You-Seop Lee

Precise temperature control and rapid thermal cycling in a micromachined DNA polymerase chain reaction chip

We have fabricated Si-based micromachined DNA polymerase chain reaction (PCR) chips with different groove depths. The platinum thin-film micro heater and the temperature sensor have been integrated on the chip. The volume of the PCR chamber in the chip is about 3.6 ?l and the chip size is 17 ? 40 mm2. The effects of groove geometry, including width, depth and position, on the thermal characteristics of the PCR chip have been investigated by numerical analysis and experimental measurement. From the results, the power consumption required for the PCR chip is reduced with the increase of groove depth. Compared with results for the case of no groove, the power consumption of the chip with a groove of 280 ?m is reduced by 24.0%, 23.3% and 25.6% with annealing, extension and denaturation, respectively. The heating rate is increased rapidly with the increase of the groove depth. In particular, it is revealed that this effect is predominant for depths in the region above 280 ?m. For a more precise control of chip temperature, the nonlinear feedback proportional-integral control scheme is used. The obtained heating and cooling rates are about 36 ?C s?1 and 22 ?C s?1, respectively. The overshoot and the steady state error are less than 0.7 ?C and ?0.1 ?C, respectively. In the experiment, the effects of the PCR buffer and the bubbles in the chamber on the temperature uniformity have also been studied. From the temperature measurement, it is revealed that the temperature difference between the thin-film sensor (on the lower plate) and the PCR buffer can be neglected if there is no air bubble in the PCR buffer. With such a high performance control scheme, we could implement a remarkable thermal cycling of conducting 30 cycles for 3 min. Finally, the chip PCR of plasmid DNA was successfully performed with no additives using the temperature control system.

A simple method for cleaning graphene surfaces with an electrostatic force.

DOI: 10.1002/adma.201303199 Graphene is a 2D conductive nanomaterial composed of a honeycomb-structured array of carbon atoms. [ 1,2 ] It is useful in a variety of fi elds due to its structural and chemical stability and because it possesses unprecedented optical and electrical properties. [ 3–7 ] Graphene can be produced by both bottom-up (chemical vapor deposition: CVD) [ 8–11 ] and top-down (chemical or mechanical exfoliation) approaches. [ 2,12–14 ] CVD techniques can yield layer-controlled graphene sheets over large areas using Ni or Cu catalyst substrates; however, the CVD process is limited to very specifi c growth templates, and graphene is seldom directly grown on insulating substrates. Graphene grown on Cu or Ni catalyst fi lms must be transferred to a target substrate for use in applications. The procedures involved in transferring these fi lms always involve a supporting polymer contact layer because the mechanical strength of the one-atom-thick graphene is insuffi cient to handle the transfer processes. The most commonly used supporting layer materials are poly(methyl methacrylate) (PMMA), polycarbonates (PCs), or conventional photoresistive (PR) layers. [ 8–11,15–19 ] After the transfer step, the supporting layer materials must be removed from the graphene surface. The chemical stripping processes involved in removing the supporting layer are routine because most supporting layer materials readily dissolve in common solvents, such as acetone; however, PMMA layers in direct contact with a graphene layer (1–5 nm from the surface) leave “PMMA-G” contamination materials that cannot be removed using simple chemical means. [ 20–22 ] The residual PMMA-G not only degrades and changes the electrical characteristics of the graphene layer, but it also introduces uncertainty into studies of the intrinsic surface properties of graphene, the development of sensor devices, the fabrication of cell culture substrates for accelerating stem cell differentiation, and the fabrication of atomic-scale honeycomb templates. [ 23–25 ] Many attempts have been made to resolve these issues, and annealing methods, such as H 2 /Ar annealing, [ 20 ] vacuum annealing, [ 21 ] and Joule heating, [ 26 ] have proven to be successful. Some approaches have “scraped the surfaces clean? using atomic force microscopy (AFM) tips. [ 27 ] Hightemperature annealing processes (>250 °C) can introduce defects into graphene surfaces, and process temperatures can be much too high for use with fl exible substrate-based devices. High-temperature annealing normally requires vacuum facilities and/or the application of electrical energy, which increases production costs. Although AFM-derived mechanical cleaning methods avoid some of these complications, their use is limited to laboratory-scale because the methods are extremely lowthroughput. Therefore, an effective cleaning method that does not involve high-temperature heat treatments would greatly benefi t practical applications that rely on transferred graphene. Traditional surface cleaning methods involve rubbing a piece of cloth over the surface. Empirical evidence from everyday life clearly shows that a combination of mechanical abrasion and detergent is much more effective toward cleaning than the simple use of detergents only; however, abrasion methods cannot be used on materials that are mechanically weak. Nanostructured materials, such as graphene or carbon nanotubes, generally have a much larger mechanical strength than conventional steel of the same dimensions; however, the aggregate strength of nanostructured materials is usually not suffi ciently large to withstand macroscopic mechanical stresses. Mechanical abrasion is generally not used to clean the surfaces of nanoscale materials. Electrostatic interactions can remove charged impurities and, therefore, present an alternative approach to direct mechanical cleaning. Rubbing a balloon or a piece of plastic on ones head to induce ones hair to rise is a familiar childhood demonstration of electrostatic interactions, in which stationary or slow-moving electric charges build up in a material. As two interfaces are rubbed against one another, electrons can cross the interface of the materials and generate a charge disparity in each of the materials. The direction of electron transfer depends on the ability of the material to accept a negative charge. For example, as a piece of rubber is rubbed against a cloth, electrons are transferred from the cloth to the rubber. The rubber becomes negatively charged and the cloth becomes positively charged. Because the presence of a static charge creates a non-uniform electrostatic fi eld that tends to attract matter toward it, a material with a large amount of static charge can be used to remove dust from a surface. In this study, extremely fi ne cloth fi bers were used to apply electrostatic-force cleaning (EFC) to a graphene surface by removing the residual PMMA-G layers, yielding an intrinsic graphene surface without causing damage to the structural integrity. The process of removing the PMMA-G layer was monitored on a step-by-step basis using optical microscopy

