Kieseok Oh

Bio-mimetic silicone cilia for microfluidic manipulation

This paper presents a bio-mimetic microfluidic device that mimics the high compliance and the beating frequency of biological cilia in order to achieve bio-compatible manipulation of microfluidics. Because the highly compliant cilia can easily collapse due to interaction energy and surface tension, the major challenge in developing a bio-mimetic device is the manufacturing of highly compliant cilia. An underwater fabrication method is developed to avoid the cilia collapse by lowering the surface energy of the cilia. Another challenge is to mimic the low beating frequency (10-100 Hz) of biological cilia. The proposed microfluidic device is excited by a piezo actuator to resonate the cilia in water. Due to the highly compliant nature of the silicone cilia, the resulting actuation frequency is in the beating frequency range of biological cilia. Simulations and experiments are presented to demonstrate microfluidic manipulation by resonance of the assembled cilia array.

Immunosensor towards low-cost, rapid diagnosis of tuberculosis

A rapid, accurate tuberculosis diagnostic tool that is compatible with the needs of tuberculosis-endemic settings is a long-sought goal. An immunofluorescence microtip sensor is described that detects Mycobacterium tuberculosis complex cells in sputum in 25 minutes. Concentration mechanisms based on flow circulation and electric field are combined at different scales to concentrate target bacteria in 1 mL samples onto the surfaces of microscale tips. Specificity is conferred by genus-specific antibodies on the microtip surface. Immunofluorescence is then used to detect the captured cells on the microtip. The detection limit in sputum is 200 CFU mL(-1) with a success rate of 96%, which is comparable to PCR.

Size-selective immunofluorescence of Mycobacterium tuberculosis cells by capillary- and viscous forces

Rapid, low cost screening of tuberculosis requires an effective enrichment method of Mycobacterium tuberculosis (MTB) cells. Currently, microfiltration and centrifugation steps are frequently used for sample preparation, which are cumbersome and time-consuming. In this study, the size-selective capturing mechanism of a microtip-sensor is presented to directly enrich MTB cells from a sample mixture. When a microtip is withdrawn from a spherical suspension in the radial direction, the cells that are concentrated by AC electroosmosis are selectively enriched to the tip due to capillary- and viscous forces. The size-selectivity is characterized by using polystyrene microspheres, which is then applied to size-selective capture of MTB from a sample mixture. Our approach yields a detection limit of 800 cells mL(-1), one of the highest-sensitivity immunosensors to date.

Added-Mass Effect in Modeling of Cilia-Based Devices for Microfluidic Systems

This article shows that the added mass due to fluid-structure interaction significantly affects the vibrational dynamics of cilia-based (vibrating cantilever-type) devices for handling microscale fluid flows. Commonly, the hydrodynamic interaction between the cilia-based actuators and fluid is modeled as a drag force that results in damping of the cilia motion. Our main contribution is to show that such damping effects cannot explain the substantial reduction in the resonant-vibrational frequency of the cilia actuator operating in liquid when compared with the natural frequency of the cilia in air. It is shown that an added-mass approach (that accounts for the inertial loading of the fluid) can explain this reduction in the resonant-vibrational frequency when operating cantilever-type devices in liquids. Additionally it is shown that the added-mass effect can explain why the cilia-vibration amplitude is not substantially reduced in a liquid by the hydrodynamic drag force. Thus, this article shows the need to model the added-mass effect, both theoretically and by using experimental results.

Semi-automated, occupationally safe immunofluorescence microtip sensor for rapid detection of Mycobacterium cells in sputum.

An occupationally safe (biosafe) sputum liquefaction protocol was developed for use with a semi-automated antibody-based microtip immunofluorescence sensor. The protocol effectively liquefied sputum and inactivated microorganisms including Mycobacterium tuberculosis, while preserving the antibody-binding activity of Mycobacterium cell surface antigens. Sputum was treated with a synergistic chemical-thermal protocol that included moderate concentrations of NaOH and detergent at 60°C for 5 to 10 min. Samples spiked with M. tuberculosis complex cells showed approximately 106-fold inactivation of the pathogen after treatment. Antibody binding was retained post-treatment, as determined by analysis with a microtip immunosensor. The sensor correctly distinguished between Mycobacterium species and other cell types naturally present in biosafe-treated sputum, with a detection limit of 100 CFU/mL for M. tuberculosis, in a 30-minute sample-to-result process. The microtip device was also semi-automated and shown to be compatible with low-cost, LED-powered fluorescence microscopy. The device and biosafe sputum liquefaction method opens the door to rapid detection of tuberculosis in settings with limited laboratory infrastructure.

