The lightest material on Earth now packs a powerful punch.
Scientists from Texas and around the world have created a material that, by density, is lighter than air yet, when electrified, instantly and powerfully contracts. Their work is detailed in this week's issue of the journal Science.
"These artificial muscles are very lightweight and can do wonderful things," said Ray Baughman, the study author from the University of Texas at Dallas.
While the artificial muscle is unlikely to be used in humans or prosthetic limbs, Baughman says "these sheets of carbon nanotubes ... are of great practical interest for LEDs, solar cells, and other applications."
Aerogels and artificial muscles have been around for decades, but both materials have largely kept to themselves until now. Astronomers have launched spacecraft containing aerogels (so called because 99.8 percent of of the material is air) to gently capture space dust and keep it safe for the return journey to Earth.
Meanwhile, material scientists have created a variety of different artificial muscles, or materials that expand and contract when an electrical charge is applied. Many artificial muscles can only operate within a limited range of temperatures, however, because they contain liquid that will either freeze or evaporate at extreme temperatures.
Separately, aerogels and artificial muscles have unusual properties, and together they make a very odd couple indeed.
According to Baughman, the new aerogel is the least dense material known to man. It is 10 times more airy than the official "Guinness Book of World Records" title-holder — an aerogel made from silicon dioxide. The new carbon nanotube aerogel is so light that one piece, spread over an entire acre, would weigh one ounce.
Despite its extreme lightness, it's no featherweight. Pushed in one direction, the nanotube part of the muscle becomes stronger than stainless steel. When pulled at a right angle, however, the aerogel in the muscle becomes more elastic than rubber.
Unlike rubber, which has a relatively small range of temperatures in which it can function, the new aerogel can operate within a huge range of temperatures, from -196 degrees Celsius to more than 1600 degrees Celcius. That's cold enough to turn nitrogen into a liquid, and hot enough to melt iron.
Paradoxically, heating the carbon nanotubes (up to a point) actually makes them stronger. Unlike materials like iron that become softer as they are heated, the carbon nanotubes become stiffer the hotter they become.
The aerogel has another rare property; its density increases as it's stressed. Usually stretching a material spreads a set number of atoms over a larger area. Rubber bands are an exception — their sides come together slightly to create a denser material when stretched.
The new aerogel reacts in a similar way. And because aerogel has so much empty space and due to its specific physical structure, stretching it causes the material to contract dramatically — about 30 times greater than rubber.
Materials like this are exceedingly rare, said Tissaphern Mirfakhrai, a researcher at the University of British Columbia who creates carbon nanotube yarn.
"It's the lowest density material in existence. As you pull it, the nanotubes get closer and closer together and create less and less space in the structure."
The aerogel's properties become even more remarkable when electricity is applied. An electrical charge of less than a volt causes the aerogel to expand 220 percent in milliseconds, many times faster than normal human muscle can move.
Even at large voltages, the muscle can expand and contract with virtually no fatigue. Only when operating at extreme temperatures will the carbon nanotube aerogel start to degrade. At more normal temperatures the artificial muscle contracts and expands indefinitely.
The new artificial muscle is also stronger than human muscle. "If a human with two arms can lift 200 pounds, then they would be able to lift 32 times that amount," said Baughman.
Baughman doesn't like comparing his artificial muscle and real muscle, however. At this point, it is unlikely that artificial muscle will be implanted into humans to replace natural muscle tissue because of its size and the voltages necessary to operate it.
Instead, the transparent artificial muscle will likely first be used in solar cells, organic light emitting diodes (OLED) and incandescent light bulbs, among a wide variety of other potential applications. The carbon nanotubes act like electrodes, able to transmit electricity, and are also transparent, allowing light to pass through them. When an electrical charge is applied they become opaque.
Before any large scale applications are realized, scientists have to create larger amounts of the aerogel. To create the aerogel in the first place, the scientists use a technique called chemical vapor depositions (CVD). Much simplified, the scientists create an extremely fine mist of individual carbon atoms. These atoms settle on an iron plate and bind to each other to create a what looks like tubes of chicken wire.
The densely packed carbon nanotubes are 30 times taller than they are wide. A cross section of the nanotubes looks like a bamboo forest, according to Baughman. To create the aerogel, scientists pull a single "tree" from the forest, which drags other "trees" with it. As the nanotubes emerge they rotate and realign themselves to create the extremely lightweight aerogel.
Run electrical current through them and they become an artificial muscle.
"Our discovery of methods for producing these carbon nanotube sheets, their strange properties, and their corresponding remarkable performance as artificial muscles is just the beginning of a story," said Baughman. "My guess is that this story will have a happy ending in terms of new products that benefit humankind."