Artificial muscles are likely to be essential components of robotics, prosthetic limbs, and a variety of micro-machinery. Quite a few designs are out there, involving materials like carbon nanotubes and silicon elastomers but, to one degree or another, these usually fail to operate as well as natural human muscles. In today’s issue of Science, University of Texas at Dallas scientists led by Ray Baughman report on a new type of muscle that dramatically outperforms biological ones in nearly every way.

Baughman’s research group created carbon nanotube aerogel sheets by pulling nanotubes from a mass of disordered tubes into organized bundles of ribbons. These bundles formed an aerogel with a surprisingly low density (about 1.5 mg/cm3), making them nearly as light as air. Just one gram of this material can cover an area of over 30 m2. Although these sheets can spread out, they are also compressible. Their thickness can be reduced 400-fold, decreasing their overall volume. Perhaps even more notable than their low density is their amazing elasticity, which is simultaneously combined with hardness.

The aerogel sheets can stretch both in thickness and width when electric current is pumped into them. The current causes adjacent bundles of nanotubes to repel each other, forcing the gel as a whole to expand.

Width-wise, it can elongate by 220 percent at a rate of 37,000 percent per second. That is superior to other carbon nanotube materials, which have a actuation rate of only 20 percent per second. To put that into perspective, natural muscles has a maximum rate of 50 percent per second, so the material outperforms muscle by a factor of over 700. The thickness of the sheets can also stretch to about 200 percent.

The hardness comes in because this material is inelastic along its length. Whenever the material stretches in width or thickness, it contracts in length, but only by a few percent. It can’t stretch in length because it's extremely rigid in that axis and can generate an isometric stress (isometric means without changing shape) of 3.2 MPa, which is 32 times more than the sustainable maximum for skeletal muscles. An accompanying perspective describes this difference in behavior between the two axes as follows: "this apparently unprecedented degree of anisotropy is akin to having diamond-like behavior in one direction and rubber-like behavior in the others."

The material can also operate in a large temperature range that would be impossible for biological materials. From 25�C to over 1200�C, the properties of the material remain relatively stable.

While Baughman’s artificial muscle is temperature-independent and superelastic and has high isometric stress-generation capability, biology isn't obsolete just yet. The new material doesn’t measure up to natural muscles in at least one way: maximum work achieved for every cycle. It can do 30 J/kg, which is lower than the 40 J/kg for real muscles.

Given the positive properties of these artificial muscles, they have the potential to allow prosthetics and future robots to operate with superior muscle elasticity and isometric stress generation, even at extreme temperatures. But Baughman and his colleagues also envision a variety of other uses for them. The new material has interesting optical properties, as it can diffract light in a direction that is perpendicular to the nanotubes' alignment and this can be modulated to different frequencies.

Finally, these muscles can be elongated and then immobilized in their stretched states at various densities. Combining that factor with the optical properties and voltage-dependent actuation, the authors propose using their material for “organic light-emitting displays, solar cells, charge stripping from ion beams, and cold electron field emission.”

Science, 2009. DOI: 10.1126/science.1168312