Kyu-youn Hwang

Replication of polyethylene nano-micro hierarchical structures using ultrasonic forming

We present the replication of polyethylene (PE) nano-micro hierarchical structures and their application for superhydrophobic surfaces. A commercial ultrasonic welding system was used to apply ultrasonic vibration energy to the forming of nano-micro hierarchical structures. To evaluate ultrasonic formability, Ni nanomold and nano-micro hierarchical mold were designed and fabricated. The optimal weld times were 1.5 s and 3.0 s for PE nanoprotrusions and nano-micro hierarchical structures, respectively. The forming process was conducted at atmospheric pressure. The PE structures were well replicated without a vacuum. The trapped air in the microcavity of the nano-micromold was dispersed and absorbed into the molten PE. Ultrasonic nano-microreplication technology showed an extremely short processing time and did not require a vacuum environment. To investigate the applicability of ultrasonic forming, the fabricated nanoprotrusions and nano-micro hierarchical structures were coated with plasma polymerized fluorocarbon (PPFC) of a hydrophobic nature and were applied to modify superhydrophobic surfaces. The contact angle was increased from 106° (smooth surface) to 125° (nanostructured surface) and finally to 160° (nano-microstructured surface) so that the surface became superhydrophobic.

Dry etching of polydimethylsiloxane using microwave plasma

This paper presents a new polydimethylsiloxane (PDMS) dry-etching method that uses microwave plasma. The applicability of the method for fabricating microstructures and removing residual PDMS is also verified. The etch rate of PDMS was dominantly influenced by the gas flux ratio of CF4/O2 and the microwave power. While the PDMS etch rate increased as the flux ratio of CF4 was increased, the etch rate decreased as the flux ratio of O2 was increased. The maximum etch rate of 4.31 µm min−1 was achieved when mixing oxygen (O2) and tetrafluoromethane (CF4) at a 1:2 ratio at 800 W power. The PDMS etch rate almost linearly increased with the microwave power. The ratio of the vertical etch rate to the lateral etch rate was in a range of 1.14–1.64 and varied with the gas fluxes. In consideration of potential applications of the proposed PDMS etching method, array-type PDMS microwells and network-type microprotrusion structures were fabricated. The contact angle was dramatically increased from 104° (non-etched PDMS surface) to 148° (etched PDMS surface) and the surface was thereby modified to be superhydrophobic. In addition, a thin PDMS skin that blocked holes and PDMS residues affixed in nickel microstructures was successively removed.

Polymer microreplication using ultrasonic vibration energy

Polymethyl methacrylate (PMMA) microstructures were fabricated by a polymeric microreplication technology using ultrasonic vibration energy. A commercial ultrasonic welder system was used to apply ultrasonic vibration energy for micromolding. Two different types of nickel micromolds, which were equipped with pillar-type and pore-type microstructures, were fabricated. PMMA was used as the polymer microreplication material, and the optimal molding times were determined to be 2 s and 2.5 s for the pillar-type and pore-type micromolds, respectively. Compared with conventional polymer microreplication technologies, the proposed ultrasonic microreplication technology showed an extremely short processing time. Heat energy generated by ultrasonic vibration locally affected the vicinity of the contact area between the micromold and the polymer substrate. Consequently, only that very limited area was melted so that the bulk material was not seriously affected by the thermal effect and thermal shrinkage could be minimized. Furthermore, although the replication process was not performed in vacuum conditions, the ultrasonic micromolding showed high fidelity in polymer microreplication using the pore-type micromold.

The effects of adhesion energy on the fabrication of high-aspect-ratio SU-8 microstructures

The effects of a Si wafer cleaning method on the adhesion behavior of SU-8 microstructures were investigated. In particular, the role of adhesion energy was clearly demonstrated for fabricating high-aspect-ratio (HAR) SU-8 micropillar arrays with reduced contact area. The surface energy values of the Si wafers prepared through different cleaning techniques were calculated using contact angle measurements, revealing that the difference of adhesion energy was mainly determined by the presence of a SiO2 layer. It was found that the adhesion energy of the SU-8 microstructure to the Si wafer was increased by approximately 72% through HF treatment that is required to remove the oxide layer. Furthermore, the dispersion component of the surface energy played a major role in the adhesion of SU-8 to various solid substrates. These results consistently indicate that thermodynamic adhesion energy needs to be high for the strong macroscopic adhesion strength of the SU-8 photoresist. Also, the calculated adhesion energy values were in good agreement with the physical measurements of macroscopic shear stress, suggesting that a simple thermodynamic approach used here can be used to examine the adhesion characteristics of the SU-8 photoresist. Using the optimized cleaning method, HAR SU micropillar arrays of 20 µm width and 300 µm height (AR = 15) were fabricated.

PDMS bonding to organically-modified solid surface using photocatalyst for fabricating low-cost plastic microchip

A novel PDMS bonding technique using photocatalyst has been developed to fabricate polymer-based DNA sample preparation microchip for molecular diagnostics. To bond PDMS substrate onto organosilane-modified pillar arrays, organosilane layer on top plane of micropillars was selectively removed through photocatalytic oxidation using TiO2 and UV irradiation. The proposed method exposed underlying SiO2 surface without deteriorating the organic film on lateral surface of the micropillars which is used for bacterial adhesion, and then resulting SiO2 layer was bonded to PDMS. The low-cost plastic DNA sample preparation chip has been successfully fabricated and it demonstrated similar performance to Glass/Si chip in terms of bacterial cell capture efficiency and PCR compatibility.

Novel Bacterial DNA Sample Preparation Method from Whole Blood using Surface-Modified Silicon Microstructures

A novel bacterial DNA sample preparation device for molecular diagnostics has been developed. Based on optimized conditions for bacterial adhesion, surface-modified silicon pillar arrays for bacterial cell capture were fabricated and their ability to capture bacterial cells was demonstrated. The capture efficiency for bacterial cell, E. coli, in buffer solution was over 90% with a flow rate of 400 mulscr/min. Moreover, the proposed method captured E. coli cells present in 50% whole blood effectively. The captured cells from whole blood were then in-situ lyzed on the surface of the microchip and the eluted DNA was successfully amplified by qPCR. These results demonstrate that the full process of pathogen capture to DNA isolation from whole blood could be automated in a single microchip.