Introduction
When I look out the window of my den at night I very often see geckos skittering around at remarkable speeds on the slick glass window pane. Of course I am delighted to have the geckos around because of their very hearty appetite for troublesome insect pests, but like many others, I have often wondered how they managed to maintain their footing on vertical surfaces and while running round upside down. An investigative trail, worthy of a Sherlock Holmes saga, has culminated in the identification of a truly novel mode of locomotion for geckos.
Over the years many different mechanisms have been suggested for the remarkable climbing abilities of geckos. One by one most of these have been ruled out. It was once thought that secreted adhesives might do the trick. However, geckos have no secretory glands on their feet. Some speculated that tiny suction cup-like structures might possibly be involved, but it was soon found that gecko feet can cling even in a vacuum. Still others speculated that tiny hooks and claws might be involved, much like a Velcro strip. This is a methodology some cockroaches employ. Even though geckos do have small claws at the end of their toes, it was shown that removal of these scarcely affected the climbing ability of these animals. Even charge/charge interactions were ruled out by observing geckos freely moving around in highly ionized chambers. Now, a research effort involving evolutionary biology, biomechanics and nanotechnology has come to the startling conclusion that geckos use van der Waals forces to keep their toes firmly attached to walls, ceilings and other surfaces.
In June 2000 a research team led by Robert Full, a professor of integrative biology at the University of California, Berkeley, and Kellar Autumn, a former post-doctoral fellow with Full and now a professor at Lewis and Clark College in Portland, Oregon, reported this remarkable proposition in a paper in Nature. To understand how they came to that conclusion one has to appreciate the detailed structure of the geckos’ feet. On the bottom of each foot are ruffled wave-like structures termed lamellae. These lamellae are in turn composed of many tiny hairs (called setae), each about one tenth the width of a human hair and scarcely one hundred micrometers long. Just like our own hairs, these gecko setae are made of the tough flexible protein keratin. There are about half a million of these setae per foot. The ends of these hairs are highly branched, looking, as Dr. Full has described it, like broccoli. Electron microscopy showed that each of the setae branches terminated in a flat pad-like structure, termed a spatula. It is these flat endings that are crucial to the generation of van der Waals interactions between geckos and walls and windows. There are between 100 to 1000 spatulae per seta, or a total of up to a billion spatulae per gecko. Drs. Full and Autumn worked with Professor Thomas Kenny and his graduate students Yiching Liang and Ben Chui at Stanford University to manipulate and measure the adhesive forces of a single gecko hair with a micromechanical sensor (MEMS). Also involved was Professor Ron Fearing and his team members, Dr. Pang Chan and Dr. Wolfgang Zesch, at Berkeley who developed a wire force gauge to assess binding strengths of the gecko hairs..
What the researchers found was that if the hairs were simply placed against a solid surface they showed very little adhesion. If, however, the setae were pushed lightly into the surface and then dragged slightly backward, the interactive forces increased dramatically (about 600 fold). These push and drag mechanisms mimic what a gecko does in real life, pushing lightly into a wall or ceiling and being dragged down by gravity. In microscopic terms what this interplay of hair setae on the geckos’ feet and the solid surface does is to very very closely align the millions of flat spatulae endings of the setae with the solid surface at atomic distances close enough for van der Waals interactions to come into play.
In fact, this was not the first suggestion that geckos might employ van der Waals interactions for adhering their feet to smooth surfaces. Back in the 1960’s the German scientist Ewe Hiller raised this possibility, but the microscopic tools for measuring the attachment forces of individual setae were unavailable at that time.
What are these van der Waals forces? The term van der Waals forces has been used to describe several types of very close range non-covalent interactions at the atomic level. These include London dispersion forces, hydrophobic interactions, dipole and induced dipole forces, as well as stacking interactions. The concept was originally introduced by the Dutch chemist Johannes van der Waals (1873-1923) in an effort to explain gas/liquid/solid transitions. The attractive forces in van der Waals interactions are transitory and result from atomic level electronic fluctuations. When electron-rich areas arise spontaneously on one object (for example on a gecko spatula) they can be close to an electron-poor (or positive) region on an adjacent surface (atoms of the ceiling, for example), providing an attractive interaction. The attractive forces involved are very weak compared with covalent interactions (less than one twentieth the strength) and they drop off dramatically with distance (r6-r8). However small these forces are individually, collectively with the half million or more setae on each gecko footpad and up to a billion total spatulae per gecko available for interaction, they provide enormous sticking power. It has been estimated that if all of the hairs on the gecko toes were fully interacting on a surface, the forces generated would be equivalent to those necessary to hold a small child to the ceiling.
You may recall that these same types of forces are involved in the stacking of the nucleotide bases in DNA and RNA, the self-assembly of phospholipids into membranes, and they play a large role in determining the folding pattern of globular proteins and are important in the binding of many substrates to the active sites of enzymes.
It is one thing for the gecko to be attached to wall surfaces with such a strong overall force, but, once adhered, how can the toes be detached? Anyone who has seen these animals moving across a window marvels at their agility. They can move at speeds of up to 3 feet per second and this requires that their feet attach and detach 15 times a second. Here high speed photography of geckos’ movements provided the answers. These films showed that when the foot hairs were levered upward about 30 degrees by a curling of the toes this allowed the spatulae to be readily pulled off the surface.
In a followup to the original research, Autumn and Full have also discovered that the gecko adhesion system is self-cleaning. They have fouled up the hairs on the animals’ feet with microscopic beads and find that within just a few steps all the beads are removed by mechanisms not yet fully understood. This amazing self-cleaning ability is yet another tantalizing prospect for potential practical applications of this overall propulsion system.
There are many possibilities for practical applications building upon the principles that geckos employ for locomotion. The researchers are already working with the company "irobotics Inc." of Somerville, Massachusetts, and an early prototype of a climbing robot incorporating some of the gecko-like features has been developed.
Much of the work carried out by the original research teams and by irobotics is supported by funding from the Office of Naval Research and from DARPA (Defense Advanced Research Projects Agency). These agencies clearly see the potential for devices such as micro-rovers for surveillance of hostile environments such as earthquake and explosion damaged buildings, for examination of places with high levels of radiation and even for exploring the surfaces of other planets. Or how about a pair of artificial gecko hair covered mittens. It could turn rock climbing into a whole new sport.
Gecko Web Site Links and Resources
General Chemistry Online - more on van der Waals forces
iRobot corperation - More about what is going on at irobotics
Dr. Full Laboratory - his lab's current research
Dr. Autumn's Laboratory - his lab's current research
Dr. Autumn has also put together a mini-presentation on a variety of adhesive mechanisms.
Kingsnake.com - Find out about the care and feeding of geckos
Scientific American - Gecko's sticky situation
Nature.com - You can learn more about the original Nature paper (Nature 405, 681-685, 2000) see some wonderful pictures of the structure of gecko feet and their adhesive structures as well as a short movie of a gecko in motion.
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