WooSeok Choi

Preparation of stable superhydrophobic mesh with a biomimetic hierarchical structure

Stable superhydrophobic meshes with a biomimetic hierarchical structure were fabricated via simple electropolymerization and chemical polymerization processes. Because the multi-scale hierarchical structure provides a stable superhydrophobic state by maintaining a Cassie state, these meshes have high static and dynamic waterproof capabilities.

Fabrication of functional micro- and nanoneedle electrodes using a carbon nanotube template and electrodeposition

Carbon nanotube (CNT) is an attractive material for needle-like conducting electrodes because it has high electrical conductivity and mechanical strength. However, CNTs cannot provide the desired properties in certain applications. To obtain micro- and nanoneedles having the desired properties, it is necessary to fabricate functional needles using various other materials. In this study, functional micro- and nanoneedle electrodes were fabricated using a tungsten tip and an atomic force microscope probe with a CNT needle template and electrodeposition. To prepare the conductive needle templates, a single-wall nanotube nanoneedle was attached onto the conductive tip using dielectrophoresis and surface tension. Through electrodeposition, Au, Ni, and polypyrrole were each coated successfully onto CNT nanoneedle electrodes to obtain the desired properties.

On steady two-fluid electroosmotic flow with full interfacial electrostatics

A two-fluid electroosmotic flow in a microchannel is studied by considering full hydrodynamic and electrostatic interactions on the interface. Jumps in electrical potential and in charge density across the interface, in particular, are found to create counterintuitive flow behavior through the electrostatic interaction of the interface with the external field imposed. The interfacial electrostatic effects are shown to induce flow reversal within physically reasonable parametric ranges. It is also shown that the electrostatic properties of the interface must be carefully considered in electroosmotic pumping lest the nonconducting fluid should stay stationary or flow in an unintended direction. A formula for quantitative control of electroosmotic pumping is provided.

Carbon-Nanotube-Modified Electrodes for Highly Efficient Acute Neural Recording

Microelectrodes are widely used for monitoring neural activities in various neurobiological studies. The size of the neural electrode is an important factor in determining the signal-to-noise ratio (SNR) of recorded neural signals and, thereby, the recording sensitivity. Here, it is demonstrated that commercial tungsten microelectrodes can be modified with carbon nanotubes (CNTs), resulting in a highly sensitive recording ability. The impedance with the respect to surface area of the CNT-modified electrodes (CNEs) is much less than that of tungsten microelectrodes because of their large electrochemical surface area (ESA). In addition, the noise level of neural signals recorded by CNEs is significantly less. Thus, the SNR is greater than that obtained using tungsten microelectrodes. Importantly, when applied in a mouse brain in vivo, the CNEs can detect action potentials five times more efficiently than tungsten microelectrodes. This technique provides a significant advance in the recording of neural signals, especially in brain regions with sparse neuronal densities.

Is free surface free in micro-scale electrokinetic flows?

When a rigid boundary is replaced by a free surface, or by a liquid-liquid interface, the shear stress is reduced near the boundary because the no-slip on the rigid boundary is relaxed. A nonzero fluid velocity exists on the free surface, and the flow rate would increase by the replacement. For electrokinetic flows, however, the replacement from rigid to free boundary can lead to an increase (decrease) in shear stress (flow rate). This counterintuitive effect of free surface is presented in this note for a generic electroosmotic flow in a microchannel.

Fabrication of Cell-Encapsulated Alginate Microfiber Scaffold Using Microfluidic Channel

Traditional approaches in tissue engineering are limited in that cell seeding is inefficient and cells cannot be located on a scaffold precisely. Moreover, the traditional methods, which rely on a random and probabilistic process, produce scaffolds with low regularity in porosity, pore size, and interconnection of pores. In this research, we propose a novel method to fabricate a scaffold for tissue engineering, which can overcome the limitations of traditional approaches. Cell-encapsulated alginate solution and cross-linker solution were laminarly flowed into a microfluidic channel. Then, the alginate solution was gelled to form a cell-encapsulated alginate microfiber by the diffusion of gelation ion from the cross-linker solution and ejected from the outlet of channel to the reservoir. The diameter of the fabricated microfiber can be controlled by the flow rate ratio of the two solutions. Moreover, this method, which has no cell seeding step, eliminates the possibility of loss of cells and the problems related to distribution of cells. We also show the feasibility of the alginate microfiber as a scaffold, which can promote chondrogenesis. The chondrogenesis in the alginate microfiber was evaluated by both histological and biochemical analyses. The increase of major markers of chondrogenesis such as glycosaminoglycan and collagen shows the potential of alginate microfiber as a scaffold for cartilage.

