But for now, Baughman and his colleagues are focusing on optical applications for the material. Because carbon nanotubes are highly conductive, the flexible sheets could perhaps be used to make electrodes for solar cells and organic light-emitting diodes with controllable transparency and conductivity. “For that application, you want to tune the density of carbon nanotubes per unit area,” Baughman says. “That determines how much transparency the sheet has.” In the Science paper, the researchers show that the ribbons can be deposited on a silicon substrate in their expanded, more transparent state. The ribbons also diffract light so that they could perhaps prove useful in optical communications. Changing their dimensions sends different wavelengths of light in different directions.
The researchers make the material by growing entangled carbon nanotubes and then pulling intertwined nanotube bundles into ribbons. When a voltage is applied to the strips, the nanotubes become charged and push each other away, making the material expand. It normally returns to its original state when the voltage is removed.
The ribbons will probably still need to generate more force before they are practical for many applications. Right now, they generate 32 times as much force per unit area as heart muscles, which is a lot for their nanoscale dimensions, says Ian Hunter, a professor of mechanical engineering at MIT. However, electroactive polymers generate up to eight times as much force per unit area as the nanotube sheets. “For artificial muscle, you need a large change in force coupled with a large change in length,” Hunter says.
Polymer actuators also need just a few volts to contract. The ribbons, in contrast, require three to five kilovolts, which Hunter says is too high for use in humans and higher than ideal for robotics. However, he adds that “the nanotube ribbons will find many important applications because they change dimensions much faster than existing polymer actuators.”
The ultralow density of the sheets could be the reason why they do not generate large forces. John Madden, an electrical- and computer-engineering professor at the University of British Columbia in Vancouver, Canada, suggests that one way to increase the force that they supply could be to make the sheets denser and increase the degree of interlocking between the nanotubes.