Particle manipulation using standing acoustic waves in the microchannel at dual-frequency excitation: Effect of power ratio
Introduction
Microparticle manipulation, accumulation, and separation play a critical role in the biological analysis and clinical diagnosis, such as the detection of circulating tumor cells in the micro-total analysis system (μTAS) [1], [2]. Till now many techniques have been developed using different mechanisms, such as electroosmosis [3], dielectrophoresis (DEP) [4], magnetophoresis [5], thermophoresis [6], [7], optical tweezers [8], optoelectronic tweezer [9], hydrodynamic manipulation [10], and acoustophoresis [11], [12], [13]. Among them, acoustic methods are appropriate for on-chip manipulation or patterning of microparticles/cells as they can manipulate theoretically many types of microparticles if their contrast is high enough with respect to the surrounding medium at the comparable size of the acoustic wavelength. Compared to their optical, electrical, and magnetic counterparts, acoustic approaches are inherently non-invasive to biological objects and work for most microparticles regardless of their optical, electrical, and magnetic properties [14]. Currently, there are several types of acoustic methods, using either bulk acoustic wave (BAW) or surface acoustic wave (SAW). Although standing bulk acoustic waves (SBAWs) method has shown promising results [13], [15], [16], their non-planar structures and complex fabrication processes make it challenging to integrate with other planar lab-on-a-chip components. The microfluidic channel used in SBAWs is usually made of silicon and glass. The generation of SBAW highly depends on complicated resonator structure and a stable temperature, and the resonant frequency is on the order of MHz. SAWs are generated by applying radio-frequency (RF) signals to the interdigital transducer (IDT) deposited on the piezoelectric substrate if its thickness is at least 1.5 times larger than the wavelength (h/λ > 1.5), propagate mostly on the surface of the substrate, and then leak into the fluid at the Rayleigh angle determined by the Snell’s law [17], [18], [19], [20], [21], [22]. A poly-dimethylsiloxane (PDMS) microchannel fabricated by the existing fast-prototyping methods (i.e., soft lithography) was aligned and bonded between a pair of parallel IDTs. SAWs are usually at high excitation frequency (i.e., > 10 MHz), and its use results in finer resolution in terms of particle manipulation compared to BAWs. When the thickness of the substrate is smaller (h/λ < 1.5), the plate waves (i.e., Lamb waves) dominate the vibration through the entire substrate [23], [24]. Two counter-propagating acoustic waves interfere with each other to establish a pseudo-standing acoustic wave field within the fluid domain in the microchannel, where the suspended particles tend to move towards nodal points by an acoustic radiation force acting on them due to the differences in density and compressibility of the cells and microparticles with respect to the surrounding fluid [25]. The strength of the acoustic force is dependent on the driving frequency, particle size, and its acoustic contrast factor [26].
The acoustic tweezers technique shares the same limitation as many other on-chip patterning ones: the manipulated pattern features cannot be modified easily. Once the IDT is fabricated with a certain period, its driving frequency can be adjusted only within the small bandwidth (or high resonant Q factor). So IDT usually operates in a stationary manner. In order to achieve more flexible manipulation and enhance the focusing and separation efficiency, different strategies have been developed to change the acoustic fields in the microchannel. In an acoustic fractionation method combined with the split-flow lateral-transport thin (SPLITT), two acoustic standing waves, working at the first and second modes, were used in different parts of the microchannel to separate particles in different sizes based on the various rates at which the particles move to the nodal lines [27]. The first and second modes of the acoustic chamber could be switched alternatively in a continuous flow for the separation of microparticles [28], [29]. Three resonant modes of the microchannel were also used for separation in a continuous flow, the fundamental mode for pre-concentration of all cells at the center of the microchannel and subsequently a switching acoustic field between the second and third resonant modes to separate cells of different sizes or contrast factors [30]. Because the piezoceramics in SBAWs is usually working in the thickness mode, the first and third resonant modes of the microchannel can be produced with high efficiency. Using a relay controlled by a rectangular control voltage, these two modes could be switched to separate the suspended constituents onto the side and center pressure nodal lines by tuning the duty cycle of the switching and was found to be robust with respect to different particle position and time offsets from the switching cycle [31]. However, because of significant differences in the electrical impedances at the first and third thickness mode of piezoceramics [32], three function generator, two impedance matching units, and an electromagnetic relay were used for the signal generation, which increases the cost and control complexity of experimental setup. In order to achieve tunable cells and microparticles patterning by varying standing SAW field, slanted-finger interdigital transducers (SFITs) were applied [20], [33]. SAWs are generated from the sub-channels where the period of the slanted fingers satisfies the resonance condition, and the bandwidth of the excited SAW is inversely proportional to the number of slanted fingers. The SFITs allow dynamic control of the specific position of the pressure node.
