Flexural wave-based soft attractor walls for trapping microparticles and cells

Abstract

Acoustic manipulation of microparticles and cells, called acoustophoresis, inside microfluidic systems has significant potential in biomedical applications. In particular, using acoustic radiation force to push microscopic objects toward the wall surfaces has an important role in enhancing immunoassays, particle sensors, and recently microrobotics. In this paper, we report a flexural-wave based acoustofluidic system for trapping micron-sized particles and cells at the soft wall boundaries. By exciting a standard microscope glass slide (1 mm thick) at its resonance frequencies <200 kHz, we show the wall-trapping action in sub-millimeter-size rectangular and circular cross-sectional channels. For such low-frequency excitation, the acoustic wavelength can range from 10-150 times the microchannel width, enabling a wide design space for choosing the channel width and position on the substrate. Using the system-level acousto-structural simulations, we confirm the acoustophoretic motion of particles near the walls, which is governed by the competing acoustic radiation and streaming forces. Finally, we investigate the performance of the wall-trapping acoustofluidic setup in attracting the motile cells, such as Chlamydomonas reinhardtii microalgae, toward the soft boundaries. Furthermore, the rotation of microalgae at the sidewalls and trap-escape events under pulsed ultrasound are demonstrated. The flexural-wave driven acoustofluidic system described here provides a biocompatible, versatile, and label-free approach to attract particles and cells toward the soft walls.

Fig. 1. Working principles of the flexural-wave driven soft attractor walls.

(a) Schematics of the experimental setup (b) wall trapping of 10 μm diameter polystyrene (PS) particles under ultrasound during 6.8 seconds (c) wall trapping of microalgae inside the rectangular channel under ultrasound during 0.57 seconds (d) admittance-phase plots of the system round 109.6 kHz resonance frequency corresponding to cosine-type displacement of the channel (e) numerical simulations of the complete setup at calculated resonance frequency of 103 kHz, the blue-red color bar indicates the transverse displacement in μm scale with a close-up in dashed green inset, and the dashed-blue inset shows the acoustic radiation force in pN range inside the rectangular channel, where the pink arrows show the direction of force (f) admittance-phase plots of the system round 154 kHz resonance frequency corresponding to sine-type displacement of the channel (g) numerical simulations of the complete setup at calculated resonance frequency of 103 kHz, the blue-red color bar indicates the transverse displacement in μm scale a close-up in dashed green inset, and the dashed-blue inset shows the acoustic radiation force in pN range inside the rectangular channel, where the pink arrows show the direction of force.

Fig. 2. Wall trapping of 10 μm PS particles in different rectangular microchannels.

(a) Time-lapse images of particles under ultrasound. The narrower 100 μm and 200 μm width channels show excellent wall trapping, whereas the wider 500 μm and 750 μm width channels have poor wall-trapping performance due to the sine-type displacement actuation. (b) The particle tracing simulations of the microchannel cross-section from dispersed to ultrasonically manipulated condition over 15 seconds, confirming the final location of the beads in the experiments (Movie S1†). The blue and red colors indicated the minimum and maximum speed of the particles, respectively. The scale bars are 50 μm. The operating frequencies are 105 kHz for 100 μm and 200 μm width channels and 155 kHz for 500 μm and 750 μm width channels.

Fig. 3. Analysis of wall trapping strength for different particle diameters in the rectangular cross-sectional channels.

(a) The acoustic radiation force shows a significant increase for the increasing particle diameters. The purple arrows indicate the direction of radiation forces. (b) The particle tracing simulations over 25 seconds by accounting both acoustic radiation and streaming forces; the strong wall trapping begins for particles approximately larger than 10 μm diameter, whereas the smaller ones follow the streaming-induce vortex. The channel width is set to 200 μm, where the operating frequency is 105 kHz. The displacement of 1 × 10⁻⁷ cos(kx) is applied on the bottom PDMS layer.

Fig. 4. Wall trapping of 10 μm PS particles in different capillary channels.

(a) Time-lapse images of particles under ultrasound for 200 μm to 500 μm diameter channels (Movie S2†) (b) schematics of the acoustofluidic setup for wall trapping inside artificial capillaries. (c) The particle tracing simulations of the microchannel cross-section from dispersed to ultrasonically manipulated condition. The traces confirm the final location of the beads observed in the experiments. The scale bars are 100 μm. The operating frequencies are 87 kHz for 200 μm diameter channel, 47 kHz for 300 μm diameter channel, 46.8 kHz for 400 μm diameter channel and 32.9 kHz for 500 μm diameter channel.

Fig. 5. Analysis of wall trapping strength for different particle diameters in the circular cross-sectional channels.

(a) The acoustic radiation force shows a significant increase for the increasing particle diameters. The purple arrows indicate the direction of radiation forces. (b) The particle tracing simulations over 27 seconds by accounting both acoustic radiation and streaming forces; the strong wall trapping begins for particles approximately larger than 10 μm diameter, whereas the smaller ones follow the streaming-induce vortex. The channel width is set to 200 μm, where the operating frequency is 87 kHz. The displacement of 1 × 10⁻⁷ cos(kx) is applied on the bottom PDMS layer.

Fig. 6. Microalgae-wall interactions under ultrasonic trapping forces.

(a) Time-lapse images show trajectory of microalgae during 0.2 seconds natural swimming interval to the wall trapping stage during the next 6.4 seconds (Movie S3†). (b) The effect of the driving voltage amplitude on the trapping of microalgae swimmers, the green and red arrows show the trapping and escaping trajectories, respectively (Movie S4†). (c) The time-lapse images of the single microalgae under pulsed ultrasound signal of 2 seconds width, with its center position in x-axis plotted in (d) (Movie S5†). (e) The rotational behavior of the microalgae at the wall boundaries, with a direction denoted by red arrows (Movie S6†). The scale bars in (a) and (b) are 50 μm, and in (c) and (e) are 25 μm.