Frog tongues are sticky like glue and that's all there is to it, right? Actually no, it turns out that things aren't quite that simple. Led by mechanical engineering Ph.D. student Alexis Noel, researchers at Georgia Tech have discovered that frog saliva switches between watery and viscous states, allowing the animals to both catch prey and then whip it back into their mouths. The findings could have implications for human technology.
Noel started by collecting saliva from 18 frogs and then placing it in a rheometer, which is a device that measures the manner in which liquids flow in response to applied forces. What she found was that the saliva changes viscosity with shear rate.
How does that help the frog out?
Well, it means that when its tongue flies forward and stops against the insect, the saliva turns thin and runny, allowing it to thoroughly make contact by flowing into all of the insect's nooks and crannies. As soon as the tongue snaps back, though, the saliva becomes much thicker and stickier, keeping the prey from falling off. Finally, once the tongue and its payload are back in the frog's mouth, the saliva becomes watery again, allowing the insect to be pulled off and swallowed.
The other part of the equation lies in the softness and stretchiness of the tongue itself. Made up of tissue that's ten times softer than that of the human tongue, the frog's tongue stretches and stores energy like a spring, plus it's very good at changing shape to conform to the contours of the prey. Combined with the "reversible" saliva, the result is a tongue that performs significantly better than manmade adhesive polymers.
"Frog tongues can attach and reattach with soft, special properties that are extremely stickier than typical materials," says Prof. David Hu, Noel's advisor. "Perhaps this technology could be used for new Bandaids. Or it could be used to create new materials in soft manufacturing."