Researchers at the Rensselaer Polytechnic Institute have developed ultra-thin flexible batteries and energy storage devices that can be powered by blood, sweat or urine.

Most current batteries are made by layering two electrodes over two charge-holding layers, with an insulating layer in the centre rendering them bulky, rigid and heavy.

The new battery is a nano-composite paper made by growing carbon nanotubes on a silicon substrate and impregnating the gaps between the tubes with cellulose, which is essentially paper.

The carbon nanotubes act as electrodes allowing the device to conduct electricity while maintaining its flexibility. The cellulose means that it can be activated by the natural electrolytes found in bodily fluids like blood and sweat.

The design allows the battery to maintain its integrity despite being bent, folded or even cut into pieces.

"It is essentially a regular piece of paper, but made in a very intelligent way. We are not putting pieces together: it is a single, integrated device," said Robert Linhardt, the paper's co-author and professor at Rensselaer Polytechnic.

"The components are molecularly attached to each other: the carbon nanotube print is embedded in the paper, and the electrolyte is soaked into the paper. The end result is a device that looks, feels and weighs the same as paper."

Although current prototypes are about the size of a postage stamp, the researchers hope that this breakthrough will help redesign the next generation of electronics and implantable medical equipment.

As well as being environmentally friendly, the battery is completely integrated and contains no water, meaning that it can function in temperatures between -75 and 150 degrees Celsius.

However, one of the major current drawbacks may be the cost of manufacture on a large scale, compared with performance.

"The advantage of a flexible device would be that you could roll it in a film or a sheet. However, carbon nanotubes are very expensive," said Peter Kofinas, an engineering professor at the University of Maryland.

"It does not look like it performs better than currently available batteries and supercapacitors in the market, so from a commercial standpoint this would be very expensive if you want to make a large sheet out of this material."

The Rensselaer team has filed a patent for the invention and is now working on ways to improve the efficiency of the batteries and boost the power output by stacking them like a ream of paper.