July 26, 2007 The prototype nanogenerator provides continuous direct-current electricity by harvesting mechanical energy from such environmental sources as ultrasonic waves, mechanical vibration or blood flow. Providing power for nanometer-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 the nanogenerator 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. This device could be in your shoes for example and when you walk you could generate your own small current to power small electronics. Anything that makes the nanowires move within the generator can be used for generating power.
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 “zig-zag” electrodes, which contain thousands of nanometer-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.
Zhong Lin Wang, Regents’ Professor in the School of Materials Science and Engineering at the Georgia Institute of Technology and his nano research group members Xudong Wang, Jinhui Song and Jin Liu expect that with optimization, their nanogenerator could produce as much as 4 watts per cubic centimeter – based on a calculation for a single nanowire. That would be enough to power a broad range of nanometer-scale defense, environmental and biomedical applications, including biosensors implanted in the body, environmental monitors – and even nanoscale robots. Additional development will be needed to optimize current production. “We need to be able to better control the growth, density and uniformity of the wires,” Wang said. “We believe we can make as many as millions or even billions of nanowires produce current simultaneously. That will allow us to optimize operation of the nanogenerator.”
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