Animals have evolved a variety of strategies for gripping, scaling and clinging to surfaces. Arguably the most powerful of these strategies is the adhesive pad. If an animal can walk upside-down on a smooth surface, there’s an excellent chance it has adhesive pads on its feet. These pads are unlike anything humans have engineered. No glue or suction cups or velcro or magnets are involved. They work sort of like tape, if tape were self-cleaning and could be attached and detached rapidly and repeatedly without any loss in performance. (So, not really like tape at all.) Many different animals have evolved adhesive pads specifically for locomotion, and these pads vary considerably in their structure and underlying mechanism.
The two basic adhesive pad structural forms we see in nature are smooth and fibrillar. Smooth and fibrillar pads can in turn be “wet” or “dry”, depending on whether the animal secretes a fluid over them.
Fibrillar pads resemble microscopic forests, or toothbrushes, and are sometimes referred to as “hairy” adhesives. Geckos, spiders, beetles and flies have fibrillar adhesives.
|Ptyodactylus hasselquistii by Anne Peattie||Gecko adhesive (Thecadactylus rapicauda) by Anne Peattie|
Smooth pads look more like balloons or rubber balls. They’re defined by their smooth, continuous surface. Animals with smooth pads include frogs, bats, grasshoppers, cockroaches, ants and stick insects.
|Tree frog, by acodring, on Flickr||Grasshopper, by Cyron, on Flickr|
How do they work? (The short answer)
Despite their differences, both types of pads can do one thing very well: adapt and conform to just about any kind of surface, down to the sub-micron scale. Regardless of their material properties, they behave like very soft, compliant structures. This compliance allows direct intermolecular bonding between the foot and the substrate. Individual hairs on fibrillar pads are composed of a stiff material, but each one can bend and adapt to meet either rough or smooth surfaces at a very local level. Smooth pads, it turns out, are actually fibrillar on the inside, but appear smooth because they are covered with an unbroken layer of soft material. Despite this extra layer of material, smooth pads are just as good at conforming to bumpy surfaces as fibrillar pads are (Bullock et al., 2008). Once this intimate contact is established, molecules making up the pad come in such close contact with molecules making up the substrate that they become attractive to each other and form intermolecular bonds, namely the ubiquitous and versatile van der Waals interactions.
Tape: a human-engineered adhesive
Tape, which also relies on intermolecular bonding for adhesion, is a layer of very soft material (the tacky side) bonded to a layer of stronger material (the non-sticky backing). When you apply tape to a wall, that soft material oozes onto the paint and in and around small bumps, forming bonds with the wall while maintaining its connection to the backing. Any loose debris (or thumbprints, or water) that gets in between the tape backing and the wall prevents the sticky layer from forming a bond with both of those surfaces, lowering the overall attachment force. When you peel tape away from a surface, it tends to leave behind a sticky residue. The softness of the sticky material makes it very weak internally. It tears apart when you peel it, some of it staying stuck to the substrate and some of it staying stuck to the backing (unless the substrate material is even weaker, in which case it carries some of the substrate away with it; either way it’s not going to work a second time). The strength of tape is dependent largely on its thickness. The thicker the tape, the bigger the bumps it can tolerate, and the stronger it will stick. Scotch tape has a thickness around 1/1000th of an inch (0.025 mm), electrical tape is about five times that (0.13 mm) and duct tape can be ten times as thick (0.25 mm) or more. Even the thickest tapes are not that thick, and tapes perform best when stuck to smooth surfaces. The strength of tape’s bond further relies on a preload force – that would be your thumb – pressing the tape into the surface, forcing the goo to flow and conform to the wall. The harder you press, the stronger the bond (to a point). For this reason, tape is known as a “pressure-sensitive adhesive” in the industry parlance.
How do they work? (The longer answer)
Animals with sticky pads on their feet can run around without worrying about their feet getting clogged with debris or tearing apart as they peel away. Some of these animals can attach and detach all of their 4, 6, 8 or more feet over 10 times per second. They don’t have the time (or energy) to press each foot into the surface and yank it off again. A tape-like mechanism is not an option during rapid locomotion. So how do sticky-footed animals do it?