In this study, a new excitation strategy in the acoustic tweezer technique, dual-frequency excitation with the fundamental frequency and third harmonic of IDTs simultaneously, was proposed and evaluated both numerically and experimentally. Because of the similar acoustic wave generation efficiencies at these two modes, excitation signals could be forwarded to IDT through the power amplifier directly without individual impedance matching units. The amplitude and distribution of resultant acoustic radiation force applied to the microparticles are dependent on the power ratio between fundamental and third harmonic components. As a result, the particle accumulation in the microchannel could be tuned. Such tunability may be valuable in many on-chip cell studies (i.e., isolating the circulating tumor cells), such as shortening the accumulation time to the position of the pressure node and changing the position of the pressure node and the percentage of particle accumulation.
Section snippets
stokes drag and acoustical radiation force
For fluid with low Reynolds number (i.e., Re = 0.55) and low Mach number, the motion of incompressible laminar flow is described by [34]where is the fluid velocity, μ is the dynamic viscosity, ρ0 is the fluid density, p is the pressure on the fluid, I is the identity matrix, is an external force vector. Because of different velocities between fluid and particle (vfluid and vparticle), the Stoke drag force produced on the object in the fluid is
Fabrication of microchannel and IDTs
The PDMS microfluidic channel was fabricated using the soft-lithography and mould-replica techniques. PDMS (Sylgard 184, Dow Corning, Midland, MI, USA) was mixed with elastomer base (Sylgard 184, Dow Corning) in a ratio of 10:1. The length, width, and height of microchannel are 2 cm, 300 μm, and 50 μm, respectively. The mixture was degassed in a vacuum oven (3608-1CE, Thermo Scientific, Waltham, MA, USA) and poured on the silicon wafer (SI8PSPD, Bonda Technology, Singapore) with negative tone
IDT and PDMS microchannel
The alignment of a PDMS microchannel with a pair of IDTs is shown in Fig. 2 a. The IDT fingers were captured by the microscope and analyzed by ImageJ, and their width was determined to be 155.1 ± 0.65 μm, which is slightly larger than the design. Using a network analyzer (HP8510B, Agilent Technologies, Santa Clara, CA, USA), the S12 frequency response of IDTs (transmission coefficient) shows several peaks in Fig. 2b, which corresponds to the different resonant modes. The values of 6.1 MHz and 17.8
Conclusions
In conclusion, we have investigated the particle motion in the microchannel by standing acoustic waves at dual-frequency excitation and developed an effectively tunable patterning technique by varying the power ratio of fundamental frequency to the total acoustic power applied to a pair of IDTs. The number and position of pressure nodes for microparticle accumulation and the percentages of microparticles at each pressure node can be adjusted dynamically without changing on-chip or off-chip
Acknowledgements
This study was financially supported by the Academic Research Fund (AcRF) Tier 1 (RG171/15), Ministry of Education, Singapore.
Yannapol Sriphutkiat received his Bachelor’s Degree in Industrial Engineering from Chulalongkorn University, Thailand in 2014. He is currently pursuing his PhD degree at Nanyang Technological University, Singapore.H is research interests focuses on the effect of ultrasound on the reduction of particle deposition/accumulation and clogging.
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Yannapol Sriphutkiat received his Bachelor’s Degree in Industrial Engineering from Chulalongkorn University, Thailand in 2014. He is currently pursuing his PhD degree at Nanyang Technological University, Singapore.H is research interests focuses on the effect of ultrasound on the reduction of particle deposition/accumulation and clogging.
Yufeng Zhou received the B.S. and M.S. degrees from the Department of Electrical Science and Engineering, National Key Laboratory of Modern Acoustics, Nanjing University, China in 1996 and 1999, respectively, and the Ph.D. degree in Bioacoustics from Duke University, USA, in 2003. He received postdoctoral training from both Duke University and the Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington. Dr. Zhou joined the School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, as an Assistant Professor in 2010. His research interests are mostly focused on the biomedical ultrasound, including the manipulation of microparticles by surface acoustic waves, high-intensity focused ultrasound (HIFU) for solid tumor ablation, extracorporeal shock wave lithotripsy (ESWL), sonothrombolysis, ultrasound-mediated drug delivery, bubble cavitation and its interaction with an acoustic burst for tissue fragmentation.