How can a gecko affect a football game? “By selling car insurance on commercials during halftime?” would be the wrong answer. Rather, instead of watching a gecko sell insurance, one would have to look carefully at a gecko hanging off the glass walls of its aquarium, much like a reptilian Spider-man, to see that football pros could learn a lot from geckos. This is exactly what researchers at the University of Dayton and Georgia Technical Institute have done (Qu et al. 2008). Researchers garnered inspiration from a paper published in Nature (Autumn et al. 2000) which examined the pad of a gecko to determine exactly how it is able to stick so effortlessly to something as slippery as glass. They then applied the sticky gecko pad concept as they created a carbon nanotube array on a small piece of silica. What researchers over the years have been able to determine is that a gecko’s foot can have upwards of 500,000 seta, which are small fibers that are composed of keratin with spatula-shaped portions at the end (see figures 1 and 2)
Keratin, a polymeric protein comprised of several units of various forms of keratin monomers, is the same material found in our hair and skin, though gecko’s cells align the keratin in a different fashion than our bodies. Mammals produce a form of keratin called alpha-keratin that has within it lots of alpha-helices (Huggins 1957), basically columns that stack together (Huggins 1977), that are found in our skin, fingernails, hair, and elsewhere in our bodies (Moll et al. 2008). Geckos, and reptiles in general, produce and assemble keratin together in a form called beta-keratin, which contains within it repeating beta-pleated sheets instead of alpha helices (Maderson 1964). The geckos with their beta-keratin also have the extra feature of these strong, rigid setae on their pads, which enable them to perform their miraculous wall-climbing feats. Researchers have determined the pad of a gecko can support up to 10 Newtons per cm2 (N cm-2) (Irschick et al. 1996); however, the carbon nanotube silica pad produced by the Dayton and Georgia Tech researchers produced 100 N cm-2! In other words, if the same gecko weighed 10 times as much, it would still be able to hang from the glass wall safely if it had the silica and carbon nanotubes on its pad!
So, one asks, how can this relate to football? How can the pad of a gecko provide an advantage to improve my favorite high school/college/NFL team’s chances at winning? Notice the gloves some of the players wear. These gloves, as one would find out quickly after wearing a pair, amplify the grip of the athlete on the football, allowing them to make even more amazing and athletic catches. The underlying concept of these gloves revolves around the material covering the fingers of the glove, which has a higher coefficient of friction to the material of the football than skin does. Basically, this means that the material on the glove ‘sticks’ to the football better than your skin. Cutters, a leading manufacturer of football receivers’ gloves, has data showing that their gloves have some of the most ‘tack’ in the field (Basic Guide to van der Waals forces 2012).
How do any of the materials, be it silica and carbon nanotubes, or material found on the football players’ gloves, work? It’s a similar question one might ask when playing with super glue: how does glue stick two different materials together? This is a question that has been posed for some time and was even addressed at the First International Congress on Adhesion Science and Technology in 1996 (Somorjai 1998). Generally speaking, adhesion can be divided into two classes: Strong adhesion and Weak adhesion. Strong adhesion deals with the strong chemical bonds of an adhesive to a material, where the adhesive’s interactions are ordering on the magnitude of, or at times stronger than, the bonds within the material the adhesive is sticking to. Weak adhesion deals primarily with van der Waals force, and it is thought that the setae on the gecko’s pads operate using these forces. Van der Waals forces are a form of weak intermolecular interactions with which molecules and atoms can interact with each other (Basic Guide to van der Waals forces 2012). Though van der Waal forces may be weaker than actual chemical bonds between atoms, if there are enough molecules present, for example on the pad of a gecko, the interactions generated between the pad of a gecko and a glass wall can be quite large.
The various strengths of intermolecular forces and interactions at the molecular level lead to the concepts of friction, lubrication, and adhesion. The concept of adhesion has industrial applications (such as motor oil as a lubricant in an engine and ‘tacky’ gloves for athletes), but in order to test the ‘adhesiveness’ of a substance, scientists must have methods to test a substance’s properties. One method used to test the ‘adhesiveness’ of a substance is atomic force microscopy (“AFM”).
AFM is conducted by using an extremely small pin, approximately one atom wide, attached to a lever. As the pin is lowered towards a surface to be tested and moved around the surface of the material, the tip starts to move in response to the molecules at the surface of the substance. The movements of the lever are recorded. When the interactions are compared to other materials, it provides insight into how the properties of different materials compare. The types of forces a pin tip can experience are chemical bonding, magnetic forces, electrostatic forces, and van der Waals forces, among others (Lang et al). A similar technique, scanning force microscopy, was invented by Gerd Binnig and Heinrich Rohrer in the 1980’s, but led to the discovery of AFM. The extreme utility of these techniques led to Binnig and Rohrer winning a Nobel Prize in Physics in 1986. Researchers all over the world have used AFM and variations of it to examine various types of materials and test them for their adhesive forces (Thomas et al. 1995) as well as to examine molecular origins of friction.
When companies start to design products like receivers’ gloves, they are not just going out and using AFM to find the stickiest substance around. A material known as Stickum, produced by Mueller Sports Medicine, was used by Lester Hayes in the NFL in the 1970’s. His liberal use of the material prompted a ban on the substance. Stickum itself has been redesigned from the original gooey, stick-to-everything substance to a new powder that is sprayable. Companies and researchers are still perfecting the ‘dry’ adhesive technologies, going from leather gloves, leather gloves treated with tackifying resins, and leather gloves treated with a silicon resin to enhance frictional forces (Kang 2002). Regardless of how, science has helped make football more competitive and exciting by providing highly advanced polymer coated gloves that, when coupled with some of the most athletic people in the world, enable breath-taking, one-handed, catches.
By: Bret Van Ausdall, University of Utah
Autumn, K., Y.A. Liang, S.T. Hsieh, W. Zesch, W.P. Chan, T. W. Kenny, R. Fearing, and R.J. Full. 2000. Adhesive force of a single gecko foot-hair. Nature 405:681.
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Cutter Fall Catalogue. 2012. Retrieved from http://www.cuttersgloves.com/catalog_fall12/
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Irschick, D. J., C.C. Austin, K. Petren, R.N. Fisher, J.B. Losos, and O. Ellers. 1996. A comparative analysis of clinging ability among pad-bearing lizards. Biological Journal of the Linnean Society 59:21.
Kang, H. S. “Athletic glove having silicone-printed surface for consistent gripping ability in various moisture conditions.” Patent 6,408,442. 25 June 2002.
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Qu, L., L. Dai, M. Stone, Z. Xia, and Z.L.Wang. 2008. Carbon nanotube arrays with strong shear binding-on and easy normal lifting-off. Science 322:238
Somorjai, G. A. 1998. The molecular surface science of adhesion. Pages 3-15 in W.J. van Ooij and H.R. Anderson, Jr., editors. First International Congress on Adhesion Science and Technology. Ridderprint bv, Ridderkerk, The Netherlands.
Thomas, R. C., J.E. Houston, R.M. Crooks, T. Kim, and T.A. Michalske. 1995. Probing adhesion forces at the molecular scale. Journal of the American Chemical Society 117:3830.
Articles by Bret Van Ausdall.