One important property of sticky feet is their orientation-dependence. Each individual adhesive pad has an active and inactive state. Unlike tape, sticky feet don’t need to be pushed into the surface, but they do have to be dragged across the surface, ever so slightly, to begin sticking. When the pad is oriented correctly, that drag pulls the adhesive pad further into its active state, creating more contact and stronger adhesion. This dragging motion is imperceptible compared to the motion of the whole foot, and adhesive pads evolved such that the animal doesn’t have to change anything about its normal climbing gait to use them. In fact, most animals with adhesive pads can adhere to a vertical surface without any effort at all; the body weight of the animal alone “loads” the adhesive, activating it. To detach, the animal simply shifts its pads in the opposite direction – or lifts them straight up – again requiring negligible effort beyond the normal movement inherent to a stride. In this sense, sticky feet resemble claws much more closely than tape. Claws only function while loaded in a particular direction, and reversing that load causes them to unhook from their purchase. (This is not a coincidence; adhesive pads evolved in the presence of claws, and the two attachment modes had to work in parallel.) Tape, once engaged, resists detachment no matter which way you pull on it. In designing an adhesive meant for permanent or static detachment, orientation dependence would be a flaw. But for rapid, dynamic attachment, it’s a feature.
Sticky feet are self-cleaning, which is probably one of the coolest and least understood things about them. We can watch it happening before our eyes in the lab but the mechanism is still something of a mystery. Part of the answer is in the orientation dependence I described above. Between each step, adhesive pads revert to their non-active state. The gecko’s dry, fibrillar adhesive branches into minute tips so small (200 nm) that each one, on its own, hardly adheres at all. Only when the animal activates millions of these tips in concert, distributed across all of its toes, does it generate enough force to support its body weight. Between steps, any debris that was in contact with just a few dozen, or even a few thousand, of these tips will suddenly find that it’s in contact with far fewer as the adhesive hairs spring into their inactive configuration, leaving the debris suddenly unattached and easily brushed away as the animal initiates its next step (Hansen & Autumn, 2005). Smooth pads also change shape between steps, contracting and radically decreasing the available surface area of the pad, but perhaps more importantly, smooth pads are always “wet”. That is, animals with smooth pads secrete a fluid over them, which could help “wash” debris away. Many animals with fibrillar adhesive pads also secrete fluids over their feet, so this potential strategy is not limited to smooth pads (Clemente, et al., 2010).
It’s important to note that fluid secretions from adhesive pads do not act like glue. They don’t dry out and harden. They are viscous (like oil) but not inherently tacky, the way honey is. (If you want to get technical about it, they are non-Newtonian emulsions; Dirks et al., 2010) Secretions seem to help fill in gaps on uneven (rough) surfaces, improving attachment, but on smooth surfaces, less fluid is better (Drechsler & Federle, 2006). Researchers are still working out their exact contributions to attachment.
Finally, animals with adhesive pads can stick to a wide variety of different surfaces, from smooth, waxy plants to rough, dusty rocks, plus a lot of things they never encounter in nature, like silicon wafers and glass slides. In general, sticky feet adhere more strongly to smooth surfaces than to rough, and there is good reason for this. All of these animals have claws, which work fine as long as there are bumps and crags to hook onto. Adhesive pads represent an overall improvement in attachment capability over claws – some animals have even lost claws after evolving adhesive pads – but their exceptional performance on smooth surfaces suggests that adhesive pads are an adaptation for climbing smooth surfaces in particular.
|Prionus californicus foot by boingr, on Flickr||Ladybug, by CharlesLam, on Flickr|
Bullock JMR, Drechsler P, Federle W (2008) Comparison of smooth and hairy attachment pads in insects: friction, adhesion and mechanisms for direction-dependence. J Exp Biol 211:3333-3343 (doi: 10.1242/jeb.020941)
Clemente CJ, Bullock JMR, Beale A, Federle W (2010) Evidence for self-cleaning in fluid-based smooth and hairy adhesive systems of insects. J Exp Biol 213:635-642 (doi: 10.1242/jeb.038232)
Dirks J-H, Clemente CJ, Federle W (2010) Insect tricks: two-phasic foot pad secretion prevents slipping. J R Soc Interface 7:587-5593 (doi: 10.1098/rsif.2009.0308)
Drechsler P, Federle W (2006) Biomechanics of smooth adhesive pads in insects: influence of tarsal secretion on attachment performance. J Comp Physiol A 192:1213-1222 (doi: 10.1007/s00359-006-0150-5)
Hansen WR, Autumn K (2005) Evidence for self-cleaning in gecko setae. Proc Natl Acad Sci USA 102:385-389 (doi: 10.1073/pnas.0408304102)