Fabric may make the first real power suit
Nanofibres made that produce power when rubbed together.
Mobile phone battery running out mid-conversation? One day you might be able to make a few vigorous arm movements while wearing a nanowire electricity-generating shirt to keep the battery going.
This is power-dressing in the real sense: nothing to do with shoulder pads or 1980s office dramas. Zhong Lin Wang and his colleagues at the Georgia Institute of Technology in Atlanta have made a yarn out of nanofibres that produce charge when they are rubbed against one another. Materials woven from these yarns could be used for self-powering clothes, shoes or biological implants such as pacemakers. The work is published in Nature.1
Wang became frustrated by the number of nanodevices being invented without the concurrent development of similarly sized powering technology. "No matter how small these devices are, they still need battery power," he says. Wang decided that a nanoscale energy-producing device was needed, and turned to mechanical ways of producing that energy — from body motion, wind or even a heartbeat. “All this mechanical energy is wasted,” he says.
Wang took standard synthetic Kevlar fibres and coated them with tetraethoxysilane, onto which they stuck a layer of zinc oxide. Crystals of zinc oxide grew outwards, forming crystalline rods protruding from the fibres like the hairs on a brush. The power comes from the zinc oxide, which is piezoelectric: when mechanical stress — such as bending, crushing or stretching — is applied to a piezoelectric material, it produces a voltage.
Wang's basic power-generating system involves two of these fibres, intertwined. One of the two has its 'brushes' coated with a thin layer of gold. When the fibres rub against each other, the stiffer gold-coated brushes bend the non-coated brushes. Because of the piezoelectric effect, charge builds up on these bristles, and is gathered up by the gold-coated ones. “The metal on the brushes collects all this charge,” says Wang. Wires attached to the ends of the gold-coated and non-gold-coated fibres can carry the current to a device, such as a lightbulb (see picture).
“It is so cheap and simple,” says Wang: the brushes can be made easily in the lab, in beakers, at 80 °C. “And it works.” When two strands were pulled and pushed against each other at a frequency of 80 times a minute, this produced 5-picoamp pulses of electricity. Although this is a tiny amount, when a number of the fibres were woven into a yarn, the effect was much bigger. A yarn made from six fibres produced a current up to 50 times larger than that measured for the two-fibre nanogenerator.
As for applications, Wang imagines a 'power shirt' being made. “We want to make a yarn, and from a yarn make a fabric.” With this, he suggests applications for soldiers to power radio equipment, for example. The material would have the advantage of harvesting energy — albeit a very small amount — from tiny movements. Even a heartbeat works to produce power, he says.
Because so far the biggest system made has comprised only six fibres, it is hard for Wang to estimate how much walking we are going to have to do to fire up our iPods, but the team estimates that a square metre of material could produce between 20 and 80 milliwatts of power from normal mechanical vibrations such as footsteps. That is still far short of the amount of power required to illuminate a standard light bulb — around 60 Watts — and a mobile phone, which needs about 1–3 Watts. Today, 5 Watt solar panels can be used to help top up mobile phone batteries in the field.
Wang is working to optimize the system. He has patents on the fibres and a number of companies interested in developing the technology, he says.
"It's certainly forward-looking," says Joe Paradiso, director of the Responsive Environments Group at the Massachusetts Institute of Technology’s media lab. He notes that it's not clear at this stage what kind of movement could generate the power needed for real applications, or whether these fibres could really work efficiently and reliably in something such as clothing or fabric.
Wang believes they will, and expects a material to be produced within three years that is fully functional, flexible and wearable — although whether it will be fashionable remains to be seen.
- Qin, Y., Wang, X. & Wang, Z. L. Nature 451, 809-813 (2008)