Intae Kim

Highly Efficient Three-Way Saturated Doherty Amplifier With Digital Feedback Predistortion

A three-way saturated Doherty power amplifier (S-DPA) based on class-F topology is proposed to improve efficiency at large backed-off output power regions. The high efficiency is demonstrated by implementing the amplifier using Eudyna EGN010MK GaN HEMTs and testing it with a continuous wave (CW) signal and a forward-link wide-band code division multiple access (WCDMA) 1-FA signal at 2.14 GHz. The proposed S-DPA has a power-added efficiency (PAE) of 50% for the CW signal at 10 dB backed-off output power region, while a two-way S-DPA and class-F PA have PAEs of 39% and 25%, respectively. Also, the proposed S-DPA has a PAE of 46.7% for the WCDMA 1-FA signal with peak-to-average power ratio of 10 dB, while the two-way S-DPA and the class-F PA have PAEs of 42.7% and 28.7%, respectively. Moreover, the proposed S-DPA, when linearized by a digital feedback predistorter, delivers an adjacent channel leakage ratio of 49.2 dBc at a 2.5 MHz offset with a PAE of 45.9%.

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.

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.

Electrohydrodynamic repulsion of droplets falling on an insulating substrate in an electric field

Control technique for charged droplets, named as electrohydrodynamic droplet repulsion (EDR), is reported. Charged droplets emitting from a capillary and falling down toward an insulating substrate are deflected from their intended impact point due to the like polarity of previously fallen droplets. The mechanism for this potentially critical phenomenon is explained by applying a theoretical model describing the balance between gravitational and Coulomb forces acting on a droplet. The parametric conditions for the EDR are subsequently presented.

Ionic liquid flow along the carbon nanotube with DC electric field

Liquid pumping can occur along the outer surface of an electrode under a DC electric field. For biological applications, a better understanding of the ionic solution pumping mechanism is required. Here, we fabricated CNT wire electrodes (CWEs) and tungsten wire electrodes (TWEs) of various diameters to assess an ionic solution pumping. A DC electric field created by a bias of several volts pumped the ionic solution in the direction of the negatively biased electrode. The resulting electro-osmotic flow was attributed to the movement of an electric double layer near the electrode, and the flow rates along the CWEs were on the order of picoliters per minute. According to electric field analysis, the z-directional electric field around the meniscus of the small electrode was more concentrated than that of the larger electrode. Thus, the pumping effect increased as the electrode diameter decreased. Interestingly in CWEs, the initiating voltage for liquid pumping did not change with increasing diameter, up to 20 μm. We classified into three pumping zones, according to the initiating voltage and faradaic reaction. Liquid pumping using the CWEs could provide a new method for biological studies with adoptable flow rates and a larger ‘Recommended pumping zone’.

Optical Detection of Paraoxon Using Single-Walled Carbon Nanotube Films with Attached Organophosphorus Hydrolase-Expressed Escherichia coli

In whole-cell based biosensors, spectrophotometry is one of the most commonly used methods for detecting organophosphates due to its simplicity and reliability. The sensor performance is directly affected by the cell immobilization method because it determines the amount of cells, the mass transfer rate, and the stability. In this study, we demonstrated that our previously-reported microbe immobilization method, a microbe-attached single-walled carbon nanotube film, can be applied to whole-cell-based organophosphate sensors. This method has many advantages over other whole-cell organophosphate sensors, including high specific activity, quick cell immobilization, and excellent stability. A device with circular electrodes was fabricated for an enlarged cell-immobilization area. Escherichia coli expressing organophosphorus hydrolase in the periplasmic space and single-walled carbon nanotubes were attached to the device by our method. Paraoxon was hydrolyzed using this device, and detected by measuring the concentration of the enzymatic reaction product, p-nitrophenol. The specific activity of our device was calculated, and was shown to be over 2.5 times that reported previously for other whole-cell organophosphate sensors. Thus, this method for generation of whole-cell-based OP biosensors might be optimal, as it overcomes many of the caveats that prevent the widespread use of other such devices.

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.

Organophosphorus Compounds Detection Using Suspended SWNT Films

We developed a one-step method for fabrication of addressable suspended SWNT films and demonstrate excellent detection performance of paraoxon based on OPH-immobilized SWNT films for environmental monitoring. For dispersed SWNT suspension, COOH-SWNT was prepared by the oxidation of carbon nanotubes using acid treatment and sonication. Suspended SWNT-film was fabricated between cantilever electrodes by dielectrophoretic force and surface tension of the water meniscus. After that, OPH were immobilized on suspended SWNT-films by nonspecific binding for enzymatic hydrolysis of paraoxon. The electrical properties of the SWNT films were measured in real time at room temperature. Structurally suspended SWNT films from substrate surface made possible rapid and highly sensitive detection of target molecules with increased convectional and diffusional fluxes of the molecules and with a large binding surface area. SWNT film FET resulted in a real-time, label-free, and electrical detection of paraoxon to the concentration of ca. 10 ǺNwith a step-wise rapid response time of several seconds.

Nitrophenol detection using suspended SWNT films for environmental monitoring

Suspended addressable SWNT film arrays between cantilever electrodes were successfully developed for environmental monitoring. Structurally suspended SWNT films from substrate floor made possible rapid and highly sensitive detection of target molecules with increased convectional and diffusional fluxes of the molecules and with a large binding surface area. SWNT film FET resulted in a real-time, label-free, and electrical detection of nitrophenol to the concentration of ca. 1µM with a step-wise rapid response time of several seconds.

Fabrication of entangled single-wall carbon nanotube films as nanoporous junctions for ion concentration polarization

Ion concentration polarization (ICP) is a distinctive electrochemical phenomenon that occurs near an ion-exchange membrane with an applied DC electric field, generating a significant concentration gradient in back and forth on the membrane. To date, however, there have been only a few attempts to introduce unconventional materials for ion transport in micro?nanofluidic systems. Here, we describe the development of a novel ICP system using an entangled single-wall carbon nanotube (SWNT) film as an ion-selective membrane instead of a Nafion membrane, for investigating the detailed relationship between electrical properties, i.e., ionic conductance through nanojunctions, and nonlinear electrokinetic behavior.