Jiseok Lim

Parallelized ultra-high throughput microfluidic emulsifier for multiplex kinetic assays

Droplet-based microfluidic technologies are powerful tools for applications requiring high-throughput, for example, in biochemistry or material sciences. Several systems have been proposed for the high-throughput production of monodisperse emulsions by parallelizing multiple droplet makers. However, these systems have two main limitations: (1) they allow the use of only a single disperse phase; (2) they are based on multiple layer microfabrication techniques. We present here a pipette-and-play solution offering the possibility of manipulating simultaneously 10 different disperse phases on a single layer device. This system allows high-throughput emulsion production using aqueous flow rates of up to 26 ml/h (>110 000 drops/s) leading to emulsions with user-defined complex chemical composition. We demonstrate the multiplex capabilities of our system by measuring the kinetics of β-galactosidase in droplets using nine different concentrations of a fluorogenic substrate.

Ultra-high throughput detection of single cell beta-galactosidase activity in droplets using micro-optical lens array.

We demonstrate the use of a hybrid microfluidic-micro-optical system for the screening of enzymatic activity at the single cell level. Escherichia coli β-galactosidase activity is revealed by a fluorogenic assay in 100 pl droplets. Individual droplets containing cells are screened by measuring their fluorescence signal using a high-speed camera. The measurement is parallelized over 100 channels equipped with microlenses and analyzed by image processing. A reinjection rate of 1 ml of emulsion per minute was reached corresponding to more than 105 droplets per second, an analytical throughput larger than those obtained using flow cytometry.

Design and fabrication of micro optical film by ultraviolet roll imprinting

With increasing demand for large-scale functional optical films with microstructure in the field of flat panel displays, a technology capable of fabricating large-scale polymeric micro-patterns has received much attention. To fabricate large-area micro-optical films, we designed and constructed an ultraviolet roll imprinting system consisting of a roll stamp, a material dispensing unit, a pair of flattening rollers, a contact roller, and a releasing roller. Two methods for fabricating roll stamps were considered: direct machining of the roll base and wrapping a flat stamp around the roll base. As practical examples of the roll imprinting process, we designed and fabricated a lenticular lens array, a pyramidal pattern, and a microlens array, and measured and analyzed their geometrical and optical properties. Our results suggest that the proposed UV roll imprinting process is a feasible method for mass producing large-scale functional optical films.

Elimination of flux loss by optimizing the groove angle in modified Fresnel lens to increase illuminance uniformity, color uniformity and flux efficiency in LED illumination

A Fresnel lens is an optical component that can be used to create systems more compact, cost-effective, and lightweight than those using conventional continuous surface optics. However, Fresnel lenses can usually cause a loss of flux efficiency and non-uniform distribution of illuminance due to secondary refraction by surface discontinuities, especially along the groove facet. We therefore proposed to modify a groove angle in the Fresnel lens and analyzed interrelation between the groove angle and multiple optical performances, such as flux efficiency and the uniformity of illuminance and color. The groove angle was optimized to maximize the uniformity and efficiency in the target viewing angle considering various weights of merit functions. Specifically, in our study, when the uniformity of illuminance had a little more weight than the flux efficiency (ratio of 0.6:0.4), final optimum groove angles of 24.7 degrees , 29.4 degrees , and 31.3 degrees were obtained at target viewing angles of 20 degrees , 30 degrees , and 40 degrees , respectively. We also fabricated a modified Fresnel lens with a groove angle of 29.4 degrees using UV-imprinting. The real optical performance of the fabricated Fresnel lens was then compared to that of a spherical lens.

Injection Molding of Nanopillars for Perpendicular Patterned Magnetic Media with Metallic Nanostamp

In this work, we focus on two important technical issues that need to be overcome before the injection molding technology can be used in the mass production of patterned media, possibly, of small form factor or non-rotating type. Firstly, we describe a fabrication method of a metallic nanostamp through the use of an ultraviolet (UV)-imprinted polymeric nanomaster. Secondly, we examine a method to use a passive heating of the nanostamp to increase the replication quality and to obtain the desired height of the injection-molded nanopillars. Finally, we show that the magnetic film on each injection-molded nanopillars with passive heating has a single magnetic domain.

