Nanotechnology that harvests energy from movement

The power shirt can convert movement such as muscle stretching and blood flow into electrical energy that may be sufficient to power nanodevices and nanosystems.

Pairs of textile fibres covered with zinc oxide nanowires generate current using the piezoelectric effect. Combining current flow from many fibre pairs woven into a shirt or jacket could allow the wearer's body movement to power a range of portable electronic devices.

The fibres could also be woven into curtains, tents or other structures to capture energy from wind, sound or other mechanical energy.

"The fibre-based nanogenerator would be a simple and economical way to harvest energy from physical movement," said Zhong Lin Wang, a Regents Professor in the School of Materials Science and Engineering at the Georgia Institute of Technology.

"If we can combine many of these fibres 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 current while walking."

The microfibre-nanowire hybrid system builds on the nanowire nanogenerator that Wang's research team announced in April 2007. That system generates current from arrays of vertically aligned zinc oxide nanowires that flex beneath an electrode containing conductive platinum tips. The nanogenerator was designed to harness energy from environmental sources such as ultrasonic waves, mechanical vibrations or blood flow.

The nanogenerators use the coupled piezoelectric properties of zinc oxide nanostructures, which produce small electrical charges when they are flexed.

After a year of development, the original nanogenerators — which are 2 x 3 mm² — can produce up to 800 nA and 20 mV.

The microfibre generators rely on the same principles, but are made from soft materials and designed to capture energy from low-frequency mechanical energy.

They consist of DuPont Kevlar fibres on which zinc oxide nanowires have been grown radially and embedded in a polymer at their roots, creating what appear to be microscopic baby bottle brushes with billions of bristles.

One of the fibres in each pair is also coated with gold to serve as the electrode and to deflect the nanowire tips.

"The two fibres rub together just like two bottle brushes with their bristles touching, and the piezoelectric-semiconductor process converts the mechanical motion into electrical energy," Wang said.

"Many of these devices could be put together to produce higher power output."

With an improved design, Wang estimates that a square metre of fabric made from the special fibres could theoretically generate as much as 80 mW.

The nanogenerator could be the foundation for exploring self-powering technology for in-situ, real-time and implantable biosensing, biomedical monitoring and biodetection, with potential for defence and civil applications.

"We are looking to power some small gas, chemical biosensors. We are also looking to power some small electronic components," Wang said.

"In the longer term, we hope to power an MP3 player, a small laptop computer and mobile phone."

As a next step, the researchers want to combine multiple fibre pairs to increase the current and voltage levels. The researchers also plan to improve conductance of the fibres.

The greatest challenge is to improve the output voltage and power, according to Wang. A key is to have high-quality samples and controlled packaging technology so that many nanowires can generate electricity continuously and simultaneously.

"The major hurdle is to improve the output power. Once we can raise the output voltage to 0.5–1 V, there are outstanding applications in many areas," Wang said.

However, one other significant challenge for the researchers is washing the power shirt. Zinc oxide is sensitive to moisture, so in real shirts or jackets, the nanowires would have to be protected from the washing machine.

"We have to protect it from direct contact with water. Therefore, the cloth made of such fibres has to be the smart shirt as well with multiple layers," Wang said.

"You put the power shirt in the middle, so that the inner and outer layers can be washed."

Wang estimates that it will be at least five years until the microfibre fabric is on the market.