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Being a Gecko is Sticky Business!

Close-up image of a gecko foot
Close-up image of a gecko foot

Have you ever seen a gecko run up a tree or fence? They are very fast, and they seem to stick to just about anything! Wouldn’t it be fun if you could hang upside down from the ceiling or a branch like a gecko?

Geckos belong to the lizard family, and they can be anywhere from 2 to 24 inches long. They help us by eating insects, and they are found in many warm climates throughout the southeastern and southwestern parts of the US, and around the world. However, their most amazing feature is the ability of their feet to stick to almost anything.

Stickiness happens when two substances are attracted to each other. If you have ever felt the pull of two magnets toward each other, you have felt one example of an attractive force. So how does a gecko stick? Is it a sticky goo, tiny suction cups, or even tiny hooks? No, it’s none of those things. The answer has to do with chemistry!

The gecko sticks with temporary attractive forces between molecules called Van der Waals forces. They are easily formed and broken again, and only occur over short distances, like when the molecules on the gecko’s feet are very close to the molecules of the wall. The distance between the molecules has to be very tiny. The molecules are approximately 0.3 – 0.6 nanometers apart. Nanometers measure very small lengths. If one of your hairs were a mile wide, a nanometer would only take up one inch of it! A gecko’s attraction to surfaces like walls, ceilings and branches depends on thousands of tiny hair-like structures called setae on the bottom of the lizard’s toes. The setae have even smaller divisions on their ends called spatulae, which are like tiny versions of the kitchen spatulas that chefs use to flip pancakes. The spatulae are like the tiny bristles at the end of a brush. The molecules that make up the spatulae are attracted to the molecules of the wall, so the gecko’s feet stick to the wall.

The gecko can break that interaction by bending its toes just right. Then, lifting its foot quickly, the gecko sticks it to the wall in a different spot. Because there are so many spatulae, the interaction is very strong. Scientists have estimated that gecko feet would be able to support up to 290 pounds if every one of the spatulae were interacting with the surface at once! That would be one huge gecko!

The gecko can’t stick to everything, though. The non-stick coating used in many cooking pans is made of a substance called Teflon, which geckos cannot stick to. Teflon has a surface rich with fluorine atoms, which do not have an attraction to the spatulae of a gecko. Geckos also have trouble on wet surfaces because the water disrupts the Van der Waals interactions between their feet and the surface.

In a branch of science called biological-inspired design, many scientists and engineers are learning from the gecko by creating new materials that can stick to other materials as efficiently as the gecko foot. Bio-inspired researchers are also currently working on making robots that can climb vertical surfaces, sticky pads for soldiers to use when climbing, and wound-sealing substances that would not require stitching. Wouldn’t it be fantastic if in the future a scientist discovered a substance that lets people hang upside down like a gecko? Maybe that future scientist will be you!

Dr. Lori Stepan is an Associate Teaching Professor of Chemistry at Penn State University in State College, PA.