Hongseok (Moses) Noh

Improved protein detection on an AC electrokinetic quartz crystal microbalance (EKQCM).

Microscale electrodes supplied with an AC field can generate rotational fluid patterns known as AC electroosmosis. In the present study, this effect was used to improve antibody binding on a biosensor surface. Antibodies, like many other large, slow moving biomolecules, tend to suffer from transport limitations during a reaction with a surface-bound receptor. Stirring such reactions with AC electroosmosis can alleviate this transport limitation by bringing fresh reagent to the surface. For the first time, the use of this phenomenon was used to improve the capture of protein on a sensor. Directly adsorbed antibodies were bound to the surface of specially modified quartz crystal microbalances, known as electrokinetic QCMs (EKQCMs) and the signal was enhanced by about 5.6 times. Modification of the QCM resulted in little reduction of quality factor (from ∼ 5.3 k to ∼ 4.6k) and an increased sensitivity to viscosity changes (151%). Full immunoassays performed on electrodes fabricated on glass surfaces were used to ensure antibody function was not significantly degraded by the enhancement technique.

Microfluidic platform for hepatitis B viral replication study

Hepatocytes, the cells responsible for the metabolic and detoxification processes in the liver, are the predominant target of hepatitis B virus (HBV) infections, a major cause of liver cancer. The limited availability of normal human hepatocytes for cell-culture based studies is a significant challenge in HBV-associated liver cancer research. Therefore, there is a need for miniaturized cell-culture systems that can serve as a platform for studying the effect of HBV infections on hepatocyte physiology. Here, we present a microfluidic platform that can be used to study HBV replication in both rat and human hepatocytes. Polydimethylsiloxane (PDMS) microchannels fabricated using soft lithography techniques served as a culture vessel for both primary rat hepatocytes (PRH) and a human hepatoblastoma cell line, HepG2. The micro cell-culture chamber was then used as a model for HBV replication studies. Cells were grown in static culture conditions and either transfected with an HBV-genome cDNA or infected with the viral genome expressed from a recombinant adenovirus. Supernatants collected from the microchannels were assayed for secreted HBV using polymerase chain reaction (PCR). We achieved approximately 40 and 10% transfection efficiencies in HepG2 cells and PRH respectively, and 80–100% adenoviral infection efficiency in PRH comparable to standard tissue culture methods. Moreover, we successfully detected replicated HBV using our novel platform. This platform can be easily extended to studies involving DNA transfection or HBV infection of primary human hepatocytes since only a small number of cells are required for studies in microfluidic chambers.

KrF excimer laser micromachining of MEMS materials: characterization and applications

Conventional photolithography-based microfabrication techniques are widely used to create microscale structures and devices nowadays. However, these techniques are limited to two-dimensional fabrication and also to particular materials. Excimer laser micromachining enables us to overcome those limitations and facilitates three-dimensional (3D) microfabrication. This paper presents a comprehensive characterization study for excimer laser micromachining of micro-electro-mechanical system (MEMS) materials. By using a 248 nm KrF excimer laser and four representative MEMS materials (Si, soda-lime glass, SU-8 photoresist and poly-dimethysiloxane (PDMS)), the relations between laser ablation parameters (fluence, frequency and number of laser pulses) and etch performance such as the etch rates in the vertical and lateral directions, aspect ratio, and surface quality were obtained. The etch rate increased almost linearly for all four materials as the fluence increased but no significant variation in etch rate was observed as the frequency of laser pulses was changed. The etch rate was also inversely proportional to the number of laser pulses. Physical deformation in the laser-machined sites on PDMS and SU-8 was investigated using SEM imaging. In order to demonstrate the 3D microfabrication capability of an excimer laser and the utility of this characterization study, two novel implantable biomedical microscale devices made of SU-8 and PDMS were successfully fabricated using the optimized laser ablation parameters obtained in this study.

Magnetically actuated micropumps using an Fe-PDMS composite membrane

In this paper we describe a novel Fe-PDMS composite that can be used to create magnetically actuated polymeric microstructures. The composite is formed by suspending <10μm iron (Fe) particles in polydimethylsiloxane (PDMS) at concentrations ranging from 25-75% by weight. Material properties and processing capabilities have been examined, and to demonstrate materials usefulness we have designed, fabricated, and tested two prototypical micropumps that utilize an Fe-PDMS actuator membrane.

Dielectrophoretic particle–particle interaction under AC electrohydrodynamic flow conditions†

We used the Maxwell stress tensor method to understand dielectrophoretic particle–particle interactions and applied the results to the interpretation of particle behaviors under alternating current (AC) electrohydrodynamic conditions such as AC electroosmosis (ACEO) and electrothermal flow (ETF). Distinct particle behaviors were observed under ACEO and ETF. Diverse particle–particle interactions observed in experiments such as particle clustering, particles keeping a certain distance from each other, chain and disc formation and their rotation, are explained based on the numerical simulation data. The improved understanding of particle behaviors in AC electrohydrodynamic flows presented here will enable researchers to design better particle manipulation strategies for lab‐on‐a‐chip applications.

