Nanotube aerogel sheets – better than real muscle?

Aerogels are incredibly lightweight (nearly lighter than air) and strong materials, and one of this blog’s most popular posts is a video demonstrating some amazing aerogel properties. Although it’s not a new material, I’ve felt that only recently have R&D techniques been able to mature enough to match aerogel’s capabilities. Indeed, now there is news of a new nanotube-aerogel application: artificial muscle. And a very strong and versatile muscle, at that. Information on this development, featured in a technical article and a brief review of the research in the new issue of Science, comes from a University of Texas team led by Ray Baughman. Baughman, director of UT’s NanoTech Institute, actually has been working in the field of nanotube-based materials for several years and developed a method of making nanotube yarn in 2005. His idea for a muscle-type application got a boost, according to a story on the UT website, by a visit from DARPA’s John Main:

During the visit, Main described his visions of a future that could include such advancements as artificial muscles for autonomous humanoid robots that protect people from danger, artificial limbs that act like natural limbs and exoskeletons that provide super-human strength to firefighters, astronauts and soldiers – all of which are able to perform lengthy missions by using shots of alcohol as a highly energetic fuel.

Baughman and his crew were then off and running, and soon reported in a 2006 issue of Science about their creation of two types of artificial muscle – one based on nanotubes and the other of wire – that could use alcohol or hydrogen to fuel dimensional changes in the material. In 2008, the group issued another report in Science about mechanical-dimensional properties of buckypaper composed of layers of single-walled nanotubes and multi-walled nanotubes. Again, from the UT website:

When most materials are pulled in one direction, they get thinner in the other direction, similar to how a rubber band behaves when it is stretched. However, specially designed carbon nanotube sheets, dubbed “buckypaper,” can increase in width when stretched. The buckypaper can also increase in both length and width when uniformly compressed.

Baughman reported that when additional layers of MWNTs were added, the buckypaper shifted from a positive Poisson’s ratio to a negative value. “This abrupt switching of the sign of Poisson’s ratio is so surprising and the structure of the nanotube sheets is so complicated that we initially believed that quantitative explanation was impossible using state-of-art theoretical capabilities,” said Baughman. Their conjecture then was that the material acts like a stretchable wine rack that can narrow as it’s stretched – unless it is constrained by other racks above and below. In their most recent discovery, Baughman’s team, building on their prior successes, found a way to make aerogel sheets by pulling nanotube into “organized bundles of ribbons.” The resultant material had a miniscule density, of about 1.5 mg/cm³. The material is also extremely elasticity and hard. Equally important is what happens when a current is applied: The aerogel expands rapidly both in thickness and width. They measured expansion rates of 37,000 percent per second, compared to the 50 percent per second for biological muscle.

At the same time, the material is inelastic along its length. This rigidity allows it to endure a stress of 3.2 MPa, 32 times more than natural muscles. Baughman and his group said, “[T]his apparently unprecedented degree of anisotropy is akin to having diamond-like behavior in one direction and rubber-like behavior in the others.” Researchers also noted that the properties of the aerogel sheets are temperature independent. The group sees obvious applications for artificial muscle in conjunction with a new generation of prosthetics and robotics. They also note that the aerogel sheets have unique tuneable optical properties that might make them useful for “organic light-emitting displays, solar cells, charge stripping from ion beams, and cold electro field emission.”