In Mission: Impossible! Ghost Protocol (the fourth installment of the series), a memorable scene has the heroic--and nigh-indestructible--Ethan Hunt (Tom Cruise) climbing the outside of the Burj Khalifa, more than 1700 feet above the ground. Because it's Mission: Impossible, Hunt of course has no harness, rope, or other climbing gear--only a pair of adhesive gloves and some rubber-soled shoes.
When tech wizard Benji (Simon Pegg) is giving Ethan the demo of how the gloves work, he emphasizes that to un-stick the glove from the window, Ethan needs to use a "rolling-off motion." Ethan, being the super-agent he is, quickly masters this rolling motion and proceeds to use the gloves to pull himself up the sheer glass wall of the skyscraper.
(It probably goes without saying that, of course, one of the gloves' batteries die once he is past the point of no return, requiring him to finish the climb with only one miracle sticky glove, which manages to catch his entire body weight from a free-fall a short time later.)
As with most action movies, willing suspension of disbelief is required to truly appreciate the stunts that punctuate Ghost Protocol at predictable intervals. But in this case, the physics isn't actually that far-fetched: it's likely that the gloves are biomemetic--technology and engineering inspired by the design and/or function of biological systems. In this case, judging by their appearance and Benji's description of how they work, it's a safe assumption that the gloves are based on the biomechanics and biophysics that help geckos, flies, and other small animals scale walls, dangle from ceilings, and perform other impossible missions.
The climbing abilities of various lizards have been an object of fascination at least since the time of Aristotle, but it's only within the last century or so that we've been able to start building a detailed understanding of how the structure of their feet enables those abilities. With the advent of advanced microscopy, computer modeling, and incredibly sensitive force detection tools, the mechanism of lizard toe adhesion has become more well understood.
A lizard climbing up a wall or across a ceiling has to have a way to "turn on" and "turn off" the stickiness of its feet. Always "on" and the lizard can't move; always "off" and it's stuck (heh) with the limitations of gravity like we poor bipeds. So how does it work?
As with many seemingly miraculous everyday occurrences, lizards can thank physics for their selectively sticky feet. Like Benji's magic sticky gloves, lizard feet have a differential stress response: they respond differently to tension than to shear forces. When under tension, the adhesive force is strong; but subject it to a shearing or bending force and it weakens enough to detach. The "rolling motion" Ethan uses to un-stick his hand from the Burj Khalifa's glass produces that shearing force, and lizards create a similar type of motion in their feet when they walk.
The notion of a material that responds differently to different kinds of force shouldn't be too unfamiliar: rope is strong under tension (pulling) but weak under compression or shear (sideways) forces, whereas many kinds of structural materials (girders etc.) are strong under compression but weaker under shear or tension forces. This is one of the reasons why earthquakes, tidal waves, and other disturbances that produce strong shear forces can cause massive amounts of structural damage, even though they generate less total force than the weight of the building itself pressing downward every day.
But these are all examples of solid objects demonstrating internal strength or weakness, which is a far cry from adhesion (stickiness) between two different objects--and this is where the physics comes in. As it turns out, gecko feet owe their stickiness to the millions of tiny setae (projections) that make up the bottom surface of each toe--more than 14,000 per square millimeter. Because of their microscopic size, each seta experiences tiny, transient molecular forces known as van der Waals forces when it contacts another surface.
Although van der Waals forces are among the weakest interactions between molecules, they nonetheless can provide a significant amount of adhesive force when applied to the millions of individual setae on each foot. In one clever study, Autumn et al calculated an adhesive force of more than five atmospheres--more than enough to hold a tiny lizard against the ceiling.
What's especially fascinating about Autumn et al's results is that they were able to demonstrate that the adhesive forces were primarily the result of the size and shape of the setae, rather than their chemical composition: when they created simulated setae out of rubber and polyester resin, they observed the same adhesive properties as were present in the gecko toes. They were also able to demonstrate that the adhesive force increases significantly as the setae get smaller--which may be part of the reason this type of setal structure has been evolutionarily selected in so many climbing lizards.
The idea that large numbers of setae may have a survival advantage in some species is supported by the genomic work of Liu et al, who used genetic data to construct phylogenetic trees showing the evolution of a number of different lizard species. Their results showed that Gekko japonicus, the Schlegel's Japanese gecko, contain more copies of genes for the proteins that form setae than do other lizard species who have fewer setae per unit area.
