An improved carbon nanotube-based material creates, for the first time, directionally varied adhesive force and has a gripping ability nearly three times the previous record and 10 times as strong as a gecko, a team of researchers from four US institutions is reporting. This close-up view shows the elastic hairs that allow a gecko to climb walls and cling to ceilings. (Image: Wikipedia Commons) Scientists have long been interested in the ability of these lizards to cling to ceilings and even polished glass by their toes. The creatures owe this amazing ability to microscopic branched elastic hairs in their toes that take advantage of atomic-scale attractive forces to grip surfaces and support surprisingly heavy loads. Several research groups have attempted to mimic those hairs, called setae, with structures made of polymers or carbon nanotubes. Because there are so many pillars so close together, they are held tightly to the surface the gecko is walking on by a molecular force called the van der Waals force. This relatively weak force causes uncharged molecules to attract each other. In a paper published in the Oct. 10 issue of Science, researchers from the University of Dayton, the Georgia Institute of Technology, the Air Force Research Laboratory and the University of Akron describe successful construction of a gecko-inspired adhesive that is 10 times stronger than a gecko, at about 100 newtons per square centimeter. The carbon nanotube array could give artificial gecko feet the ability to tightly grip vertical surfaces while being easily lifted off when desired. Scanning electron microscope image of the vertically aligned multiwalled carbon nanotubes grown for this research. (Image courtesy Liangti Qu) "The resistance to shear force keeps the nanotube adhesive attached very strongly to the vertical surface, but you can still remove it from the surface by pulling away from the surface in a normal direction," said Liming Dai, the Wright Brothers Institute Endowed Chair in the School of Engineering at the University of Dayton. "This directional difference in the adhesion force is a significant improvement that could help make this material useful as a transient adhesive." The key to the new material is the use of rationally-designed multiwalled carbon nanotubes formed into arrays with "curly entangled tops," said Zhong Lin Wang, a Regents' Professor in the Georgia Tech School of Materials Science and Engineering. The tops, which Wang compared to spaghetti or a jungle of vines, mimic the hierarchical structure of real gecko feet, which include branching setae of different diameters. When pressed onto a vertical surface, the tangled portion of the nanotubes becomes aligned in contact with the surface. That dramatically increases the amount of contact between the nanotubes and the surface, maximizing the van der Waals forces that occur at the atomic scale. When lifted off the surface in a direction parallel to the main body of the nanotubes, only the tips remain in contact, minimizing the attraction forces, Wang said. Beyond the ability to walk on walls, the material could have many technological applications, including connecting electronic devices and substituting for conventional adhesives in the dry vacuum of space. "The contact surface area matters a lot," he said. "When you have line contact along, you have van der Waals forces acting along the entire length of the nanotubes, but when you have a point contact, the van der Waals forces act only at the tip of the nanotubes. That allows us to truly mimic what the gecko does naturally." In tests done on a variety of surfaces including glass, a polymer sheet, Teflon and even rough sandpaper, the researchers measured adhesive forces of up 100 Newtons per square centimeter in the shear direction. In the normal direction, the adhesive forces were 10 Newtons per square centimeter – about the same as a real gecko. Schematic shows the change in vertically-aligned multiwalled carbon nanotubes during adhesion measurements. (Image courtesy Liangti Qu) The resistance to shear increased with the length of the nanotubes, while the resistance to normal force was independent of tube length. Though the material might seem most appropriate for use by Spider-Man, the real applications may be less glamorous. Because carbon nanotubes conduct heat and electrical current, the dry adhesive arrays could be used to connect electronic devices. "Thermal management is a real problem today in electronics, and if you could use a nanotube dry adhesive, you could simply apply the devices and allow van der Waals forces to hold them together," Wang said. "That would eliminate the heat required for soldering." Another application might be for adhesives that work long-term in space. "In space, there is a vacuum and traditional kinds of adhesives dry out," Dai said. "But nanotube dry adhesives would not be bothered by the space environment." Liangti Qu, a research assistant in Dai's laboratory, grew the nanotube arrays with a low-pressure chemical vapor deposition process on a silicon wafer. During the pyrolytic growth of the vertically aligned multiwalled nanotubes, the initial segments grew in random directions and formed a top layer of coiled and entangled nanotubes. This layer helped to increase the nanotube area available for contacting a surface. Qu noted that sample purity was another key factor in ensuring strong adhesion for the carbon nanotube dry adhesive. For the future, the researchers hope to learn more about the surface interactions so they can further increase the adhesive force. They also want to study the long-term durability of the adhesive, which in a small number of tests became stronger with each attachment. And they may also determine how much adhesive might be necessary to support a human wearing tights and red mask. "Because the surfaces may not be uniform, the adhesive force produced by a larger patch may not increase linearly with the size," Dai said. "There is much we still need to learn about the contact between nanotubes and different surfaces." The research is sponsored by the National Science Foundation and the US Air Force Research Lab at Wright-Patterson Air Force Base near Dayton, Ohio. For more information, visit: www.gatech.edu