I’m itching to write a little about my recent trip to Cozumel, but I’m a bit bogged down by homework for the next few days. Even so, the homework in my Principles of Nanotechnology course overlaps with some interesting observations about geckos I made while in Mexico (that they can walk on walls and ceilings in various conditions), so I thought I’d share. Unfortunately, my camera broke, so you’ll have to do with a picture I took at the California Academy of Sciences. Anyway, the homework assignment is all about the nanotechnology of gecko feet, and what makes them stick. Hit the read more link for the rest of the goodies.
Before we get into the nitty gritty of the assignment, if you’re adventurous, have a look at the source we were given as a reference: A reversible wet/dry adhesive inspired by mussels and geckos. Additionally, I’ll be pulling some extra information from Biological Adhesion for Locomotion on Rough Surfaces, which may require a subscription. Now, against my better judgement, I’m just going to copy and paste the questions and answers for the first three problems on my assignment (the others are about the polarity dependence of STM images, see Atom-selective imaging of the GaAs(110) surface). Here goes!
- A gecko walks up walls using physical adsorption forces. Would the gecko’s ability to walk be altered if the wall was wet? What about if the wall had a layer of very fine dust on it?
The gecko’s ability to walk up walls stems from special structures on their feet called setae which interact with surfaces via Van der Waals forces (physical adsorption forces). Each seta is roughly 1mm long and 5µm in diameter, terminating in hundreds of fine structures called spatula which are 200nm long and 10nm in diameter. Since these structures are flexible, they can conform to a surface effectively and increase the overall surface area available for interaction.
A gecko’s ability to walk on the wall when wet depends on the force available to each seta. In , their artificial “gecko” structure composed of nanopillar array of PDMS 600nm long and 400nm in diameter. These structures had an adhesive force per pillar of 39.8±0.2nN in air, reducing to 5.9±0.2nN in the presence of water. This overall reduction in force per setae by a factor of 7 means that a typical gecko weighing 25g, will have its shear force parallel to the surface of 10N in air reduced to 1.5N in wetness (assuming reduction in force per setae is approximately the same as the artificial structure). Since the gecko weights 25g or 0.025kg, the force due to gravity on a vertical surface is only 0.025kg*9.8N/kg=0.245N, and the gecko will continue to stick. Its ability to walk on a wet surface will be weakened, but not eliminated.
However, actual gecko setae do not suffer the same decrease in force due to thin layers of water because of their ability to squeeze water out between setae . In fact, in , they cite another source that claims the pull-off force for a gecko’s foot is reduced by a factor of six when submerged in water. In either case, the gecko’s ability to walk on wet surfaces is diminished, but not eliminated.
In , the question of whether a gecko can walk on a contaminated surface is questioned. The answer, it turns out, is that gecko’s can walk on dirty/dusty surfaces for a special reason. It was proposed that small particles would passivate the gecko’s feet, but it was found that geckos never need to clean their feet; this suggests that these small solid particles bind more strongly to the wall than the gecko’s feet. The author of  finds this explanation unsatisfactory since adhesive tapes lose their adhesive properties after repeated applications to dusty surfaces. He proposes that the lateral movements of the gecko’s feet scratch these particles off the setae, maintaining their adhesive properties. Therefore, we must assume that the ability of gecko’s to walk on dusty surfaces is diminished, but not eliminated.
- Evolution has created tiny and fairly large (gecko) animals that can walk up walls. Is it just a matter of time before bigger animals develop similar capabilities? Or is there some limit that prevents that?
We must consider the ratio of surface area available for adhesion to the weight of an animal. Assuming an animal walks on a vertical surface, the adhesion force must be greater than the force due to gravity (neglecting whether an animal could remain rigid enough to walk vertically, poor giraffe…). For a gecko, each foot is about 100mm2, for a total of 400mm2, and setae have a density of about 10,000setae/mm2. Each seta can withstand 10-5N shear force, which corresponds to about 0.1N/mm2. Thus, a gecko has an available shear force of 0.1N/mm2*400mm2=40N. (Note, however, that the value quoted in the literature is only 10N. We will stick with the ideal case). The weight of a gecko, calculated about is 0.245N, and therefore the ratio of shear force to weight is 40N/0.245N=163, which we require to be above 1. We need our animal to obey this relation (Surface Area of Feet[mm2]*0.1N/mm2) / (Weight [kg] * 9.8N/kg) > 1, or rearranging Surface Area of Feet[mm2] / Weight [kg] > 98mm2/kg.
So a gecko has 400mm2/0.025kg=16,000mm2/kg. An elephant has feet approximately 50cm in diameter (no references), with the largest recorded weight being 12,000kg. So an elephant would have (50cm*10mm/1cm)2/12,000kg=200,000mm2/12,000kg=17mm2/kg (yes, I realise that elephant’s feet aren’t square, I’m just too lazy to divide 50cm by two, convert to mm, square it, and multiply by pi, since the answer will only make up for the fact that I used the heaviest elephant on record). Since 17mm2/kg is less than the 98mm2/kg we require, we are forced (sadly) to admit that a geckelephant cannot walk up walls.
The limiting factor is the surface area of adhesive contact to weight ratio of an animal, which we require to be 98mm2/kg. Since our geckelephant doesn’t meet this criteria (even within a factor of 5), we are forced to admit that there are some current animals which would magically benefit from the addition of gecko-like structures to their feet/hands. Since this is not how evolution works (also, quite sadly) we can assume that the only way a species would develop the ability to walk up walls is if (i) there is an evolutionary pressure to do so, and (ii) the species’ body is within the current limits of a gecko-like foot structure or (iii) the species could evolve to be within the constraints of the gecko-like foot structure or (iv) the species could develop a better adhesive and so beat the constraints of the gecko-like foot structure.
The rest of the questions are based on reading , but the most interesting question is the applicability of this science, to which I wrote:
Indeed, there will be practical products based on this science. Cited in , for starters are: wet temporary adhesives for medical industrial, consumer and military settings. If the products are reliable, they would be better than current adhesives because (i) they are reusable over may cycles (ii) they work in wet/dry conditions and (iii) they are likely to work in conditions where other adhesives have limited functionality (i.e. hot/cold, low/high pressures…etc.). Just like the buttons/zippers/velcro, these products would find their way easily into daily life.
As with any approach, expanding the limits of functionality by testing in other conditions would be an improvement. They geometry of these structures also needs to be explored, i.e. varying the length/width of the structures, spacing between pillars, adhesive layer thickness…etc. Additionally, a biomimetic structure with a setae/spatula refinement may further improve the structure. In all, this research could be expanded substantially.
Wow, I’m seriously impressed if you read all of that. Have a gecko five!
 Haeshin Lee, Bruce P. Lee, Phillip B. Messersmith, “A reversible wet/dry adhesive inspired by mussels and geckos”, Nature, Vol 448, p338-341, (2007).
 B.N.J. Persson “Biological Adhesion for Locomotion on Rough Surfaces: Basic Principles and A Theorist’s View”, MRS Bulletin, Vol 32, p486-490, (2007).