How cool would it be to climb smooth vertical walls with ease? Geckos accomplish this phenomenal feat with their phenomenal feet. On the bottoms of gecko toes one can observe row upon row of overlapping pads known as lamellae. Magnification reveals that each lamella consists of arrays of incredibly densely packed stalks known as setae. Impressively, the structural hierarchy does not stop here. An individual seta is branched at its tip into hundreds of 200-nm spatulae. It is the interaction between these many flattened spatula tips and the substrate that leads to adhesion. Previous work by Kellar Autumn and colleagues has demonstrated that it is likely van der Waals forces (yes,the same van der Waals you might remember from general chemistry) between the spatulae and substrate that underlie adhesion. End of story, right? Wrong. Autumn continues to ask intriguing questions concerning the workings of this amazing anatomical system.
Recently, along with Wendy Hansen, Autumn has started investigating how geckos keep their `sticky' feet clean. After all, if smooth glass walls stick to these toes, shouldn't just about everything else, including all the microscopic gunk that surely covers any realistic substratum in nature? To investigate this question, Hansen and Autumn have been purposefully contaminating gecko feet with lots of dirt and seeing what happens to their adhesive capacity.
The biologists used two approaches to get at the answers to their questions. The first involved using live Tokay geckos to measure whole-digit forces on a glass force plate before and after clogging the toe pads with tiny ceramic microspheres, or `standardized dirt'. The authors found a considerable reduction in the stress applied by a gecko's toe to the glass plate following application of the `dirt'; the dirty toes had lost some of their stickiness. However, repeated usage (i.e. simulated steps) of the dirtied toe led to a recovery of shear force. So simply by using their dirty feet, geckos can soon scamper up walls again.
The second approach involved removing the gecko from the anatomical system to see if setae can manage these sticky feats on their own. The team isolated strips of toe setae and affixed them to acetate strips (setae are readily re-grown in a few months). Hansen and Autumn then used a micromanipulator to force each setal array to adhere to a glass force plate. Using the micromanipulator to pull down on the setal strip, they measured the shear forces between the setae and the glass. The authors first performed adhere-pull trials to characterize a `clean' force; then they `dirtied' setae and subjected them to additional trials. The first pull with dirty setae revealed a 60% reduction in shear force, but with each additional cycle,Hansen and Autumn observed increasing levels of shear force.
The authors reached three conclusions based on their results: (1) dirt clearly muddles the adhesive performance of gecko toes, (2) setae become cleaner the more they're used and (3) self-cleaning is an intrinsic property of setae and does not actually require a gecko. So what cleans the setae?Amazingly, the answer seems to be the wall itself. The substrate actually`pulls' particles off setae, assuming each individual particle is attached to relatively few spatulae (an assumption that seems merited based on the authors' observations).
I for one am hoping we can put this information to use and come up with kitchen counters that, like gecko's feet, actually get cleaner the more you use them.