Precisely move objects using only sound waves

Researchers at EPFL have developed an innovative method that uses sound waves to guide floating objects around obstacles in water. This new approach, inspired by optics, has the potential for important biomedical applications such as non-invasive targeted drug delivery.

The 2018 Nobel Prize in Physics was awarded to Arthur Ashkin for the creation of optical tweezers, which are laser beams capable of manipulating microscopic particles. While optical tweezers are beneficial for a variety of biological purposes, they require highly regulated and unchanging conditions to work effectively.

According to Romain Fleury, head of EPFL's Wave Engineering Laboratory in the School of Engineering, optical tweezers work by generating a light “hotspot” to trap particles, similar to a ball falling into a hole. However, if there are other objects nearby, setting up and manipulating this “hole” becomes challenging.

For the past four years, Fleury and postdoctoral researchers Bakhtiar Orazbayev and Mathieu Maleczak have been using sound waves to manipulate objects in unpredictable, dynamic settings. The team's approach, known as wave motion shaping, does not rely on the object's environment or physical characteristics. Just the object's position is needed, and the sound waves take care of the rest.

“In our experiments, instead of trapping objects, we gently pushed them around, like you would push a puck with a hockey stick,” Fleury explains.

In laboratory experiments, audible sound waves emanating from a speaker array guided a floating ping-pong ball along a predetermined path. A second array of microphones captured the response as the sound waves interacted with the ball, allowing the researchers to calculate the optimal speed of the sound waves in real time.

This method, rooted in momentum conservation, is simple and promising. Inspired by the optical technique of wavefront shaping, it represents the first application of its kind to move an object. Moreover, the team's method is versatile, as it is not limited to moving spherical objects along a path, but it can also control rotation and move complex floaters such as origami lotus.

Experimental setup with speakers and microphones placed at either end of a water tank, and vertical scattering objects in the middle.
Experimental setup, with speakers and microphones at either end of a water tank, and vertical scattering objects in the middle. Credit: EPFL/LWE CC-BY-SA 4.0

After successfully guiding the ping-pong ball, the scientists conducted further experiments using both static and dynamic obstacles to introduce complexity into the system. Moving the ball around these objects demonstrated the effectiveness of shaping wave motion in a dynamic, uncontrolled environment such as the human body. Fleury emphasizes that sound represents a highly promising tool for biomedical applications due to its non-invasive and harmless nature.

“Some drug delivery methods already use sound waves to release encapsulated drugs, so this technique is particularly attractive for pushing a drug directly toward tumor cells, for example,” Fleury says.

The potential applications of this method are truly unprecedented, particularly in the fields of biological analysis and tissue engineering. Manipulating cells using sound waves rather than physically touching them significantly reduces the risk of damage or contamination. Additionally, the possibility of using this method with light in the future opens up even more exciting opportunities.

The researchers' next goal is to take their sound-based experiments from the macroscopic to the microscopic level. With funding from the SNSF, they are ready to conduct experiments under the microscope, taking advantage of ultrasonic waves to precisely manipulate cells at the microscopic level.

Journal Reference:

  1. Bakhtiar Orazbayev, Mathieu Malejac, Nicolas Bachelard, Stefan Rotter and Romain Fleury. Wave-motion shaping for moving objects in heterogeneous and dynamic media. Nature Physics, 2024; DOI: 10.1038/s41567-024-02538-5

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