Kentaro Iwami

Bio rapid prototyping by extruding/aspirating/refilling thermoreversible hydrogel

This paper reports a method for rapid prototyping of cell tissues, which is based on a system that extrudes, aspirates and refills a mixture of cells and thermoreversible hydrogel as a scaffold. In the extruding mode, a cell-mixed scaffold solution in the sol state is extruded from a cooled micronozzle into a temperature-controlled substrate, which keeps the scaffold in the gel state. In the aspiration mode, the opposite process is performed by Bernoulli suction. In the refilling mode, the solution is extruded into a groove created in the aspiration mode. The minimum width of extruded hydrogel pattern is 114 +/- 15 microm by employing a nozzle of diameter 100 microm, and that of aspirated groove was 355 +/- 10 microm using a 500 microm-diameter nozzle. Gum arabic is mixed with the scaffold solution to avoid peeling-off of the gel pattern from the substrate. Patterning of Sf-9 cell tissue is demonstrated, and the stability of the patterned cell is investigated. This system offers a procedure for rapid prototyping and local modification of cell scaffolds for tissue engineering.

Optical Near-Field Probe Integrated With Self-Aligned Bow-Tie Antenna and Electrostatic Actuator for Local Field Enhancement

Microelectromechanical systems (MEMS)-based near-field scanning optical microscopy (NSOM) probes with a bow-tie antenna structure consisting of two metal triangular electrodes separated by a narrow gap have been designed and fabricated. An electrostatic actuator is integrated on this bow-tie probe to decrease the gap width for enhancing the optical near-field intensity. A self-alignment process based on deep reactive ion etching and wet anisotropic etching is established to fabricate the symmetric bow-tie structure. The static and dynamic actuations of electrostatic actuators are examined. With the mechanical resonance of the antenna structure to lateral direction, NSOM imaging is performed in the visible range, and the subwavelength resolution beyond the diffraction limit of light is demonstrated

An Au nanofin array for high efficiency plasmonic optical retarders at visible wavelengths

An Au nanofin array was designed and fabricated for in a microscale optical retarder with high transmittance at visible wavelengths. The array was designed on the basis of the theory of waveguides. The adopted nanocoating process realized a high aspect ratio Au structure with a period of 400 nm and a height of 800 nm. The transmittance of transverse magnetic polarized light at visible to near-infrared wavelengths exceeded 40% and a retardation of 170° was achieved at 633 nm.

Electron field emission from a gold tip under laser irradiation at the plasmon-resonant wavelength

Electron field emission properties of a sharp gold tip under continuous wave laser irradiation at the plasmon-resonant wavelength are investigated. A gold-coated tungsten emitter with a tip radius of 100 nm is used. The plasmon-resonant wavelength of the tip obtained by a finite-difference time-domain calculation is around 530 nm. Thus, I-V characteristics of the tip under laser irradiations at 442, 532, and 633 nm were measured. The intensity of 10 mW, 532 nm irradiation at the focal point was 13.9 kW/cm2. TE-polarized 532 nm irradiation shows the maximum current enhancement despite its smaller photon energy and intensity compared with the irradiation at 442 nm. The current enhancement factor ranged from 13.7 to 217. From the energy dispersive x-ray analysis, these results can be attributed to plasmon resonance. The possible mechanisms of plasmon-resonant current enhancement are considered to be the effective photoassisted field emission and thermal field emission.

Measurement of nanoparticle sizes by conventional optical microscopy with standing evanescent field illumination

The size of a particle smaller than the diffraction limit is measured using a conventional optical microscope by adopting a standing evanescent field illumination. The scattering intensity from a nanoparticle is periodically modulated by shifting the intensity fringes of the standing evanescent field. By measuring contrast of scattering intensity variation during one cycle of modulation, particle sizes can be estimated easily. Furthermore, material dependence can be canceled using contrast as an evaluation factor. From the experimental results, particle sizes ranging from 20 to 250 nm are successfully determined. This technique offers a low-cost size measurement for nanoparticles.

