It may be light as air, but new artificial muscle is no wuss

It's far lighter than a featherweight but packs a powerful punch — relatively speaking. The new artificial muscle invented by U.S. researchers is, at once, light as air, stiffer than steel and more flexible than rubber, and has one other quality that makes it an ideal candidate for space travel.

Material could be used for efficient solar cells and LEDs in near future

At left is the artificial muscle with no voltage applied. When a voltage is applied, the material expands in width, as shown in the middle (room temperature) and right (high temperature) images. ((University of Texas at Dallas))
It's far lighter than a featherweight but packs a powerful punch — relatively speaking.

The new artificial muscle invented by researchers at the University of Texas at Dallas is, at once, light as air, stiffer than steel and more flexible than rubber, and has one other quality that makes it an ideal candidate for space travel.

"The big strength with the present muscles is that they can operate over this enormous temperature range," said Ray Baughman, an author of a paper describing the material, which will be published in Friday's issue of Science.

That range begins below the extreme cold of liquid nitrogen and stretches all the way to the extremely hot melting point of iron, observed University of British Columbia professor John Madden, in a commentary published in the same issue of Science.

Artificial muscles, including some invented by Baughman about 25 years ago, already exist and are used commercially to do things like control the focus of cameras, Baughman told in an interview Thursday.

But those were based on an electrochemical process that slows down dramatically at low temperatures, making them unsuitable for many industrial and aerospace applications.

The new material is made of carbon nanotubes — carbon molecules that are extremely long, thin, and, like most molecules, microscopic — where the tubes are aligned in the same direction. More well-known forms of carbon include graphite, diamond and soot.

"Forests" of the tubes are grown at high temperatures, using acetylene gas as a starting material, and they are drawn into transparent, electricity-conducting sheets so thin that an ounce would cover an acre, said a news release from the University of Texas at Dallas.

The "muscle" is a type of material known as an aerogel.

Mostly made of air

This scanning electron microscope image of a carbon nanotube aerogel sheet densified using alcohol was taken perpendicular to the sheet plane. ((Courtesy of Ray Baughman/University of Texas at Dallas) )
"An aerogel just means a material that is largely air," Baughman said.

When a voltage is applied, charge is injected into the material, causing repulsion that pushes the nanotubes apart. That causes the material to expand dramatically in width and thickness, but contract in the length direction.

The change in length is only two per cent, but can generate more than 30 times higher stress than natural muscle for its weight, Baughman said.

"Think of a wine rack.… If the wine rack is almost fully collapsed, you pull a little bit in the direction which you elongate when it's fully collapsed and you get a giant strain perpendicular, in terms of per cent."

If all the air is taken out of it, the material is as dense as air before a voltage is applied.

"If we actuate them [make them move] … you get a material that is 10 times less dense than air and 10 times less dense than the lowest density previous aerogel that's reported in the Guinness Book of World Records," Baughman said. "It just rises like smoke."

The material is very stiff in the length direction (parallel to the nanotubes) but extremely flexible in the width direction.

Madden, who also studies artificial muscles, likened that to "having diamond-like behaviour in one direction and rubber-like behaviour in the others."

No nanotube airplanes yet

He suggested that the material is well suited for creating thin, stiff beams and plates for aerospace applications. In the future, if the material becomes capable of higher forces, it could be used in medical devices, robots or implants, he added.

Baughman said his new material is still a long way from being used in aircraft or spacecraft due to the difficulty of manufacturing carbon nanotubes on a large scale.

"If you want to use the carbon nanotube sheets for reinforcement material in the shell of an aircraft, you got a problem — there you need tons," he said. "It's not that you can't scale up to tons, it's that we're very far away from doing that."

He suggested that a more realistic, near-term application would be as an electrode material for efficient organic light-emitting diodes and solar cells. For that application, very little material would be needed.

In such devices, the weight per unit area determines how transparent the electrode is, and because light needs to enter and leave the device through the electrode, making it more transparent improves efficiency. In order to create an ultra-transparent electrode, a sheet of artificial muscle could be actuated to reduce its density by a factor of three. It could then be frozen in that ultra-light configuration.

Members of Ray Baughman's research team are, left to right: Anvar Zakhidov, Raquel Ovalle, Ray Baughman, Márcio Lima, Jiyoung Oh, Yuri Gartstein, Shaoli Fang, Ali Aliev, Alexander Kuznetsov, and Mikhail Kozlov. ((University of Texas at Dallas))