Sound waves rotate droplets to concentrate, separate nanoparticles

Mechanical engineers at Duke University have devised a method for spinning individual liquid droplets to concentrate and separate nanoparticles for biomedical purposes. The technique is much more efficient than traditional centrifuge approaches, working its magic in less than a minute instead of taking hours or days and requires only a small part of the typical sample size. The invention could highlight new approaches to applications ranging from precision biological tests to cancer diagnosis.

The results appear online in the journal on December 18 Scientific advances.

“This idea was born out of a very 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 from Duke to William Bevan. We decided to investigate whether we could use this method to create a point-of-care system that can separate and enrich nanoparticles quickly and efficiently.

Huang and his doctoral student Yuyang Gu began the investigation by building a device capable of rotating individual drops of liquid. In the center of a piezoelectric surface is a ring of polydimethylsiloxane, a type of silicon commonly used in microfluidic technologies, which limits the drop limits and keeps it in position. The researchers then placed a sound wave generator called an interdigitated transducer (IDT) on each side and tilted them so that sound waves with different frequencies traveled through the piezoelectric surface to enter the drop.

When turned on, IDTs create surface acoustic waves that push on the sides of the droplets, such as Donald Duck, which is blown by a giant pair of speakers. At low power settings, the top of the drop begins to sway around the ring, like a muffin top from Jell-O. But when the power reaches 11, the balance between the surface tension of the drop and its centrifugal force causes it to take the shape of a pill and begin to rotate in place.

The researchers then investigated how fluorescent nanoparticles of different sizes behaved in spinning droplets. As the drop rotates, the nanoparticles themselves were also dragged along a helical pattern. Depending on their size and frequency of sound, they were also pushed to the center of the drop due to the input force of sound waves and hydrodynamics.

The researchers found that using different frequencies, they could specifically concentrate particles as small as tens of nanometers. These dimensions correlate with biologically important molecules, such as DNA and exosomes – biological nanoparticles released from each cell type in the body, which are thought to play an important role in cell-to-cell communication and disease transmission.

But they were still facing another problem. As the nanoparticles of one size moved toward the center of the drop, the nanoparticles of other sizes still flew at random, making it difficult to access the concentrated reward.

Their solution? A second spinning drop.

“We set two drops of different sizes next to each other so that they rotated at different speeds,” Gu said. “By connecting them to a small channel, any nanoparticles that do not concentrate in the first end up spinning and getting trapped in the second.”

To further show how useful their double-drop centrifugal system could be, the researchers showed that they could successfully separate subpopulations of exosomes from a sample. And unlike regular centrifugation methods that require large amounts of samples and can take overnight to work, their solution only required a much smaller sample volume – such as five microliters – and less than a minute.

“We are considering this work by simplifying and accelerating sample processing, detection and reactive reactions in various applications, such as point of care diagnosis, biological tests and liquid biopsies,” said Gu.

The ability to separate and enrich exosomal subpopulations and other biological nanoparticles is extremely important. Huang added. “For example, while the recent discovery of exosomal subpopulations has excited biologists and researchers because of their potential to revolutionize the field of noninvasive diagnosis, exosome subpopulations have not yet been used in clinical settings. associated with the separation of exosome subpopulations due to their small size. Our approach provides a simple and automatic approach to separating exosome subpopulations in a fast and biocompatible manner. Therefore, we believe that it is essential to unblock the clinical utility of exosomal subpopulations. . ”