1898 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 62, NO. 9, SEPTEMBER 2014 RF-Activated Standing Surface Acoustic Wave for On-Chip Particle Manipulation Jinhong Guo, Member, IEEE, Joshua L. W. Li, Fellow, IEEE, Yu Chen, Leslie Y. Yeo, James R. Friend, and Yuejun Kang Abstract—On-chip flow cytometry provides a powerful tool to characterize cell samples for point-of-care diagnosis. In particular, sample focusing at specific locations along the microchannel is crucial to ensure the accuracy of detection. In this paper, we present a simple strategy of interfacing an RF-activated standing surface acoustic wave (SSAW) substrate with a microfluidic channel, and use this device to study the dynamic process of particle aggregation along the microchannel. Specifically, the SSAW generated by two parallel interdigital transducers induces an acoustic radiation force that propels particles suspended in the liquid toward the pressure nodes whose locations are tunable by judicious choice of the applied SSAW frequency. We also carry out a theoretical analysis that provides an estimation of the time for the particle assembly, which is validated by experimental results. This SSAW transducer can therefore be easily integrated into a microfluidic chip with moderate energy consumption, offering a convenient and effective solution in the development of on-chip flow cytometry. Index Terms—Acoustic, acoustic wave components, RF/mi- crowaves, surface acoustic wave (SAW) measurement, SAW devices. I. INTRODUCTION T HE DETECTION and characterization of biological particles, such as cells and biomolecules, is a funda- mental technique in biology and medical biotechnology. Since Manuscript received March 21, 2014; revised May 21, 2014 and July 16, 2014; accepted July 18, 2014. Date of publication August 01, 2014; date of current version September 02, 2014. The work of Y. Kang was supported by the Ministry of Education of Singapore (RG 26/11) under a Tier-1 Academic Research Fund. This paper is an expanded version from the IEEE MTT-S In- ternational Microwave Workshop Series on RF and Wireless Technologies for Biomedical and Healthcare Applications, Singapore, Dec. 9–11, 2013. J. Guo and Y. Kang are with the School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459 (e-mail: jguo002@e.ntu.e du.sg; yuejun.kang@ntu.edu.sg). J. L. W. Li is with the Institute of Electromagnetics, University of Electronic Science and Technology of China, Sichuan 611731, China (e-mail: lwli@ieee. org). Y. Chen is with the A*STAR Institute of Microelectronics, Singapore 117685 (e-mail: cheny1@ime.a-star.edu.sg). L. Y. Yeo and J. R. Friend are with the School of Civil, Environmental and Chemical Engineering and the School of Electrical and Computer Engineering, RMIT University, Melbourne, Vic. 3000, Australia (e-mail: leslie.yeo@rmit. edu.au; james.friend@rmit.edu.au). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. This paper has supplementary downloadable multimedia material available at http://ieeexplore.ieee.org provided by the authors. This includes a video of standing surface acoustic waves in a microfluidic channel measured by a micro- scanning Doppler vibrometer. This video is 0.5 MB in size. Windows Media Player or QuickTime required for viewing. Digital Object Identifier 10.1109/TMTT.2014.2342667 bioparticles usually exist in fluidic environment, microflu- idics-based lab-on-a-chip devices provide excellent platforms for various biomedical manipulations and assays. The amaz- ingly fast development of lab-on-a-chip technology in recent decades has had a profound impact on the food and healthcare industries [1]–[3], as well as novel applications in bio-defense against bioterrorism and bio-warfare [4]. Many experimental techniques for microparticle manipulation have been exten- sively reported in prior studies, such as using biochemical [5], electrokinetic [6], optical [7], and magnetic [8] methods. Compared to these conventional techniques, another popular method that utilizes acoustics to drive microfluidic actuation has shown distinct advantage as an easy tool for manipulation of colloidal particles [9]–[11]. Using various types of ultrasonic transducers, acoustic energy can be easily transmitted into colloidal particle suspensions in confined micro-geometry. The acoustic radiation force due to the standing wave in the carrier medium drives the particles to the local pressure nodes. This unique phenomenon can be applied for particle concentration, positioning, and fractionation. Compared with other popular techniques, acoustic particle manipulation does not require fluorescence or magnetic labels; avoids direct coupling of the electric field into the fluid, therefore circumventing undesirable electrochemical reaction and joule heating effects; and does not affect the bioelectricity, and thus causes less stress on the biological cell membrane. Consequently, acoustic methods have higher biocompatibility for biomedical applications. A typical and interesting application is to use acoustic focusing of biological cells into a thin stream for sample preparation and for micro flow cytometry [12]. Most previous devices that employ acoustic fields to manip- ulate microparticles create bulk standing waves through piezo- electric transducers. Recently, surface acoustic wave (SAW) de- vices have become more popular because of their design flex- ibility, ability for further downscaling, and on-chip integration through the use of interdigitated transducers (IDTs) [13]. Subtle positioning in 1-D or 2-D arrays with finer resolution down to the size of a single cell can be achieved by controlling the ex- citation frequency and configuration of the IDTs [14]–[19]. A sinusoidal pressure wave in the suspending medium is gener- ated from the fluid–substrate interaction. One of the major chal- lenges when interfacing the SAW substrate and the microfluidic chip is how to transmit the acoustic energy in desired locations inside the microchannel efficiently. However, most polymeric materials commonly used for microfabrication, such as polydimethylsiloxane (PDMS) due to their low cost and ease of rapid prototyping, are unfortunately 0018-9480 © 2014 IEEE. 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