There are hundreds of species of geckos worldwide, varying in size from 1.5-35cm, with this unusual climbing ability. They live in the tropical regions of the world, and are primarily nocturnal. Geckos are also one of the few lizards with a voice, and usually chirp or click.
Past hypotheses, now all disproved, have ranged from microscopic suction cups, adhesive secretions, chemical bonding, capillary adhesion, and tiny hooks. Many of these were simple to disprove: tiny hooks would not work on a molecularly smooth surface such as polished glass, which geckos can climb with ease; suction cups would require a lot of force to break the seal and also wouldn't work in vacuum, while geckos lift their feet as easily as if they weren't sticky at all and their feet still stick under vacuum; and geckos have no glands on their feet to produce any secretions.
The bottom of a gecko foot is covered with tiny hairs called setae, each 100μm long and tipped with about a thousand even smaller hairs with spatula-shaped tips called spatulae, each 200nm around.
These setae are the key to the gecko's climbing ability.
In 2000, researchers took a single seta and attempted to measure its adhesive force using a microscopic, 2-dimensional force sensor. The forces on the setae can be parallel or perpendicular to the surface the gecko is climbing, depending on the orientation of the surface, so a 1-dimensional sensor wouldn't accurately measure the adhesive force.
Simply touching the seta to the sensor got no results; the seta wouldn't stick at all. After observing a gecko on a treadmill, the researchers noted that the gecko placed its palm first, then uncurled its toes to set them down and apply its grip. They then tried to touch the seta to the sensor at the same angle the gecko used when placing its setae on the treadmill's surface, pushing into the surface then pulling back along it the same way the gecko does to increase contact and adhesion. It stuck.
It also adhered with a force of 200μN, 10 times more than they had expected based on studies of adhesive force done on the whole gecko, and right in the middle of the range of van der Waals forces. A single seta adheres with enough force to support an ant; if all setae in a large gecko such as the Tokay gecko, 35cm long and the gecko used in the experiments, were in full use simultaneously, they would adhere with enough force to support an adult human. The stability of the surface they adhere to, however, is another matter entirely.
Also based on their observations of the walking gecko, they found that the gecko curled its toes to lift its feet, in effect peeling the rows of setae off the surface. Applying that observation to the microscopic force sensor, they found that if they increased the angle of a "stuck" seta to approximately 30 degrees, it released easily and without needing to overcome the seta's maximum adhesion, explaining why a gecko can stick incredibly well to any surface, and yet lift its feet easily and quickly enough to run at speeds of up to 1m/s on a vertical polished glass wall.
At this point, the researchers had strong evidence for the van der Waals hypothesis - but still couldn't rule out capillary adhesion.
In August of 2002, after further studies, capillary adhesion was finally disproved. The researchers tested adhesion on hydrophobic and hydrophilic materials, and the seta stuck to both with the same force. They also noted that the seta itself was hydrophobic, making the use of water's surface tension as an adhesive even more unlikely.
They also made two setae of different synthetic materials, silicone rubber and polyester, and tested their adhesion. The material forming the setae did not affect the adhesive force, confirming again that it was van der Waals forces and not chemical bonding or capillary effects, which would be dependant on the chemical composition of the setae and the surface, that provided the adhesion. The materials chosen for the artificial setae were selected because they can be easily molded, an important consideration in potential future use of this technology.
The setae and their spatulate tips provide the most intimate contact possible with a surface short of actual chemical bonding, which is what allows van der Waals forces to come into play. A computer model was created to determine the ideal shape for a tip, assuming van der Waals forces as the adhesive; the final model was nearly identical to the natural setae tips found on gecko feet.
Now that artificial setae tips have been successfully made and proven to stick as effectively as natural ones, the door is open for the development of a "gecko tape" that would have potential applications in nearly every industry, as well as in the home. The manufacture of any quantity of setae is currently beyond our capabilities, but that could change now that we know it will work. And if it doesn't, we can harvest setae from geckos as they shed their skin and setae every few months. It's not mass-production, but it would provide plenty of setae for the micro-applications.
A "gecko tape", like gecko feet, would be strongly adhesive yet easy to remove; would leave no residue; would be self-cleaning and reusable; and would stick to any surface (except teflon, which has such a low surface energy even van der Waals forces don't work on it), no matter how smooth or rough and under any conditions, including under water and in vacuum.
A tape with those properties would be useful in many ways for many reasons. A clean-room environment would appreciate the lack of residue; a micromanufacturer or microsurgeon would appreciate the ability to manipulate tiny objects by attaching a single setae, and easily releasing it when finished; astronauts would appreciate the adhesion in vacuum; robots with sticky feet could be made to negotiate difficult terrain, to help search and rescue teams or scientists exploring unstable or difficult to navigate areas such as collapsed buildings, lava tubes, or underwater caves, or even to explore the lunar or martian surface.
In a move potentially disconcerting to duct tape enthusiasts, 3M is already negotiating to license the patent, so it can manufacture its own "gecko tape."
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