The biology of a gecko’s foot that gives the lizard its remarkable climbing ability has been used by engineers at Stanford University to create a robot that can climb smooth surfaces including a wall of slick glass. With feet modeled on the intricate design of gecko toes, the Stickybot could lead to the development of robots that can scale vertical surfaces to access dangerous or hard to reach places.
Mark Cutkosky, the lead designer of the Stickybot, a professor of mechanical engineering and co-director of the Center for Design Research, has been collaborating with scientists around the U.S. for the last five years to build climbing robots. Having designed a robot that could climb rough surfaces such as brick wall and concrete, he turned to the gecko for ideas on how to tackle smooth surfaces such as glass and metal.
"Unless you use suction cups, which are kind of slow and inefficient, the other solution out there is to use dry adhesion, which is the technique the gecko uses," Cutkosky said.
Every square millimeter of a gecko’s footpad contains about 14,000 hair-like structures called setae. Each seta has a diameter of five micrometers and is in turn tipped with between 100 and 1,000 spatulae. These spatulae are only 0.2 micrometers long (or just below the wavelength of visible light) and interact with the molecules of the climbing surface with a molecular attraction called van der Waals force.
Because van der Waals force interactions involve no fluids the gecko is able to adhere to most surfaces without the use of liquids or surface tension. It also enables a gecko to hang and support its whole weight on one toe by placing it on the glass and then pulling it back. It only sticks you you pull in one direction – their toes are a kind of one-way adhesive, Cutkosky said.
"It's very different from Scotch tape or duct tape, where, if you press it on, you then have to peel it off. You can lightly brush a directional adhesive against the surface and then pull in a certain direction, and it sticks itself. But if you pull in a different direction, it comes right off without any effort," he said.
It is this property that makes a one-way adhesive so important for climbing because it requires little effort to attach and detach a robot’s foot.
"Other adhesives are sort of like walking around with chewing gum on your feet: You have to press it into the surface and then you have to work to pull it off. But with directional adhesion, it's almost like you can sort of hook and unhook yourself from the surface," Cutkosky said.
Headed in the right direction
Once they realized that direction matters, Cutkosky and his team were able to come up with a rubber-like material with ting polymer hairs made from a micro-scale mold. They then attached a layer of the adhesive cut to the shape of Stickybot’s four feet, which are about the size of a child’s hand, enabling it to peel and stick its feet to the surface of a wall with ease.
The newest versions of the adhesive, developed in 2009, have a two-layer system that mimics the gecko’s flap like ridges, called lamellae, and setae. At about 20 micrometers wide, the “hairs” are even smaller than the ones on the first version and support higher loads, allowing Stickybot to climb surfaces such as wood paneling, painted metal and glass. The material is also strong and reusable and leaves behind no residue or damage.
As well as working on a Stickybot successor with rotating ankles that will allow it to turn in the middle of a climb – a task made difficult because the adhesive sticks in only one direction – the Stanford University team is currently working on scaling up the adhesive material for humans. A technology called Z-Man (Spiderman was already taken), which would allow humans to climb with gecko adhesive is in the works.
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