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


- Objective

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.

- Results

(a) Schematic diagram of the F-TSAW-based microfluidic chemical concentration gradient generator. The stimulant and buffer solutions were injected through separate inlets in the PDMS microfluidic channel. The F-TSAW originated from the interdigitated Au electrodes and propagated along the positive x-axis direction. As the F-TSAW interacted with the fluid, the induced acoustic streaming flow formed symmetrical vortices that controlled the mixing of the stimulant and buffer solutions. Tunable gradient profiles were obtained downstream. (b) PIV measurements using 1 lm polymer particles to trace the flow: (i) symmetrical vortices formation in a stationary fluid when subjected to 14.7 VRMS F-TSAW; (ii)-(iv) the interaction of vortices with the uniform flow (500 µL/h ) inside the channel input voltages of 9.8, 14.7, and 19.6 VRMS.

Characterization of the chemical concentration gradient profiles. (a) On the extreme left, an input voltage of 0 VRMS produced separate laminar flows of the stimulant (green) and buffer (black) solutions, with only diffusion-limited mixing. A voltage of 14.7 VRMS produced ASF via F-TSAW, which perturbed the laminar flow and induced moderate mixing of the stimulant and buffer solutions. A higher input voltage in the range 14.7–17.9 VRMS produced an adjustable gradient profile, as measured at the dotted lines, shown at positions 1–6 downstream. (b) The normalized concentrations of the stimulant solution measured at positions 1–6 in (a), given each of the input voltages, are plotted against the microfluidic channel width along the x-axis direction. The net flow rate was maintained at 1100 µL/h (stimulant 100 µL/h and buffer 1000 µL/h). (c) For a flow rate of 1100 µL/h and input voltage of 14.7 VRMS, the normalized concentration of stimulant is measured at ROI shown in (a). The inset corresponds to a lower flow rate of 550 µL/h (stimulant 50 µL/h and buffer 500 µL/h) and input voltage of 12.2 VRMS.

Temporal control over the gradient profile is demonstrated with a fast switching frequency of 0.25 Hz and a flow rate of 1100 µL/h and 550 µL/h. (a) A full cycle, with ON and OFF intervals of 2 s, respectively, is shown. The input voltage during the ON state was set to 17.9 VRMS. The dashed lines indicate the position of PDMS microchannel walls. (b) The normalized concentration measure at ROI in (a) is plotted against time for input voltages of 16.3 VRMS and 17.9 VRMS, while the flow rate was 1100 µL/h. (c) A constant voltage of 17.9 VRMS switching ON/OFF every second and flow rate 1100 µL/h yielded the plotted chemical concentrations as a function of time. The inset corresponds to voltage of 12.2 VRMS switching ON/OFF every 2 s and flow rate 550 µL/h.

- Device Fabrication

The fabrication of the focused travelling surface acoustic waves (F-TSAW) based device is carried out in two stages as shown in Fig. S1. A PDMS (polydimethylsiloxane) microfluidic channel is fabricated using soft lithography and mold replica process, while metal electrodes are being deposited on a piezoelectric substrate (LiNbO3). Oxygen plasma bonding is used to permanently bond the PDMS microchannel with the LiNbO3 substrate.

The fabrication process for the F-TSAW based gradient generator.

- Design in the CAGG

The cross-type acoustic gradient generator (CAGG) has a pair of circular electrodes known as focusing interdigitated transducer (F-IDT) which is designed to focus the acoustic energy in a narrow beam. The F-TSAW is very effective in generating acoustic streaming flow (ASF) inside the microchannel which is used to mix the fluids. Fig. S2 (a) delineates important dimensions of the CAGG whereas Fig. S2 (b) shows a close-up view of the interdigitated electrodes. The design of the F-IDT is based on well-known single phase unidirectional transducer (SPUDT) design as shown in the inset in Fig. S2 (b). The width of the electrodes and the spacing between them are carefully adjusted to propagate the maximum acoustic energy in the forward direction.

(a) The design and dimensions of the CAGG (b) Close-up of view of the electrodes with the inset showing the SPUDT design.

Acoustic streaming flow

Polymer particles of diameter 1 m suspended in DI water are used to trace acoustic streaming flow. The flow rate is 0 L/h and 500 L/h while the input voltage is 17.9 VRMS and 19.6 VRMS, respectively.

Fast switching gradient generation

The stimulant and buffer solutions are infused into the microfluidic channel at flow rates of 100 L/h and 1000 L/h, respectively. The input voltage of 17.9 VRMS is turned ON and OFF every 2 seconds interval.

Acoustothermal heating of polydimethylsiloxane microfluidic system

Summary We report an observation of rapid (exceeding 2,000 Ks-1) 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 m to 1290 m, 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.


Left, Infrared movie showing the heating and cooling of the 200 m-thick PDMS layer by SAWs from two focused IDTs. Right, Infrared movie showing the heating of the 200 m-thick PDMS layer mounted on the slanted finger IDT. An AC RF signal time-shared by five frequencies (18, 20, 22.5, 25.5, and 30 MHz) is given to heat five locations in the PDMS.

Submicron Separation of Microspheres via Travelling Surface Acoustic Waves

- Abstract

The separation of targeted malignant or diseased cells from a whole blood sample is critical to numerous biochemical studies. A variety of the miniaturized separation devices use polystyrene (PS) microspheres for their characterization. It is important to study the continuous label-free separation of PS particles based on their acoustofluidic parameters such as size, density, compressibility and shape. Submicron separation is the segregation of the particles having diameter difference of less than one micrometre. The submicron separation of PS microspheres is achieved inside a polydimethylsiloxane (PDMS) microfluidic channel via travelling surface acoustic waves (TSAW). The TSAW of different frequencies, 200, 192, 155, and 129 MHz, realized continuous separation of particles with diameter 3.0, 3.0, 3.4 and 4.2 µm from those of diameters 3.2, 3.4, 4.2 and 5.0 µm, respectively. The separation of 3.4, 4.2, and 4.5 µm PS particles from 4.2, 4.5, and 5.0 µm particles, respectively, is also demonstrated using a 129 MHz TSAW with power inputs of 104.3, 34.1, and 13.3 mW, respectively. Furthermore, the 155 MHz TSAW continuously separated 3.2 µm fluorescent particles from 4.2 µm particles at a flow rate as high as 1,250 µL/h. This study could be useful for isolation of cells that are insignificantly different in size than others like diabetic fat cells from healthy fat cells.