Geckos are the superheroes of the lizard family. Equipped with sticky toe pads capable of supporting the weight of two humans, they cling to walls and scurry across ceilings with ease. But like any superhero, the reptiles have their kryptonite. A new study shows that soaked surfaces and wet feet cause them to lose their grip.
The key to the gecko's sticking success lies in tiny hairlike structures, called setae, found on the base of its toes. Each of these microscopic bristles can split into hundreds of nano-sized tips called septulae. Septulae create so-called van der Waals interactions between their molecules and the molecules of the surface that a lizard is clinging to. Such interactions are normally weak, but because there are millions of septulae on each of a gecko's toes, each tiny bristle adds a small grip, which together creates a secure hold. A million setae, which would fit neatly on a dime, could support the weight of a child.
Previous research had tested geckos' adhesive abilities on dry surfaces, such as smooth glass and rough walls, but they had not investigated how the reptiles react to wet surfaces, which are common in the rainforest environments in which many geckos live. So in the new study, researchers at the University of Akron in Ohio fit a group of Tokay geckos (Gekko gecko) with a tiny harness hooked up to a force sensor and placed the reptiles on dry, misted, and water covered glass surfaces. As the harness pulled back on the geckos, the sensor measured how much force it took to get the well-attached reptiles unstuck. The team found that as the surfaces became wetter, the geckos struggled to maintain their grip.
The researchers then tested how the geckos dealt with wet feet on different surfaces. They submerged the lizards' feet in water for 90 minutes and then strapped them to the harnesses and tested them on the same surfaces again. This time the team found a significant reduction in adhesive ability. It takes 20N of force, or about 20 times its own body weight, to get a gecko to slip when its feet are dry and on a dry surface. But when a gecko's feet were wet, they became unstuck with less than 1N of force. A gecko that had its feet submerged slid down the same dry surfaces that it would normally have no problem scrambling up (see video). A gecko's ability was most impaired when their feet and the surface were soaked, slipping with less than 0.5N of force applied, the team reports online today in The Journal of Experimental Biology.
Although scientists don't know the answer yet, one hypothesis for why the submerged geckos lost their stickiness is that their toes lost their superhydrophobic, or water-resistant, properties as they became oversaturated with water.
"It's interesting because we think of geckos generally as being really great at sticking," says lead author and integrative biologist Alyssa Stark. "This experiment shows that there are some limitations to their ability."
The results highlight the importance of understanding the ecology of an animal and the surfaces that it normally encounters in the wild, especially when looking at that organism's biomechanics, says Timothy Higham, a biomechanist from the University of California, Riverside, who was not involved in the work. "Before this study we probably would not have guessed that water would have impacted adhesion this much. In fact we might have even predicted that it wouldn't impact adhesion at all, given that the gecko's hydrophobic pads are supposed to work fairly well in water."
Duncan Irschick, a functional biologist at the University of Massachusetts, Amherst, says that he was not shocked by the results. "It's not unexpected that if you dunk lizards into water that they're going to have their adhesive performance reduced." What is startling, he says, is the extent to which the gecko's adhesive performance declined.
Stark says that her team's research will enable biologists to better understand how geckos behave when they are approached by a predator while wet and how they keep a grip on a drenched branch. It also has implications for scientists looking to create new adhesives that can support immense weights by mimicking the gecko's mechanics. Ali Dhinojwala, a polymer scientist from the University of Akron, says that by understanding what causes the gecko's adhesion to fail, scientists can avoid the same pitfalls when they design synthetic gecko-like adhesives, which could be used in medicine, robotics, and new super glues. By studying the limitations of the gecko's natural adhesive system, scientists might get their hands on adhesives that are reusable, hold enormous amounts of weight, and stay strong for long periods of time—even when wet.