Putting “Life” in Order with Acoustical Tweezers Designed for Widespread Use

Whether you’re pulling out a splinter of wood or an eyebrow hair, tweezers are the go-to tool. For these and many other situations that involve moving an object too small to grasp with human hands, a $1.49 pair of metal tweezers is good enough. However, moving an object too small to see requires a much more complicated and expensive kind of tweezers.

In work that will soon be published in the journal Physical Review Applied, a team of French researchers from the Institute of Electronics, Microelectronics and Nanotechnology and the Paris Institute of Nanosciences demonstrated a new design for acoustical tweezers. Acoustical tweezers use sound waves to move tiny objects like biological cells. This new design is easier and cheaper to mass produce than previous designs, which means the potentially game-changing technology could soon be in labs around the world.

The researchers manipulated randomly scattered beads 30μm in size to spell the word “LIFE” using their acoustic tweezers. Image Credit: Riaud et. al /APS.

Like a photographer posing a family portrait, if you want to image cells or other tiny objects through a microscope in a useful way, you often need to physically arrange them. You also need to arrange such objects when carrying out experiments and building small devices. However, this can be much more difficult than it sounds. Cells are finicky, sensitive to heat and easily destroyed by touch.

While scientists have explored many approaches for moving tiny objects in the last ten years, acoustic tweezers are leading the way when it comes to biological cells. The approach is powerful, doesn’t destroy cells, and can be integrated with disposable parts to avoid contamination. It doesn’t require that cells or the substrates they sit on have specific optical properties, as is the case with optical tweezers. Acoustic tweezers work well for moving individual particles or groups and for varying sizes.

The new design works like this. The cells or objects are suspended in a water-based solution, sandwiched between a top layer of silicone and a glass slide. The slide sits on top of a very thin layer of oil that covers a spiral-shaped transducer, a device which generates an ultrasound beam. Like the gel a technician smears on a patient’s skin before an ultrasound, the oil acts as a lubricant and helps transmit the sound waves into the subject.

The transducer sends a beam of ultrasound waves up through the oil and into the slide. This generates a kind of swirling vibration, which creates something called an “acoustic vortex” in the water. The result is what the researchers describe as an “intense ultrasonic ring”—a high-pressure region surrounding a low-pressure one that can be precisely positioned around a cell or group of cells, trapping them inside. By moving the beam, the researchers can move the trapped cells to the desired location. The cells are released by turning off the beam.

Acoustic tweezers have been around for a while, but this new design is significantly more user friendly than previous versions. It uses a single transducer (other designs require more), is easily miniaturized, is well-suited for mass production, and can be easily integrated into various devices like microscopes. Its success comes from immersing the particles in a liquid and using a spiral-shaped transducer to generate the acoustic vortex.

The French team had previously built a different system for creating acoustic vortices and trapping objects, among other functions. It worked well, but was cumbersome to use and required complicated electronics, according to the researchers. This led to the question: How do you create a cheap, miniaturized, simple system, accessible to anyone using a standard microscope?

“The idea came to design transducers spiraling like the waves that we wanted to create,” says Antoine Riaud, who performed this work as part of his PhD thesis. “Nobody had explored this possibility.” There were challenges along the way, but it took less than three months and ten prototypes to get a working device. The test results were excellent; the researchers were able to pattern dozens of individual particles within minutes.

Acoustic tweezers have the potential to significantly advance the technology for printing cells, building microscopic electronic and mechanical systems, assembling nanosystems, and studying how cells respond to mechanical stimuli, among other possibilities. “When we wrote the paper, we decided to offer the opportunity to every researcher in the world to build their own tweezers for research purposes and created a free program that enables anyone to implement their own design and thus contribute to the advances of this promising technology,” says Riaud.

There is a little something for the artists too. With acoustic tweezers we can look at things in new ways and assemble objects in ways they haven’t been assembled before. Perhaps that will lead to some artistic creations along the lines of the disturbingly beautiful petri dish handprint Tasha Sturm created of her son’s dirty hand, or the gorgeous artwork Klaus Kemp creates with microscopic algae.

Kendra Redmond

You may also read these articles