The next not-so-big thing: Nanogenerators

Just as a machine won't run without a source of power, a device built at the nano-scale is of little use if it doesn't have the energy to work as a sensor or drug delivery particle. Since there isn't a battery in existence yet that's small enough to couple with a nanodevice, researchers have sought to power such microscopic creations with energy drawn from their surroundings. This energy so far has come primarily from chemical reactions (such as the oxidation of hydrogen peroxide), but that may change. A new study published today in Science describes the creation of a nanogenerator that transforms kinetic energy into a continuous flow of direct-current (DC) electrical energy.

Zhong Lin Wang, director of the Center for Nanostructure Characterization at the Georgia Institute of Technology, says the kinetic energy can come from a variety of sources, including ultrasonic waves, mechanical vibration or blood flowing through the body. (Wang wrote about self-powered nanotech machines for the January 2008 issue of Scientific American.)

Wang's nanogenerator consists of an array of vertically aligned zinc oxide nanowires (each about one micron long) standing about a half-micron apart on a flat piece of gallium arsenide, sapphire or a flexible polymer substrate. On top of that nanowire forest, Wang and his colleagues lowered a tiny plate containing thousands of silicon electrodes until they were touching the tips of the nanowires (sandwiching these wires between the substrate and the electrode plate). When the plate—which is wavy like a piece of corrugated cardboard—is pressed down on the wires, this flexing produces small electrical charges. "The elastic bending of the zinc oxide nanowires is a major advantage," Wang says. Carbon nanotubes, for example, are much less flexible.

Each nanowire can generate only about a 50 millivolt charge. That means that each generator would have to contain hundreds or thousands of nanowires, and many of these generators would have to be linked in parallel to produce a meaningful amount of energy that could power even the tiniest sensor. Wang says his short-term goal is to create a nanogenerator that can produce a 0.50 volt charge.

Such generators could be used to power sensors for detecting cancer or measuring blood sugar level for diabetics, Wang says. He adds that within five to 10 years, the technology will mature to the point that these generators could be placed in the soles of shoes or the fabric of clothes so that people will be able to power their iPods and cell phones using the mechanical energy created by the rustling of their clothes or compression of their shoe insoles as they walk.

The design for Wang's new mini-generator, however, poses several challenges. When nanowires are grown, they don't all end up the same length. Some wires might not be long enough to make contact with the plate and those that are will produce different levels of energy when pressed by the electrode plate.

Still, Wang sees a need to for his nanogenerator because no batteries exist that are small enough to power technology at the nano scale. Another issue, Wang points out, is that batteries tend to use toxic materials such as lithium and cadmium, which cannot be implanted in the body as part of a biomedical device. Zinc oxide, he notes, is non-toxic.

The schematic above shows the direct current nanogenerator built using aligned zinc oxide nanowire arrays with a "zigzag" top electrode. The nanogenerator is driven by an external ultrasonic wave or mechanical vibration and the output current is continuous. The lower plot is the output from a nanogenerator when the ultrasonic wave was on and off. Schematic courtesy of Zhong Lin Wang, Georgia Tech