What You Need to Know: Researchers from MIT, Pennsylvania State University, and Carnegie Mellon University have devised a new way to separate cells by exposing them to sound waves as they flow through a tiny channel. Their device, about the size of a dime, could be used to detect the extremely rare tumor cells that circulate in cancer patients’ blood, helping doctors predict whether a tumor is going to spread. Separating cells with sound offers a gentler alternative to existing cell-sorting technologies, which require tagging the cells with chemicals or exposing them to stronger mechanical forces that may damage them.
“Acoustic pressure is very mild and much smaller in terms of forces and disturbance to the cell. This is a most gentle way to separate cells, and there’s no artificial labeling necessary,” says Ming Dao, a principal research scientist in MIT’s Department of Materials Science and Engineering and one of the senior authors of the paper, which appears this week in the Proceedings of the National Academy of Sciences.’
How They Do It: To sort cells using sound waves, scientists have previously built microfluidic devices with two acoustic transducers, which produce sound waves on either side of a microchannel. When the two waves meet, they combine to form a standing wave (a wave that remains in constant position). This wave produces a pressure node, or line of low pressure, running parallel to the direction of cell flow. Cells that encounter this node are pushed to the side of the channel; the distance of cell movement depends on their size and other properties such as compressibility.
However, these existing devices are inefficient: Because there is only one pressure node, cells can be pushed aside only short distances.
The new device overcomes that obstacle by tilting the sound waves so they run across the microchannel at an angle — meaning that each cell encounters several pressure nodes as it flows through the channel. Each time it encounters a node, the pressure guides the cell a little further off center, making it easier to capture cells of different sizes by the time they reach the end of the channel.
This simple modification dramatically boosts the efficiency of such devices, says Taher Saif, a professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign. “That is just enough to make cells of different sizes and properties separate from each other without causing any damage or harm to them,” says Saif, who was not involved in this work.
My Take: Circulating tumor cells in clinical settings are very rare: A 1-milliliter sample of blood may contain only a few tumor cells. If we can detect these rare circulating tumor cells, we may be able to study their biology and determine whether the patient is at a high risk for the cancer taking hold in organs such as the liver, lungs, bones, or brain. While not ready for clinical use, this method is a move in the direction of better detecting circulating tumor cells in the body. I’m Dr. Michael Hunter.
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