Sound waves spin droplets to focus, separating nanoparticles

Mechanical engineers at Duke University have devised a method for spinning individual droplets of liquid to target and separate nanoparticles for biochemical purposes. This method is much more efficient than traditional centrifuge methods, working its magic within minutes instead of taking hours or days, and requires very little of the normal sample size. The innovation could solidify new approaches to applications ranging from precision bioassays to cancer diagnosis.

The results will appear online on December 18 in the magazine Advances in science.

“This idea came from an interesting recent discovery that you can use surface acoustic waves to spin a drop of liquid,” said Tony Jun Huang, Distinguished Professor of Mechanical Engineering and Materials Science at Duke. ” we will explore whether we could use this method to create a point – of – care system that can dispense and enrich nanoparticles quickly and efficiently. “

Huang and doctoral student Yuyang Gu began their study by building a machine capable of spinning individual drops of liquid. At the center of a piezoelectric surface is a ring of polydimethylsiloxane, a type of silicon commonly used in microfluidic technologies, which confines the boundaries of the droplet and holds it in place. The researchers then placed a sound wave generator called an interstellar transducer (IDT) on each side and dragged it so that sound waves with different frequencies travel through the piezoelectric surface to pass through. into the droplet.

When turned on, the IDTs create surface acoustic waves that push on the sides of the droplets as Donald Duck gets a beat from a couple of speakers. At low power conditions, the top of the droplet begins to move around the ring like a muffin top made of Jell-O. But when the power turns up to 11, the balance between the surface tension of the droplet and its centrifugal force causes it to take on a pill shape and start spinning instead.

The researchers then studied how fluorescent nanoparticles of different sizes behaved within the spinning droplets. As the droplet spins, the nanoparticles themselves were pulled in a helical pattern. Depending on the magnitude and frequency of the sound, they were pushed toward the center of the droplet as a result of force entering the sound waves and hydrodynamics.

The researchers found that, by using different frequencies, they could collect grains in particular as small as tens of nanometers. These sizes are associated with biologically important molecules such as DNA and exosomes – biological nanoparticles released from all cell types in the body that are thought to be important in cell-to-cell communication and proliferation. diseases.

But they still had another problem. While nanoparticles of one size went to the center of the droplet, nanoparticles of other sizes were still flying at random, making it difficult to get to the thick toilet.

Their solution? The second spinning drop.

“We set up two droplets of different sizes next to each other so that they would spin at different speeds,” Gu said. “By connecting them with a small channel, any nanoparticles that are not focused in the first end spin off and get caught in the second.”

To further demonstrate the usefulness of their two-droplet centrifugal system, the researchers showed that it could separate subunits of exosomes from a sample. And unlike common centrifugation methods that require a large number of samples and can be taken overnight for work, their solution required only a much smaller sample size – such as five microliters – and less than minute.

“We expect this work to simplify and accelerate sample processing, detection and repeatability in a number of applications such as point-of-care diagnostics, bioassays and melt biopsies,” Gu said.

“The ability to separate and enrich exosome subpopulations and other biological nanoparticles is extremely important.” Huang added. “For example, while the recent discovery of exosome subpopulations has inspired biologists and researchers because of their ability to regenerate the field of noninvasive diagnosis, exosome subpopulations have not yet been used. in clinical settings.related to separating exosome subpopulations due to their small size.Our approach offers a simple, automated approach to separating subpopulations exosome in a rapid and biodegradable manner. As a result, we believe it is critical to the clinical utility of exosome sub-forces. ”