Georgia Tech researchers have taken an important step toward creating fabrics that could generate power from the wearer’s walking, breathing, and heartbeats. The researchers, led by materials-science professor Zhong Lin Wang, have made a flexible fiber coated with zinc oxide nanowires that can convert mechanical energy into electricity. The fibers, the researchers say, should be able to harvest any kind of vibration or motion for electric current.
The zinc oxide nanowires grow vertically from the surface of the polymer fiber. When one fiber brushes against another, the nanowires flex and generate electric current. The researchers described a proof-of-concept yarn in a paper published this week in the journal Nature. They show that the output current increases by entwining multiple fibers to make the yarn.
By the researchers’ calculations, a square meter of fabric made from the fibers could put out as much as 80 milliwatts–enough to power portable electronics. The development could make shirts and shoes that power iPods and medical implants, curtains that generate power when they flap in the wind, and tents that power portable electronics devices.
In 2007, Wang and his colleague the 2007 TR 35 winner Xudong Wang (no relation) built a zinc oxide nanowire array that generated direct current when exposed to ultrasonic vibrations. The piezoelectric nanowires stood on an electrically conducting substrate that acted as an electrode. The other electrode was a platinum-coated silicon plate with parallel peaks and trenches carved on its surface. (See “Nanogenerator Fueled by Vibrations.”) When the ultrasonic waves pushed the electrodes together, the nanowires bent and produced current.
In the new work, the researchers have substituted the rigid, zigzag electrode with a flexible one. They convert some of the bendable fibers into electrodes by applying a thin layer of gold to them. These gold-plated fibers act as flexible electrodes.
The researchers entangle a gold-coated fiber with an uncoated fiber. When the fibers are pulled back and forth with respect to each other, the individual gold-plated nanowires push and bend the uncoated nanowires, generating current.
The flexibility of the fibers brings the idea of wearable, foldable energy sources closer to fruition, says Charles Lieber, a chemistry professor at Harvard University. The flexibility is also crucial for harvesting energy from extremely small ambient motion, says Thomas Thundat, who studies nanoscale biological sensors at Oak Ridge National Laboratory. Entwining the flexible fibers, he explains, leads to very close contact between the gold-coated and the uncoated nanowires. As a result, small motions, such as a light wind or walking movements, make the coated and uncoated nanowires brush against each other and generate current.
“The idea is ingenious,” says Min-Feng Yu, a mechanical-science and engineering professor at the University of Illinois at Urbana-Champaign. “It’s like you have millions of nanogenerators outputting electricity simultaneously, each at maximum performance.”.
The generator’s ability to capture small movements makes it especially useful for powering biological sensors, Thundat says. Microscale sensors can be implanted in the body to measure such things as cancer biomarkers and glucose. But chemical batteries are bulky compared with the tiny sensors, and they have a limited lifetime. “Implanted sensors based on [the fiber nanogenerator] concept could use blood pressure or muscle movement for operation,” Thundat says.
The Georgia Tech advance would not be possible without the simple but highly innovative process the researchers have used to make the fibers, Lieber points out. Zhong Lin Wang and his colleagues first cover a polymer fiber with a 100-nanometer-thick zinc oxide layer. They immerse the fiber in a reactant solution at 80 °C, which results in nanowires growing vertically from the surface. Then the researchers use a final trick to keep the nanowires firmly attached to the fibers while keeping the fibers flexible. They dip the fibers in tetraethoxysilane, a liquid used in weatherproofing and protective coatings. The tetraethoxysilane forms two coatings: one between the fiber and the zinc oxide layer, and another on top of the zinc oxide layer.
This tetraethoxysilane coating makes the fiber robust. The zinc oxide layer did not crack or peel off even when the fiber was twisted. The nanowires also stayed put after the researchers continuously brushed two fibers against each other for 30 minutes. The fibers will have to last even longer and have higher output power in order to be used practically, Wang says.
Power-generating shirts might still be out of reach for most. At this point, the fabric might be affordable for the military for use in tents and shoes, says Wang, but “it is probably too expensive for you and me to buy.”