Frog Tongues Hold Secret to Better Adhesives

Biomimicry has become such a common word in the design community that it's easy to forget what a profound idea it is: Instead of designing solutions to problems from the ground up, we can investigate how millions of years of evolution have solved similar problems. From paints that repel water like a plant's leaves do, to swimsuits that mimic shark skin for the ultimate in hydrodynamics.

So when seeking ways to make a better adhesive, scientists looked to a logical place for clues: A frog's tongue. While we might assume that frogs use their tongues to capture prey that is much smaller and lighter than they are (say flies or crickets), some frogs successfully capture bigger prey. To do that, they use a force to capture their food that can exceed the weight of their own bodies. Frogs are fairly lightweight — which makes swimming and springing easier — so to be able to keep that lightness while still bringing down larger prey is a huge advantage. That's where their extra-sticky and soft tongues come in, as the video below explains.

Key to what helps frogs' tongues catch — and hold onto — this prey is a special mucus that functions as a "pressure-sensitive adhesive," according to an Oregon State University news release. "This mucus is able to generate large adhesive forces in response to the high strain of retraction," said Dr. Joe Baio, assistant professor of bioengineering at Oregon State University.

Baio and researchers from the University of Aarhus, Denmark, University of Kiel, Germany, and the National Institute of Standards and Technology worked together on a recent study to determine how the chemical structure of the mucus changes after a frog strikes out with its tongue. This hadn't been looked at before, though there is plenty of research being done around how frog tongues work so quickly and effectively.

To accomplish this deep dive into the chemical structure of tongue mucus, the University of Kiel researchers simply got three adult horny frogs together, and held up crickets behind a glass plate. When the frogs struck at the crickets, the glass in between caught their fresh tongue mucus.

The mucus on frog tongues is different from what we produce when we have a stuffy nose; frog mucins (proteins) form chains that have coiled structures. When scientists looked closely at them, they could see that these protein chains twisted together around an axis, a structure called a fibril, and this is the key to the stickiness of frog tongues. The amazing part is that the fibrils formed in response to the frog tongue retracting — a very fast chemical process that means the adhesive on their tongues is basically activated only when needed. "It is these fibrils that allow the mucus to generate strain-responsive adhesive forces by acting as molecular shock absorbers for the tongue," said Baio.

An adhesive that utilized these same properties — becoming extra-sticky only when subjected to a certain level of force — seems like it could help us out of some sticky situations.