Lessons from a gecko
If you’ve spent some time in the southern United States you may have seen a gecko explode out of seemingly nowhere, scuttle across a ceiling, and then disappear, as if what they just did wasn’t some “Mission Impossible”-esque feat. They are able to do this because of the structure of their feet, and millions of little branch-like filaments on them called setae. The seta break down further into smaller bristles called spatula that are able to get so close to the surfaces the gecko interacts with that the electrons in the gecko hair interact with the electrons in the wall or whatever surface they are working their way across, and an electromagnetic attraction is created. These Van Der Walls forces are part of what allows the gecko to spider man its way around.
Shear force is the secret to the gecko’s ability to attach and detach with ease. The setae branch out at oblique angles from the feet, which allows for a greater surface area contact between the spindles and the surface the gecko will stick to, and therefore a greater shear force to support the gecko. In fact, if the angle between the toe hairs and the surface they stick to is increased beyond 30 degrees, the gecko will detach. This is sort of like peeling a piece of tape off a wall, or ripping off a bandage. It requires less force to peel both off from the edge than it requires to pull off with a force perpendicular to the surface. This allows them to run across surfaces with such ease, as their foot will detach with ease as the gecko lifts it off the surface to run further.
In addition, the geckos don’t have to clean their feet, and they don’t leave any additional residue. The particles of contaminants, like dirt, tend to cling to the surfaces, not the gecko feet, because the energy required to counterbalance the attraction between the dirt particle and the surface demands a higher number of spatulae on the feet to interact with the dirt particle than what is possible.
These properties have inspired a lot of research, much of which is ongoing, to develop products mimicking gecko feet. The collective of these products are called synthetic setae. This image to the right is part of a research paper from University of California, and you can find the whole paper here. Image B gives a closer view of the setae on the gecko feet, images E-H are images of synthetic setae of different widths, and images C and D are closer views of image F, which is synthetic setae of width 100 µm.
Early on, polymers alone were initially used to mimic the gecko feet for their flexibility and ease of fabrication. Recently, carbon nanotubes have been added into the mix as they have the highest strength to weight ratio of any known material. They also have a greater ratio between length and diameter compared to the polymers alone, and that will result in a greater surface area and larger shear force capability. These tiny gecko foot imitation surfaces are fabricated using MEMS and NEMS techniques, including photolithography and electron beam lithography, plasma etching, micro molding, and more.
Gecko tape was one of the earliest applications, it was made of polyimide fibers, and was able to support a 40 g spider man toy hanging by its hand. One study done at Stanford showed us that an adult human could in fact scale a wall like spider man, and you can see the researchers use synthetic setae to do their own stunt here. Synthetic gecko foot hair made of carbon nanotubes and PMMA polymer was shown to be capable of withstanding a shear stress four times higher than that of the gecko foot, even if only on small surface areas. Many different applications of the gecko feet have been produced, and some have even been developed into commercial applications to be used for applications in orthotics, surgical tools, manufacturing, safety equipment, and sports equipment.
The gecko’s abilities are not limited to their sticky feet. They have large cones in their eyes that allow them to focus light with varying wavelengths at once on their retina. This allows them to focus on objects at different distances. Nocturnal geckos are also able to see colors at night. This is because the gecko evolved from lizards that did not have night vision rods, and the geckos eventually developed color receptors in their eyes that are more sensitive, in particular to blue and green hues. The hope is to eventually be able to develop better cameras, and multi-focal contact lenses by studying the gecko’s multifocal eyes.
These little guys have proven that there is still more inspiration that we can draw from them, and they challenge us to think of ways to find inspiration in unconventional places.