If you thought that electricity-generating knee brace thing announced last week in the journal Science by researchers at Simon Fraser University in British Columbia looked like too much work, take heart. With any luck, we may soon be able to get watts out of our waistcoats. Or sweaters. Or shirts, all without moving a muscle. It's the best thing to happen to textiles since wash-‘n-wear.
The knee-mounted generator captures energy from leg muscles. But you still have to walk to generate the five watts of electricity per leg that the scientists calculate as the device’s output, or 13 watts if your get a move on—enough for 30 minutes of talk time on a mobile phone. The inventors, led by Max Donelan, think the device could replace the 30-odd pounds of batteries that U.S. soldiers typically carry to operate their electronic gear. But as I say, you still gotta walk.
Now nanotechnology researchers are announcing fabrics that scavenge mechanical energy from sources as leisurely as heartbeats and ambient noise, and turn it into electricity. Call it the ultimate power suit.
It works like this: start with synthetic Kevlar fibers from DuPont and coat them with tetraethoxysilane and crystals of zinc oxide. Zinc oxide is what’s called piezoelectric: when stressed by moving, it produces a voltage. Zhong Lin Wang of the Georgia Institute of Technology and colleagues intertwine the fibers into yarn, so that when the fibers rub against one another charge, builds up on the bristles. “The two fibers scrub together just like two bottle brushes with their bristles touching, and the piezoelectric-semiconductor process converts the mechanical motion into electrical energy,” he explains. An output wire carries the resulting current to any device you like, including a battery if you want to store it.
Here’s the beauty part, as Wang and colleagues report today in the journal Nature. Because the fibers are so small (nano, after all), even the slightest oscillation moves them—a heartbeat, a light breeze, reaching out for a poolside drink. The whole thing can be scaled up into power tents or curtains. How much juice are we talking about? Wang estimates the output at up to 80 milliwatts per square meter of fabric, enough to run personal electronics off your shirt.
“The fiber-based nanogenerator would be a simple and economical way to harvest energy from physical movement,” said Wang. “If we can combine many of these fibers in double or triple layers in clothing, we could provide a flexible, foldable and wearable power source that, for example, would allow people to generate their own electrical current.” He added, “. . . while walking”—but clearly that much exertion is not necessary. Just position yourself poolside in a light breeze, and you’re good to go.
Biggest challenge LabNotes sees? Washing your power shirt. Zinc oxide doesn’t like to get wet, so the fabric would have to be dry-cleaned.