Can you outline your academic background in a few words?
I took a university degree that ended with a PhD in mechanical engineering. Since 2014, I have been teaching and doing research at Georgia Tech. My area of expertise is tribology, a technological science that studies the effects induced by two immobile or animate systems (or materials) coming into contact. Tribology is therefore concerned with the phenomena of friction, wear and also lubrication.
My laboratory, the Tribology & Surface Engineering Lab, is dedicated exclusively to this science. Part of our work has very concrete applications for many industries, particularly those in the mechanical sector. It goes without saying that knowledge of materials is indispensable! Another part of our work is devoted to fundamental research and the development of new materials that can help our science evolve.
Like many laboratories, you are fascinated by the feet of the gecko, the little lizard that can cling to any surface. Reproducing their characteristics seems to be the Holy Grail for many researchers. Why is that?
The gecko is able to walk on a ceiling! Of course, it is far from being the only one and other lizards do so just as well, and that is not even mentioning insects. However, the gecko is the only one that can bear a weight of more than 40 kg, without falling down, despite weighing less than 100 g. This can be explained by the nature of its fingers, which are composed of millions of keratin hairs whose diameter is only a few microns at their base. At their tips, these hairs split into even finer hairs that end in a spatula structure. At the molecular level, a force comes into play - Van der Walls' force. Very briefly, it is a force capable of binding atoms or molecules together. In short, the gecko’s secret lies in the microscopic structure of its hairs, which interact with surfaces at an intermolecular level. Depending on the orientation of the animal's hairs, they either adhere to or detach from a surface. This is what enables it to walk around on a ceiling.
Can you tell us how long you have been working on the subject?
I was exposed to tribology research in biology and bionics as early as 2004, when I began my postdoctoral training at the Max Planck Institute for Metal Research in Stuttgart, Germany. There we studied artificial mushroom-like adhesive microstructures inspired by the fibrillar attachment hairs of male beetles. It is something I have been working on ever since.
Now, thanks to flexible polymers, you have successfully reproduced the hairs on a gecko’s foot. How did you go about doing this?
Briefly, we mix different polymers that are soft but capable of hardening over time (a bit like simple modelling clay that hardens under the action of air). Then we dip razor blades into this curing mixture and remove them, stretching the polymer to make several small streaks. They reproduce the hairs of the gecko at the micron scale, which is a prerequisite to make a material adhere using Van der Walls’ force.
This solution of stretching using razor blades is more affordable than developing a mould and above all much more efficient because the demoulding operation is susceptible of disrupting the surface quality of the polymer.
One of the great innovations of our method is that with a small quantity of an easy-to-gather material, it would be possible to manufacture this new adhesive in a simple kitchen with no need to be in a clean room. It is therefore within everyone's reach, or at least that of many industrialists.
Which polymers did you use?
So far, we have used a combination of PVS (polyvinyl siloxane), a polymer used in dental moulding, PDMS or dimethicone (Polydimethylsiloxane), a polymer that can be liquid like water or have the density of a gum depending on its degree of polymerisation and which is used in the composition of silicone rubbers. Finally, polyurethane (PU), a polymer well known in particular for its foams. However, I would like to say that other flexible elastomers can also be used.
What were the main difficulties you encountered?
There were many, and it took us many months and even years to overcome them. Although our method is simpler than moulding, it took a year to develop it. We had to find the right polymer, the right soaking time for the blades, the right shape to give the striations. Then we had to bring the theoretical computer model to life. It was far from straightforward as there were so many parameters to control: viscosity, polymer temperature, speed of insertion and removal of the blades, and more. There is still room for improvement and we are currently working on refining the elasticity of our polymer in order to improve its performance. Its elasticity is very important because it is the shear effect* that will determine whether or not it adheres to a surface!
What will be the industrial applications of your work?
First of all, I think it could be used in robot arms that operate in high-precision industrial environments, as is the case for the manufacture of microprocessors. Handling a microchip is a very delicate operation. Currently, these robot arms have ceramic "hands" that use air vacuum grippers to manipulate the silicon wafers. As the ceramic wears down due to friction, it releases particles that can contaminate the wafer, which can lead to lithographic defects. Using a hand made with our "gecko foot polymer" would be preferable as it does not crumble and therefore cannot damage the wafers.
This material could also be used to attach a painting on a wall or any type of small objects on the ceiling. There are many potential applications for both industrial and domestic use.
* A state of internal stress in a structure, in which each part tends, under the effect of forces acting in opposite directions, to slide in relation to the neighbouring part.