James Jungho Pak

Electrochemistry at the Edge of a Single Graphene Layer in a Nanopore

We study the electrochemistry of single layer graphene edges using a nanopore-based structure consisting of stacked graphene and Al(2)O(3) dielectric layers. Nanopores, with diameters ranging from 5 to 20 nm, are formed by an electron beam sculpting process on the stacked layers. This leads to a unique edge structure which, along with the atomically thin nature of the embedded graphene electrode, demonstrates electrochemical current densities as high as 1.2 × 10(4) A/cm(2). The graphene edge embedded structure offers a unique capability to study the electrochemical exchange at an individual graphene edge, isolated from the basal plane electrochemical activity. We also report ionic current modulation in the nanopore by biasing the embedded graphene terminal with respect to the electrodes in the fluid. The high electrochemical specific current density for a graphene nanopore-based device can have many applications in sensitive chemical and biological sensing, and energy storage devices.

Fabrication of ionic-polymer-metal-composite (IPMC) micropump using a commercial Nafion

This paper describes the fabrication and characteristics of an ionic polymer-metal composite (IPMC) membrane-shaped micro-actuator and its application to the fabrication of a micro-pump. After fabricating two 8mm×8mm IPMC membrane-shaped actuators using a Nafion film, their displacements were measured. The fabricated IPMC membrane-shaped micro-actuators showed displacement of 14~27μ at the applied voltage ranging from 4VP-P to 10VP-P at 0.5Hz. Displacement of the IPMC actuator fabricated with a commercially available Nafion is large enough to make the IPMC actuator a membrane-shaped micro-actuator for fabricating an IPMC micro-pump. IPMC micro-pump was fabricated by assembling IPMC membrane-shaped micro-actuator and PDMS(polydimethylsiloxane) micro-channel together. PDMS micro-channel was designed to have nozzle/diffuser structures which make the fluids flow from inlet to outlet when the IPMC membrane-shaped micro-actuator is deflected up and down by the applied voltages. The measured flow rate of the fabricated IPMC micro-pump was about 9.9μℓ/min at 0.5Hz when the input voltage and duty ratio were 8V P-P and 50%, respectively. The test results illustrate that the fabricated IPMC micro-pump is suitable for pumping fluid through micro-channel on a PDMS substrate. Mechanical performances of beam-shaped and bridge-shaped conductive polymer actuator in aqueous solution and in solid electrolyte have been measured and analyzed. The optimum thickness of polypyrrole for the best bending performance is about 17-19 μm which has been polymerized at the current density of 5.4 μA/mm2 for 120 minutes. For the application of conductive polymer actuator to a micropump, silicon bulk micromachining process has been combined.

Zinc oxide inverse opal enzymatic biosensor

We report ZnO inverse opal- and nanowire (NW)-based enzymatic glucose biosensors with extended linear detection ranges. The ZnO inverse opal sensors have 0.01–18 mM linear detection range, which is 2.5 times greater than that of ZnO NW sensors and 1.5 times greater than that of other reported ZnO sensors. This larger range is because of reduced glucose diffusivity through the inverse opal geometry. The ZnO inverse opal sensors have an average sensitivity of 22.5 μA/(mM cm2), which diminished by 10% after 35 days, are more stable than ZnO NW sensors whose sensitivity decreased by 10% after 7 days.

Fitting Improvement Using a New Electrical Circuit Model for the Electrode-Electrolyte Interface

The characteristics of impedance for the electrode-electrolyte interface are important in the electrode researches for biomedical applications. So, the equivalent circuit models for the interface have been researched and developed. However, the applications of such previous models are limited in terms of the frequency range, type of electrode or electrolyte. In this paper, a new electrical circuit model was proposed and demonstrated its capability of fitting the experimental results more accurately than before. A new electrical circuit model consists of three resistors and two constant phase elements. Electrochemical impedance spectroscopy was used to characterize the interface for several materials of Au, Pt, and stainless steel electrode in 0.9% NaCl solution. The new model and the previous model were applied to fit the measured impedance results, and were compared their goodness of fit

Dry Jet-Wet Spinning of Cellulose/N-Methylmorpholine N-oxide Hydrate Solutions and Physical Properties of Lyocell Fibers

In dry jet-wet spinning of a cellulose/N-methylmorpholine N-oxide hydrate solution, the effects of the hydration number n in NMMO hydrates and the concentration and molecular weight of cellulose are investigated in terms of the physical properties of the fibers. Dry jet-wet spinning of lyocell fibers is also investigated using three different set-ups; a piston type, an N2 gas pressure type, and spinning equipment with an extruder. The effects of spinning conditions such as the spin draw ratio, air gap distance, and composition of the coagulation bath are investigated. The physical properties of the fibers such as birefringence, initial modulus, and tensile strength increase with a decrease in n and an increase in the air gap distance and spin draw ratio. The relationship between the physical properties and the fiber denier is newly suggested in this spinning system. The tensile fracture morphology reveals that fibers from the NMMO hydrate containing less water have more fibrils due to their higher molecular orientation. Further, the orientation structure of the cellulose becomes more noticeable with the decreased hydration levels of the solvent because it produces thicker and longer fibrils when the cellulose fibers are treated with an ultrasonic generator. The crystallite size of the cellulose depends on the composition of NMMO in the coagulation bath. The crystallite size also decreases with the increased air gap distance.

