Liming Dai, a professor of materials engineering at the University of Dayton, and Zhong Wang, director of the Center for Nanostructure Characterization at Georgia Tech have developed an adhesive made of carbon nanotubes whose structure closely mimics that of gecko feet. It’s 10 times more adhesive than the lizards’ feet and, like the natural adhesive, easy to lift back up. And it works on a variety of surfaces, including glass and sandpaper.
The adhesive force of these nanotube arrays is about 100 newtons per square centimeter–enough for a four-by-four-millimeter square of the material to hold up a 1,480-gram textbook. [four-by-four-centimeters should hold 148 kilograms. So part of one hand or foot would be able support an average persons weight] And its adhesive properties were the same when tested on very different surfaces, including glass plates, polymer films, and rough sandpaper. One advantage of this adhesive over others is that its strength is strongly direction dependent. When it’s pulled in a direction parallel to its surface, it’s very strong. That’s because the branched nanotubes become aligned, says Dai. But when it’s pulled up with little force, as one would peel a piece of Scotch tape, the nanotubes lose contact one by one.
The adhesive sticks best when it is pulled down parallel to the surface it is sticking to–this is called shear adhesion. This action arranges the tips of the curly nanotubes so they have maximum contact with the substrate, thereby maximizing the Van der Waals force. Pulling the adhesive off in a motion perpendicular to the substrate is much easier–at this angle the sticking force is ten times weaker.
Other problems to solve before they’re commercially viable:
– Wall-climbing robots will require adhesives that work again and again without wearing out or getting clogged with dirt
– nanotube adhesives will need to be grown on different substrates [not just silicon] than those used so far
Dai says that carbon nanotubes’ versatility may help overcome the dirt problem. These structures can readily be functionalized with proteins and other polymers. Dai is developing adhesive nanotube arrays coated with proteins that change their shape in response to temperature changes. A robot could have feet that heat up when they get clogged, shedding dirt so that it can keep walking.
The ability of gecko lizards to adhere to a vertical solid surface comes from their remarkable feet with aligned microscopic elastic hairs. By using carbon nanotube arrays that are dominated by a straight body segment but with curly entangled top, we have created gecko-foot–mimetic dry adhesives that show macroscopic adhesive forces of 100 newtons per square centimeter, almost 10 times that of a gecko foot, and a much stronger shear adhesion force than the normal adhesion force, to ensure strong binding along the shear direction and easy lifting in the normal direction. This anisotropic force distribution is due to the shear-induced alignments of the curly segments of the nanotubes. The mimetic adhesives can be alternatively binding-on and lifting-off over various substrates for simulating the walking of a living gecko.
Separate research on mimicking gecko’s was a hybrid metal/polymer hierarchical structure that mimics all aspects of a gecko’s foot The hybrid nickel/polymer adhesive used an electric field to help release the adhesive. The nickel/polymer material had adhesive forces of roughly 1.4N/cm**2 in the normal direction. [the carbon nanotubbes have 100N/cm**2 adhesion force]