Discovering Geckos’ Sticky Secrets
By Tara Roberts
About 1,400 species of geckos roam the Earth. Their lineage dates back 200 million years, making them the oldest group of living lizards. They’re even older than the dinosaurs.
But scientists have only recently begun to understand their remarkable abilities to stick to – and easily release from – surfaces such as rocks, trees, and ceilings.
“Geckos’ toe pads allow them to move through an environment in the way no other organism can,” says gecko–feet expert Travis Hagey of the University of Idaho.
Hagey’s most recent research – funded by the National Science Foundation’s BEACON Center for Evolution in Action – focuses on building the most finely detailed models of gecko toe pads ever created.
Hagey began studying gecko feet as a doctoral student in the UI Department of Biological Sciences, and he’s continuing his work as a postdoctoral researcher in the Department of Mechanical Engineering. His work seeks to understand how gecko species’ various foot designs evolved, how they allow geckos to interact with their environments, and how their unique structures could help humans design better adhesives.
Shape, not material, causes the “stickiness” of gecko feet. The lines on the bottoms of a gecko’s toes are actually flaps of skin covered in tiny hair–like structures, which have even tinier spatula–shaped structures on their tips.
These tiny structures are made of beta–keratin, the same hard protein that makes up scales and feathers.
The flaps are visible to human eyes, but the hairs are about 100 microns long, roughly the thickness of an average human hair. The tip structures are about 200 nanometers long – smaller than a wavelength of visible light.
They’re so small, they interact with whatever material the gecko is climbing at a molecular level, through weak forces called van der Waals forces.
“Just the fact that the structures and the surfaces both have electrons spinning around means they’re mildly attracted to each other,” Hagey says.
The flaps, hairs and tips work together to allow gecko toe pads to not only stick strongly, but also to release easily, quickly, and without damaging surfaces – something human–made adhesives can’t do, which is why engineers are so keen to create materials that imitate their abilities.
“They can generate really strong adhesion, and they can detach quickly and easily,” Hagey says. “There are no human adhesives that can do that.”
Scientists discovered the molecular forces at work in gecko feet in the late 1990s, but most of the research focused on one gecko species, the familiar blue–and–orange Tokay gecko.
But geckos are a diverse group, and their toe–pad arrangements and characteristics vary depending on species’ habitats and lineage. The differences extend all the way to the flaps and hairs.
“Some hairs are short, some are long, some are thick, some are skinny – and those are just the characteristics we can measure easily with a scanning electron microscope,” Hagey says. “It’s complicated at every possible dimension.”
Scanning electron microscopes – which can create images of objects at the nanoscale – can be used to see details of gecko toe pads, but do a poor job of imaging how the hairs are arranged as a whole, so Hagey has been hunting down new tools for his research.
He’s traveled the country with his gecko test–subjects to use micro–CT scanners, which at the right settings can create a 3D image of gecko toe–hair arrangements. He’s also working with Argonne National Laboratory in Chicago to use some of the world’s most detailed x–ray imaging tools to capture the details of individual hairs, down to the nanometer scale.
Combining these two methods, and measuring a variety of gecko species’ feet, will allow him to build models that can simulate the full range of possibilities allowed by gecko toe–pad structures.
Biologists could use the simulations to discover, for example, the most ideal toe–pad structures for geckos living in trees – then go out and study wild, tree–dwelling geckos to find out how their toes compare. The differences would reveal the geckos’ biological limits and offer clues to their evolutionary paths.
“We can really start to get a better idea of why animals are shaped the way they’re shaped,” Hagey says.
Engineers who are designing adhesives could input their desired materials and parameters into the simulation and produce a gecko–like model to meet their needs.
“The same simulation approach that can help us understand how animals have evolved can help us develop synthetics, too,” Hagey says. “Once I get that running, I can answer lots of cool questions.”
Image courtesy of Prof. Kellar Autumn.
© 2009 Kellar Autumn