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First Artificial Muscle Arrays From Carbon Nanotubes


New developed sheets of single-walled nanotubes were shown by scientists from the Max Planck Institute for Solid State Research in Stuttgart/Germany, from the USA, Australia and Italy, to generate higher stresses than natural muscle and higher strains than high-modulus ferroelectrics (Science 21 May 1999).

What looks like a plate of spaghettis is an electron micrograph of the bucky paper. The noodles are not individual nanotubes but bundles or ropes of several hundred tubes. These ropes are entangled to form the paper. The bar at the bottom of the figure indicates the scale. It is 100 nanometers long. This particular bucky paper was made by Catherine Journet in Montpellier, France. The bucky paper which has been used for the actuator experiments looks similar in the electron microscope and has been made by Andrew Rinzler and his coworkers in Gainesville, Florida.

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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.

The strip actuator: Strips of bucky paper are on both sides of a scotch tape. Electrochemically, one side is charged negatively (with positive sodium counterions), and the other positively (with negative chloride counterions). Both sides expand (in neutral bucky paper chemical bonding is optimized), but the positive side expands more than the negative and the whole structure bends, like a human arm bends by contracting a muscle on one side and expanding its antagonist on the opposite side.

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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.

Length change of a straight strip of bucky paper (the double line is a hysteretic effect, i.e. there is some retardation when loading and unloadig the paper). The length changes by about 0.04%, if the potential of 1 Volt is applied. This seems to be a very small change. But compared to other actuator materials (piezoelectric devices) it is fairly large, and the voltage needed is fairly small. Moreover, the nanotube actuators work in aqueous environment, and in particular in salt water - like in sea water or under physiologic conditions.

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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.


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