Effects of substrate conductivity on cell morphogenesis and proliferation using tailored, atomic layer deposition-grown ZnO thin films

We demonstrate that ZnO films grown by atomic layer deposition (ALD) can be employed as a substrate to explore the effects of electrical conductivity on cell adhesion, proliferation, and morphogenesis. ZnO substrates with precisely tunable electrical conductivity were fabricated on glass substrates using ALD deposition. The electrical conductivity of the film increased linearly with increasing duration of the ZnO deposition cycle (thickness), whereas other physical characteristics, such as surface energy and roughness, tended to saturate at a certain value. Differences in conductivity dramatically affected the behavior of SF295 glioblastoma cells grown on ZnO films, with high conductivity (thick) ZnO films causing growth arrest and producing SF295 cell morphologies distinct from those cultured on insulating substrates. Based on simple electrostatic calculations, we propose that cells grown on highly conductive substrates may strongly adhere to the substrate without focal-adhesion complex formation, owing to the enhanced electrostatic interaction between cells and the substrate. Thus, the inactivation of focal adhesions leads to cell proliferation arrest. Taken together, the work presented here confirms that substrates with high conductivity disturb the cell-substrate interaction, producing cascading effects on cellular morphogenesis and disrupting proliferation, and suggests that ALD-grown ZnO offers a single-variable method for uniquely tailoring conductivity.

Lumped Modeling of Crosstalk Behavior of Thermal Inkjet Print Heads

This paper presents a lumped model to predict crosstalk characteristics of thermally driven inkjet print heads. The model is based on a heat conduction equation, an empirical pressure-temperature equation, and a nonlinear hydraulic flow-pressure equation. It has been simulated through the construction of a Kirchhoffian R-L-C network, and subsequently analyzed using SIMULINK and an electronic circuit simulation tool. Using the lumped R-C model, heating characteristics of the head are predicted to be in agreement with IR temperature measurements. The inter-channel crosstalk is simulated using the lumped R-L network. The values of viscous flow resistance, R and flow inertance, L of the inter-channels are adjusted to accord with the 3-D numerical simulation results of three adjacent jets. The crosstalk behaviors of a back shooter head as well as a top shooter head have been investigated. Predictions of the proposed lumped model of the meniscus oscillations are consistent with numerical simulations. Comparison of the lumped model with experimental results identifies that abnormal two-drop ejection phenomena are related to the increased meniscus oscillations because of the more severe crosstalk effects at higher printing speeds. Our model can be used as a design tool for a better design of thermal inkjet print heads to minimize crosstalk effects.Copyright

Numerical Simulation of Hydro-Acoustic Flow of Liquid in Piezo Inkjet Print Head

This paper presents numerical and theoretical studies of acoustic wave interactions in slightly compressible liquids within piezoelectrically driven inkjet print heads. The interconnected flow channels may cause jet cross-talk, resulting in poor printing quality. Thus, it should be reduced by modifying the channel structure with the acoustic wave interactions considered. Compressible gas flow driven by the sudden movement of a top wall in the channel is calculated using Flow3D and is validated with the result obtained by using the narrow gap theory. For the calculation of pressure waves in the ink flow, a limited compressibility model of the Flow3D is used. It is found that reducing the restrictor width can damp out the jet cross-talk by inhibiting the pressure wave propagation. The degree of cross-talk has been quantified using the maximum values of cross-correlations between neighboring channels and a critical channel dimension for acceptable cross-talk has been proposed. This finding is verified by drop visualization experiments using silicon-micromachined piezo inkjet print heads that are fabricated by our group.Copyright

Effects of Meniscus Motion on Ejection Performance in Thermal Inkjet Print Heads by 3-D Numerical Simulation and 1-D Lumped Modeling

Undesired meniscus motion at the nozzle exti can cause detrimental effects on ejection performance of an inkjet print head, resulting in degradation of printing quality at high frequency operation. In this study, visulization of droplet ejection and meniscus motion was performed experimentally, and the results were compared with those of numerical simulations. Effects of design factors on themeniscus motion, such as material properties of ink and geometric dimensions of head structure, were investigated by three-dimensional (3-D) numerical simulations. The result demonstrated that the ejection performance and the following menicus motion might be affected significantly due to changes in the design factors. For simple and fast computation, one-dimensional (1-D) lumped model was constructed, and its results were meaningful for the intuitive understanding of ejection performance and meniscus oscillation. Also, effects of different meniscus conditions on the subsequent ejection were investigated by 3-D numerical computations. The results showed that a stabilizing time, e.g., 70μs, was needed for uniform reproducible ejection. The results of this study will be helpful for development of the inkjet print heads of higher performance.Copyright