Geckos use dry adhesion, involving microscopic hairs on their toe pads, as well as other aspects of internal anatomy, to climb vertical walls and run across ceilings, a skill that has long fascinated scientists. It’s a particular mystery how some species as much as 100 times heavier than others can use adhesion so effectively.
Geckos range in body size from about 2 grams to over 250 grams, a difference in scale greater than 100-fold. It had been assumed that larger toe pads account for the ability of larger geckos to climb as well as smaller ones. But now a team of biologists and polymer scientists at the University of Massachusetts Amherst show that increasing the surface area of gecko toe pads alone cannot explain this.
Instead, in a series of animal and synthetic experiments, they suggest that the bodies of geckos act much like a spring, and that as geckos become larger, they also become stiffer, thus enabling them to climb as well as small geckos.
Synthetic Gecko Feet
functional morphologist Duncan J. Irschick, polymer scientist Alfred J. Crosby and colleagues show that geckos have a spring-like mechanism in their bodies to enhance adhesion as they become larger.
In 2012, four of the authors invented the flexible adhesive Geckskin. It mimics a gecko’s ability to strongly yet easily attach and detach their feet to walk on walls and ceilings.
The lead author of the current work, doctoral student Casey A. Gilman, measured the ability of five gecko species varying in weight from about 0.7 to 3.5 ounces (2 to 100 grams) to adhere to glass on a force-extension instrument that measured geckos’ clinging force. She also measured changes in the stiffness of gecko anatomy.
Further, the polymer scientists in the group created synthetic three-toed gecko feet from fabrics and soft elastomers to model adhesive performance.
Gilman and colleagues found that as gecko body size increased, their complete adhesive system, that is, the tendons, skin, connective tissue and setae, became stiffer, resulting in the larger animals’ legs and feet being far stiffer than in smaller geckos.
Gecko Adhesive System
The increased stiffness plays an important role in enabling larger geckos to produce sufficient adhesive forces to climb, the authors say.
Credit: UMass Amherst
By incorporating both the area of the gecko toe pad and stiffness in their model, the biologists found it was possible to explain 92 percent of the variation in adhesive ability among a variety of gecko species.
“As predicted,” the researchers note, “the gecko adhesive system becomes less compliant (stiffer) as geckos become larger.”
The team’s materials scientists constructed a model gecko system using a synthetic adhesive pad and tendon, or mechanical spring, and ran experiments to simulate the dynamic motion of climbing. These experiments with synthetic gecko feet showed the same pattern, confirming results observed in live geckos.
Crosby says, “These findings not only help us to understand the natural world around us, but they also provide the physical ingredients for engineers to build new, better adhesives.”
Irschick says the increased stiffness enhances adhesion because it enables the surfaces forces at the surface, produced by van der Waals bonds, to be stored and distributed efficiently.
“Our analysis shows that simple mechanical changes in geckos explains a large portion of the adhesive ability of geckos.”
On and Off Switch
A gecko’s adhesion is tested on a wet surface. Credit: Image courtesy of University of Akron
In a separate study, Oregon State University researchers found the little lizards can turn the “stickiness” of toe hairs on the bottom of their feet on and off, which enables them to run at great speeds or even cling to ceilings without expending much energy.
Alex Greaney, co-author and an assistant professor of engineering at OSU, said:
“Since the time of the ancient Greeks, people have wondered how geckos are able to stick to walls — even Archimedes is known to have pondered this problem. It was only very recently, in 2000, that Kellar Autumn and colleagues proved unequivocally that geckos stick using van der Waals forces.”
Van der Waals forces are weak atomistic level forces, but geckos are able to take advantage of them because of a remarkable system of branched hairs called ‘seta’ on their toes, Greaney explained:
“These seta and their hierarchy can deform to make intimate contact with even very rough surfaces — resulting in millions of contact points that each are able to carry a small load.
Understanding the subtleties of the process for switching stickiness on and off is groundbreaking,” said Greaney. “By using mathematical modeling, we’ve found a simple, but ingenious, mechanism allows the gecko to switch back and forth between being sticky or not. Geckos’ feet are by default nonsticky, and this stickiness is activated through application of a small shear force. Gecko adhesion can be thought of as the opposite of friction.”
Greaney and colleagues also found that the entire process is quite subtle, so a synergistic combination of angle, flexibility, and extensibility of the hairs exists that results in incredibly robust and tough adhesion — but still allow geckos to unstick without expending energy.
Any Practical Applications?
Credit: Alyssa Stark
So, what kinds of applications will these findings enable?
For the past 10 years, many researchers have been trying to create ‘synthetic dry-adhesives‘ to replicate the gecko. In fact, these types of adhesives are already being used in climbing robots that can search through earthquake rubble in search of survivors.
“While we don’t envision Mission Impossible sticky gloves, which are inspired by or based on the concept of gecko adhesion, we envision that robots will use gecko adhesion in extreme environments in the future,” Greaney said.
“One of the really cool things that we’ve found is the way seta can absorb a large amount of energy, but also can recover it,” Greaney said. “Absorbing energy makes for a tough adhesive joint — for the gecko, it means it can catch itself after jumping or falling and also enables a gecko to rapidly dart off in different directions to avoid predation.”
It’s surprising that the easy detachment mechanism can recover this stored energy, so the researchers are interested in studying whether this is coupled with other aspects of the gecko’s physiology to enable it to take advantage of the recovered energy — much like a kangaroo does when bounding.
“We’re also interested in exploring how this robust, but switchable behavior, has the collective behavior of seta in a hierarchical system,” Greaney added.