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Sticking together: Lessons from the tree frog
11 OCTOBER 2007

Adhesion: The amphibian solution
When it comes to sticking things together, evolution is an amazing technologist. The invention of Velcro was patterned on the hooks of the burdock, whose sticky seeds spread by grabbing fur. The hairs on a spider's foot allow it to climb smooth leaves.

And now we hear that the tree frog has served as the model for a new type of reusable adhesive. Tree frogs can climb trees with remarkable agility, grabbing and releasing as needed, leaving not a trace of sticky goo. Try that with packing tape! In fact, completely reusable adhesive is rare -- and usually rather weak (think Post-It notes).

Illustration shows the pads of insect and frog and how they are sticky. Indian engineers created a "microfluidic adhesion" material that was inspired by the fluid-filled vessels in the feet of insects and vertebrates such as the tree frog. Image courtesy Journal of Comparative Physiology A, 2006, 186, 821-831.

Adhesive: Calling Mr. Natural!
Adhesion occurs when molecules get close enough together for inter-molecular attraction to occur, wrote Animangsu Ghatak, of the Indian Institute of Technology (Kanpur, India), who collaborated on the new, biologically-inspired adhesive with Abhijit Majumder and Ashutosh Sharma.

Due to the time difference, we "talked" with Ghatak via email. He told us that nature is a fine source of adhesives. "In the animal world, the mechanism of adhesion has evolved for millions of years and is very elegant. With a single foot these animals not only create an adhesion strength 50 to 100 times their own body weight, but also allow very quick de-bonding when required. Furthermore, these adhesives do not get contaminated by dust or particulate matter, unlike most man-made adhesives."

Conventional adhesives are made of a gummy "visco-elastic" material (stuff that stretches and flows slowly). When you try to separate two objects, much of your pulling energy gets dissipated in the surrounding adhesive, away from the actual separation point. This dissipation of energy makes the adhesive stronger, Ghatak says.

But once your pulling adds enough energy to the adhesive, Bingo! The two pieces separate.

One key to separation is the appearance of cracks at the line where the bond fails, which then spread through the adhesive or along the interface of adhesive and whatever it's stuck to.

Adhere-engineers (what exactly do you call somebody who studies adhesive, anyway?) have learned to strengthen adhesives by placing it in multiple, separate blocks. When the separation force causes cracks, they stop at the edge of the block instead of spreading through the whole bond. Because it takes more energy to start another crack, this increases the amount of force needed to rip the adhesive apart.

Neon green frog with white-lined eyes and mouth grasps a tree branch, close to trunk
The adhesive pads on the feet of a white-lipped tree frog are clearly visible. Dig the groovy white smile? Photo: Wet Tropics, North Queensland, Australia

Cracked up
To create a strong adhesive, Ghatak and colleagues mimicked the multilevel structure found in the pads of tree frogs, insects and lizards. Instead of using blocks, they formed tiny channels beneath the adhesive surface and filled them with liquid, and found that this prevented the cracks from spreading. "At the location of the channels, due to the liquid pressure, the tensile stresses do not develop easily," Ghatak wrote. "In essence, in a patterned microfluidic adhesive such as ours, as also in many biological natural adhesives, the adhesion strength is enhanced by a mechanism that inhibits crack propagation."

A half gray, half blue box has blue rods running through its center
The fluid-filled channels beneath the adhesive layer make a new adhesive that can be removed and reattached. The channels help ensure good surface contact. Image courtesy Journal of Comparative Physiology A, 2006, 186, 821-831.

More energy is required to initiate a crack than to spread it, so if cracks are continually nipped in the bud as they reach a channel, the overall energy required to destroy the bond will be higher. That makes the adhesive stronger.

Once you monkey with the geometry and arrangement of the channels within the adhesive, many possibilities arise. "By intelligent arrangement of channels and controlling their geometric parameters, the strength of an adhesive can be controlled over a wide range," Ghatak wrote. "Moreover, the adhesive remains reusable. The visco-elastic material in conventional adhesive gets deformed permanently, but in this new adhesive we trap a viscous liquid inside channels made of elastic material. So overall the adhesive remains elastic and comes back to its original form after use. So while liquid inside the channel supplies adhesive strength, bulk elastic material gives it reusability."

To date, the adhesive "is not yet as strong as the everyday adhesive that we use," Ghatak added. However, the technology's benefits "lie in its reusability and the control of adhesion strength over a wide range."

Six feet, toes splayed wide, press down to show ridged, adhesive surface
The foot of the gecko, a lizard that climbs walls and ceilings with aplomb. Geckos adhere with van der Waals force, the weak attraction between neutral atoms and molecules arising from their different electrical charges. Geckoes have also inspired new synthetic adhesives. Image courtesy Kellar Autumn and NSF.

We had to ask: What causes that ripping sound when you rip off a piece of Scotch tape? Ghatak told us that the visco-elastic adhesive is forming filaments that quickly break, and the rest of the elastic adhesive relaxes back into position. "This whole process happens very fast and when its frequency lies within the audible range, we hear the ripping sound," Ghatak says. "However, when the same tape is peeled off slowly, the fracture primarily occurs at the interface of the adhesive and the adherent, so the ripping sound does not occur."

Re-usable, peel-off adhesives would be helpful for a robot that could crawl up a wall, and for many other applications that require temporary adhesion and a clean removal, Ghatak wrote. And releasable wallpaper might be helpful for those whose esthetic preferences are constantly changing.

Finally, think about those infernal stickers on fruits and veggies. Next time you're swearing silently as you jam your thumbnail under a "can't-quit-sticky" sticker on a ripe, tender peach, silently urge Ghatak on. The peel-off label: Bring it on!

- David Tenenbaum

Related Why Files
Composite Materials
Secrets of a Spider's Foot
Disappearing Frogs

Bibliography
• Microfluidic Adhesion Induced by Subsurface Microstructures, by A. Majumder, A. Ghatak, A. Sharma, Science, Oct. 12, 2007.


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