Oh-Sun Kwon

Active digital microfluidic paper chips with inkjet-printed patterned electrodes.

Active, paper-based, microfluidic chips driven by electrowetting are fabricated and demonstrated for reagent transport and mixing. Instead of using the passive capillary force on the pulp to actuate a flow of a liquid, a group of digital drops are transported along programmed trajectories above the electrodes printed on low-cost paper, which should allow point-of-care production and diagnostic activities in the future.

Paper-Based Bimodal Sensor for Electronic Skin Applications

We present the development of a flexible bimodal sensor using a paper platform and inkjet printing method, which are suited for low-cost fabrication processes and realization of flexible devices. In this study, we employed a vertically stacked bimodal device architecture in which a temperature sensor is stacked on top of a pressure sensor and operated on different principles, allowing the minimization of interference effects. For the temperature sensor placed in the top layer, we used the thermoelectric effect and formed a closed-loop thermocouple composed of two different printable inks (conductive PEDOT:PSS and silver nanoparticles on a flexible paper platform) and obtained temperature-sensing capability over a wide range (150 °C). For the pressure sensor positioned in the bottom layer, we used microdimensional pyramid-structured poly(dimethylsiloxane) coated with multiwall carbon nanotube conducting ink. Our pressure sensor exhibits a high-pressure sensitivity over a wide range (100 Pa to 5 kPa) and high-endurance characteristics of 105. Our 5 × 5 bimodal sensor array demonstrates negligible interference, high-speed responsivity, and robust sensing characteristics. We believe that the material, process, two-terminal device, and integration scheme developed in this study have a great value that can be widely applied to electronic skin.

Application of paper EWOD (electrowetting-on-dielectrics) chip: Protein tryptic digestion and its detection using MALDI-TOF mass spectrometry

A paper-based open EWOD chip was used for protein tryptic digestion, and the resulting peptides were analyzed using MALDI-TOF mass spectrometry to identify the proteins. Although on-chip protein digestion was previously demonstrated on a glass-based EWOD platform, this is the first report to show that a paper-based EWOD can also be used for protein digestion and further analysis by mass spectrometry. A number of protein digestion processes, i.e., disulfide bond reduction, alkylation, buffering, and tryptic digestion, can be successfully conducted on the paper-based EWOD chip. Furthermore, a “Y”-shaped junction design was shown to be effective in successfully manipulating a droplet for protein digestion. A paper-based EWOD platform offers a cheap and versatile avenue for biological applications.

Control-fluidic CoDesign for paper-based digital microfluidic biochips

Paper-based digital microfluidic biochips (P-DMFBs) have recently emerged as a promising low-cost and fast-responsive platform for biochemical assays. In P-DMFBs, electrodes and control lines are printed on a piece of photo paper using inkjet printer and conductive ink of carbon nanotubes (CNTs). Compared with traditional digital microfluidic biochips (DMFBs), P-DMFBs enjoy notable advantages, such as faster in-place fabrication with printer and ink, lower costs, better disposability, etc. Because electrodes and CNT control lines are printed on the same side of a paper, a new design challenge for P-DMFB is to prevent the interference between moving droplets and the voltages on CNT control lines. These interactions may result in unexpected droplet movements and thus incorrect assay outputs. To address the new challenges in automated design of P-DMFBs, this paper proposes the first control-fluidic codesign flow, which simultaneously adjusts the control line routing and fluidic droplet scheduling to achieve an optimized solution. As the control line routing may not be able to address all the interferences between moving droplets and the voltages on control lines, droplet rescheduling is performed to effectively deal with the remaining interferences in the routing solution. Computational simulation results on real-life bioassays show that the proposed codesign method successfully eliminates all the interferences, while a state-of-the-art maze routing method cannot solve any of the benchmarks without conflicts.

Effects of Silicone Oil on Electrowetting to Actuate a Digital Microfluidic Drop on Paper

The effects of an immiscible, lubricating polydimethylsiloxane fluid, referred to as silicone oil, on the static deformation and on the dynamic motion of a water drop on paper induced by electrowetting were investigated. The deformation of a drop on a hydrophobic film of amorphous fluoropolymers top-coated with less hydrophobic silicone oil was much more predictable, reversible and reproducible than on the uncoated surface. In the dynamic tribological experiment for a sliding drop along an inclined surface, a significant decrease in the friction coefficient, with an unexpected dependency of the contact area, was observed. Based on the curve fitting analysis, the shear stress and the net friction force were estimated quantitatively. Because of the tribological effect and the reduced shear friction force of the oil film, the static and the dynamic electrowetting states of the water drop were enhanced.

Charge-selective membrane protein patterning with proteoliposomes

A novel method to fabricate transmembrane protein (TP) embedded lipid bilayers using microcontact printing and applying proteoliposomes to different types of substrates, has been developed. The electrostatic interaction between the negatively charged proteoliposome and the substrate, which had been positively functionalized by microcontact printing, allowed the formation of TP-embedded, patterned lipid bilayers. The positively charged amino functional group on the substrate did effectively attract the negatively charged vesicles, inducing them to be adsorbed and subsequently ruptured to form a giant mosaic lipid bilayer, resulting in an immobilized TP-embedded lipid layer precisely on the targeted patterns, which were backfilled with a zwitterionic lipid bilayer. The rapid and highly selective recognition of the charged liposomes was visualized, and the biological functions of the TPs in the lipid matrix were also observed.

Patterning of gold nanoparticles on fluoropolymer films by using patterned surface grafting and layer-by-layer deposition techniques.

The patterning of gold nanoparticles (GNPs) on the surface of a fluoropolymer substrate by using patterned surface grafting and layer-by-layer deposition techniques is described. The surface of a poly(tetrafluoroethylene-co-perfluorovinyl ether) (PFA) substrate was selectively implanted with 150 keV proton ions. Peroxide groups were successfully formed on the implanted PFA surface, and their concentration depended on the fluence. Acrylic acid was graft polymerized onto the implanted regions of the PFA substrate, resulting in well-defined patterns of poly(acrylic acid) (PAA) on the PFA substrate. The surface properties of the PAA-patterned PFA surface, such as chemical compositions, wettability, and morphology, were investigated. The surface analysis results revealed that PAA was definitely present on the implanted regions of the PFA surface, and the degree of grafting was dependent on three factors: fluence, grafting time, and monomer concentration. Furthermore, GNP patterns were generated on the prepared PAA-patterned PFA surface by layer-by-layer deposition of GNPs and poly(diallyldimethyl ammonium chloride). The multilayers of GNPs were deposited only onto the PAA-grafted regions separated by bare PFA regions, and the resulting GNP patterns exhibited good electrical conductivity.