Researchers have demonstrated a nanoscale generator that produces continuous direct current electricity by harvesting mechanical energy from environmental sources. The device could produce current from ultrasonic waves, mechanical vibration or blood flow.

Based on arrays of vertically aligned zinc oxide nanowires that move inside a novel ‘zigzag’ plate electrode, the nanogenerators could provide a new way to power nanoscale devices without batteries or other external power sources.

‘This is a major step toward a portable, adaptable and cost-effective technology for powering nanoscale devices,’ said Zhong Lin Wang, a professor of materials science at the Georgia Institute of Technology. ‘There has been a lot of interest in making nanodevices, but we have tended not to think about how to power them. Our nanogenerator allows us to harvest or recycle energy from many sources to power these devices.’

The nanogenerators take advantage of the unique coupled piezoelectric and semi conducting properties of zinc oxide nanostructures, which produce small electrical charges when they are flexed.

Fabrication begins with growing an array of vertically aligned nanowires approximately a half-micron apart on gallium arsenide, sapphire or a flexible polymer substrate. A layer of zinc oxide is grown on top of substrate to collect the current. The researchers also fabricate silicon ‘zigzag’ electrodes, which contain thousands of scale-scale tips made conductive by a platinum coating.

The electrode is then lowered on top of the nanowire array, leaving just enough space so that a significant number of the nanowires are free to flex within the gaps created by the tips. Moved by mechanical energy such as waves or vibration, the nanowires periodically contact the tips, transferring their electrical charges. By capturing the tiny amounts of current produced by hundreds of nanowires kept in motion, the generators produce a direct current output in the nano-Ampere range.

The researchers expect their nanogenerator could produce as much as 4 watts per cubic centimetre, based on a calculation for a single nanowire. That would be enough to power a broad range of scale-scale defence, environmental and biomedical applications, including biosensors implanted in the body, environmental monitors and nanoscale robots.

Providing power for scale-scale devices has long been a challenge. Batteries and other traditional sources are too large, and tend to negate the size advantages of nanodevices. And since batteries contain toxic materials such as lithium and cadmium, they cannot be implanted into the body as part of biomedical applications.

Because zinc oxide is non-toxic and compatible with the body, the new nanogenerators could be integrated into implantable biomedical devices to wirelessly measure blood flow and blood pressure within the body. And they could also find more ordinary applications.

‘If you had a device like this in your shoes when you walked, you would be able to generate your own small current to power small electronics,’ Wang said. ‘Anything that makes the nanowires move within the generator can be used for generating power. Very little force is required to move them.’