Direct inkjet printing of micro-scale silver electrodes on polydimethylsiloxane (PDMS) microchip

Recently, direct inkjet printing of conductive solutions has received much attention in the microfluidics and lab-on-a-chip community because of its low-cost and mask-free deposition of electrodes on various substrates. However, the investigation of micro-scale direct inkjet printing on the polydimethylsiloxane (PDMS) substrate has not been completed. Here we present a direct inkjet printing technique to produce narrow (40–90 µm) silver microelectrodes on PDMS. Extensive experimental characterization studies on the pattern uniformity and electrical properties of the printed silver lines are presented. The effect of major printing parameters such as drop spacing, sintering temperature and duration, platen temperature, and nozzle temperature have been thoroughly investigated. We also investigated multiple layer printing as well as the effects of thermal expansion and mechanical bending. In order to demonstrate the utility of the inkjet-printed silver microelectrode, we fabricated both quadruple and castellated electrodes, and conducted dielectrophoretic manipulation of microbeads. The results clearly show that the printed silver electrodes can be used for electrokinetic applications in PDMS microchip devices. We believe that the direct inkjet printing of silver ink on PDMS presented here can provide a very convenient way of creating microelectrodes on PDMS devices for a variety of applications in the MEMS, microfluidics, and lab-on-a-chip communities.

Flow-induced voltage generation over monolayer graphene in the presence of herringbone grooves

While flow-induced voltage over a graphene layer has been reported, its origin remains unclear. In our previous study, we suggested different mechanisms for different experimental configurations: phonon dragging effect for the parallel alignment and an enhanced out-of-plane phonon mode for the perpendicular alignment (Appl. Phys. Lett. 102:063116, 2011). In order to further examine the origin of flow-induced voltage, we introduced a transverse flow component by integrating staggered herringbone grooves in the microchannel. We found that the flow-induced voltage decreased significantly in the presence of herringbone grooves in both parallel and perpendicular alignments. These results support our previous interpretation.

Design and Fabrication of a PDMS/Parylene Microvalve for the Treatment of Hydrocephalus

We present a novel microvalve for the treatment of a pathological condition, i.e., hydrocephalus. This microvalve is made of polydimethylsiloxane/Parylene composite layer which has a 3-D dome petal shape. This geometry enables the microvalve to rectify fluid flow in the forward and backward directions. New microfabrication techniques such as dome-shaped SU-8 mold fabrication and excimer laser machining for valve opening have been investigated to build the proposed microvalve. The pressure drop versus flow rate characteristics of the fabricated microvalve was investigated through in vitro flow tests. The flow test results showed that a 10 × 10 microvalve array with a cross-cut opening shape (200 × 60 μm) was found to be optimal for the treatment of hydrocephalus.

A novel reflectance-based aptasensor using gold nanoparticles for the detection of oxytetracycline.

We present a novel reflectance-based colorimetric aptasensor using gold nanoparticles for the detection of oxytetracycline for the first time. It was found that the reflectance-based measurement at two wavelengths (650 and 520 nm) can generate more stable and sensitive signals than absorbance-based sensors to determine the aggregation of AuNPs, even at high AuNP concentrations. One of the most common antibacterial agents, oxytetracycline (OTC), was detected at concentrations as low as 1 nM in both buffer solution and tap water, which was 25-fold more sensitive, compared to the previous absorbance-based colorimetric aptasensors. This reflectance-based colorimetric aptasensor using gold nanoparticles is considered to be a better platform for portable sensing of small molecules using aptamers.

The Physical Foundation of Vasoocclusion in Sickle Cell Disease

The pathology of sickle cell disease arises from the occlusion of small blood vessels because of polymerization of the sickle hemoglobin within the red cells. We present measurements using a microfluidic method we have developed to determine the pressure required to eject individual red cells from a capillary-sized channel after the cell has sickled. We find that the maximum pressure is only ∼100 Pa, much smaller than typically found in the microcirculation. This explains why experiments using animal models have not observed occlusion beginning in capillaries. The magnitude of the pressure and its dependence on intracellular concentration are both well described as consequences of sickle hemoglobin polymerization acting as a Brownian ratchet. Given the recently determined stiffness of sickle hemoglobin gels, the observed obstruction seen in sickle cell disease as mediated by adherent cells can now be rationalized, and surprisingly suggests a window of maximum vulnerability during circulation of sickle cells.