Elimination of nanovoids induced during electroforming of metallic nanostamps with high-aspect-ratio nanostructures by the pulse reverse current electroforming process

We studied a technique for fabricating metallic nanostamps with void-free, high-aspect-ratio nanostructures, using a pulse reverse current (PRC) electroforming process. During conventional electroforming of high-aspect-ratio nanostructures, a high current distribution is concentrated at the top and bottom corners, resulting in relatively fast growth of the nickel electroformed layer. This phenomenon leads to the formation of nanovoids in a metallic nanostamp, causing degradation of the stamp performance. To prevent the formation of nanovoids, we controlled the current waveform during the electroforming process. In this way, the process suppressed the formation of nanovoids, while effectively achieving a uniform current distribution. As practical examples, two types of metallic nanostamps were fabricated via direct current and PRC electroforming processes, one with a pitch of 0.8 µm and a height of 1.8 µm, and another with a pitch of 350 nm and a height of 525 nm. The internal nanovoids developed during the electroforming process were measured and analyzed.

Fabrication of Hybrid Microoptics Using UV Imprinting Process with Shrinkage Compensation Method

With increasing demand for compact aspherical optics in the field of imaging and optical data storage, fabrication technology for low cost micro aspherical optics has become a research priority. There are various types of micro aspherical optics, such as injection molded lens, glass molded lens, glass and hybrid lens. Among these types of lens, the hybrid is regarded as one of the most suitable because it combines good optical properties with low cost. The hybrid lens is fabricated by fabricating an aspherical layer on a spherical glass lens. To fabricate the hybrid lens at low cost, a UV imprinting process is preferred for its simplicity. However, in the conventional UV imprinting process volumetric shrinkage of the photopolymer causes various problems such as surface wrinkling and asymmetric local shrinkage. To overcome such limitations of the UV imprinting process, a shrinkage compensation method using an iris diaphragm to control the direction of polymerization is proposed and analyzed experimentally. To evaluate the proposed UV imprinting process, a hybrid lens was designed and fabricated, and its geometrical property was measured and compared with the design value.

An Amplex Red-based fluorometric and spectrophotometric assay for L-asparaginase using its natural substrate.

We report on the development of a sensitive real-time assay for monitoring the activity of L-asparaginase that hydrolyzes L-asparagine to L-aspartate and ammonia. In this method, L-aspartate is oxidized by L-aspartate oxidase to iminoaspartate and hydrogen peroxide (H2O2), and in the detection step horseradish peroxidase uses H2O2 to convert the colorless, nonfluorescent reagent Amplex Red to the red-colored and highly fluorescent product resorufin. The assay was validated in both the absorbance and the fluorescence modes. We show that, due to its high sensitivity and substrate selectivity, this assay can be used to measure enzymatic activity in human serum containing L-asparaginase.

Nanoimprinting of conductive tracks using metal nanopowders

A method for metal nanopowder imprinting is proposed as a patterning process for conductive tracks, which is inexpensive and scalable down to submicrometers. To examine the feasibility of the proposed method, conductive tracks with widths of submicrometers to a few tens of micrometers were formed using a thermal nanoimprinting system, and the effects of processing conditions on the degree of replication and the degree of sintering and the electric conductivity of imprinted conductive tracks were analyzed.

Design of microlens illuminated aperture array fabricated by aligned ultraviolet imprinting process for optical read only memory card system

A microlens illuminated aperture array (MLIAA) was designed for the multiprobe optical read only memory card system. As a method to integrate the microlens array (MLA) with the aperture array containing 1000×1000 apertures, an aligned ultraviolet imprinting process was used to satisfy the system objectives of reduced focal length and reduced spot size. An analysis using diffraction theory was performed to obtain the design tolerances for both MLIAA and the imprinting process. The maximum intensity of the optical probes at the Talbot [Philos. Mag. 9, 401 (1836)] position from the fabricated MLIAA was 12 times higher than that from the aperture array without a MLA.