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IMAGE: SEAN LANGELIER/UNIVERSITY OF MICHIGAN
This story was corrected on 22 July 2009.
21 July 2009—Researchers at the University of Michigan have found a way to control the movement of tiny droplets of fluid in a microelectromechanical (MEMS) device—with sound. The scientists use several musical tones to move droplets along different channels on a chip. By combining tones or applying them at appropriate times, they can move liquids along multiple channels and even mix, split, and sort the liquids.
The advance, published this week in Proceedings of the National Academy of Sciences, holds promise for making microfluidic devices compact and simple. Microfluidic devices are glass or plastic chips with etched channels through which you can maneuver nano- or even picoliters of liquids. Thousands of chemical or biological reactions can be performed on the chips simultaneously, promising to speed up medical diagnostics, chemical synthesis, and drug discovery.
Despite their name, the chips are not really ”micro” yet. They still need multiple connections to bulky off-chip pressurized gas pumping systems. State-of-the-art microfluidic devices contain thousands of polymer microvalves—a technology pioneered by Stanford University bioengineering professor Stephen Quake who founded the startup Fluidigm Corporation—that are opened and closed to move the liquids. Each valve may sometimes need its own pressure source.
The new acoustic technique is designed to minimize the external infrastructure needed to control droplet flow on the chip. Chemical engineering professor Mark Burns and his colleagues feed musical tones from a speaker into four cylinders of precise lengths cut from borosilicate tubes. Each cylinder resonates at a specific frequency and amplifies those vibrations, which builds up pressure inside it. The cylinders are each connected to one of four channels in a silicone chip, and the pressure drives the droplets in the channels. ”You apply this one signal and have four independently controlled outputs,” says Sean Langelier, a graduate student in Burns’s laboratory. ”So we are reducing the number of device interconnects to essentially one.”
Making the sound louder moves the droplets faster, while making it longer moves them farther. The researchers also demonstrated mixing and splitting liquids on chips they fabricated with branched channels.
Burns’s group isn’t the first to try to move droplets on chips using sound. Other researchers have used sound waves to vibrate the surface of a chip and move droplets sitting on it. In another technique, known as acoustic streaming, Langelier says, ”you blast a really high-intensity sound wave at a liquid.”
The new method is more sophisticated, yet still simple. ”The benefit of this is that they’re using a single driving force to drive multiple flows on the chip,” says Jerome Ferrance, a chemistry professor at the University of Virginia. ”It’s very good work [and] presents a good way to control flow in microdevices.”
To give the mechanism a bigger edge over today’s technologies, the researchers will need to shrink it further. The resonance cylinders—which are between 11 and 19 centimeters long and about 5 cm wide—are smaller than the syringe pumps that are used to manipulate fluids on many microfluidic chips, says Ferrance, but they are still large compared with the chip itself.
Langelier says that the team is already trying to make smaller resonance cavities that can be integrated into the chip itself. ”Once you start shrinking down cavities, the only thing that changes is the frequency: The smaller the cavity, the higher the frequency it will resonate at,” he says. This means a more practical device would need sounds in the kilohertz range; the proof-of-concept demonstration unveiled this week uses 400- to 600-hertz sounds.
Eventually, the acoustic input could come from a small piezoelectric transducer, which converts electrical signals into sound waves. That would make the device really compact and portable by getting rid of the bulky speaker.
About the Author
Prachi Patel is a contributing editor to IEEE Spectrum . In her recent work for IEEE Spectrum Radio, she notes that although Wall Street ”quants” are blamed for the financial meltdown, they are still in demand. And in the June 2009 issue, she wrote about how experts expect resume fraud to rise during this period of high unemployment.