The inertial migration of particles in micro-scale flows has received much attention due to its promising applications, such as the membrane-free passive separation of particles or cells. The particles suspended in rectangular channels are known to be focused near the center of each channel face as the channel Reynolds number (R_C) increases due to the lift force balance and the hydrodynamic interactions of the particles with the wall. In this study, the three-dimensional positions of neutrally buoyant spherical particles inside a square microchannel are measured using the digital holographic microscopy technique, and a transition from the lateral tubular pinch to the cross-lateral focusing with increasing R_C is reported. The particles are found to migrate first in the lateral direction and then cross-laterally toward the four equilibrium positions. A general criterion that can be used to secure the fully developed state of particle focusing in Lab-on-a-Chip applications is also derived. This criterion could be helpful for the accurate estimation of the design parameters of inertial microfluidic devices, such as R_C, channel length and width, and particle diameter.

The elasto-migration of microparticles across a streamline induced by elasticity and inertia in microscale pipe flow of viscoelastic fluids was investigated using a holographic technique. The effects of blockage ratio (d/D), flow rate (Q), and entry length (L) on particle migration induced by fluid elasticity were evaluated. Single-line particle focusing was demonstrated for an elasticity-dominant fluid. Furthermore, shear-thinning effect on particle migration was evaluated. Finally, we propose the focusing number, a non-dimensional parameter, to estimate the focusing state of particles in elasticity-dominant fluids. This criterion can be used to accurately estimate the design parameters, such as rheological properties, channel length, and particle diameter, for microfluidic devices using elasto-migration.

Measurement of blood flow with high spatial and temporal resolutions in a three-dimensional (3D) volume is a challenge in biomedical research fields. In this study, digital holographic microscopy is used to measure the 3D motion of human red blood cells (RBCs) in a microscale volume. The cinematographic holography technique, which uses a high-speed camera, enabled the continuous tracking of individual RBCs in a microtube flow. Several autofocus functions that quantify the sharpness of reconstructed RBC images are evaluated to locate the accurate depthwise position of RBCs. In this study, the squared Laplacian function yields the smallest depth of focus and locates the depthwise positions of RBCs with a root mean square error of $2.3\mathrm{\mu m}$. By applying this method, we demonstrate the measurement of four-dimensional (space and time) trajectories as well as 3D velocity profiles of RBCs. The measurement uncertainties of the present method are also discussed.

Subcutaneous injection of drug solution is widely used for continuous and low dose drug treatment. Although the drug injections have been administered for a long time, challenges in the design of injection devices are still needed to minimize the variability, pain, or skin disorder by repeated drug injections. To avoid these adverse effects, systematic study on the effects of injection conditions should be conducted to improve the predictability of drug effect. Here, the effects of injection conditions on the drug permeation in tissues were investigated using X-ray imaging technique which provides real-time images of drug permeation with high spatial resolution. The shape and concentration distribution of the injected drug solution in the porcine subcutaneous and muscle tissues are visualized. Dynamic movements of the wetting front (WF) and temporal variations of water contents in the two tissues are quantitatively analyzed. Based on the quantitative analysis of the experimental data, the permeability of drug solution through the tissues are estimated according to permeation direction, injection speed, and tissue. The present results would be helpful for improving the performance of drug injection devices and for predicting the drug efficacy in tissues using biomedical simulation.

Recently, there has been significant progress in the field of soft- and hard-X-ray imaging for a wide range of applications, both technically and scientifically, via developments in sources, optics and imaging methodologies. While one community is pursuing extensive applications of available X-ray tools, others are investigating improvements in techniques, including new optics, higher spatial resolutions and brighter compact sources. For increased image quality and more exquisite investigation on characteristic biological phenomena, contrast agents have been employed extensively in imaging technologies. Heavy metal nanoparticles are excellent absorbers of X-rays and can offer excellent improvements in medical diagnosis and X-ray imaging. In this context, the role of gold (Au) is important for advanced X-ray imaging applications. Au has a long-history in a wide range of medical applications and exhibits characteristic interactions with X-rays. Therefore, Au can offer a particular advantage as a tracer and a contrast enhancer in X-ray imaging technologies by sensing the variation in X-ray attenuation in a given sample volume. This review summarizes basic understanding on X-ray imaging from device set-up to technologies. Then this review covers recent studies in the development of X-ray imaging techniques utilizing gold nanoparticles (AuNPs) and their relevant applications, including two- and three-dimensional biological imaging, dynamical processes in a living system, single cell-based imaging and quantitative analysis of circulatory systems and so on. In addition to conventional medical applications, various novel research areas have been developed and are expected to be further developed through AuNP-based X-ray imaging technologies.

Fabrication of superhydrophobic surfaces is an area of great interest because it can be applicable to various engineering fields. A simple, safe and inexpensive fabrication process is required to fabricate applicable superhydrophobic surfaces. In this study, we developed a facile fabrication method of nearly perfect superhydrophobic surfaces through plasma treatment with argon and oxygen gases. A polytetrafluoroethylene (PTFE) sheet was selected as a substrate material. We optimized the fabrication parameters to produce superhydrophobic surfaces of superior performance using the Taguchi method. The contact angle of the pristine PTFE surface is approximately 111.0° ± 2.4°, with a sliding angle of 12.3° ± 6.4°. After the plasma treatment, nano-sized spherical tips, which looked like crown-structures, were created. This PTFE sheet exhibits the maximum contact angle of 178.9°, with a sliding angle less than 1°. As a result, this superhydrophobic surface requires a small external force to detach water droplets dripped on the surface. The contact angle of the fabricated superhydrophobic surface is almost retained, even after performing an air-aging test for 80 days and a droplet impacting test for 6 h. This fabrication method can provide superb superhydrophobic surface using simple one-step plasma etching.