Jong Hyeon Chang

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

Effect of contact angle hysteresis on electrowetting threshold for droplet transport

Abstract This paper describes the effect of the contact angle hysteresis caused by the adhesion between the liquid droplet and the solid surface on the threshold voltage for droplet transport in three different ground-type electrowetting configurations. It was found that the threshold voltage equals the hysteresis voltages in both single-plate and twin-plate configurations and the square root of 2 times the hysteresis voltage in the two-plate configuration by analyzing the contact angles at electrowetting threshold theoretically. The fabricated devices using 2.5 um thick polyimide (ϵr = 3.3) as the dielectric layer and thin Teflon AF as the hydrophobic layer (θ0 = 116° and α = 4–6°) showed the threshold voltages for droplet transport initiation of 28, 40 and 28 V in the single-plate, two-plate and twin-plate devices, respectively, and the threshold voltages for stable droplet transport of 35, 50 and 35 V, respectively. All the results are in very good agreement with theoretical values.

Microfabirated Clark-type Sensor for Measuing Dissolved Oxygen

This paper presents a microfabricated Clark-type sensor which exactly can measure dissolved oxygen in the cell containing solution. We designed and fabricated a microfabricated Clark-type sensor for measuring the oxygen respiration level of few cells. The microfabricated Clark-type sensor is composed of 3-electrodes on a glass substrate, a FEP (Fluorinated ethylene propylene) oxygen-permeable membrane, and PDMS (Polydimethylsiloxane) reservoir for storing few cells containing solution. The microfabricated Clark-type sensor was fabricated using MEMS technology. Thin-film Ag/AgCl was employed as a reference electrode and its durability was verified by obtaining a stable open circuit potential versus a commercial Ag/AgCl electrode for 2 hours. Response time, reproducibility and linearity of the fabricated oxygen sensor were examined by amperometry. The fabricated Clark-type sensor showed 40 sec of 90% response time, reproducibility with 1.37 nA standard deviation of 97.03 nA mean in the full-oxygen state and 0.26 nA standard deviation of 3.25 nA mean in the zero-oxygen state, and an excellent linearity with a correlation coefficient of 0.994.

Low voltage electrowetting on atomic-layer-deposited aluminum oxide

Electrowetting on dielectric (EWOD) is useful in manipulating droplets for digital (droplet-based) microfluidics, but its high driving voltage over several tens of volts has been a barrier to overcome. This paper presents the characteristics of EWOD device with aluminum oxide (Al2O3, εr≈10), deposited by atomic layer deposition (ALD), as the high-k dielectric for lowering the EWOD driving voltage substantially. The EWOD device of the single-plate configuration was fabricated by several steps for the control electrode array of 1 mm × 1 mm squares with 50μm space, the dielectric layer of 127nm thick ALD Al2O3, the reference electrode of 20μm wide line electrode, and the hydrophobic surface treatment by Teflon-AF coating, respectively. We observed the movement of a 2μl water droplet in an air environment, applying a voltage between one of the control electrodes and the reference electrode in contact with the droplet. Exponentially increasing droplet velocity with the applied voltage was obtained below 15V. The measured threshold voltage to move the droplet was as low as 3V which is the lowest voltage reported so far in the EWOD researches. This result opens a possibility of manipulating droplets, without any surfactant or oil treatment, at only a few volts by EWOD using ALD Al2O3 as the dielectric.

Simplified Ground-type Single-plate Electrowetting Device for Droplet Transport

The current paper describes a simpler ground-type, single-plate electrowetting configuration for droplet transport in digital microfluidics without performance degradation. The simplified fabrication process is achieved with two photolithography steps. The first step simultaneously patterns both a control electrode array and a reference electrode on a substrate. The second step patterns a dielectric layer at the top to expose the reference electrode for grounding the liquid droplet. In the experiment, a 5 ㎛ thick photo-imageable polyimide, with a 3.3 dielectric constant, is used as the dielectric layer. A 10 ㎚ Teflon-AF is coated to obtain a hydrophobic surface with a high water advancing angle of 116° and a small contact angle hysteresis of 5°. The droplet movement of 1 mM methylene blue on this simplified device is successfully demonstrated at control voltages above the required 45 V to overcome the contact angle hysteresis.

Rapidly dissolving silk protein microneedles for transdermal drug delivery

This paper presents rapidly dissolving fibroin microneedles for the first time. A reverse PDMS microneedles mold was first created and drug-contained fibroin solution was poured into this reverse PDMS microneedles mold. Fibroin microneedles were successfully fabricated after drying and detaching the solidified fibroin structure from the PDMS mold. These fibroin microneedles serve as a matrix to incorporate drug molecules while maintaining the drug activity. The dimensions of the fabricated fibroin microneedles are 500 μm in length, 200 μm in diameter at the base, and 5 μm in radius at the tip. These fibroin matrix microneedles can dissolve within one minute under the skin to release the drug molecules and the dissolved fibroin in the skin generates noninflammatory amino acid degradation products usable in cell metabolic functions. The fibroin microneedles containing methylene blue as a drug were fabricated and their surface morphology, internal layered structure, mechanical property, and the dissolving characteristics were examined. These rapidly dissolving fibroin microneedles provide more benefit than conventional syringes for painless transdermal drug delivery.

Application of polypyrrole(PPy) as the media material for the release and the detection of a neurotransmitter

This paper presents the application of polypyrrole(PPy) as a medium material for the release and the detection of a neurotransmitter, i.e. epinephrine, using its electrically stimulated ion exchange property. Neuron signals are transmitted in a synapse, which is composed of releasing and detecting parts of neurotransmitters. PPy was electrochemically polymerized with NaDBS as dopants on Au electrode and then was incorporated with epinephrine by cation exchange process. The incorporated epinephrine was released by applying a controlled voltage and the released amount of epinephrine was determined using an ultraviolet (UV) spectrometry. Experimental results of the releasing part show that the released amount of epinephrine depended not only on the thickness and the size of PPy film but also on the releasing time. Spontaneously diffused epinephrine amount was measured to be only 18% of the voltage driven release amount. The absorbance change of epinephrine due to the applied potential during releasing process is negligible compared with that of the released epinephrine. Overoxidized PPy(OxPPy) for the detecting part shows a good cation permselectivity for the detection of epinephrine and the current is also higher than that at the Au electrode in the same concentration of the epinephrine. The current level is different with dopants with which the OxPPy film is polymerized and the sensitivity of the OxPPy electrode depends on the thickness of PPy film.