The first type of device employed a carbon nanotube cantilever. Dubbed a cantilever-based nanotube fuel-cell muscle, the system converted chemical energy (in the form of hydrogen fuel) to electrical energy, which it could then use for movement, other needs or storage.

The cantilever consisted of a strip of nanotube sheet covered with platinum-coated carbon and the ionic polymer Nafion. As well as functioning as the muscle actuator, this structure formed the cathode of the fuel cell and was immersed in sulphuric acid electrolyte. Also in contact with the electrolyte was a second electrode formed from a platinum-carbon-Nafion layer on a Nafion-117 membrane. This membrane separated the hydrogen fuel from the electrolyte.

Actuation of the nanotube sheet resulted from the injection of holes that took place when oxygen was reduced at the nanotube electrode. Hole injection produced a dimensional change in the sheet because of a combination of quantum mechanical and electrostatic effects. The 3 cm-long nanotube cantilever deflected by around 2 mm in roughly 5 s. Shorting the electrodes returned the cantilever to its original position in around 1 s.

The team's continuously shorted fuel-cell muscle, on the other hand, incorporated a shape-memory wire coated with nanoparticles of platinum catalyst. This type of muscle converted the chemical energy of the fuel into thermal energy, which caused actuation.

Made from nickel titanium alloy, the shape-memory wire acted as a single electrode, functioning as a shorted electrode pair. Adding oxygen or air to fuel in the form of hydrogen, methanol vapour or formic acid vapour caused the wire to heat above its austenitic phase transition temperature and contract. When the fuel was interrupted the wire cooled to below its martensitic phase transition temperature and returned to its original length.

Typically the wire contracted by around 5% and supported stress of 150 MPa. That gave it a stress-generation capability around 500 times that of human skeletal muscle, although it contracted by only a quarter of the amount of natural muscle. As a result, the continuously shorted fuel-cell muscle's work capability was 100 times that of skeletal muscle. Coiling the shape-memory wire would enable greater contractions, although this would be likely to also decrease the stress generation.

The artificial muscles could have applications in robotics, freeing up robots from being tethered to heavy battery packs. They could also find a use in prosthetic limbs, smart sensors, dynamic Braille displays, and smart skins for aerospace vehicles. Ultimately it may even be possible to use artificial muscles in the human body, by replacing the metal catalyst with tethered enzymes that can exploit food-derived fuels.

"The shorted fuel-cell muscles are especially easy to deploy in robotic devices, since they comprise commercially available shape-memory wires that are coated with a nanoparticle catalyst," said Ray Baughman of the University of Texas at Dallas. "The major challenges have been in attaching the catalyst to the shape-memory wire to provide long muscle lifetimes, and in controlling muscle actuation rate and stroke."

The researchers reported their work in Science.