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Researchers have devised a way to fabricate tiny electrodes from glass, harnessing a phenomenon by which nanoscale glass walls can be transformed from insulators to conductors and back again. At larger scales, that phenomenon, known as "dielectric breakdown," leads to excess heating and structural damage, but at the nanoscale the process appears to be harmless and reversible.
Sanghyun Lee of the Pohang University of Science and Technology in South Korea and Ran An and Alan Hunt of the University of Michigan at Ann Arbor announced their finding in a paper published online May 16 in Nature Nanotechnology, along with a prototype application in what may be the smallest man-made pump in existence. (Scientific American is part of Nature Publishing Group.)
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Using lasers, the group machined glass channels just 600 nanometers wide onto a substrate. (A nanometer is one billionth of a meter.) Two electrolyte-filled channels were placed end to end, with a thin glass wall separating them. Ordinarily the wall would serve as a dam, blocking the flow of both electrolyte and electric current.
But dielectric breakdown, induced by extreme electric fields, can change that, allowing current to pass through the glass wall even though the wall remains structurally intact and continues to prevent electrolyte flow. The researchers found that at such small scales, even an electric potential of 10 volts would suffice to transform the glass insulator into a conducting electrode. And the heat accompanying dielectric breakdown, which can fry larger devices, dissipates so quickly on the nanoscale that the glass structure appears to suffer no permanent damage.
As a demonstration, the authors fabricated a fluid pump using one of their glass electrodes to drive flows on the smallest scales, on the order of a quadrillionth of a liter per second. The pump makes use of electroosmosis, whereby electricity pushes fluids along—in this case from one end of the pump to the other. The heart of the device is just four microns across—roughly the size of a red blood cell, the authors note—although the plumbing leading into and out of the pump extends much farther. (A micron is 1,000 nanometers, or one millionth of a meter.)
"Although smaller pumps exist in nature (for example, ion pumps)," the authors wrote, "to the best of our knowledge this is the smallest pump of any kind ever fabricated and integrated onto a microchip de novo."
Image credit: Alan Hunt/Sanghyun Lee