Tristan Gilet and his colleagues from the Mircofluidics lab (UR A&M - Faculty of Applied Sciences), in collaboration with the Laboratory of Functional and Evolutionary Morphology (UR FOCUS) and the ULB's TIPs, have just discovered a new secret in the walking mechanisms of the dock beetle. This research has just been published in the journal Royal Society Interface (1) in which they describe the movements of the tips of the beetle's legs. Understanding these mechanisms can be particularly interesting for the future of microrobotics.
The dock beetle is a fairly sedentary insect, widely spread throughout the world and which remains most of its time attached to a stem or leaf." Like many other insects (including flies) the dock beetle has hundreds of setae, tiny hairs (length 50µm, diameter 2µm) at the ends of each of their little legs, which generate the adhesion necessary for their locomotion, explains Sophie Gernay, a PhD student at the Microfluidics Lab." During each movement, a tiny amount of liquid arrives at the tip of each seta.
In August 2016, researchers at the Microfluidics Lab revealed - in a study published in the same journal Royal Society Interface( (2) - the role of this liquid in adhesion." In our previous publication, we quantified the effect of this liquid, which generates an adhesive capillary force, the same as the one we use when wetting our fingers to turn the pages of a book,"says Sophie Gernay. This force allows the tip of each hair to bend so that it follows more closely the roughness of the surface on which the insect is walking. Multiplied by the large number of setae, the adhesive force is then sufficient to allow an insect to walk upside down on any surface.
The dock beetle is a small beetle that weighs only 11mg.
Until now, researchers had no choice but to immobilize the insect in order to decipher its adhesion mechanisms."We blocked the beetle's leg, then gently brought it into contact with the substrate," explains Tristan Gilet, head of the Microfluidics Lab. But it had to be noted that this forced movement of the insect was sometimes not very realistic. The reason is that the insect's walking movements had never been properly quantified." So we decided to take the time. We left the dock beetle free to walk on a microscope slide. Then we waited until it decides to place a leg in the tiny field of view of the microscope. So we were able to film and study the movements of the insect's microscopic adhesive structures."
Microscopic view of the ventral face of the leg of a dock beetle walking upside down. Each yellow spot represents the adhesive tip of a hair (a seta) in contact with the overlying substrate.
Researchers have learned many lessons from these measurements. The first is that despite walking upside down, the dock beetle does not need to use the full potential of its adhesive mechanism. « Most of the time, the insect uses only half of its setae," notes Sophie Gernay. « It is certainly easier to detach the leg when it doesn't stick too much ». Second conclusion: the setae are dimensioned so as not to bend (except at their tip) when the leg strikes the substrate. It is enough for the insect to use only a few setae to completely block the movements of the leg. "Attachment and detachment are then made by rotating the leg, causing each seta to position itself like a piece of tape." Researchers also observed that during attachment, the insect slightly pulls its leg towards its body, so that the hairs make good contact, as if it were painting the surface to better apprehend it. Detachment, on the other hand, is twenty times faster than attachment and lasts only a few milliseconds." The setae resist to the movement of the leg until the last moment. When these last irreducible setae capitulate, the leg snaps and moves away like an arrow, with an acceleration of almost 50g! This rapid detachment seems to minimize the amount of precious liquid the insect leaves behind where it walks,"says the young researcher.
Researchers have been studying and watching insects walking for a long time and the first study dates back more than a century. The novelty in this study is that for the first time it was possible to film simultaneously the global movement of each leg and the local movement of each seta." The first takes place at the millimetre scale, in a few tenths of a second, and is controlled by the leg muscles. On the other hand, the second happens at the micrometer scale in milliseconds, and it's uncontrolled." In other words, it is the global movement of the leg that passively induces the local movement of the setae, so that they generate the desired adhesion.
Detaching a leg. The setae are first gradually detached, starting with those closest to the body. When the last setae surrender, the leg snaps and moves away. The leg is approximately 200µm wide, and the sequence lasts about 0.05s.
There are an infinite number of ways to bring an insect's leg or a microrobotic gripper into contact with an object, and then detach it. The gripper could arrive from above or from the side, it could slide or not, more or less quickly, with or without rotations, etc. Optimizing this movement in microrobotics would require a colossal amount of engineering work. « Our effort to miniaturize robots runs up against a severe limit: it seems impossible with current technology to grasp an object of less than a few tenths of a millimetre in size, and to deposit it elsewhere ». Insects have already found a solution to this problem after several hundred million years of evolution. The tip of their setae, however fifty times smaller, attaches and detaches from the surface on which they walk, at a rate exceeding our best microrobots. This is why their adhesive microstructures is inspiring engineers today.
« The movements that we quantified in the dock beetle had already been qualitatively observed in the fly, whose legs also have setae. When several species independently evolve towards the same solution to a problem (here fast and robust walking on any surface), one can think that this solution is effective, and thus an excellent source of inspiration, » concludes Sophie Gernay, the first author of the publication.
(1) Multiscale tarsal adhesion kinematics of freely-walking dock beetles, Sophie GERNAY et al., Journal of the Royal Society Interface, novembre 2017.
(2) Elastocapillarity in insect fibrillar adhesion, Sophie Gernay et al. Journal of the Royal Society Interface, Août 2016. (Read Hold on ! )
Sophie GERNAY et Tristan GILET
Microfluidics Lab I A&M Research Unit I Faculty of Applied Sciences
+32(0)4 366 91 66 - Tristan.Gilet@uliege.be
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