Fabrication and Calibration of pH Sensor Using Suspended CNT Nanosheet

In this research, the pH sensor was developed using CNT nanosheet with Nafion coating for the advanced medical sensor such as a blood gas analyzer. The CNT nanosheet was formed by dielectrophoresis and water-meniscus between cantilever-type electrodes. Then, the process of the heat annealing and the Nafion coating was conducted for reducing contact resistance and giving proton selectivity respectively. We measured the response of the pH sensor as the electrolyte-gated CNT-nanosheet field effect transistor. The sensor showed a linear current ratio in a similar range of the normal blood pH. A calibration method for decreasing of the response variation among sensors has also been introduced. Coefficient of variance of the pH sensor was decreased by applying the calibration method. A linear relation between the calibrated response of the sensors and pH variance was also obtained. Finally, the pH sensor with a high resolution was fabricated and we verify the feasibility of the sensor by applying the calibration method.

Site-specific immobilization of microbes using carbon nanotubes and dielectrophoretic force for microfluidic applications

We developed a microbial immobilization method for successful applications in microfluidic devices. Single-walled nanotubes and Escherichia coli were aligned between two cantilever electrodes by a positive dielectrophoretic force resulting in a film of single-walled nanotubes with attached Escherichia coli. Because this film has a suspended and porous structure, it has a larger reaction area and higher reactant transfer efficiency than film attached to the substrate surface. The cell density of film was easily controlled by varying the cell concentration of the suspension and varying the electric field. The film showed excellent stability of enzyme activity, as demonstrated by measuring continuous reaction and long-term storage times using recombinant Escherichia coli that expressed organophosphorus hydrolase.

Fabrication of Conducting Polymer Nanowires

The advancement of nanotechnology provides opportunities for fabrication of nanoscale materials and higher performance devices using nanomaterials with high precision. Currently, various nanomaterials and nanostructures in the range of 1 to 100 nm have been produced by chemical and physical methods. Among various nanostructured materials, one-dimensional (1D) materials, such as nanowires, nanotubes, nanorods, and nanobelts, have potential applications in nanoscale electronics (Cui & Lieber, 2001), optoelectronics (Duan et al., 2001), photonics (Gudiksen et al., 2002; Huang et al., 2001), sensors (Cui et al., 2001), and solar cells (Law et al., 2005) due to their unique electrical, chemical, and optical properties (Li et al., 2006; Thelander et al., 2006; Wang et al., 2008). Nanowires are useful in chemical or biological sensors for detecting single molecules because they have a high surfaceto-volume ratio and a highly sensitive 1D nanostructure that gives rise to large conductivity change associated with binding molecules (Cui et al., 2001; Ramanathan et al., 2005). Conducting polymers, such as polypyrrole, polyaniline, polythiophene, and their derivatives, are promising materials for synthesis of nanostructured materials and devices (Langea et al., 2008; Malinauskas et al., 2005). Compared with other materials, conducting polymers have some unique electrical, chemical, and optical properties because of their conjugated structures, and they are easily synthesized using chemical or electrochemical synthetic methods at room temperatures with low cost (Aleshin, 2006; Briseno et al., 2008; Guimard et al., 2007; Xia et al., 2010). Conducting polymers have electrical and optical properties similar to those of metals and semiconductors, while maintaining the flexibility and properties commonly associated with conventional polymer substances (Dai et al., 2002; Heeger, 2002; Shirakawa, 2002). For example, the electrical conductivity of these polymers can be adjusted from an insulator to traditional metals by varying the species and concentrations of doping ions. Undoped conducting polymers with conductivities of 10-10 to 10-5 S cm-1 can be changed into semiconducting or metallic materials with conductivities of 1 to 104 S cm-1 through a chemical or electrochemical doping process (MacDiarmid, 2002). Also, optical absorption bands and mechanical volume of conducting polymers can be changed by entrapped doping ions. Therefore, they have been used for various applications such as electronic devices (Hashizume, 2006), optoelectronic devices (Noy et al., 2002), actuators (Berdichevsky & Lo, 2006), transistors (Alam et al., 2005), and chemical sensors (Bangar et al., 2009; Garcia-Aljaro et al., 2010). In recent years, 1D conducting polymer nanostructures have been demonstrated to have improved performance with low dimensionality. Many different fabrication methods have

Organic electrochemical transistors based on a dielectrophoretically aligned nanowire array.

In this study, we synthesized an organic electrochemical transistor (OECT) using dielectrophoresis of a carbon nanotube-Nafion (CNT-Nafion) suspension. Dielectrophoretically aligned nanowires formed a one-dimensional submicron bundle between triangular electrodes. The CNT-Nafion composite nanowire bundles showed p-type semiconductor characteristics. The drain-source current decreased with increasing gate voltage. The nanowire bundles showed potential as pH sensor because the drain-source current ratio varied linearly according to the gate voltage in pH buffers.