Resonant behavior and microfluidic manipulation of silicone cilia due to an added mass effect

In a microfluidic device, cilia are good candidates to generate complex flow in solution by stirring the microscale fluid. To realize the potential as a novel microfluidic system, it is essential to understand the resonant behavior of cilia in solution and optimize the performance of a cilia-based device. In this paper, the resonant behavior of polydimethylsiloxane (PDMS) cilia in water is investigated by a novel computational method in order to understand the underlying physics and to suggest the optimal design of a cilia-based device. The resonant frequency of a single cilium in water is quantitatively compared to both analytical solutions and experimental results considering an added mass effect. Also, to propose the optimal design of cilia-based device as a microfluidic mixer, the fluid velocity and the pressure in multiple cilia are analysed according to the spacing between neighboring cilia. When the spacing increases from 100 µm to 600 µm, the flow pattern generated by the tips of cilia is changed from a large vortex to multiple small vortexes, which can be utilized for transport, separation and reaction of biomolecules. The particle dispersion with the mixing efficiency in multiple cilia is analysed to predict the performance as a microfluidic device.

Nanotips for single-step preparation of DNA for qPCR analysis

A single-step concentration and elution method is developed for detection of DNA in buffer, saliva, and blood. A nanotip capturing DNA using an electric field and capillary action is directly dissolved in buffer for qPCR analysis. The concentration yield and the relative parameters are compared with those of a commercial kit.

Fluid Manipulation by Bio-Mimetic Cilia

This paper describes the fabrication and actuation of bio-mimetic cilia for fluid manipulation. High aspect ratio cilia made of polydimethylsiloxane (PDMS) were successfully assembled in a microfluidic device by our novel fabrication method. This method was to release the PDMS cilia from a Si mold and assemble the cilia in a device. All the process was performed under water in order to avoid the stiction and pairing of the PDMS cilia. The underwater assembly method enabled a high aspect ratio PDMS structure assembly in a fluidic device. The PDMS cilia were actuated in air and water by lead-zirconate-titanate (PZT) microstage. In the fabricated device, the maximum displacement of the cilia was observed at 120Hz in air and at 50Hz in de-ionized (DI) water with our experimental condition. The actuated cilia in a solution produced convective and propulsive fluid flow near the cilia structure. The developed device can be used for precise handling of small volume sample (e.g., 1 μL).© 2007 ASME

Review: Rod-Shaped Nanoparticle Assembly Using an Electric Field

Various nanowire or nanotube-based devices have been demonstrated to fulfill the future demands on semiconductor industries and bio/chemical sensors. To fabricate such devices, an electric field-based assembly method has demonstrated a great potential for parallel- and one dimensional assembly of nanowires. In this review paper, the future direction of electric field guided assembly of nanowires is discussed with our recent results. The challenges and opportunities of the assembly are also introduced with the current trends of the nanowire assembly.Copyright

Added-mass effect in modeling of cilia-based devices for microfluidic systems

This article shows that the added mass due to fluid structure interaction significantly affects the vibrational dynamics of cilia-based devices. Our main contribution is to show that such damping effects cannot explain the substantial reduction in the resonant vibrational frequency of the cilia operating in liquid when compared to the natural frequency of the cilia in air. It is shown that an added-mass approach (that accounts for the inertial loading of the fluid) can explain this reduction in the resonant vibrational frequency when operating the cantilever-type devices in liquids. Additionally, it is shown that the added-mass effect can explain why the cilia-vibration amplitude is not substantially reduced in a liquid by the hydrodynamic drag force. Thus, this article shows the need to model the added-mass effect, both, theoretically and by using experimental results.Copyright