup +/ and As/sup +/ ions to 10/sup 15/ cm/sup -1/ dose where TRIM-simulated projection ranges (R/sub p/) are 1.16 and 0.95 /spl mu/m, respectively. Cl/sup +/ ion implantation is expected to generate the lower strain field within the sapphire substrate, since it has smaller ionic radius (r/sub Cl+/ < r/sub As+/) and deeper R/sub p/ (R/sub p/ /sub Cl+/ > R/sub p/ /sub As+/) than the As/sup +/. To recover the disordered sapphire surface caused by implants, rapid thermal annealing was performed. After that, GaN epilayers were deposited on the low temperature GaN buffer layers grown by metal organic chemical vapor deposition (MOCVD) technique.

Evaluation of the accuracy of dose delivery for IMRT based on transit dosimetry.

This study was designed to investigate the mechanism by which electric fields affect cell function, and to determine the optimal conditions for electric field inhibition of cancer cell proliferation. Low-intensity (<2 V/cm) and intermediate-frequency (100–300 kHz) alternating electric fields were applied to glioblastoma cell lines. These electric fields inhibited cell proliferation by inducing cell cycle arrest and abnormal mitosis due to the malformation of microtubules. These effects were significantly dependent on the intensity and frequency of applied electric fields.

Influence of intentionally strained sapphire substrate on GaN epilayers

Considering for the concept of power storage systems, such as those used to supply power to microelectronic devices, ferritins have aroused a lot of interests for applications in bioelectrochemical devices. And electron transfer rates from the proteins to electrode surface are key determinants of overall performance and efficiency of the ferritin-based devices. Here we have investigated the electron transport mechanism of ferritin layer which was immobilized on an Au electrode. The current-voltage (I-V) curves are obtained by a conductive atomic force microscope (c-AFM) as a function of contact area between AFM tip and the ferritin layer. In the low voltage region, I-V curves are affected by both Fowler-Nordheim tunneling and image force. On the other hand, the experimental results are consistent with a Simmons model in a high voltage region, indicating that, as the voltage increases, the image potential has a dominant effect on the electron transport mechanism. These results are attributed to the film-like character of the ferritin layer, which generates an image potential to lower the barrier height in proportion to the voltage increment.

Inhibition of brain tumor cell proliferation by alternating electric fields

In this work, top-gate single ZnO nanowire-based FETs embedded with Au nanoparticles as the charge storage were fabricated and their memory effects were characterized.

The Effect of Image Potential on the Current-Voltage Characteristics of a Ferritin-layer

The main goal of this work is to minimize stress fields due to the mismatch in lattice constants and thermal expansion coefficients between GaN and sapphire by using the chemical and physical changes of the sapphire (0001) substrate. Effects of N/sup +/-implanted sapphire (0001) substrate on GaN epilayer by metal organic chemical vapor deposition (MOCVD) were investigated. It is our intention to examine the possibility of employing the N/sup +/-implantation treatment of the sapphire (0001) substrate to improve the properties of GaN epilayer grown by MOCVD.