And using the same nanotechnology, Georgia Institute of Technology researchers have also generated electrical current from a tapping finger – moving the users of BlackBerry devices, cell phones and other handhelds one step closer to powering them with their own typing.
“Using nanotechnology, we have demonstrated ways to convert even irregular biomechanical energy into electricity,” said Zhong Lin Wang, a Regent’s professor in the Georgia Tech School of Materials Science and Engineering. “This technology can convert any mechanical disturbance into electrical energy.”
The demonstrations of harnessing biomechanical energy to produce electricity were reported February 9 in the online version of the American Chemical Society journal Nano Letters. The research was supported by the Defense Advanced Research Projects Agency (DARPA), the U.S. Department of Energy, the U.S. Air Force, and the Emory-Georgia Tech Center for Cancer Nanotechnology Excellence.
The study demonstrates that nanogenerators – which Wang’s team has been developing since 2005 – can be driven by irregular mechanical motion, such as the vibration of vocal cords, flapping of a flag in the breeze, tapping of fingers or hamsters running on exercise wheels. Scavenging such low-frequency energy from irregular motion is significant because much biomechanical energy is variable, unlike the regular mechanical motion used to generate most large-scale electricity today.
The nanogenerator power is produced by the piezoelectric effect, a phenomenon in which certain materials – such as zinc oxide wires – produce electrical charges when they are bent and then relaxed. The wires are between 100 and 800 nanometers in diameter, and between 100 and 500 microns in length.
To make their generators, Wang’s research team encapsulated single zinc oxide wires in a flexible polymer substrate, the wires anchored at each end with an electrical contact, and with a Shottky Barrier at one end to control current flow. They then attached one of these single-wire generators to the joint area of an index finger, or combined four of the single-wire devices on a “yellow jacket” worn by the hamster.
The running and scratching of the hamster – and the tapping of the finger – flexed the substrate in which the nanowires were encapsulated, producing tiny amounts of alternating electrical current. Integrating four nanogenerators on the hamster’s jacket generated up to 0.5 nanoamps; less current was produced by the single generator on the finger.
Wang estimates that powering a handheld device such as a Bluetooth headset would require at least thousands of these single-wire generators, which could be built up in three-dimensional modules.
Beyond the finger-tapping and hamster-running, Wang believe his modules could be implanted into the body to harvest energy from such sources as muscle movements or pulsating blood vessels. In the body, they could be used to power nanodevices to measure blood pressure or other vital signs.
Because the devices produce alternating current, synchronizing the four generators on the hamster’s back was vital to maximizing current production. Without the synchronization, current flow from one generator could cancel out the flow from another.
The research team – which also included Rusen Yang, Yong Qin, Cheng Li and Guang Zhu – solved that problem by using a substrate that was flexible in only one direction, forcing the generators to flex together. Still, there was substantial variation in the output from each generator. The differences result from variations in the amount of flexing and from inconsistencies in the hand-built devices.
“The nanogenerators have to be synchronized, with the output of all of them coordinated so the current adds up constructively,” Wang noted. “Through engineering, we would expect this can be resolved in the future through improved design and more consistent manufacturing.”
To ensure that the current measured was actually produced by the generators, the researchers took several precautions. For instance, they substituted carbon fibers – which are not piezoelectric – for the zinc oxide nanowires and measured no output electrical signal.
The research team encountered a number of obstacles related to its four-legged subjects. Wang’s team first tried to outfit a rat with the power-generating jacket, but found that the creature wasn’t very interested in running.
At the suggestion of Wang’s daughter, Melissa, the researchers found that hamsters are more active creatures – but only after 11 p.m. They had to experiment with a jacket configuration that was tight enough to stay on and to wrinkle the nanogenerator substrate – but not so tight as to make the hamster uncomfortable.
“We believe this is the first demonstration of using a live animal to produce current with nanogenerators,” Wang added. “This study shows that we really can harness human or animal motion to generate current.”
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