Dancing with the Cells: Acoustic Microflows Generated by Oscillating Cells

Abstract

The interaction of a sound or ultrasound wave with an elastic object, such as a microbubble, can give rise to a steady‐state microstreaming flow in its surrounding liquid. Many microfluidic strategies for cell and particle manipulation, and analyte mixing, are based on this type of flow. In addition, there are reports that acoustic streaming can be generated in biological systems, for instance, in a mammalian inner ear. Here, new observations are reported that individual cells are able to induce microstreaming flow, when they are excited by controlled acoustic waves in vitro. Single adherent cells are exposed to an acoustic field inside a microfluidic device. The cell‐induced microstreaming is then investigated by monitoring flow tracers around the cell, while the structure and extracellular environment of the cell are altered using different chemicals. The observations suggest that the maximum streaming flow induced by an MDA‐MB‐231 breast cancer cell can reach velocities on the order of mm s−1, and this maximum velocity is primarily governed by the overall cell stiffness. Therefore, such cell‐induced microstreaming measurements, including flow pattern and velocity magnitude, may be used as label‐free proxies of cellular mechanical properties, such as stiffness.

Acoustic Characterization of Polydimethylsiloxane for Microscale Acoustofluidics

“…Working frequencies of commonly used BAW and SAW devices can range from 2 to 8 MHz and from 5 to 50 MHz [9,39,40], respectively. However, due to the availability of transducers, the measurements are only conducted within the frequency band 3.0-7.0 MHz here, which overlap with typical BAW frequencies, but only cover the lower part of the SAW bands.…”

An ultrafast enzyme-free acoustic technique for detaching adhered cells in microchannels

“…18 In our device, when the PZT is actuated at relatively low power (lower than $20 W), cells can undergo a stable oscillation, which can give rise to microstreaming ow. 15 However, at higher actuation powers, especially above $50 W, cells gradually lose their adhesion to the substrate over time (Fig. 1a).…”

“…Thin substrates were used to maximize the contribution of Lamb waves to the propagation of sound waves through the substrate. 15,26 As a proof-of-concept demonstration and for the ease of culturing, we use a small population of cells (i.e., a few thousand cells in a region of interest (ROI)) inside a polydimethylsiloxane (PDMS) microuidic channel bonded to a $70 mm glass substrate. Once the cells are seeded into a bronectincoated microuidic channel and cultured for at least two days, we expose them to short (in the range of 200-500 ms) pulses of acoustic waves using a piezoelectric (PZT) transducer bonded to the glass surface near the microuidic channel (see Fig.…”

“…The regions of maximum acoustic pressures on the substrate can vary depending on the device geometry, substrate material, and actuation frequency. 15,16 We hypothesize that the substrate vibration and the resultant acoustic forces, as well as the microstreaming ow, are responsible for the detachment of the cells. We conrm the vibrations (i.e., exural waves) of the substrate within the ROI, located inside the microuidic channel near the PZT, using optical coherence tomography (OCT) (see Fig.…”

“…Recently, we reported that exposure of cells to acoustic waves at low and moderate excitation powers (i.e., smaller than $20 W) could generate cell-induced microstreaming 15 and facilitate the delivery of extracellular materials to the cell. 16 The actuation power used in these two works was high enough to generate acoustic microstreaming only, and therefore, most cells retained their attachment to the substrate of the microuidic channel even during relatively long (e.g., $20 min) exposures.…”