Byung ng Ha

Microchannel Anechoic Corner for Size-Selective Separation and Medium Exchange via Traveling Surface Acoustic Waves

We demonstrate a miniaturized acoustofluidic device composed of a pair of slanted interdigitated transducers (SIDTs) and a polydimethylsiloxane microchannel for achieving size-selective separation and exchange of medium around polystyrene particles in a continuous, label-free, and contactless fashion. The SIDTs, deposited parallel to each other, produce tunable traveling surface acoustic waves (TSAWs) at desired locations, which, in turn, yield an anechoic corner inside the microchannel that is used to selectively deflect particles of choice from their streamlines. The TSAWs with frequency fR originating from the right SIDT and propagating left toward the microchannel normal to the fluid flow direction, laterally deflect larger particles with diameter d1 from the hydrodynamically focused sample fluid that carries other particles as well with diameters d2 and d3, such that d1 > d2 > d3. The deflected particles (d1) are pushed into the top-left corner of the microchannel. Downstream, the TSAWs with frequency fL, such that fL > fR, disseminating from the left SIDT, deflect the medium-sized particles (d2) rightward, leaving behind the larger particles (d1) unaffected in the top-left anechoic corner and the smaller particles (d3) in the middle of the microchannel, thereby achieving particle separation. A particle not present in the anechoic corner could be deflected rightward to realize twice the medium exchange. In this work, the three-way separation of polystyrene particles with diameters of 3, 4.2, and 5 μm and 3, 5, and 7 μm is achieved using two separate devices. Moreover, these devices are used to demonstrate multimedium exchange around polystyrene particles ∼5 μm and 7 μm in diameter.

Adjustable, rapidly switching microfluidic gradient generation using focused travelling surface acoustic waves

We demonstrate a simple device to generate chemical concentration gradients in a microfluidic channel using focused travelling surface acoustic waves (F-TSAW). A pair of curved interdigitated metal electrodes deposited on the surface of a piezoelectric (LiNbO3) substrate disseminate high frequency sound waves when actuated by an alternating current source. The F-TSAW produces chaotic acoustic streaming flow upon its interaction with the fluid inside a microfluidic channel, which mixes confluent streams of chemicals in a controlled fashion for an adjustable and rapidly switching gradient generation.

Acoustothermal heating of polydimethylsiloxane microfluidic system

We report an observation of rapid (exceeding 2,000 K/s) heating of polydimethylsiloxane (PDMS), one of the most popular microchannel materials, under cyclic loadings at high (~MHz) frequencies. A microheater was developed based on the finding. The heating mechanism utilized vibration damping in PDMS induced by sound waves that were generated and precisely controlled using a conventional surface acoustic wave (SAW) microfluidic system. The refraction of SAW into the PDMS microchip, called the leaky SAW, takes a form of bulk wave and rapidly heats the microchannels in a volumetric manner. The penetration depths were measured to range from 210 μm to 1290 μm, enough to cover most sizes of microchannels. The energy conversion efficiency was SAW frequency-dependent and measured to be the highest at around 30 MHz. Independent actuation of each interdigital transducer (IDT) enabled independent manipulation of SAWs, permitting spatiotemporal control of temperature on the microchip. All the advantages of this microheater facilitated a two-step continuous flow polymerase chain reaction (CFPCR) to achieve the billion-fold amplification of a 134 bp DNA amplicon in less than 3 min.

Generation of Dynamic Free-Form Temperature Gradients in a Disposable Microchip

Temperature gradients (TGs) provide an effective approach to controlling solvated molecules and creating spatiotemporally varying thermal stimuli for biochemical research. Methods developed to date for generating TGs can only create a limited set of static temperature profiles. This article describes a method for establishing dynamic free-form TGs in polydimethylsiloxane (PDMS) as well as in gases and liquids in contact with the PDMS. The heating mechanism relies on the efficient acoustic absorption by the PDMS of high-frequency (5-200 MHz) surface acoustic waves (SAWs). MATLAB-aided actuation of a transducer enabled the generation and propagation of SAWs in a controlled fashion, which permitted spatiotemporal control over the temperature in the PDMS microstructures. This technique is exploited to perform one-shot high-resolution melting (HRM) analysis to detect single nucleotide polymorphisms (SNPs) in DNA. The experimental results displayed a 10-fold higher resolution and an enhanced signal-to-noise ratio compared to the results obtained using a conventional real-time PCR machine.

