Developing a self-cleaning adhesive is one of the adhesive industry's Holy Grails. ‘If you think about a piece of sticky tape or some sort of glue, you can't reuse it and if you get a bit of dirt on it, it doesn't work anymore’, says Niall Crawford from the University of Glasgow, UK. But nature invented self-cleaning adhesives aeons before humans began bonding objects together. Explaining that animals have evolved two different attachment systems – wet and dry – Crawford points out that sticky tape is most analogous to the smooth wet attachment systems employed by some insects and tree frogs. Yet tree frogs' sticky toe pads remain clean after thousands of
First the team tested how well contaminated and uncontaminated White's tree frogs clung to a glass plate as they rotated it from horizontal, through vertical to upside down. Explaining that the frogs tended to want to jump off when they began to feel insecure, Crawford and Endlein gently encouraged the frogs to hold tight by shielding them with their hands and caught them when the plate became too steep and the frog's hold failed. Monitoring the plate's angle, the team found that the frogs with uncontaminated feet only began to slip as the plate tipped over (106 deg) and finally lost their grip at 142 deg. However, when they dusted the frog's feet with microscopic glass beads, the animals began slipping soon after the glass plate began to tilt. They had lost adhesion, so how could they recover?
Knowing that dry-footed geckos and wet-footed insects shuck dirt from their feet while walking, the team set out to determine whether tree frogs use the same approach. Holding a tree frog on a computer-controlled stage and carefully applying a single layer of glass beads to one of its toe pads, Crawford and Endlein carefully pressed the contaminated toe onto a glass coverslip and then pulled it free. Measuring the adhesion force as they pulled the frog away, they found that it had fallen to zero: the frog was completely incapable of clinging on to the smooth surface with its contaminated feet. The situation hardly improved after the duo repeated the manoeuvre multiple times and when they checked the coverslip surface they saw that barely any of the beads had been shed from the amphibian's foot. Simply dabbing the toe onto a surface was not sufficient to clean it.
However, when they simulated real tree frog footsteps, by gently dragging the toe pad sideways after contact with the coverslip, the situation was completely different. Over the course of eight simulated footsteps the toe pad gradually recovered adhesion, slowly at first (only recovering 20% of adhesion after the first five steps) but returning to normal by the final contact. In addition, when the team checked the cover slip at the end of each toe pad step, they found large numbers of beads deposited on the surface.
So the keys to the tree frog's self-cleaning success are the sliding motion – which shears particles away from the toe pad and increases the contact area with the surface – and the sticky mucous secretions – which help flush away contaminants – although Crawford suspects that it will be a while before the adhesive industry successfully produces tree-frog-inspired sticky tape.
