Prosthetic limbs could soon be covered in an electronic skin that mimics human touch, researchers say.

This week's issue of Science reports on the development of a flexible plastic skin that can communicate a sense of pressure to brain cells.

"Skin is probably one of the best wearable technologies that nature ever developed," said researchers Dr Benjamin Tee, an electrical engineer who carried out the research as part of a PhD at Stanford University in the Bao Reseach Group.

Dr Tee was inspired to study engineering as a child when he saw a robot rebuild Luke Skywalker's lost hand in Star Wars.

"The hand actually had skin that could feel," he said.

"I thought that was kind of cool.

"In this research we're interested in the cells that sense the force," said Dr Tee, who is now at the Agency for Science Technology and Research Singapore (A*STAR).

Skin on our fingertips contains pressure sensors that produce voltage pulses when we touch things.

The frequency at which these pulses are created gives the brain information about what we're touching.

"I tried to create an artificial system that mimics the biological one," Dr Tee said.

Dr Tee and colleagues made their electronic skin by coupling a layer of tiny rubber pyramids containing millions of conductive carbon nanotubes with a layer of plastic that had an electrical circuit printed on it.

A close up picture of the stretchable skin with artificial mechanoreceptors ( Bao Research Group, Stanford University )

The plastic skin generates voltage pulses when pressure is applied. The higher the pressure, the closer the carbon nanotubes are pushed together. This increases the conductivity frequency of the voltage pulses.

Importantly, unlike previous developments, this is the first artificial touch system to produce digital signals, which enables force information received by our fingertips to be encoded in the frequency of the pulse.

Dr Tee and colleagues report the electronic skin is capable of generating the same frequency of voltage pulses as human skin - up to 200 hertz.

Being digital also means the system is low powered, which is also important since, to mimic human touch, prosthetic skin will need to have thousands of sensors in the space of a fingertip to feel properly.

The researchers hooked their electronic skin it up to mouse brain neurones and a computer to show that when pressure was applied to the skin, the mouse neurones communicated this information.

They also used a computer to show a robot hand covered by the artificial skin could also feel pressure.

Fully functional artificial skin in five years?

At the moment the electronic skin can only detect static pressure rather than moving pressure, for example a brushing action.

However Dr Tee said he hopes to develop a range of different sensors within five years that can give artificial skin the full range of features involved in touch, including vibration, texture and temperature.

A battery layer will need to be added to the electronic skin, he added, and actual prosthetic skin would need to be connected to a person's nerves directly using "neuroprobes".

Dr Tee said a prosthesis that enabled the wearer to feel could also help relieve phantom limb pain.

The work was supported by the Xerox Foundation, Samsung Electronics as well as the Korean and Singaporean governments.