Now that I've mentioned The Gecko's Foot, this is a good time to talk about why this appendage is an icon for cool science and bioinspiration. Why is the gecko so interesting to material scientists? Because it can do this! That photo hasn't been rotated. The gecko is literally sticking to glass upside down, and the best part is that it's barely expending any energy! This is an often sought-after skill among espionage experts. Science is getting closer to making those gloves a reality as we unravel and replicate the gecko's secrets. One of the most alluring properties of the adhesion of gecko toes is that they stay completely dry. There's no goo on their feet and no residue left behind on the surface. This is good for spies that don't want to leave a trace, it's a more solid grip than suction cups, and works on more surfaces than magnets. So what is the mechanism behind this completely clean stickiness? It has to do with physical forces that only work on the nano-scale. The gecko's foot is a much more complex surface when you put it under a microscope. It has rows of lamellae on its toepads that are made up of small hairs, which split off into even tinier setae that end in flat spatulas. Why so many minuscule bristles? As I said, the adhesive forces operate at the nano-scale, so the foot must make as many contact points as possible at this scale. What we see as flat surfaces with our unaided eyes are in fact rugged terrains of peaks and valleys when you zoom in: This densely packed array of setae offers a lot more flexibility to the surface of the foot than normal skin would. All of those teeny hairs can bend and compress until they precisely match the topography of the whatever they're trying to stick to. Once you get lizard and glass into such close contact, a phenomenon known as van der Waals forces takes over. This is an attractive force that exists between all atoms that get within a few nanometers of each other. It's not an excessively strong force, but when it's maximized over a large scale, the power adds up quickly. Van der Waals adhesion was first noticed in the gecko, but it's an important mechanism in many other animals. Hm, what else climbs on smooth, often vertical surfaces... Spiders have structures called scopulae that are similarly made up of tiny bristles. They generally have fewer setae than geckos, however, since there is an inverse relationship between the weight of the animal and the number of hair-contacts needed to support it. A spider is lighter than a gecko, so it doesn't require as much van der Waals force to sustain its adhesion to a surface. The first Spider-Man movie (pictured above) got close to addressing the source of Peter Parker's stickiness correctly, but they just missed the target. One scene shortly after his genetic transformation zooms in on his thumb and shows small hairs with intricate structures emerging from his skin. It's a nice try, but those hairs are too widely spaced apart to have any meaningful nano-scale interactions with the brick buildings of New York City. The movie seemed as though it were going for more of a barb-like projection, but that idea falls short as well. Those spines are far to tiny to maintain enough strength to support a man's weight over such a small surface area. And besides, Peter just covers up his sticky hands with gloves anyway! Not to mention the shoes he wears that thwart any possibility of additional adhesion with his feet. That's why X-Men nemesis and member of the Brotherhood of Mutants villain Toad is a more logically sticky character. Toad's gloves only cover his palms, leaving his adhesive fingers plenty of freedom to van der Waals where they please. His palms would offer added stability, but hilarious experiments with geckos have shown that they have enough excess stick-to-it-iveness in their pads that just a single toe is sufficient suspend their entire body. Amphibians do have a slightly different adhesive style however, and a more wet one. Wet adhesion means that we're not as interested in adapting tree frog toepads into industrial nanomaterials because they require a constant reapplication of fluids, whereas gecko feet are more clean and can operate without a living gecko. Recent research has shown that tree frogs do appear to employ some close contact attractive forces with surfaces. Those hexagonal epithelial cells on its toes provide the same flexibility as gecko setae, but at a slightly larger scale. The wetness comes from the fluid mucus that flows in the channels between those flat-ended pegs. Going back to industrial applications, where is my gecko tape?? Rest assured, engineers are working on it as we speak. Andre Geim has had success in replicating the functional nanostructures of gecko feet and making workable gecko tape. The process still needs to be refined, however, before it can be made on a bigger scale and as durable as the gecko feet themselves. The dream of nearly infinitely reusable and mess-free tape is nearly upon us! In reality, most household tapes are good enough already that we won't be seeing gecko tape in our homes too soon. The medical field will certainly jump at the chance to use a strong, removeable adhesive that doesn't introduce sticky chemicals into the body though. So if gecko tape can be successfully fabricated, would it be possible to make those gloves Tom Cruise is using to climb the Burj Khalifa? Yes! Spiderman can become a reality in the near future. Although while I don't doubt the ability of the gloves to stick to walls, how do you keep said gloves from slipping off your hands...? In the meantime, we've adapted these adhesive And since I mentioned Toad, I need to share with you my favorite superhero science lesson of all time:
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Alanna DurkinExploring the realm of biologically inspired design one superhero example at a time, with some other natural sciences mixed in. Archives
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