In Science today, Animangsu Ghatak and colleagues at the Indian Institute of Technology in Kanpur describe a type of reusable adhesive film that was inspired by the way that animals and insects cling to surfaces¹. The material has the ‘lift-and-stick-again’ stickiness that enables geckos and flies to walk up walls and across ceilings.

Haven’t people already made those?

Yes and no. Various other biomimetic artificial sticky coatings have been described^2,3. The adhesive properties of such glues come from bringing two surfaces as close as possible to each other, so that an attractive force comes into play. That can be done by making the gluey surface soft, so it flows into crevices when squeezed down — but biomimetic glues copy the strategy of the gecko's foot, which has pads coated with lots of tiny hairs that penetrate every nook and cranny.

Ghatak's glue relies not so much on getting two things to stick together, but in making it harder to peel them away from each other.

What is this attractive force?

It's called the van der Waals force, and it acts between any two objects at distances of a few nanometres (millionths of a millimetre) from each other — even individual molecules are pulled together by this force.

How does that work?

It’s an electrical attraction. Objects have ‘floppy’ clouds of electrons, which slop around to create temporary patches of positive and negative charge on their surfaces. When two surfaces are close to each other, a patch of negative charge on one will induce a patch of positive on the other, creating an attractive force — the van der Waals force. The fluctuations are ultimately quantum-mechanical, so you could say that it’s quantum physics that sticks a fly to the ceiling.

So is that how the new adhesive works?

Yes... but the polymer films of the Indian team are only moderately soft, and so only mildly tacky — like a soft plastic film pressed onto a window, say. The trick to getting stronger adhesion in this case is to make it harder to separate the two surfaces.

How have they done that?

The team buried microchannels in the polymer layers, like underground train lines, filled with air or oil and running parallel to each other. When a flexible plate is peeled off this sticky polymer film, the force needed to keep peeling jumps up every time the peel contact line runs over one of these microchannels.

The gap created by the peeling apart of the two materials is like a crack opening up between the two surfaces. The separation can be slowed if its energy gets absorbed. That’s how plywood gets its strength: a crack running perpendicular to the wood's bonded layers gets deflected into the weak interfaces, slowing down the propagation of the original crack. The microchannels have a similar effect — when the peeling apart hits one, the energy of the cracking goes into deforming the channel instead of peeling off the adhesive.

How big is the effect?

It depends on the ratio of the layer thickness to the channel width. By twiddling this, the researchers found they could double the force required to peel off the microchannelled adhesive as compared to a smooth one.

The effect is even bigger if the channels are partially filled with silicone oil instead of air. In that case, there is a negative pressure inside the channels — a kind of pull that opposes the stretching of the polymer film as the peel front moves over it. This sucks up even more energy from the unpeeling. Then the adhesion strength can be boosted by a factor of up to 25.

Different combinations of microchannels at different depths in the polymer layer can create different kinds of stickiness. An upper tier of oil-filled channels and a lower tier of air-filled ones, for example, creates a low-tack adhesive material for easy release, like Post-It Notes. Reverse this arrangement and you get a very strongly-sticking material.

Does this copy anything in nature?

Possibly – it seems that ants and bees have networks of fluid-filled vessels inside their sticky feet pads that might exploit this effect⁴.

So which is better — adhesives inspired by gecko feet or this new stuff?

Ghatak's film should be robust, cheap and easy to make, whereas the gecko hairs are fragile and tend to clump together, neutralizing their stickiness. But how they compare for adhesive strength remains to be seen. 

• References

  1. Majumder, A., Ghatak, A. & Sharma, A. Science, 318 258 -261 2007
  2. Geim, A.K. et al. Nature Mater, 2 461 - 463 2003 | Article | ISI | ChemPort |
  3. Ge, L., Sethi, S., Ajayan, P.M. & Dhinojwala, A. Proc. Natl Acad. Sci. USA , 104 10792 - 10795 2007 | Article | PubMed | ChemPort |
  4. Federle, W., Brainerd, E.L., McMahon, T.A. & Holldobler, B. , 98 6215 - 6220 2001