And they could eventually be used to make more advanced prosthetic limbs, say researchers at University of Texas.

Writing in Science, they say these artificial muscles are 100 times more powerful than the body's own.

They said they could even be used in "exoskeletons" to give superhuman strength to certain professions such as firefighters, soldiers and astronauts.

The approach could transform the way complex mechanical systems were built

Dr John Madden, University of British Columbia

Two types of muscle are being investigated by US researchers at the Nanotech Institute at the University of Texas in Dallas, working with colleagues from South Korea.

Both release the chemical energy of fuels, such as hydrogen and alcohol, while consuming oxygen.

In effect they are replicating the first stage in "breathing" - by taking in oxygen. The existing form of artificial muscles are driven by batteries.

However, neither of the types developed by the Texan researchers resembles a normal muscle - being made up of wires, cantilevers and glass bottles.

'Mimicking nature'

The most powerful type, "shorted fuel cell muscles" convert chemical energy into heat, causing a special shape-memory metal alloy to contract.

Turning down the heat allows the muscle to relax.

Lab tests showed that these devices had a lifting strength more than 100 times that of normal skeletal muscle.

Another kind of muscle being developed by the team converted chemical energy into electrical energy which caused a material made from carbon nanotube electrodes to bend.

Dr John Madden, from the University of British Columbia in Vancouver, Canada, writing in Science, said "the approach could transform the way complex mechanical systems were built".

He said the artificial muscles mimicked nature in a number of ways.

"The muscle consumes oxygen and fuel that can be transported via a circulation system; the muscle itself supports the chemical reaction that leads to mechanical work; electrochemical circuits can act as nerves, controlling actuation; some energy is stored locally in the muscle itself; and, like natural muscle, the materials studied contract linearly."

But he said the challenge now was to create a circulation system like that of humans that replaces the wires in the artificial muscles.

Dr Madden said pressures needed to be generated so that waste gases could be produced, and the artificial muscles could truly be described as "breathing".