A shirt that harvests energy from the wearer's physical motion converts it into electricity for powering personal electronic devices. Georgia Institute of Technology nanotechnology researchers have shown how pairs of textile fibers covered with zinc oxide nanowires generate electricity in response to applied mechanical stress. Known as "the piezoelectric effect," the resulting current flow from many fiber pairs woven into a shirt or jacket could allow the wearer's body movement to power a range of portable electronic devices such as MP3 players, GPS navigators or military equipment. The fibers could also be woven into curtains, tents or other structures to capture energy from wind motion, sound vibration or other mechanical energy. A magnified SEM image of two microfiber brushes meeting "teeth-to-teeth." The top one is coated with gold and the bottom one is covered with zinc oxide nanowires. Electrical current is created when the fibers scrub together. (Image courtesy Z.L. Wang and X.D. Wang, Georgia Tech) The microfiber-nanowire hybrid system builds on the nanowire nanogenerator that Wang’s research team announced in April 2007 (See also: Nanogenerator Converts Tiny Movements to Electric Current). That system generates current from arrays of vertically-aligned zinc oxide (ZnO) nanowires that flex beneath an electrode containing conductive platinum tips. The nanowire nanogenerator was designed to harness energy from environmental sources such as ultrasonic waves, mechanical vibrations or blood flow. "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," said Zhong Lin Wang, a professor of materials science and engineering at the Georgia Tech. "Many of these devices could be put together to produce higher-power output." “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 while walking.” The concept behind a microfiber nanogenerator. Microfibers coated with gold (yellow fibers) scrub the nanofibers that are not coated with gold (green) to produce electricity through a coupled piezoelectric-semiconductor process. (Image courtesy Z.L. Wang and X.D. Wang, Georgia Tech) Wang and collaborators Xudong Wang and Yong Qin have made more than 200 of the fiber nanogenerators. Each is tested on an apparatus that uses a spring and wheel to move one fiber against the other. The fibers are rubbed together for up to 30 minutes to test their durability and power production. They have measured current of about four nanoamperes and output voltage of about four millivolts from a nanogenerator that included two fibers, each one centimeter long. With a much improved design, Wang estimates that a square meter of fabric made from the special fibers could theoretically generate as much as 80 mWs. Fabrication of the microfiber nanogenerator begins with coating a 100-nm seed layer of zinc oxide onto the Kevlar using magnetron sputtering. The fibers are then immersed in a reactant solution for approximately 12 hours, which causes nanowires to grow from the seed layer at a temperature of 80 °C. The growth produces uniform coverage of the fibers, with typical lengths of about 3.5 µm and several hundred nanometers between each fiber. Close up image shows a pair of entangled fibers that make up a microfiber nanogenerator. Both fibers are coated with zinc oxide nanowires; one fiber is also coated with gold. When rubbed together, they generate electrical current. (Georgia Tech Photo: Gary Meek) To help maintain the nanowires’ connection to the Kevlar, the researchers apply two layers of tetraethoxysilane (TEOS) to the fiber. “First we coat the fiber with the polymer, then with a zinc oxide layer,” Wang explained. “Then we grow the nanowires and re-infiltrate the fiber with the polymer. This helps to avoid scrubbing off the nanowires when the fibers rub together.” Finally, the researchers apply a 300-nm layer of gold to some of the nanowire-covered Kevlar. The two different fibers are then paired up and entangled to ensure that a gold-coated fiber contacts a fiber covered only with zinc oxide nanowires. The gold fibers serve as a Shottky barrier with the zinc oxide, substituting for the platinum-tipped electrode used in the original nanogenerator. Georgia Tech Professor Zhong Lin Wang shows a microfiber nanogenerator composed of a pair of entangled fibers. Both fibers are coated with zinc oxide nanowires; one fiber is additionally coated with gold. When rubbed together, they generate electrical current. (Georgia Tech Photo: Gary Meek) To ensure that the current they measured was produced by the piezoelectric-semiconductor effect and not just static electricity, the researchers conducted several tests. They tried rubbing gold fibers together, and zinc oxide fibers together -- neither of which produced current. They also reversed the polarity of the connections, which changed the output current and voltage. By allowing nanowire growth to take place at temperatures as low as 80 °C, the new fabrication technique would allow the nanostructures to be grown on virtually any shape or substrate. As a next step, the researchers want to combine multiple fiber pairs to increase the current and voltage levels. They also plan to improve conductance of their fibers. So far, there is just one wrinkle, so to speak: how to wash the fabric. Zinc oxide is sensitive to moisture, so in real shirts or jackets, the nanowires would have to be protected. The research, published in today's issue of Nature, was funded by NSF's materials research division; other sponsors are the US Department of Energy and the Emory-Georgia Tech Nanotechnology Center for Personalized and Predictive Oncology. For more information, visit: gatech.edu