Moving objects precisely using only soundwaves

The researchers at EPFL have developed an innovative method that uses soundwaves to direct floating objects around obstacles in water. This new approach, inspired by optics, has the potential for important biomedical applications such as noninvasive targeted drug delivery.

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

According to Romain Fleury, the head of EPFL’s Laboratory of Wave Engineering in the School of Engineering, optical tweezers operate by generating a light “hotspot” to ensnare particles, similar to a ball dropping into a hole. However, if there are other objects nearby, it becomes challenging to establish and maneuver this “hole.”

Over the past four years, Fleury and postdoctoral researchers Bakhtiyar Orazbayev and Matthieu Malléjac have been using soundwaves to manipulate objects in unpredictable, dynamic settings. The team’s approach, known as wave momentum shaping, does not depend on the object’s environment or physical characteristics. All that is needed is the object’s position, and the soundwaves take care of the rest.

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

In the lab’s experiments, audible soundwaves emitted from a speaker array guided a floating ping-pong ball along a predetermined path. A second array of microphones captured the feedback as the soundwaves interacted with the ball, allowing researchers to calculate the optimal momentum of the soundwaves 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 can also control rotations and move complex floaters like an origami lotus.

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

After successfully guiding a ping-pong ball, scientists conducted further experiments using both stationary and moving obstacles to introduce complexity to the system. Maneuvering the ball around these objects showcased the effectiveness of wave momentum shaping in dynamic, uncontrolled environments, such as the human body. Fleury emphasizes that sound represents a highly promising tool for biomedical applications due to its noninvasive and harmless nature.

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

The potential applications of this method are truly groundbreaking, particularly in the fields of biological analysis and tissue engineering. Using sound waves to manipulate cells instead of 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 aim is to transition their sound-based experiments from the macro- to micro-scale. With SNSF funding secured, they are poised to conduct experiments under a microscope, leveraging ultrasonic waves to precisely manipulate cells at a microscopic level.

Journal reference:

Bakhtiyar Orazbayev, Matthieu Malléjac, Nicolas Bachelard, Stefan Rotter & Romain Fleury. Wave-momentum shaping for moving objects in heterogeneous and dynamic media. Nature Physics, 2024; DOI: 10.1038/s41567-024-02538-5

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