Geckos have always been a subject of great interest for zoologists for many number of years. Few years back, this humble lizard family started captivating the minds of materials scientists and nanotechnologists due to their amazing ability stick in to number of different surfaces. It all started when scientists took a close look at gecko feet, in searching for the origin of this seemingly gravity defying ability.
Material scientists had many good reasons to be interested on geckos. Only the two front feet of an ordinary gecko can support the weight of a gecko, 40 times over. A gecko surface with an area of a palm can withstand approximately 180 kg of weight which amounts to approximately three times the weight of a grown man.
What makes geckos stick to surfaces
The science behind this amazing property of gecko feet was completely explained by a group of scientists led by biologist Kellar Autumn of Lewis & Clark College in Portland, Oregon and Robert Full at the University of California Berkeley. They showed that intermolecular forces between the gecko feet and the surface give geckos this phenomenal ability. A close look to the gecko feet reveals tiny hair-like structures called setae, which are mostly found in the toe area. These structures further split into few hundreds of even smaller tips that have nanoscale sizes. These so called Septulae structures, provide large surface area to create van der Wall interactions between the hair molecules and the molecules of the surface that gecko is clinging in to. In fact, van der Waals forces are among the weakest forces in the chemical world. Yet, surface area boost given by nanoscale septulae make it significant enough and amounts to a small grip at each setae. There are millions of these small bristles in each gecko foot, which together adds up a firm adhesive bond.
Since the discovery of the origin of gecko adhesive property, many researchers have attempted to make synthetic setae; advance textured material that can mimic the adhesive ability of a gecko surface. The main driving force behind these investigations were to find a new class of adhesives that do not rely on a glue or a gum to stick but on the structural features of the surface. However, the research work on this field didn’t get enough traction in the early years, mainly because of the limitations in nanoscale fabrication techniques. Thanks to latest developments of nano materials and nanoscale fabrication techniques, scientists are now able to create structures similar to a gecko surface.
The very first to make news was the “Gecko tape”. The inventers of this product attempted to mimic the natural structures on the gecko foot on a synthetic material made of polyimide using series of techniques including, photolithography, electron beam lithography and plasma etching. However, performance wise, gecko tape was very far from the gecko foot which showed an adhesion coefficient of only 0.06 which is very low compared to what was measured on gecko foot (9 – 15). The interest and the innovations in gecko type like surfaces are growing, as seen by the increasing amount of publications and patents over the years. Researchers have mainly used flexible polymeric materials such as polyimide, polypropylene and polydimethylsiloxane. Latest research papers also show frequent use of carbon nanotubes in these structures.
Spider man gloves
The recent development in gecko surfaces that attracted lot of attention is the work carried out at Mark Cutkosky‘s Biomimetics and Dextrous Manipulation Lab at Stanford University. This is because they managed to solve one of the biggest limitations of gecko pads which was the scale up. Researchers in this field have observed that making the gecko pads larger than a certain size would make them less effective in adhering to surfaces. The team solved this problem by using a series degressive spring. Unlike, normal springs that we are used to, these springs become less stiff when they are extended. Researchers attached these springs to 24 identical adhesive tiles with special nanostructures that mimic the gecko feet. When these springs are pulled, they apply an even force on each adhesive tiles distributing the loads very evenly across the whole gecko pad. Stanford scientists have built this in to a handheld pad which effectively make it a spider man glove. Just like the spider man, they also demonstrated that they could climb the vertical surfaces with these spider man gloves on.
There are many number of applications for synthetic gecko surfaces in areas such as robotics, military, health and sports. Some researchers have already developed robots that can climb walls and walk on horizontal surfaces upside down. These robots mimic the movement of geckos and are fitted with gecko surfaces. Commercially applicable, high volume easy to scale up methods to fabricate gecko surfaces are being investigated by several research groups around the world. Although there are many challenges to overcome, many experts agree that gecko surfaces will see real world applications in next few years to come.
PS: gecko feet structure image credit: stanford.edu