"Conductive polymers have been around for a long time, but processing them to make something useful degraded their ability to conduct electricity," said Professor Yueh-Lin Loo of Princeton University.

"People didn't understand what was happening."

"We discovered that in making the polymers mouldable, their structures are trapped in a rigid form, which prevented electrical current from travelling through them."

"We have figured out how to avoid this trade-off."

"We can shape the plastics into a useful form while maintaining high conductivity."

Professor Loo and her colleagues developed a technique to relax the structure of the plastics by treating them with an acid after they were processed into the desired form.

They were then able to make a plastic transistor by printing the plastic onto a surface -- a fast and cheap method similar to the way an ink-jet printer produces a pattern on a piece of paper.

"Being able to essentially paint on electronics is a big deal," Professor Loo said.

"You could distribute the plastics in cartridges the way printer ink is sold, and you wouldn't need exotic machines to print the patterns."

Conductive polymers could represent a low-cost alternative to indium tin oxide (ITO), an expensive conducting material currently used in solar panels.

Currently, the electricity generated by plastic solar cells is collected by a transparent metal conductor made of ITO.

This conductor must be transparent so that sunlight can pass through it to the materials in solar cells that absorb the light energy.

"The cost of indium tin oxide is skyrocketing," said Professor Loo.

"To bring down the costs of plastic solar cells, we need to find a replacement for ITO."

"Our conducting plastics allow sunlight to pass through them, making them a viable alternative."

The researchers are also investigating the use of plastics as cheap diagnostic tools in medicine.

For example, there are plastics that turn from yellow to green when exposed to nitric oxide -- a chemical compound produced during ear infections in children.

"You wouldn't need any fancy machines or lab equipment to diagnose an infection," Professor Loo said.

"All you would need is your eyes to see the colour change in the plastics."