In other words, the species that have lots of very tiny setae have more copies of the gene than the species with fewer, larger setae or no setae at all, and the density of setae is closely related to the species' habit: G. japonicus is a typical gecko, with sticky feet that it uses to climb trees and walls and catch prey, and it has the most copies of the gene and the largest setal density; Anolis carolinensis, the green anole, has fewer copies of the gene and a lower setal density, but still demonstrates some wall-climbing ability; and Alligator sinensis, the Yangtze alligator, has only two copies of the gene and no setae--it lives primarily in water and does not climb. These patterns, together with the inferred timing of the genetic differentiation between the species, support the hypothesis that selection pressure drove the increase in the number of setae (and correspondingly stickier feet).
Liu et al also investigated the evolutionary history of another well-known trait of many lizard species: caudal autotomy, or the ability to "shed" a still-wriggling tail to distract predators and buy time for an escape. Species that have this ability, such as G. japonicus and An. carolinensis, show positive selection pressure for genes associated with wound healing and cell growth, whereas species that don't shed their tails lack those positively selected genes.
And now we come full-circle, because it turns out that there are a lot of similarities between the physics of gecko toes and the physics of tail autotomy: both rely on the characteristic strong-under-tension-but-weak-under-shear strain responses associated with the microscopic interfaces between surfaces.
Just as a gecko's foot is covered in millions of tiny setae, which generate adhesive forces, the point at which its tail connects to its body also contains large numbers of tiny pillars of muscle that hold the tail in place but also allow for its quick release. At the point where it detaches during autotomy, the tail contains a roughly conical arrangement of tiny, mushroom-shaped pillars. The "cone" shape at the end of the tail interfaces with a corresponding "socket" shape on the lizard's body--in other words, the tail attaches to the body the way a phone cable attaches to its charging socket.
Unlike a charging cable, though, the lizard's tail attachment is quite strong under tension--it won't detach if it's pulled straight backward away from its body. This helps prevent the tail from falling off under regular lizard behavior. But if the lizard moves its tail in just the right type of side-to-side motion, its tail separates from its body.
Baban et al used a biomemetic fracture model to elucidate how this works in detail. They demonstrated that the geometry of the tail-body attachment, along with the specific type of motion the lizard uses to initiate detachment, allows a "fracture plane" to propagate through the tail attachment surface. As the plane propagates, the tail muscles detach from their corresponding partners in the lizard's body, and eventually the entire tail has detached.
By creating silicon-based models of the tail and its attachment socket, Baban et al were able to demonstrate that the arrangement of the micropillars contributes to the tail's behavior under different kinds of stress. In most situations, the flexibility and nanoscale adhesion between the micropillars and their corresponding sockets helps to stop any incipient fractures from propagating. As Ghatak describes in a summary of the Baban article, the flexibility ensures the stresses never build up enough to cause the tail to detach completely.
It's probably unlikely that Ethan Hunt will ever dangle from the Tokyo Skytree by a detachable rope-tail...but one never knows.
Autumn, Kellar, Metin Sitti, Yiching A. Lang, Anne M. Peattie, Wendy R. Hansen, Simon Sponberg, Thomas W. Kenny, Ronald Fearing, Jacob N. Israelachvili, and Robert J. Full. 2002. "Evidence for van der Waals adhesion in gecko setae." Proc. Natl. Acad. Sci. 99 (19): 12252-12256. https://doi.org/10.1073/pnas.192252799.
Baban, Navajit S., Ajymurat Orozaliev, Sebastian Kirchhof, Christopher J. Stubbs, and Yong-Ak Song. 2022. "Biomimetic fracture model of lizard tail autotomy." Science 375 (6582): 770-774. https://doi.org/10.1126/science.abh1614.
Ghatak, Animangsu. 2022. "How does a lizard shed its tail?" Science 375 (6582): 721-722. https://doi.org/10.1126/science.abn4949.
Liu, Yan, Qian Zhou, Yonjun Wang, Longhai Luo, Jian Yang, Linfeng Yang, Mei Liu, Yingrui Li, Tianmei Qian, Yuan Zheng, et. al. 2015. "Gekko japonicus genome reveals evolution of adhesive toe pads and tail regeneration." Nature Communications 6, 10033. https://doi.org/10.1038/ncomms10033.
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