by Katie Galloway
Imagine putting tons of super glue on your hands and feet and trying to climb the Empire State Building like Spider-man. Your dynamic display of agility doesn’t get you too far. You’re stuck! That’s because the chemical bonds between the sticky glue and the building surface are stronger than the force of your weight. If the bonds on your feet and hands hold your weight, you will have to pull off with at least a fourth the force of your weight for each new grip-not a very satisfying solution if you have to climb 102 stories to the top. And by the way, you will probably have to reapply glue for each step since the glue will not stay sticky. Clearly this represents a poor design strategy for scaling sheer surfaces-so how do geckos do it?
Geckos have the amazing ability to adhere to sheer surfaces, but it’s their capacity to control their adhesion and not stay stuck that allows them to run straight up walls. Their feet aren’t sticky or else they would get stuck just like you and your super glue. Instead, geckos adhere to surfaces using spatulae composed of billions of setae, hard bristle structures that look like split-ends gone crazy. Adhesion is promoted via Van der Waals forces, the sum of the attractive intermolecular interactions between the surface and the setae.
To put Van der Waals forces into effect, the gecko employs shear force (the force applied parallel to the surface) to get its setae in close contact with the surface. By regulating the shear force, the gecko controls the area of the setae in contact with the surface of a tree or wall. The more force applied, the more the otherwise perpendicular setae bend, creating a larger surface area for the Van der Waals interactions that create adhesion. Fortunately, this process is reversible. To take another step, the gecko reduces (unloads) the shear force on the foot so the setae are no longer induced to bend. The contact area between the gecko’s foot and the wall decreases until the gecko can easily lift and replace the foot.
To see this principle in action, slide the tips of your fingers down a window. They slide pretty easily, but when you put your entire hand on the glass and pull down you encounter greater resistance. This is due to friction created by the interaction of atoms in the glass with atoms in your hand. With only your fingertips, the small area of contact results in a small frictional force. More Van der Waals forces are generated with the larger surface area of your hand, requiring more sliding force to overcome. Additionally, you can observe that even more force is required to slide your hand as you increase the pressure. This is because increasing the pressure puts more of your hand in contact with the glass, creating more intermolecular bonds. So why can’t you scale the Empire State Building like a gecko?
From your perspective, it appears that your hand is in complete contact with the glass, but in actuality, very little of your hand is touching the surface. Due to the roughness of the glass and your hand, the contact area is still relatively small, and the atoms close enough for Van der Waals interactions number very few. Putting more pressure on the surface achieves greater contact, but it also causes you to push away from the wall, which is the opposite of the direction you want to go. Without flexible setae, humans do not have much control over their degree of contact with surfaces like glass or the sides of buildings. Our ability to enhance or reduce this contact area without pushing off the surface is very limited, unlike the wieldy gecko.
While humans lack natural setae, wings, sonar, and a host of other biologically inspired devices, we are endowed with a set of powerful tools to unlock the mysteries of Earth’s creatures and harvest knowledge of the natural realm to benefit mankind. As reported in the January 30, 2008, issue of Science Daily, researchers at UC Berkeley put their curiosity and brains to work in designing a gecko-mimetic tape that adheres when loaded with shear force, yet can be easily removed from a surface without leaving a residue1. Synthetic stiff polymer microfiber arrays were fabricated from polypropylene. Forty-two million stiff plastic fibres 20um in length and only 0.6 um in diameter were arranged in 1 cm2. Two cm2 of this tape were able to hold up 400 g, or nearly 1 pound. The team from Berkeley reported that:
“In the absence of shear forces, these fibres show minimal normal adhesion. However, sliding parallel to the substrate with a spherical probe produces a frictional adhesion effect which is not seen in the flat control.”2
By harnessing the geckos’ mechanical strategy to induce Van der Waal interactions via shear force, the researchers demonstrated that stiff polymers can be used to attain sliding-induced adhesion. While this tape marks a step forward for gecko-inspired synthetic adhesives (GSA)gecko-inspired adhesives, more research into a system for nonfouling adhesives is necessary before anyone “Spider-mans” up a skyscraper.
Scientists have been studying gecko adhesion for decades, but even before formal science picked up interest, the Bible was cluing in readers to the gecko’s unique abilities. In Proverbs 30:24-28, the Bible says, “A lizard can be caught with the hand, yet it is found in kings’ palaces.” Today we have greater insight into how these little guys can be so stealthy!