Interconnection of Multichannel Polyimide Electrodes Using Anisotropic Conductive Films (ACFs) for Biomedical Applications

In this paper, we propose a method for interconnecting soft polyimide (PI) electrodes using anisotropic conductive films (ACFs). Reliable and automated bonding was achieved through development of a desktop thermocompressive bonding device that could simultaneously deliver appropriate temperatures and pressures to the interconnection area. The bonding conditions were optimized by changing the bonding temperature and bonding pressure. The electrical properties were characterized by measuring the contact resistance of the ACF bonding area, yielding a measure that was used to optimize the applied pressure and temperature. The optimal conditions consisted of applying a pressure of 4 kgf/cm2 and a temperature of 180 °C for 20 s. Although ACF base bonding is widely used in industry (e.g., liquid crystal display manufacturing), this study constitutes the first trial of a biomedical application. We performed a preliminary in vivo biocompatibility investigation of ACF bonded area. Using the optimized temperature and pressure conditions, we interconnected a 40-channel PI multielectrode device for measuring electroencephalography (EEG) signals from the skulls of mice. The electrical properties of electrode were characterized by measuring the impedance. Finally, EEG signals were measured from the mice skulls using the fabricated devices to investigate suitability for application to biomedical devices.

A bridge-type piezoresistive accelerometer using merged epitaxial lateral overgrowth for thin silicon beam formation

Abstract Merged epitaxial lateral overgrowth (MELO) of silicon has been used to form 10 μm ± 0.5 μm thick, 420 μm long, and 170 μm wide single-crystal silicon beams for a four-bridge piezoresistive accelerometer. Buried SiO2 stripes are used to produce the near-perfect backside etch-stop for the silicon membrane, while the topside thickness control is established by the growth rate of 0.1 μm min−1. The MELO membrane technique produces an accelerometer that has low-doped high-quality single-crystal silicon beams. The measured sensitivity is 287 μV V−1 g−1, which is about twice that for a similar device, and the non-linearity is less than 4% up to 30g of acceleration.

Copper metallization for crystalline Si solar cells

Cu metallization for crystalline Si solar cells was investigated using either Ti or Ti/TiN diffusion barriers. The resistivity and the specific contact resistance change were measured for both Ti(30 nm)/Cu(100 nm) and Ti(30 nm)/TiN(30 nm)/Cu(100 nm) contact structures under various annealing conditions. As the annealing temperature increased, the efficiency of the cells increased mainly due to the increase in fill-factor and ISC, which was correlated with the series resistance (RS) of the metal layer. The solar cells with Ti/TiN/Cu contacts generally showed the higher efficiencies than those with Ti/Cu, because in Ti/Cu contacts Cu diffused through Ti and increased RS.

Biomedical applications of electroactive polymers and shape-memory alloys

Among many kinds of polymer materials, electronic conductive material, that is polypyrrole, shows potential possibility for bio-relate actuator materials. However, it may be an impediment for practical use in polypyrrole actuator that polypyrrole usually requires electrolyte solution for actuation. Our first research theme is focused on this problem solving. We have investigated many kinds of solid polymer electrolyes for the substitution of electrolyte solution. Our goals are to find the stable solid electrolyte in the air, to establish the reliable fabrication process of it and to apply it for micropump application. Besides actuators, the reduction and oxidation property of polypyrrole can be exploited for active drug delivery systems by the control of structural deformation of it. We have investigated this kind of new and bio-related possibility of polypyrrole. Shape memory alloy has another possibility in the biomedical field. Due to its inherent excellent advantages as actuator materials, it can be used for micro active intravascular catheter. We have developed thin tube type bending actuator using shape memory alloy and characterized its performance by in-vivo test.

Soldering-based easy packaging of thin polyimide multichannel electrodes for neuro-signal recording

We propose a novel packaging method for preparing thin polyimide (PI) multichannel microelectrodes. The electrodes were connected simply by making a via-hole at the interconnection pad of a thin PI electrode, and a nickel (Ni) ring was constructed by electroplating through the via-hole to permit stable soldering with strong adhesion to the electrode and the printed circuit board. The electroplating conditions were optimized for the construction of a well-organized Ni ring. The electrical properties of the packaged electrode were evaluated by fabricating and packaging a 40-channel thin PI electrode. Animal experiments were performed using the packaged electrode for high-resolution recording of somatosensory evoked potential from the skull of a rat. The in vivo and in vitro tests demonstrated that the packaged PI electrode may be used broadly for the continuous measurement of bio-signals or for neural prosthetics.