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Biophysics

Moved by sound

Acoustic tweezers nudge cells or particles with sound waves.

Pushing around particles, cells or small organisms such as Caenorhabditis elegans in a laboratory dish requires a gentle touch. A team at Pennsylvania State University has recently shown how to move specimens with sound waves, creating a less invasive alternative to optical tweezer–based manipulation, which risks photodamage, or magnetic methods, which require magnetically labeled particles.

The dime-sized acoustic tweezers are made up of a microchannel on a piezoelectric substrate flanked by four transducers, such as those found in cell phones to convert sound into electronic signals and back again. “There is nothing new about our transducers, but the way we use them, that is new,” says biomedical engineer Tony Jun Huang, who co-led the collaboration with biochemist Stephen Benkovic.

In the device, a jolt of current is applied to the orthogonally arranged transducers to generate mechanical vibrations, leading to an acoustic wave with the frequency of a low bass note. The wave creates pressure differences in the fluid, allowing the team to move polystyrene beads, HeLa cells and C. elegans in multiple directions. Whereas a fixed frequency would only trap an object, the scientists could push the specimens around by tuning frequencies. “That's how the manipulation works,” Huang says.

Huang highlights the non-invasive quality of sound waves. Exposing cells to a sound wave lasting 10 minutes affected neither cell viability nor growth. Other advantages of acoustic over optical tweezers are that sound waves can be directed in ways not possible with light and that an acoustic system is less bulky than an optical one, Huang says.

The team is working to use acoustic tweezers on ever smaller particles, for example, to move nanoscale objects using high-frequency acoustic waves. “We want to make it even more precise, on a nanometer scale, which is a challenge,” he says.

Several companies have expressed interest in the device. Acoustic tweezers can be used to move single cells and multiple particles, even tens of thousands of them and at high speed, leading the team to also see its potential for biomedical cell separation and sorting.

High school students who recently visited his lab felt “they had come to a toy store to play,” Huang says. The event sparked the idea to create a toolbox for kids with acoustic tweezers and beads to help hold their interest in science.

References

  1. Ding, X. et al. On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves. Proc. Natl. Acad. Sci. USA 109, 11105–11109 (2012).

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Marx, V. Moved by sound. Nat Methods 9, 867 (2012). https://doi.org/10.1038/nmeth.2150

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