Spatiotemporally controllable acoustothermal heating and its application to disposable thermochromic displays

Polydimethylsiloxane, the most common microchannel material, effectively absorbs acoustic waves and converts the acoustic energy into thermal energy. Here, we quantitatively characterize this phenomenon and develop a heating platform that offers spatiotemporal temperature control in two dimensions. We demonstrate the use of this heating platform to innovate thermochromic displays (TCDs). A TCD comprises a display layer covered with a thermochromic substance and a heater attached to the bottom of the display layer. The thermochromic substance is opaque at room temperature and becomes translucent when heated beyond a transition temperature. Consequently, the TCDs deliver visual information concealed beneath the thermochromic substance by controlling the temperature of the display layer. The previously reported TCDs have limitations including restricted flexibility in display information and cumbersome spatiotemporal temperature control. We address these limitations by developing a disposable TCD system using our spatiotemporally controllable heating platform. The utility of the proposed system is demonstrated in shutter-type TCDs for coloured, intricate picture displays, as well as in segment-type TCDs for on-demand information displays of alphanumeric characters. Finally, we propose a new type of TCD that can represent colour gradients based on the ability of the heating technique to generate free-form, continuous temperature gradients.

Photosynthesis of cyanobacteria in a miniaturized optofluidic waveguide platform

We investigated the effect of increasing the optical penetration length, inside polydimethylsiloxane (PDMS)-based photobioreactors (PBRs), upon the photosynthetic cell growth of cyanobacteria. A thin layer of Teflon amorphous fluoropolymer (Teflon AF) was applied inside the PDMS-based PBRs to prevent light loss at the solid–liquid interface. The Teflon AF layer, with a refractive index (nTeflon = 1.31) lower than PDMS (nPDMS = 1.442) and higher than the culture medium (nmedium = 1.332), constructed the light waveguide in the PBRs via the total internal reflection. Such a combination of refractive indices led to the prevention of light loss at the interface. The cell growth rate and the optical cell density were measured periodically for 5 days under different light power and Teflon AF-coating conditions. The local or global auto-fluorescence signal and the optical density at 450 nm wavelength (OD450) were measured in parallel by a fluorescence microscope and a micro plate reader, respectively. The optofluidic waveguide-based PBR improved the photosynthetic cell growth up to ∼9% compared to a regular PBR.

Refractive-index-based optofluidic particle manipulation

This letter describes optofluidic particle manipulation based on the refractive index contrast between the particle and the surrounding medium. A laser beam propagated along one sidewall of a microfluidic channel will introduce a force that pushes a high-refractive-index particle toward the Gaussian-shaped laser beam center axis. By contrast, a low-refractive-index particle will be pushed away from the beam center axis and toward the other sidewall of the channel because the direction of the gradient forces acting on such a particle is opposite the direction of the forces acting on a high-refractive-index particle. The gradient forces acting on a particle were calculated to predict and interpret the particle behavior. High-refractive-index and low-refractive-index particles, prepared from polystyrene latex (PSL) and hollow glass particles with refractive indices of 1.59 and 1.22, respectively, were employed. The PSL and hollow glass particles could be separated based on their refractive indices. Doubly attached identical particles behaved as a single particle.

Optofluidic debubbling via a negative optical gradient force

This Letter describes the generation and removal of air bubbles from a fluid using an optofluidic platform. A T-junction geometry was used to generate air bubbles, and a negative optical gradient force subsequently removed the generated bubbles from the main stream. A numerical analysis was performed to predict and interpret the system performance. The optical gradient force was calculated using geometric optics models. A modified viscous drag force was applied when the bubble size was comparable to the channels geometric dimensions. The Dulbeccos phosphate buffered saline and air flow rates were adjusted to control the air bubble size and bubble generation frequency. Despite displaying a substantial increase in the viscous drag force as the bubble size approached the channel dimensions, the bubbles could be readily removed from the main fluid stream under appropriate optofluidic circumstances.

Dynamic manipulation of particles via transformative optofluidic waveguides

Optofluidics is one of the most remarkable areas in the field of microfluidic research. Particle manipulation with optofluidic platforms has become central to optical chromatography, biotechnology, and μ-total analysis systems. Optical manipulation of particles depends on their sizes and refractive indices (n), which occasionally leads to undesirable separation consequences when their optical mobilities are identical. Here, we demonstrate rapid and dynamic particle manipulation according to n, regardless of size. Integrated liquid-core/solid-cladding (LS) and liquid-core/liquid-cladding (L2) waveguides were fabricated and their characteristics were experimentally and theoretically determined. The high and low n particles showed the opposite behaviors by controlling the contrast of their n values to those of the working fluids. The LS waveguide was found to successfully manipulate particles according to n, and the L2 waveguide was found to provide additional system stability and flexibility, compared to the LS system.

