Robust acoustic trapping and perturbation of single-cell microswimmers illuminate three-dimensional swimming and ciliary coordination

Abstract

We report a label-free acoustic microfluidic method to confine single, cilia-driven swimming cells in space without limiting their rotational degrees of freedom. Our platform integrates a surface acoustic wave (SAW) actuator and bulk acoustic wave (BAW) trapping array to enable multiplexed analysis with high spatial resolution and trapping forces that are strong enough to hold individual microswimmers. The hybrid BAW/SAW acoustic tweezers employ high-efficiency mode conversion to achieve submicron image resolution while compensating for parasitic system losses to immersion oil in contact with the microfluidic chip. We use the platform to quantify cilia and cell body motion for wildtype biciliate cells, investigating effects of environmental variables like temperature and viscosity on ciliary beating, synchronization, and three-dimensional helical swimming. We confirm and expand upon the existing understanding of these phenomena, for example determining that increasing viscosity promotes asynchronous beating. Motile cilia are subcellular organelles that propel microorganisms or direct fluid and particulate flow. Thus, cilia are critical to cell survival and human health. The unicellular alga Chlamydomonas reinhardtii is widely used to investigate the mechanisms underlying ciliary beating and coordination. However, freely swimming cells are difficult to image with sufficient resolution to capture cilia motion, necessitating that the cell body be held during experiments. Acoustic confinement is a compelling alternative to use of a micropipette, or to magnetic, electrical, and optical trapping that may modify the cells and affect their behavior. Beyond establishing our approach to studying microswimmers, we demonstrate a unique ability to mechanically perturb cells via rapid acoustic positioning.

Acoustofluidics

“…Moreover, active particles-motile cells-have been explored as interesting measurement devices for describing the pattern of acoustic waves in devices (Kim et al, 2021a) and to determine the amplitude of the acoustic forces in two (Kim et al, 2021b) and three dimensions (Cui et al, 2023). Motile particles have been shown to strongly affect the rheology of fluids via experiments enabled by acoustically-driven jetting (McDonnell et al, 2015), in agreement with analytical predictions (Saintillan, 2010).…”

Sonic snares: Trapping microorganisms for deeper insights into swimming behavior and ciliary dynamics

“…However, freely swimming microorganisms can be challenging to control and image with sufficient resolution to study their locomotion. In this issue of PNAS, Cui et al ( 5 ) present an innovative label-free acoustofluidic device which enables the confinement of individual cilia-driven cells in a controlled space. This cutting-edge methodology offered the temporal and spatial resolution to characterize three-dimensional body motion and cilia waveform of the unicellular alga Chlamydomonas reinhardtii , with no constraints in body rotation, swimming characteristics, and cellular phenotype expression.…”

“…In their study, Cui et al ( 5 ) employed a modified version of acoustic trapping, commonly referred to as acoustic tweezers, to confine swimming microorganisms. Acoustic tweezers represent a promising and emerging technology that allows for the precise manipulation of particles by utilizing the interaction of sound waves with various media, including solids, liquids, and gases ( 16 ).…”

“…On the other hand, BAW tweezers are advantageous when manipulation throughout the bulk of the fluid is desired, and they are suitable for larger volumes and higher particle concentrations ( 20 ). Here, Cui et al ( 5 ) cleverly combined the advantages of both SAW actuation and BAW trapping array to develop a hybrid BAW/SAW acoustic tweezers platform which can benefit from the precise control of SAW and the larger trapping forces of BAW ( Fig. 1 ).…”

“… ( A ) Schematic of the hybrid BAW/SAW acoustofluidic device from Cui et al ( 5 ). A one-dimensional (1D) standing wave is generated by linearly coupled interdigitated transducers (IDTs) to develop a two-dimensional (2D) bulk acoustic field in a square glass microchamber oriented at 45° to the SAW propagation.…”