However, much work remains to be done in developing the materials. For one thing, as greater loads are applied to actuators, they can start to exhibit “creep”–that is, they do not completely return to their original state with successive cycles. Baughman says that before these actuators can be useful, creep must be eliminated. “Under load, the cycle is not reversible–you’ve got a little creep. In most actuator applications, you don’t want any creep.”
Another key issue is scaling up from thin individual threads. Although the carbon-nanotube muscles can outperform natural muscles on a per-area basis, exerting 100 times the force, natural muscles are much larger, making them stronger. This scale-up issue has been a challenge for artificial muscles in general, which is why they still can’t beat human muscles in such functions as arm wrestling, Bar-Cohen says.
Despite the challenges, Baughman’s work so far represents important advances for carbon nanotube-based artificial muscles. “[Baughman] has really taken these very far in terms of processing,” says Elizabeth Smela, professor of mechanical engineering at the University of Maryland. “The fact that he can form transparent conducting sheets, yarns, and other materials out of these carbon nanotubes is attractive. Processing is very important. You can have a promising material, but if you can’t figure out how to process it to make things out of it, it doesn’t do you much good.”