Treadmills for micro swimmers allow a closer look at behavior


June 13, 2023

(Nanowerk News) A team from the McKelvey School of Engineering at Washington University in St. Louis and the Massachusetts Institute of Technology have created an acoustic microfluidic method that offers new opportunities for conducting experiments with swimming cells and microorganisms.

“The cells our collaborators studied are strong swimmers for their size, so the force needed to trap them is enormous,” said J. Mark Meacham, professor of mechanical engineering and materials science at the McKelvey School of Engineering and senior author of the paper, published in Proceedings of the National Academy of Sciences (“Strong acoustic trapping and perturbation of single-cell microswimmers illuminates three-dimensional swimming and cilia coordination”). “In our device, ultrasonic waves such as those used for imaging are able to hold the cell body in place without affecting the way the cell swims.”

The cells used in the research are single-celled algae Chlamydomonas reinhardtiimodel organism used to study the movement of cilia, the tiny hair-like structures that move fluids and propel cells.

This new approach was motivated by previous work in the lab of Philip Bayly, Lee Hunter Honorary Professor and chair of the Department of Mechanical Engineering & Materials Science, who researches ciliary movement, and Susan Dutcher, a professor of genetics at the Washington University School of Medicine, an expert in structure and function. cilia, both are co-authors of this paper. Meacham developed the method and device with first author Mingyang Cui, who earned his master’s and doctorate degrees in mechanical engineering at McKelvey Engineering in 2017 and 2021, and is now a postdoctoral researcher at the Massachusetts Institute of Technology. C. reinhardtii cells are microscopic – and their cilia are even smaller – but they swim about 10 body lengths per second. Their cilia also beat about 60 to 70 times per second. bacterial acoustic trap The video panel on the left shows the acoustic trap and release with 1 second before and 1 second after the trapping period of 2 seconds. The two video panels on the right show synchronous beating (top) and asynchronous beating (bottom). (Credit: Meacham Laboratories)

“The field of view at the resolution needed to see cilia movement is such that cells swim out and away from where you’re looking at very quickly,” says Meacham. “It’s hard to study their swimming behavior without trapping the cells in some way.”

Cui overcomes the entrapment problem by using a combination of two types of acoustic waves. Surface acoustic waves create vibrations that travel along the surface of the material, and bulk acoustic waves are generated by surface vibrations in the liquid in which the cells are located.

“Cells are held up by acoustic waves in the fluid in places called nodes or areas of low pressure,” says Meacham. “We like to use surface acoustic waves because they allow for a higher frequency which provides smaller traps with a shorter distance between them, and provides better control of the cell you’re trying to manipulate.”

Unfortunately, conventional surface acoustic wave devices are not as efficient as their bulk acoustic wave counterparts, and efficiency is required to generate enough trapping force in these cells to withstand them without the device overheating.

“Any inefficiency causes heating, and that kills cells,” says Meacham. “Mingyang came up with a device structure in which a glass microchannel is used, which can convert surface acoustic waves into bulk acoustic waves to improve efficiency. Using glass also allows us to use high-resolution oil immersion microscopes.”

“Having solved these practical challenges, we can focus on other advantages of acoustofluidic trapping,” said Meacham. “The main need of our collaborators is to trap these cells without limiting their rotation. Acoustic trapping makes that possible because it doesn’t touch cells directly.”

Previously, to learn this swimming C. reinhardtii cells, the researchers used a suction pipette to hold the cells in place while the cilia were imaged. However, this does not allow the cell body to move in the slightest in response to ciliary strokes, especially limiting cell rotation, which is the natural movement during swimming.

“Think of it as a treadmill for these micro-swimmers, and the acoustic fields provide a way to hold cells in place without affecting ciliary motion or swimming in three-dimensional space,” says Meacham.

This device also has the added benefit of experimental work with micro-swimmers.

“We can set up 25 to 30 traps at a time and do all the analysis of trapped cells in parallel,” says Meacham. “You can’t do it with a micropipette — it’s physically impossible. This way you can take measurements quickly on a larger number of cells.”

Bayly said he was excited about the implications of this work for understanding cell motility.

“Mingyang’s results show that this method does not affect swimming in any way, but its big impact may lie in the flexibility of the approach to trap swimming cells or microorganisms in this size range,” said Bayly. “You can now do some new experiments to answer unanswered biological questions by using acoustic traps to provide a controlled environment in which to do those experiments.”


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