Today’s portable electronics (except for self-winding watches and crank radios) depend on batteries for power. Now researchers have demonstrated that easy-to-make, inexpensive nanowires can harvest mechanical energy, possibly leading to such advances as medical implants that run on electricity generated from pulsing blood vessels and cell phones powered by nanowires in the soles of shoes.
“When you walk, you generate 67 watts. Your finger movement is 0.1 watt. Your breathing is one watt. If you can convert a fraction of that, you can power a device. From the concept we’ve demonstrated, we can convert 17-30 percent of that,” says Zhong Lin Wang, professor of materials science at Georgia Tech and one of the researchers of the work, published in the journal Science.
Their results confirm a theory: zinc oxide nanowires will show a powerful piezoelectric effect, which is the production of electricity in response to mechanical pressure. Ordinarily the positive and negative charges of zinc and oxygen ions in these crystalline nanowires cancel each other out. But when the wires, which are chemically grown to stand on end on top of an electrode, bend in response to, say, a vibration, the ions are displaced. This unbalances the charges and creates an electric field that produces a current when the nanowire is connected to a circuit.
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Although each nanowire alone produces very little power, Wang says, “with simultaneous output from many nanowires, we can generate high power,” enough to run a small medical implant. The work reported in Science involved only single nanowires, but Wang says his lab has already developed technology to harvest power from multiple nanowires.
Because the chemical process by which the wires can be grown is inexpensive, at some point it may be practical to produce large arrays that are capable of providing enough power for consumer electronics. “We can grow these on polymer substrates at very low cost,” Wang says. “Our goal is to one day put these into people’s shoes so you can generate electricity when you’re walking.”
Before any devices powered by nanowires can be developed, though, researchers will need to find ways to connect all the nanowires to circuits. That, says Yi Cui, professor of materials science and engineering at Stanford University, will be a challenge but should be feasible. Indeed, Wang estimates that based on his current progress, prototype devices will be working within five years.
One early application of the “nanogenerators” is providing power for a glucose sensor implanted under the skin of the arm. Such a sensor would transmit blood sugar readings to a wrist watch and, says Cui says, one day the sensor implant could automatically releases insulin when needed.
Piezoelectric materials are frequently used in microscale devices. What’s new about this application is the ease with which nanogenerators can be made at the nanoscale, says Jun Liu, researcher at the Pacific Northwest National Laboratory. Such thin wires can be bent more than bulk zinc oxide without breaking – making it possible to apply more strain and so generate more electricity. “I think it’s a very significant piece of work,” Liu says. “[Wang] has done things that people suspected were possible, but never made work.”