The miniscule devices that fall under the title of nanotechnology have huge potential, so long as they can utilize a power source better than a battery. The successful nanotech product will rely on its ability to scavenge “waste energy” ad infinitum (as Jonathan Swift would have said). In a recent article in the Scientific American (January, 2008), Zhong Lin Wang (Director of the Centre for Nanostructure Characterization, Georgia Institute of Technology) gives the example of the 1920’s watchmaker who invented the self-winding watch. Mechanically harvesting energy from the wearer’s moving arm and putting it to work rewinding the watch was a brilliant concept. This so called “waste energy” may appear in the form of vibrations, (even human pulse), acoustic waves, temperature differences, etc.

With the ability to make power on a minuscule scale, we can entertain the notion of a myriad of miraculous achievements like: implantable biosensors that continuously monitor a patient’s blood glucose levels; autonomous strain sensors for bridges, or environmental sensors to detect toxins. This would only work if replacement batteries weren’t needed. Enter the nanogenerator, a very small energy harvester of “waste energy” that supplies electrical power to a nanoscale device. Wang’s nanogenerators consist of an array of vertical zinc oxide nanowires, hexagonal crystals with both piezoelectric and semiconducting properties. An electrode with a ridged underside sits atop the nanowires and moves from side to side in response to some vibration (the human pulse or acoustic waves). As they move from side to side, the piezoelectric nanowires develop a voltage from the compressive and tensile strains on their sides. The semiconductor nanowires and conductive electrode rectify the alternating voltage and release it as direct current.

The body produces a variety of sources for “waste energy” that nanogenerators can tap into. For instance, blood flow produces 0.93 watts of mechanical energy (0.16 W electrical energy); exhalation produces 1.0 W of mechanical energy; walking produces 67.0 W of mechanical energy; and my fingers typing this blog post produce from 6.9 to 19.0 milliwatts of mechanical energy. Okay, that actually isn’t a lot of energy. But nanogenerators don’t need to power our homes. But in the future, nanogenerators may be used to harvest and recycle the energy wasted in our daily lives.