Ultrasound Used to Control Orientation of Small Particles

Researchers already have tapped acoustic waves like the ones used in ultrasound to direct the motion of tiny robots that can self-propel through liquid at high speeds. Now they're applying a similar principal to control how particles sort themselves, something that until now has been a difficult challenge for scientists.

Scientists from Penn State University paired ultrasound with a nozzle to develop technology that can separate, control, and eject different particles based on their shape and various properties, they said.

The group—led by Igor Aronson, a professor of biomedical engineering, chemistry, and mathematics, worked with tiny materials called nanorods, which are well-studied, synthetic self-propelled particles that are a similar size and swimming speed to bacteria. Because of the similarities between the two, nanorods are often used as a proof of concept for research that can be applied to bacteria movement, the researchers said.

Specifically, the team engineered a microchannel nozzle and applied ultrasound energy to the system to successfully control the particles, Aronson said. “The nozzle plays two roles," he explained in a post on Penn State's news page. "It concentrates fluid flow, which is something other researchers have done. But in addition to that, the walls of the nozzle reflect the acoustic waves of the ultrasound energy.”

Separating the Swimmers

The team worked with nanorods composed half of platinum and half of gold, placing them in a nozzle shaped like a tiny syringe. They added hydrogen peroxide to the mix, burning it on the platinum half of each of the nanorods, which forced the particle to swim in an imitation of how bacteria behave, the researchers said.

Ultrasound was then applied to the nozzle, producing acoustic waves that, along with the flow of fluid, succeeded in separating, aggregating, or extruding the waves from the nozzle, the team reported in a paper on their work published in the journal, Small.

"The separation concept relies on the fact that nanorods and spherical particles have different responses to acoustic radiation and generated fluid flow,” Aronson explained. “By controlling the nozzle shape and the frequency and amplitude of the acoustic radiation, we can coerce particles of different shapes and material properties to behave differently."

Historically, it's been challenging to control active particles like nanorods, which can swim autonomously; thus, the research represents a level of control in the separation of these particles that is unprecedented, he added.

Applications and Future Research in Acoustics

Applications for the research are various, including 3D printing and drug delivery, Aronson said. For the former, scientists can leverage the idea of adding certain particles to ink, such as nanorods, that can be separated from other particles to create different properties in the printed object, he said.

"So now, we could separate nanorods from spherical particles to deposit only some in the printout, such as depositing polymers without nanorods and so on, all to change the property of the printout," Aronson said.

Bioprinting also can benefit from the research through the design of acoustic nozzles for bio-inkjet-like printers, he said. Researchers can control the acoustic radiation in the nozzle to extrude certain types of cells, such as stem cells, and trap other types, such as bacteria. This method can be used as an additional control for bioprints, he said.

Another application could be to separate bacteria from cells for targeted drug delivery. In fact, the team plans to continue its research by mixing live bacteria and cells in a lab setting and then separating and controlling them, which could inform this application, Aronson added.