A sticky problem?

White's Tree Frog (Pic: Diana Samuel)

Frogs that use their sticky feet to cling to trees could have practical applications in medicine.

That's according to University of Glasgow-based scientist Niall Crawford who presented this work yesterday.

"Tree frog feet may provide a design for self-cleaning sticky surfaces, which could be useful for a wide range of products especially in contaminating environments - medical bandages, tyre performance, and even long lasting adhesives," says Crawford.

Tree frogs have sticky pads on their toes that they use to cling on in difficult situations, but until now it was unclear how they prevent these pads from picking up dirt.

"Interestingly the same factors that allow tree frogs to cling on also provide a self cleaning service. To make their feet sticky tree frogs secrete mucus, they can then increase their adhesion by moving their feet against the surface to create friction. We have now shown that the mucus combined with this movement allows the frogs to clean their feet as they walk," says Crawford.

The work is part of a European-wide project on biological and technological adhesives which aims to "provide more elegant solutions to contemporary engineering and biomedical adhesive requirements and will additionally provide a platform for future technological innovation that requires adhesion in hostile conditions as a prerequisite".

The researchers put White's tree frogs (Litoria caerulea) on a rotating platform and measured the angel at which they lost their grip.

When the experiment was repeated with frogs whose feet were contaminated with dust they initially lost grip but if they took a few steps their adhesive forces were recovered. "When the frogs did not move the adhesive forces recovered much more slowly," says Mr. Crawford. "This shows that just taking a step enables frogs to clean their feet and restore their adhesion ability."

White's tree frogs have tiny hexagonal patterns on their feet, which allow some parts of the pad to remain in contact with the surface and create friction, whilst the channels between allow the mucus to spread throughout the pad. This mucus at once allows the frog to stick and then, when they move, also carries away any dirt. If this can be translated into a man-made design it could provide a re-useable, effective adhesive.