First Artificial Muscle Arrays From Carbon Nanotubes

The dominating material in the second half of the 20th century is silicon. Silicon is the most investigated material, and because of our thorough knowledge, silicon is not only the basis of electronics, but also of micromechanics. Carbon has been mainly studied in the context of organic chemistry. But in the last few years the various modifications of pure carbon have attracted the attention of materials scientists. In the form of diamond, carbon is the hardest material known, and diamond also has the highest thermal conductivity. Graphite, on the other hand, is a good electrical conductor. Fullerenes ("Bucky Balls") and nanotubes ("Bucky Tubes") are new versions of graphitic carbon.

Carbon nanotubes are very thin and long tubes. Their diameter is only one or a few nanometers, which means that they are not thicker than typical molecules, and their length can be several micrometers and even millimeters. In electronics, these nanotubes are discussed as quantum wires and quantum dots. They can be used as components of nanostructured field effect transistors and single electron transistors. Their mechanical strength enables applications in nanocomposite materials and, because of the large surface area per weight, nanotubes are good candidates for all sorts of gas adsorption, including hydrogen storage for sustainable energy supplies.

From X-ray investigations of graphite we know that the honeycomb lattice expands if the graphitic sheets are electrically charged. Quantumchemical calculations predict that this is also true for carbon nanotubes: The tubes will increase their length if we change the number of electrons sitting on a tube. This effect can be used for electromechanical actuators. Actuators are the moving parts in robotics ("artificial muscles"). The relatively large length change and the high elastic modulus lead to very large figures of merit for carbon nanotube actuators. At the moment, mechanical experiments with individual nanotubes are still difficult, but the effect can easily be demonstrated with "Bucky Paper". Bucky paper is a free standing film of bundles of nanotubes.

The actuator effect of carbon nanotubes has been demonstrated by a multinational cooperation, funded by DARPA and lead by Ray Baughman, involving teams at AlliedSignal Inc. in Morristown, USA, at the University of Wollongong in Australia, the University of Pisa in Italy, the University of Florida in Gainesville, Fl, USA, the Georgetown University in Washington DC, and the Max Planck Institute for Solid State Research in Stuttgart, Germany. For this purpose, a strip of bucky paper has been dipped in salt water and electrochemically charged by changing the potential to +/- 1V versus a standard electrode. The length expansion has been made visible by sticking the bucky paper on a piece of inert material (e. g. scotch tape). If the bucky paper expands, the bi-strip bends and the motion can easily be seen by the naked eye.

Therefore, the new actuators open a vast field of potential applications, both by using bucky paper as a macroscopic material, and by using ropes or even individual carbon nanotubes for micro and nano actuator devices.