The unique ability of geckos to scale walls and suspend from ceilings has attracted the interest of naturalists for ages, dating back to Aristotle’s observations in his History of Animals of the creature’s ability to “run up and down a tree in any way.” It is only recently that scientists have unlocked the secret behind the lizards’ perplexing mobility and begun engineering synthetic materials mimicking their abilities.
Gecko Feet Structure and Intermolecular Forces
A gecko’s foot has toepads consisting of about half a million setae made of keratin. Each of these fine hairs has hundreds of even smaller projections of nanoscale diameters called spatulae protruding from their ends. While many interactions had been hypothesized as the origin of the adhesion, such as suction, friction, and electrostatic forces, it was not until 2000 that Robert Full of the University of California, Berkeley, discovered that the adhesion was due to van der Waals forces created between the spatulae and the surface. Van der Waals forces are intermolecular forces created by induced polarizations of molecules. Though weak and negligible in most considerations, van der Waals forces become significant on the micro and nanoscale. In the case of gecko feet, the spatulae are so small and get so close to the surface that an attractive van der Waals force of around 0.4 µN develops between a single spatula and a surface. While a seemingly insignificant number, the combined force of the millions of spatulae on a single gecko foot produce an adhesion force of around 10 N, or around 2.25 lbs. Considering a gecko foot has an area around only 100 mm2, it was inevitable that scientists would attempt to mimic the power and efficiency of such a material.
Animation adapted from Shah
It was later discovered that van der Waals forces are not the only forces responsible for a gecko’s ability. Andre Geim, who is responsible for synthesizing gecko setae, discovered that capillary forces also contribute to the adhesion. Capillary forces are attractive forces created by the surface tension of a molecular layer of absorbed water that forms between two surfaces. When a gecko is climbing on a hydrophilic surface, capillary forces combine with van der Waals forces to keep the gecko in place. On hydrophobic surfaces, however, van der Waals forces play the primary role.
In 2003, Andre Geim and fellow researchers at the University of Manchester succeeded in creating a synthetic material that mimics gecko feet called gecko tape. While composed of a different material, gecko tape has a similar structure to the toepads of the lizards. The fabrication process of the tape involves many cutting edge nanotechnology methods. First, a polyimide film substrate is prepared on a silicon wafer. Then an aluminum mask is created through electron beam lithography, a process where a beam of electrons is used to create nanoscale patterns on a surface. This mask is then transferred to the polyimide film through dry etching, a procedure where ions are bombarded against the metal to remove the mask, leaving only the substrate and the projecting polyimide hairs. The material is then removed from the silicon wafer and attached to a flexible base, which allows the material to adhere better to surface, which is usually not flat due to microscopic imperfections. In order to test the adhesive force of the resulting array of polyimide hairs, Geim used an atomic force microscope (AFM) with a cantilever tip and measured the deflection of the tip. As hypothesized, each hair had around the same adhesive force as a single gecko seta.
The Future of Biomimetic Adhesives
While many problems still obstruct immediate commercial applications of gecko tape, such as poor durability and high fabrication costs, the prospects of the technology are still generating excitement in a variety of fields. An adhesive material that exploits intermolecular forces could be crucial in certain environments where conventional adhesion tools such as suctions and glues cannot function; for instance, a descendent of gecko tape might enable astronauts to perform spacewalks with the tape affixing the astronaut’s boots to the spacecraft, eliminating the need for complex harnesses. In more typical circumstances, the incredible strength of gecko tape could be used in a myriad of applications, such as increased mobility for humans in construction, inspection, and military situations. The fact remains that as we progress in our understanding and utilization of the most fundamental forces of nature, we will be able to approach problems once though unsolved from a new direction, specifically, from the bottom-up.
 Autumn, K., Liang, Y., Hsieh, S., Zesch, W., Chan, W., Kenny, T., Fearing, R., Full, R.