The research, described in Thursday's edition of the journal Nature, combines the precision of ultra-small nanotechnology with the elegant principle known as the piezoelectric effect, in which electricity is generated when pressure is applied to certain materials.
While the piezoelectric effect has been understood at least as far back as the 19th century, it is getting creative new looks now, as concerns about energy supplies are inspiring quests for alternative power sources.
For example, a Japanese railway has experimented with mats, placed under turnstiles, that translate the pressure from thousands of commuters' footfalls into usable power. French scientists have proposed capturing energy from raindrops hitting a structure with piezoelectric properties.
For the research described in Nature, Zhong Lin Wang and colleagues at the Georgia Institute of Technology covered individual fibers of fabric with nanowires made of zinc oxide. These wires are only 50 nanometers in diameter -- 1,800 times thinner than a human hair.
Alternating fibers are coated with gold. As one strand of the fabric is stretched against another, the nanowires on one fiber rub against the gold-coated ones on the other, like the teeth of two bottle brushes. The resulting tension and pressure generates a piezoelectric charge that is captured by the gold and can be fed into a circuit.
The allure of the idea is that it doesn't take unusual movement to generate usable electricity. Pretty much anything someone does while wearing a piezoelectric shirt would be productive.
"The beauty of this work is that if you have wind, or you have sonic waves, or you have vibrations, that works for you," Wang said. "You do not need a very large force for that."
Wang has coaxed the wires to grow around strands of yarn in a few square millimeters of fabric, but has not made sizable pieces yet. But he estimates that one square meter of nanowire-infused fabric would produce around 80 milliwatts of electricity, enough to recharge portable music players.
"This work represents a significant achievement," said Charles Lieber, a Harvard University researcher who also is pursuing nanotech power generation and was not involved in Wang's project.
Lieber noted that the research also could lead to biological sensors and other nanoscale devices that produce their own power from movement or sound waves. For such nanodevices to be feasible, "harvesting energy from the environment is a key technology," Lieber said.
Although Wang used gold in the research, he expects less expensive metals would work just as well as conductors. Whatever metal is used, it would be laid down in such tiny increments that he does not believe it would substantially increase the weight of an article of clothing.
However, there is one big hurdle to the advent of power shirts.
Though zinc oxide makes a nice sunscreen, it's not really
waterproof. The Georgia Tech team must figure out how to
protectively coat the nanowires -- or else one trip through the
washing machine or one rainy day would rob these fabrics of their
magic.