Highly-efficient and angle-independent zero-order half waveplate at broad visible wavelength based on Au nanofin array embedded in dielectric

A Au nanofin array embedded in SiO2 was designed and fabricated to achieve an achromatic half waveplate with high transmittance at visible wavelengths. On the basis of the waveguide theory of nanogaps and the Fresnel reflection theory, nanofin array is calculated to have ideal properties for an achromatic half-waveplate in the visible band from 560 to 660 nm with the transmittance of around 50%. A Au nanofin array with a height of 830 nm and a period of 400 nm was fabricated through a sidewall-deposition process and overcoating with spin on glass. The polarization microscopy results showed that both transmittance greater than 50% and retardation of 165° at broadband wavelengths ranging from 600 to 800 nm were simultaneously achieved. It was also demonstrated that retardation had little dependence on the incident angle.

Development of a pH sensor based on a nanostructured filter adding pH-sensitive fluorescent dye for detecting acetic acid in photovoltaic modules

Acetic acid formed via the hydrolysis of ethylene vinyl acetate (EVA) as an encapsulant in photovoltaic (PV) modules causes a decrease in the conversion efficiency of such modules by grid corrosion. Here, a nondestructive and simple optical method for evaluating the condition of PV modules is proposed. This method uses a dual-wavelength pH-sensitive fluorescent dye to detect acetic acid in PV modules using a change in pH. The change in pH induced by the formation of acetic acid is detected by the change in the ratio of the fluorescent intensities of two peaks of the dye. A pH-sensitive fluorescent dye showed sensitivity for small amounts of acetic acid such as that produced from EVA. Furthermore, a membrane filter dyed with a pH-sensitive fluorescent dye was confirmed to detect acetic acid in aged EVA after a damp-heat test (85 °C, 85%) for 5000 h in PV modules.

pH Measurement Using Dual-Wavelength Fluorescent Ratio by Two-Photon Excitation for Mitochondrial Activity

A mitochondrion has a pH gradient between the two sides of its inner membrane in order to produce adenosine triphosphate (ATP). Because ATP depletion causes numerous diseases, the measurement of the pH value around the mitochondrion is expected to clarify the mechanism of these diseases. In this study, a dual-wavelength pH-sensitive dye was excited by two-photon absorption initiated using a femtosecond pulse laser. In addition, fluorescence from the dye was directly collected from the fluorescent point using the collection-mode probe of a scanning near-field optical microscope. By this proposed method, a pH calibration curve was obtained from the fluorescent intensity ratio of the dye solution, and temporal pH variations with 0.1 s time resolution following the addition of acid were observed. Moreover, mitochondrial activity on the basis of the pH changes was successfully observed in three different mitochondrial densities.

Development of a visual encryption device using higher-order birefringence

We propose and demonstrate a novel visual encryption device composed of higher-order birefringent elements. When an optical material with higher-order birefringence is placed between a pair of polarizers and illuminated by white light, it appears only white. In contrast, when it is illuminated by monochromatic light, the transmitted intensity varies depending cosinusoidally on the wavelength. An array of such materials can express information (e.g., letters and/or images) by controlling the birefringence of each pixel. If birefringence phase retardation can be adjusted for a specific wavelength, the information will be clearly displayed when it is illuminated at this wavelength. We denote this wavelength a key wavelength. The encryption device was fabricated by controlling the amount of higher-order birefringence to achieve high contrast only by using polarized illumination at the key wavelength. Thus, the information stored in the encryption device can be decoded only by illuminating it at the key wavelength. To demonstrate the validity of this encryption principle, we constructed a 3 × 3 pixel device in which commercial retarder films were laminated. The device was illuminated by a monochromatic light. When a readout experiment was performed using the monochromatic light at the key wavelength, the stored letter was clearly visible. On the other hand, when pixel brightness was randomly distributed with illumination at the wavelength other than the key wavelength, the letter could not be recognized. Furthermore, the stored information can be easily distributed to multiple physical keys that display arbitrary images. In this case, the birefringence phase retardation is obtained by summing the values of retardation of each pixel of the physical keys. In the experimental device, the observed image was decoded by superimposing the two images using different physical keys.

Direct Evaluation of Anchoring Effects and Vertical Orientation Profiling of Liquid Crystal Films by Near-Field Birefringence Measurement

A method for evaluating both anchoring effects and molecular orientation profile of liquid crystal (LC) films simultaneously is proposed. The local birefringence changes in homeotropic aligned LC molecules are detected and profiled along the depth direction by scanning near-field optical microscopy (SNOM). From molecular responses to the reorientation caused by a SNOM probe, the anchoring strength of the alignment layer is observed. The methods reliability is confirmed through the evaluation of three homeotropic alignment layers with different anchoring strengths.