Batteries not included

New sensor uses movement to power transmission of wireless signals

Imagine charging your iPod just by shouting into it, or powering a blood glucose monitor with your pulse. Such technologies aren’t far off: In the latest rendition of tiny, energy-scavenging devices, scientists have developed a prototype sensor that produces enough electrical charge when flexed mechanically to transmit a wireless signal several meters.

FLEX YOUR POWER When bent or flexed, the zinc oxide nanowires in this little generator produce enough electrical charge to power a sensor and wireless transmitter, no batteries necessary. Such sensors might be used to monitor the integrity of buildings or bridges. Y. Hu et al./Nano Letters/2011

With further improvements, the new device might be part of an array of sensors that can monitor the strength of a bridge, for example, while powering itself with the vibrations of trucks rumbling overhead, says Zhong Lin Wang of Georgia Tech, who led the new work.

The device is powered by zinc oxide nanowires that generate charge when bent, a property found in some crystals such as quartz and even in cane sugar. Such piezoelectric materials (from the Greek piezein, for “squeeze” or “push”) have already found their way into everyday devices — some cars, for example have piezoelectric crash sensors in the airbag wiring.

Wang and his colleagues put layers of their piezoelectric nanowires, on either side of a flexible piece of polyester and sandwiched that between two metal electrodes. Then the researchers wired the penny-sized device to a little capacitor connected to a radio transmitter. When flexed between two fingers, the nanogenerator produces charge and stores it in the capacitor. The device has an output of about 10 volts and an output current of more than 0.6 microamps, Wang and his team report in the June 8 Nano Letters. That’s enough to send out a wireless signal every five minutes that’s detectable more than 10 meters away, Wang says.

Energy scavenging isn’t new, of course — windmills, waterwheels and solar panels all capture power from the environment. Today scientists are refining and optimizing different strategies and materials, trying to figure out which energy-harnessing devices work best where, says electrical engineer Joseph Paradiso of MIT.

“I’m sure there’s a niche for this,” he says of the new research.

Other researchers are focused on piezoelectric devices that use body power. A team led by Princeton University’s Michael McAlpine recently reported creating nanoribbons that might harvest the motion of lungs breathing to charge a pacemaker battery, for example. Nanofibers that generate charge when pulled and twisted could eventually be spun into clothes for powering personal electronic devices with routine movement, the University of California, Berkeley’s Liwei Lin and colleagues reported last year. And researchers in Korea are harnessing the voice’s vibrational energy to charge cell phones. You might even charge a phone with button-power while banging out texts.

Plain old pushing power has demonstrated potential. Paradiso and colleagues put piezoelectric devices into sneakers more than a decade ago to capture energy from each heel strike. “Without interfering with gait,” he says, “you can extract about a watt from a foot.”

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