Youngkyu Cho

Vertical alignment of printed carbon nanotubes by multiple field emission cycles

The effect of field emission cycles on printed carbon nanotubes was investigated using scanning electron microscopy and current–voltage measurement. After multiple emission cycles, the printed nanotubes irreversibly deformed to orient themselves parallel to the field direction and, from them, remarkably enhanced emission image with good uniformity was demonstrated. Corresponding gradual decrease in the field emission threshold and increase of field enhancement factor (β) were also observed during field emission cycles.

Sound source localization for robot auditory systems

Sound source localization (SSL) is a major function of robot auditory systems for intelligent home robots. The steered response power-phase transform (SRP-PHAT) is a widely used method for robust SSL. However, it is too slow to run in real time, since SRP-PHAT searches a large number of candidate sound source locations. This paper proposes a search space clustering method designed to speed up the SRP-PHAT based sound source localization algorithm for intelligent home robots equipped with small scale microphone arrays. The proposed method reduces the number of candidate sound source locations by 30.6% and achieves 46.7% error reduction compared to conventional methods.

Fast Sound Source Localization Using Two-Level Search Space Clustering

Steered response power phase transform (SRP-PHAT) is a method that is widely used for robust sound source localization (SSL). However, since SRP-PHAT searches over a large number of candidate locations, it is too slow to run in real-time for large-scale microphone array systems. In this paper, we propose a robust two-level search space clustering method to speed-up SRP-PHAT-based SSL. The proposed method divides the candidate locations of the sound source into a set of groups and finds a small number of groups that are likely to contain the maximum power location. By searching within the small number of groups, the computational costs are reduced by 61.8% compared to a previously proposed method without loss of accuracy.

Microfluidic in-reservoir pre-concentration using a buffer drain technique

Pre-concentration methods are essential for detecting low concentrations of influenza virus in biological samples from patients. Here, we describe a new method for draining buffer from solution in the reservoir of a microfluidic device to increase the concentration of virus in the reservoir. Viruses were captured in the reservoir by an ion depletion barrier from connected ion selective microfluidic channels. 75 μl of buffer was successfully drained from a 100 μl sample, resulting in a 4-fold increase in influenza hemagglutinin concentration in the reservoir. The volume of the final concentrated sample was suitable for detection of influenza hemagglutinin by the enzyme-linked immunosorbent assay, demonstrating the usefulness of the developed platform for enhanced sensitivity of virus detection in a conventional analysis.

Ion concentration polarization for pre-concentration of biological samples without pH change

In this paper, a method was developed for pre-concentrating large-volume biological samples for subsequent analysis. We previously developed another pre-concentration device, but it unfortunately altered the pH of the sample when an electric field was applied to the sample reservoir. Changes in the pH are not suitable for subsequent antibody-antigen reactions because of the stability issues that arise based on the target molecules isoelectric point (pI). Here, this problem was overcome using ion concentration polarization (ICP) with a cation selective membrane (Nafion). Phosphate buffered saline was used as a test solution for the sample. The sample was contained in a reservoir that was not affected by the electric field, and an ICP barrier was formed in front of the reservoir. This device could concentrate microliter-scale samples without changing the pH because the biomolecules were blocked from passing through the ICP barrier while the sample (phosphate buffered saline) was drained. A 40 μL sample was successfully pre-concentrated to 20 μL in a single channel device and 10 μL in a dual channel device, resulting in 2.1-fold and 3.3-fold increases, respectively, in influenza hemagglutinin concentrations. These changes in the concentration were confirmed by ELISA.

Speaker localization in noisy environments using steered response voice power

Many devices, including smart TVs and humanoid robots, can be operated through speech interface. Since a user can interact with such a device at a distance, speech-operated devices must be able to process speech signals from a distance. Although many methods exist to localize speakers via sound source localization, it is very difficult to reliably find the location of a speaker in a noisy environment. In particular, conventional sound source localization methods only find the loudest sound source within a given area, and such a sound source may not necessarily be related to human speech. This can be problematic in real environments where loud noises frequently occur, and the performance of speech-based interfaces for a variety of devices could be negatively impacted as a result. In this paper, a new speaker localization method is proposed. It identifies the location associated with the maximum voice power from all candidate locations. The proposed method is tested under a variety of conditions using both simulation data and real data, and the results indicate that the performance of the proposed method is superior to that of a conventional algorithm for various types of noises1.

Tunable sheathless microfluidic focusing using ion concentration polarization

In this study, we developed a tunable sheathless focusing method for focusing micrometer- and nanometer-sized particles, using ion concentration polarization (ICP) in an ion-selective, resin-coated channel. The particle movement was regulated using an electric field, and by varying the flow rate and ionic strength of the liquid solution; various phenomena were observed, depending on the particle properties. Here, we provide insights into the physical basis of the ICP-focusing phenomena, and a statistical approach for analyzing the particle movement. This ICP-focusing technology is an approach that could be applied for the separation and sorting of various particles, including cells, proteins, and bacteria.

Construction of a 3D mammary duct based on spatial localization of the extracellular matrix

Extracellular matrix (ECM)-based hydrogels can serve as scaffolds in reconstruction of physiologically relevant three-dimensional (3D) in vitro models. Biocompatible or integrated hydrogels can be used to modulate ECM properties such as stiffness and composition for studies of cell−ECM interactions and morphogenesis. To this end, we developed a new type of spatially modified collagen type 1 hydrogel by convective addition of collagen type 1 solution. The matrix displayed properties that were distinct from those of a collagen type 1 hydrogel and recapitulated the morphology and function of mammary epithelium in a 3D microfluidic platform. In this ECM, mammary epithelial cells secreted laminin and exhibited self-assembly into a basement membrane. Thus, this spatially modified ECM offers biophysical features that can facilitate the construction of mammary epithelium and, by extension, that of various other epithelial types. Additionally, our reconstructed mammary duct can be used as an in vitro model for the study of early-stage breast cancer.In this study, we present a new type of spatially modified ECM composed of type 1 collagen whose properties differ from those of a collagen matrix. Using this ECM, we developed a stable human mammary epithelium with a lumen structure that exhibited a barrier function in a microfluidic platform, produced laminin, and formed a basement membrane. This reconstructed mammary duct can serve as a model for investigating breast cancer and may be adapted to other types of epithelium for in vitro studies.Biomaterials: constructing 3D mammary ductsA biomaterial that mimics human mammary ducts has been developed by scientists in South Korea and could be used to investigate early-stage breast cancer. Hydrogels are networks of polymer molecules that have a physical texture similar to that of the extracellular matrix surrounding and supporting cells in the human body, making them ideal scaffolds for building artificial tissue. Seok Chung from Korea University in Seoul, Su Hyun Lee from the Seoul National University Hospital and their co-workers made a hydrogel with the structure of mammary ducts using collagen, and then deposited mammary epithelial cells throughout it. The hydrogel was incorporated into a microfluidic device, providing a way of introducing the various biological components found in a natural extracellular matrix into it, and thereby a method to test how these components interact with the cells.