Optical Manipulation of Droplets in a Microfluidic Platform

Abstract. In the present study, the optofluidic droplet manipulation in a microfluidic platform was demonstrated via theoretical and experimental approaches. Optical scattering force and gradient force were used to separate and trap droplets. Two types of droplets were generated by a T-junction method in the microfluidic channel. While they approach a test region where the optical beam illuminates the droplets, they were pushed by the optical scattering beam. The displacement by the laser beam is dependent on the refractive index of the droplets. By using the optical gradient force, the droplets can be trapped and coalesced. In order to bring the droplets in a direct contact, the optical gradient force was used to trap the droplets. A theoretical modeling of the coalescence was derived by combining the optical force and drag force on the droplet. Key Words: Droplet (액적), optofluidics (광유체역학), two-phase flow (다상유동), optical tweezer (광집게) 1. 서 론 2) 최근 마이크로 채널 내에서 이루어지는 여러 제어 기법에 의한 실험들을 통하여 면역학, 약물 전달, 암 조기 진단 등의 여러 분야로의 확장이 이루어지고 있다. 특히 이러한 마이크로 채널에서의 제어 방법 중 액적을 이용한 미세 유체적 기술이 주목 받고 있는데, 그 이유는 액적을 물질 대사의 기본 단위로 사용하여 여러 화학 물질을 합성 또는 검증하는 용도로 사용하는데 많은 이점을 가지고 있기 때문이다.[1] 액적이란 상이 다른 두 유체가 섞이지 않는 상태로 혼재하는 것을 말한다. 이러한 혼합 되지 않는 성질을 이용하여 실험 및 관찰의 대상이 되는 물질을 pico liter 단위로 분리하여 세밀하게 관찰할 수 있으며, 여러 화학 종의 오염 방지 등 미세 유체에서의 실험에 사용하기에 용이한 장점들을 가지고 있다[2]. 따라서 액적 기반의 미세 유체 플랫폼을 제작하여 기존의 생 화학 실험을 대체하여 증대된 효율을 가지도록 하는 연구가 많이 진행되고 있다.[3] 그러나 액적을 이용한 실험이 복잡한 공정을 거치는 기존의 실험을 대체하기 위해서는 액적을 조작하여 원하는 곳으로 보내거나 합쳐지게 하는 연구가 필연적이다. 이를 위하여, 미세 구조물[4], 전극[5], 음향 진동[6], 극부적인 열 전달[7] 등을 이용한 방법들이 제시 되었다. 그러나, 이러한 연구들은 수동적으로 쓸 수 밖에 없거나, 미세유체 칩 공정 비용이 비싼 등 한계점을 가지고 있다. 따라서 본 연구에서는 샘플을 오염시키지 않으며, 미세유체 칩 공정 가격을 낮추면서 해당 물질들을 제어할 수 있는 장점을 가진 광 집게 (optical tweezer)를 이용하여 액적을 제어하는 기법을 구현하였다. 광력은 Ashkin 의 연구를 통하여 dielectric 물질들을 제어할 수 있다는 사실이 알려졌으며[8], 샘플을 오염시키지 않으면서 마이크로 단위의 물체들을 옮길 수 있다는 점에서 여러 연구에 적용되어 왔다. 이러한 광력은 굴절률과 해당 물체의 표면에 따라 많은 영향을 받는다. 높은 굴절률의 물체가 낮은 굴절률을 가지는 주변 유체에 존재할 경우, 레이저 빔의 입사 방향으로 , 축 방향으로는 물체를 끌어당기는 광구배력(gradient force)을, 레이저 빔 방향으로는 물체를 밀어내는 광산란력(scattering force)을 받게 된다.본 연구에서는 광 구배력과 산란력을 이용하여 , 액적을 분리하거나 포획할 수 있는 마이크로 장치를 구현하였다 . 먼저, 서로 다른 두 액적을 표면 지표 없이 광력만으로 분리하기 위해서 미세 채널 내부에서 T-junction 기법을 이† Corresponding Author. KAIST, 291 Daehak-ro, Yuseong-gu, Daeje on 305-701, Korea, hjsung@kaist.ac.kr, Tel: 042-350-3027