Even the most advanced glues can’t match the remarkable toe of the gecko: Its impressive stickiness can be quickly deactivated and can support much more than the animal’s weight as it runs across surfaces smooth or rough, wet or dry, clean or dirty. Scientists have known since 2000 that this bond relies on weak attractions between uncharged molecules known as van der Waals forces. Already, researchers have designed synthetic adhesives made of polymers or carbon nanotubes to mimic those interactions.
But a recent paper in Science Advances shows that’s not the whole story. “There are forces in addition to van der Waals forces that could be playing a role in adhesion,” says materials scientist and co-first author Saranshu Singla, a postdoc at the University of Akron in Ohio.
The skin folds on gecko toes contain millions of hairs called setae, which branch and splay at the tips. Thus, the toes can make millions of individual connections with a surface. When two uncharged molecules are close to one another, random, transient variations in the electron clouds of each create van der Waals forces. But for about 20 years, the lab of senior author Ali Dhinojwala has uncovered results that couldn’t be explained by van der Waals interactions. For example, the setae stick better to some surfaces, such as glass, than others, and their adhesion changes with humidity.
Singla and colleagues used spectroscopy to investigate the interaction between gecko toes and the surface of a sapphire prism. The gem offers a glasslike surface, but is transparent to the red and infrared laser beams that researchers shined through in order to bounce off the air-gem interface. Conveniently, reptiles shed their skins every month or so, so the scientists simply collected shed toe pads and glued them to glass rods. They measured changes in the wavelength of light returning from the sapphire’s surface when they pressed the toe pads to the mineral.
Weak van der Waals forces alone would create a small shift in the frequency of the reflected light, compared to the no-toe scenario. But the larger shifts they observed indicated additional, stronger forces also contributed to the bond. These other forces are a broad category known as acid-base interactions, such as hydrogen bonds, which rely more directly on a natural unevenness in electron distribution in the partner molecules. The results help explain why gecko toes stick better to surfaces with such an electron configuration, such as glass, says Dhinojwala.
The different forces may help geckos adapt to different surfaces, says Dhinojwala. For example, rocks with charges on their surfaces might create more acid-base interactions, while a waxy leaf would likely contribute to more van der Waals attraction.
The results illuminate the ways chemical bonds don’t always fit into neat textbook categories, says Jonathan Puthoff, a materials scientist at California State Polytechnic University, Pomona. In this case, the adhesion between gecko and surface incorporates at least two types of interactions.
Singla and Dhinojwala also probed the puzzle of gecko footprints. The lab has found that gecko steps leave behind a fatty residue. It’s not an oil or grease; gecko feet have no pores or glands with which to exude such a substance. Rather, it’s a solid lipid material that seems to come from the same process that generates the setae hairs themselves.
To analyze why the lipid layer is left behind, Singla chemically removed it from the shed gecko toe pads, then pressed them against the sapphire. In this case, although the toes still stuck tightly, there was no change to the reflected light. “It seemed like the sapphire was not in contact with anything,” says Singla. “I kept repeating those experiments.”
The researchers suspect that in any given step, only a fraction of the lipid-free setae engage with the surface, creating too little contact for the experiment to measure the forces involved. But when the lipids are present, they’re spread with each step as the gecko drags its foot a bit. The spectroscopy studies indicated that the lipids are arranged with their polar heads facing outward, where they can contribute to the acid-base interactions.
The authors speculate that the lipids may also protect the delicate setae. When the gecko peels up its foot, it leaves a bit of lipid behind as a sort of sacrificial layer, but the individual hairs are undamaged.
All this information could help materials scientists enhance adhesive designs, says Dhinojwala. Puthoff adds, “It’s important to keep working with the natural system, to keep trying to understand the role of the different various materials in the synthetic systems.” For example, while this work focused on the toes of Tokay geckoes, which are found across eastern and southeastern Asia, geckos in other environments—habitats ranging from deserts to rainforests—might have adapted their toe pads differently, notes Puthoff. Singla now plans to investigate how humidity influences the natural gecko-toe bond.