Adhesion, Geckos, and Technology

One of our earlier experiences with science comes in kindergarten. The teacher brings out the Elmer's glue and we put macaroni on construction paper. The white viscous stuff acts to permanently bond the paper and pasta. As children we're ignorant of the science beneath this and view glue as magic stuff. But if we could zoom in to resolve the detail, we’d see the long squiggly polymer molecules in the glue are grabbing onto the fibers in the paper as well as the starch fibers in the macaroni. The glue acts as an adhesive.

Adhesion refers to whether or not two different surfaces stick together. [Aside: cohesion refers to how molecules within a substance are attracted to one another, one might think of it as ‘self-stickiness.’] In our glue example, there are actually two interfaces of adhesion, the glue/paper and the glue/macaroni. There are different varieties of adhesion. There is chemical adhesion, which involves actually forming a new bond between the molecules of each surface. There is the physical, grabbing adhesion of glue mentioned before. There is adhesion due to electric charge. That static cling sticker dedicated to your alma mater on your car window uses this kind of adhesion. There is even yet another type of adhesion. Even if there is no chemical bond, polymer grabbing, or charge attracting two surfaces, there is still likely to be a force of adhesion called “van der Waals” adhesion. The precise details of van der Waals adhesion will not be explored here, but it is the same van der Waals interaction (sometimes referred to as ‘London dispersion force’) many learn about in high school chemistry. Van der Waals adhesion can be surprisingly strong, as we will see.

Geckos are quite amazing in their ability to perform seemingly impossible feats of climbing. They can even easily climb up and down a smooth glass aquarium cage, a surface with no obvious footholds! Yet if you hold a gecko in your hand their feet don’t feel especially sticky. Each of their toe pads is covered with about 100,000 setae, basically short hairs. The setae then split into hundreds of tips, each about 200 nanometers in diameter. These setae tips are what touch the glass, so there must be some type of adhesion between the tips and the surface to support the gecko’s weight. Autumn and coworkers [1] cleverly demonstrated the exact nature of this adhesion. Since they produce no glue, chemical and physical mechanisms were not on the table. However, there were two dry adhesion candidates: charged and van der Waals. The team let the geckos crawl up and down smooth surfaces that were charged differently. They performed equally well on both surfaces, pointing to van der Waals adhesion as the culprit.

However, since this adhesion is strong enough to keep the gecko firmly affixed to a wall, then how do they unstick? Geckos can crawl quite quickly, so there must be an unsticking mechanism in place. The same group came forth [2] to answer this question. With a series of experiments (including forcing a gecko to hang by one toe) the team showed that the setae tips are angled. This has the net effect of increasing the adhesion forces when at they are on surfaces at steeper angle: the setae tips become stickier as needed! Further, the angled attachment affords easy detachment: there is essentially no force required to move the foot up. They also have a “reverse gear” in their back feet for moving down surfaces, where the tips are oriented in the opposite direction.

Recently, Suh and coworkers [3] have been trying to recreate this remarkable gecko adhesion synthetically. After all, this adhesion has many desirable qualities. It’s strong, yet completely reversible. It can adhere to various degrees of surface roughness and any surface orientation. These qualities are especially suited to precise tasks such as moving and manipulating sensitive electronic components. The group was able to fabricate 200 nanometer plastic ‘hairs’ on a flat surface, resembling the setae tips of the gecko. These hairs were angled as well, and a 1 inch patch of them was capable of transporting a piece of glass that was 300 times that large! However, this system relies on needing to hit the hairs at the correct angle, and so is really a passive adhesion mechanism. While the gecko uses the same basic mechanism, it is actively controlling the contact with its muscles. Unfortunately, the synthetic version would not be suited to any complex task.

So while originally inspired by the gecko, the group ultimately developed a better system for practical use [4]. They stretched out a thick sheet of rubber and treated it with a reactive gas, so only the top surface was modified and became stiffer. When the sheet was allowed to relax, it went back to its original size. However, the top surface was no longer happy in this state, and created a wrinkly pattern on the surface, like a shallow egg crate. They then stretched it again, and put small (1/50^th of a millimeter) pillars on the surface. So, when the sheet is relaxed, the pillars do not stick straight up, rather they are at odd angles due to the wrinkling, and when stretched they stick straight up. This creates a simple on/off adhesion mechanism. In the stretched state, the top of every pillar can make good contact with another surface, and in the relaxed state, none of the pillars can. They were able to pick up and move the same piece of glass in a complex choreography, all without leaving the even the slightest trace on the surface.

[1] Autumn, K. et al. PNAS 99, 12252 (2002)

[2] Autumn, K. et al. Journal of Experimental Biology 209, 3569 (2006)

[3] Suh, K.Y. et al. PNAS 106, 5639 (2009)

[4] Suh, K.Y. et al. Langmuir 26, 2223 (2010)