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Glass slivers conduct current at the nanoscale

U. MICHIGAN (US)—On a very small scale, physics can get a little peculiar. Biomedical engineers have discovered such a nanoscale phenomenon and think it could lead to faster, less expensive portable diagnostic devices.

The new technology may advance building micro-mechanical and “lab on a chip” devices that could be used for instant home tests for illnesses, food contaminants, and toxic gases.

Conductors in a macroscale world effectively transmit electricity, and materials called insulators or dielectrics don’t, unless they are jolted with an extremely high voltage.

Under such “dielectric breakdown” circumstances, as when a bolt of lightening hits a rooftop, the dielectric (the rooftop in this example) suffers irreversible damage.

Alan Hunt, associate professor of biomedical engineering at the University of Michigan, says this isn’t true at the nanoscale. He was able to get an electric current to pass nondestructively through a sliver of glass, which isn’t usually a conductor.

A paper on the research is published in the journal Nature Nanotechnology.

“This is a new, truly nanoscale physical phenomenon,” Hunt says. “At larger scales, it doesn’t work. You get extreme heating and damage.

“What matters is how steep the voltage drop is across the distance of the dielectric. When you get down to the nanoscale and you make your dielectric exceedingly thin, you can achieve the breakdown with modest voltages that batteries can provide. You don’t get the damage because you’re at such a small scale that heat dissipates extraordinarily quickly.”

These conducting nanoscale dielectric slivers are what Hunt calls liquid glass electrodes, fabricated with a femtosecond laser that emits light pulses only quadrillionths of a second long.

The glass electrodes are ideal for use in lab-on-a-chip devices that integrate multiple laboratory functions onto one chip just millimeters or centimeters in size.

While the devices could be used for instant home tests, most of them need a power source to operate, and right now they rely on wires to route the power. It’s often difficult for engineers to insert wires into the tiny machines, Hunt says.

“The design of microfluidic devices is constrained because of the power problem,” Hunt explains. “But we can machine electrodes right into the device.”

Instead of using wires to route electricity, Hunt’s team etches channels through which ionic fluid can transmit electricity. These channels, 10 thousand times thinner than the dot of this “i,” physically dead-end at their intersections with the microfluidic or nanofluidic channels in which analysis is being conducted on the lab-on a-chip (this is important to avoid contamination).

But the electricity in the ionic channels can zip through the thin glass dead-end without harming the device in the process.

Hunt’s discovery is actually the result of an accident. Two channels in an experimental nanofluidic device didn’t line up properly, Hunt ays, but the researchers found that electricity did pass through the device.

“We were surprised by this, as it runs counter to accepted thinking about the behavior of nonconductive materials,” Hunt explains. “Upon further study we were able to understand why this could happen, but only at the nanometer scale.”

As for electronics applications, Hunt says that the wiring necessary in integrated circuits fundamentally limits their size.

“If you could utilize reversible dielectric breakdown to work for you instead of against you, that might significantly change things,” Hunt says.

More news from the University of Michigan: www.ns.umich.edu/

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