Sometimes we get so used to our surroundings that we ignore the obvious. The common house lizard (gecko) has been a regular part of my life, much to my dislike. Yet, it was a while before I realized this obvious conundrum: the gecko runs on walls, or a particularly adventurous one might even walk upside down on the ceiling. This means that it can attach quite strongly to these surfaces, so as to not fall off. However, it also needs to detach easily to maintain quick mobility along the surface. How does it manage this gravity defying attachment, followed by quick detached movement?
The attachment mechanism of the gecko is by gripping, not sticking. The toe-pads of a gecko do not secrete any sticky glue-like material. In fact, they consist of millions of tiny hairs called setae. Each seta further branches into about 100-1000 spatulae at its end, resembling a make-up brush. The setae are normally curled up; they uncurl upon slight sliding motion to attach to the wall surface. The attachment occurs through forces arising out of direct interaction between molecules, in this case setae and wall. These interactive forces, called Van der waals forces, get weaker over distance, which means that if the contact between the setae and wall is lost, the attachment force diminishes, and the toe can move forward.
Zoomed in images of gecko toe (a), showing rows of setae (b), a single seta (c), and spatulae (d) (Results from ref. 1)
One group directly measured the force exerted by an individual seta. This was done by carefully removing the seta from the animal and gluing it onto a platform. The spatulae-bearing ‘sticky’ end of the seta was allowed to interact with a spring-like surface termed cantilever. When the seta pulled with some force, the springy cantilever bent; when this force was no longer applied, the cantilever returned to its original position. The researchers noted that the seta starts exerting force after sliding for a short distance, then reaches its maximum attachment force of 200 microN. Considering that the animal has about 6 million setae, it should be able to lift an object of about 130 kg, IF all the setae engage at once.
In reality, millions of setae are not engaged simultaneously in the gecko. In fact, the even distribution of forces, cleaning of the adhesive (gecko hairs are self cleansing), and right choice of material were few of the many challenges in making scaled-up artificial adhesives mimicking this mechanism. Several groups have nevertheless overcome some of these obstacles and succeeded in producing competent alternatives. A recent leap (or slide) was achieved in 2014, when a research group from Stanford demonstrated their ‘gecko-like’ pads to climb up a wall.
Although we have a long way to go, its heartening to know that Mission: Impossible-like wall scaling is not impossible after all.
Research groups working on gecko-like adhesives: