Microparticle Manipulation by Standing Surface Acoustic Waves with Dual-frequency Excitations

Summary

A protocol for manipulating the microparticles in a microfluidic channel with a dual-frequency excitation is presented.

Abstract

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip (LOC) system. The simultaneous excitation of the fundamental frequency and its third harmonic, which is termed as dual-frequency excitation, to a pair of interdigital transducers (IDTs) could generate a new type of standing acoustic waves in a microfluidic channel. Varying the power and the phase in the dual-frequency excitation signals results in a reconfigurable field of the acoustic radiation force applied to the microparticles across the microchannel (e.g., the number and location of the pressure nodes and the microparticle concentrations at the corresponding pressure nodes). This article demonstrates that the motion time of the microparticle to only one pressure node can be reduced ~2-fold at the power ratio of the fundamental frequency greater than ~90%. In contrast, there are three pressure nodes in the microchannel if less than this threshold. Furthermore, adjusting the initial phase between the fundamental frequency and the third harmonic results in different motion rates of the three SSAW pressure nodes, as well as in the percentage of microparticles at each pressure node in the microchannel. There is a good agreement between the experimental observation and the numerical predictions. This novel excitation method can easily and non-invasively integrate into the LOC system, with a wide tenability and only a few changes to the experimental set-up.

Introduction

LOC technology integrates one or several functions on a microchip for biology, chemistry, biophysics, and biomedical processes. LOC allows a laboratory set-up on a scale smaller than sub-millimeters, fast reaction rates, a short response time, a high process control, a low volume consumption (less waste, lower reagents cost, and less required sample volume), a high throughput due to parallelization, a low cost in the future mass production and cost-effective disposables, a high safety for chemical, radioactive, or biological studies, and the advantages of a compact and portable device^1,2. Precise cell manipula....

Protocol

1. Preparation of the Microfluidic Channel

1. Mix poly-dimethylsiloxane (PDMS) with an elastomer base in a ratio of 10:1.
2. Degas the mixture in a vacuum oven and pour it on a silicon wafer with a negative tone photoresist pattern on the top.
3. Degas the patterned silicon wafer again and heat it at 70 °C for 3 h in an incubator for solidification.

2. Fabrication of the Interdigital Transducers

1. Deposit 20 nm of Cr and 400 nm of Al on a LiNbO₃ wafer; pattern 20 strips with a width of 150 µm and an aperture of 2 cm on a plastic mask for photolithography by depositing the positive photoresi....

Results

The distributions of the acoustic pressure and the acoustic radiation force of an SSAW at the dual-frequency excitation (6.2 and 18.6 MHz) are shown in Figure 1. Here, the dual-frequency excitation occurs on polystyrene microparticles (4 µm in diameter) in a microchannel with a width of 300 µm at an acoustic power of 146 mW. The resultant acoustic pressure is always in phase when P₁ > 90% so that only one pressure node is present at

Discussion

The microparticle motion in the microchannel by an SSAW at the dual-frequency excitation was extensively investigated in this study, and an effectively tunable patterning technique by varying the dual-frequency excitation signals was developed and tested. The production of such a waveform is easily realized by most function generators, and the adjusting approach is very convenient. Both the S₁₂- and S₁₁-frequency responses of the fabricated IDTs illustrate several resonant modes^34<.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
poly-dimethylsiloxane	Dow Corning	Sylgard 184
poly-dimethylsiloxane elastomer base	Dow Corning	Sylgard 184
silicon wafer	Bonda Technology	SI8PSPD
negative tone photoresist	Microchem	SU-8
double-side polished LiNbO3 wafer	University Wafer	Y-128°
positive photoresist	Nicolaus-Otto-Straße	AZ 9260
oxygen plasma	Harrick Plasma
plastic mask	Infinite Graphics