Imagine a super-adhesive material that’s so strong, an index-card sized piece could hold more than 300 kilograms against a flat wall.
How do geckos do it?
For over a century, scientists around the world have tried to figure out how the tiny gecko, a lizard weighing barely 140 grams, can climb up a smooth vertical surface, or saunter nonchalantly across a ceiling, with a grip that would take a 4 kilogram bag of apples to break.
Developing a material that can mimic the adhesive strength of the gecko’s tiny footpads would be very desirable, and have almost unlimited commercial and military applications.
Key to the gecko's climbing and clinging ability are the millions of microscopic hairs, or setae, that cover the bottom of the lizard's flexible footpads. The setae are so fine that they actually form an electrostatic bond with the molecules of whatever object the lizards are scaling. And as powerful as their foot-hold is, geckos can release it with just a bend of their wrist as they move along a surface.
Researchers have managed to simulate the gripping power of the tiny hairs using nanotechnology. But Duncan Irschick, a functional morphologist -- or physical structures expert -- at the University of Massachusetts, Amherst (U. Mass), says they haven’t yet been able to make artificial setae work at the scale needed to hold onto really heavy objects.
So Irschick and his colleagues used a different strategy to make the adhesive they call "Geckskin." They theorized that the easily reversible adhesion of the gecko's footpads is not dependent solely on those microscopic hairs, but also on the unique internal anatomy of the lizard's foot. So they set out to simulate that anatomy in an adhesive material:
“To actually recreate the tendon and the skin, which we believe play a really important role in adhesion. And then we actually used what is called a draping adhesive. So the Geckskin actually uses some elements of what geckos have. It uses their anatomy, but doesn’t actually recreate these hairs that geckos have," Irschick explained. "The bottom line is, our adhesive can then hold really large weights on smooth surfaces -- much like a gecko -- and can be peeled (off) -- much like a gecko.”
To recreate the unique arrangement of tendons that connect bone to skin in the gecko's foot, Alfred Crosby, a polymer scientist at U. Mass., says researchers wove a soft pad into stiff fabric and bound it together with a caulk-like rubber compound. He says the pad mimics the way a gecko's foot-pad "drapes" over a surface for maximum contact.
Small size, tremendous strength
“Another important development was that we understood in order for it to maintain stiffness to the force that’s being applied, you needed a tendon. So we actually stitched the tendon into the skin. The tendon is just another piece of fabric,” Crosby stated.
The result is a 406-square millimeter piece of Geckskin -- about the size of an index card -- that can hold more than 317 kilograms against a vertical surface -- and be removed with a simple tug without leaving a sticky residue like tape. The material can easily secure a large flat-screen television against a wall.
Funding for the Geckskin project was provided by the US government’s Defense Advanced Research Projects Agency, or DARPA. DARPA hopes to use Geckskin in its Z-Man project, which is studying ways to allow soldiers and other special operatives to climb walls -- like Spiderman.
Duncan Irshick and Alfred Crosby describe the development of Geckskin in the Journal Advanced Materials.