Powerful Threads

Amid last week's London Fashion Week hoopla came the announcement of a decidedly more utilitarian fiber—one that can harvest energy from friction or low-frequency vibration. These fibers could be woven into fabrics that scavenge energy from body movements such as footsteps, providing power on the go (Nature 2008, 451, 809).

The practical threads, made by materials scientist Zhong L. Wang and colleagues at Georgia Tech, are flexible Kevlar fibers covered with zinc oxide (ZnO) nanowires. Wang's team entangled two of these fibers, which resemble tiny cylindrical brushes, and then applied gentle friction. Because ZnO is a semiconductor and has piezoelectric properties—it can generate a voltage in response to mechanical stress—the energy from the applied friction was converted to electricity.

Nanodevice expert Charles M. Lieber of Harvard University praises the "highly innovative materials chemistry" Wang's team employed to make Kevlar fibers that were still supple even after they were ensheathed in crystalline ZnO, calling it "a first." The key was to surround the fiber with a thin layer of seed ZnO crystals from which nanowires could sprout, Wang explains. "The thinner the seed crystal is, the more flexibility the fiber has." He emphasizes that the procedure is inexpensive and can be easily scaled up.

To generate electricity, the team wound together two ZnO nanowire-covered fibers, one of which they first coated with gold. Pulling the fibers using a tiny spring-loaded device makes the ZnO nanowires and their gold-coated counterparts grind together at the interfaces, like the gnashing of millions of microscopic teeth. The specialized junction between the gold metal and the ZnO semiconductor allows current to flow only in one direction, so that all of the currents add up constructively even as nanowires bend in different directions.

But just rubbing two fibers together doesn't generate enough power to be useful, Wang says. He now hopes to find an efficient way to weave fibers together that maximizes the power that they can generate.