tag:theconversation.com,2011:/ca-fr/topics/adhesives-33706/articlesAdhesives – La Conversation2019-06-05T14:03:09Ztag:theconversation.com,2011:article/1077732019-06-05T14:03:09Z2019-06-05T14:03:09ZSpider glue’s sticky secret revealed by new genetic research<figure><img src="https://images.theconversation.com/files/277969/original/file-20190604-69071-x7jm95.jpg?ixlib=rb-1.1.0&rect=844%2C62%2C4778%2C3592&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Spider glue is actually a specialized silk protein.</span> <span class="attribution"><span class="source">Sarah Stellwagen</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>What do all of the <a href="https://wsc.nmbe.ch/">over 45,000 described spider species</a> on Earth have in common? Each makes at least one type of silk. And there are an awful lot of types out there.</p>
<p>An individual orb weaving spider – the kind that spins the classic two-dimensional aerial spiral webs that seem to always be suspended at human face-height – can produce seven different silks, each with unique material properties.</p>
<p>Dragline silk forms the frame of an orb web and is famous for its strength and toughness, <a href="https://www.sciencemag.org/news/2018/11/spider-silk-five-times-stronger-steel-now-scientists-know-why">comparable to that of steel</a>. The capture spiral is made of a highly stretchy version called flagelliform silk. Orb weaving spiders use an additional type of silk to wrap prey and create web decorations.</p>
<p>But there’s another kind that, on the surface, doesn’t resemble silk at all: the sticky glue with which some spiders cover their silk capture threads. It doesn’t look like the classic threads that come to mind when thinking of spider silk, but the gluey substance from these webs is in fact a silk protein.</p>
<p>For many years, researchers have been uncovering the secrets of spider glue, which stays wet in its open air environment and sticky over many rounds of attachment and release. Its genetic blueprint has remained elusive, however, meaning scientists haven’t been able to think about setting up large-scale production of this potentially useful biomaterial. </p>
<p>Using new technology, my colleague and <a href="https://scholar.google.com/citations?user=gWWab2oAAAAJ&hl=en&oi=ao">I have been able</a> to sequence the <a href="https://doi.org/10.1534/g3.119.400065">first full genetic sequences</a> that code for spider glue proteins.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/277968/original/file-20190604-69059-a0pc48.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/277968/original/file-20190604-69059-a0pc48.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/277968/original/file-20190604-69059-a0pc48.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=354&fit=crop&dpr=1 600w, https://images.theconversation.com/files/277968/original/file-20190604-69059-a0pc48.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=354&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/277968/original/file-20190604-69059-a0pc48.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=354&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/277968/original/file-20190604-69059-a0pc48.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=445&fit=crop&dpr=1 754w, https://images.theconversation.com/files/277968/original/file-20190604-69059-a0pc48.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=445&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/277968/original/file-20190604-69059-a0pc48.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=445&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Spider glue drops spread along a strand of capture spiral silk.</span>
<span class="attribution"><span class="source">Sarah Stellwagen</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>A silk that’s really a sticky glue</h2>
<p>Under a microscope, orb weaver glue resembles beads on a string – little glistening spheres along a strand of stretchy support silk. Instead of being spun into a fiber as it leaves the spider’s body like other silks, the glue proteins are extruded as a jumbled mass. Their job is to stickily retain prey that get caught in the web.</p>
<p>Different spider species produce glue tailored to their habitat’s conditions and prey.</p>
<p>The glue of tropical orb weaving species is sticky in the spider’s wet habitat, but downgrades to just tacky in low humidity. The glue of orb weavers from dry regions becomes dilute and thin if the humidity is too high.</p>
<p>Bolas spiders forgo the orb web, and instead produce a large globule of glue at the end of a long strand of silk that they whirl rapidly through the air. The glue of this sticky snare is specialized for capturing moths covered with loose scales.</p>
<p>Widow spiders produce vertical, glue-covered trip lines that detach from the ground when encountered by an unsuspecting victim, springing the prey into the air where it hangs suspended. Unlike orb weaver glue, widow glue is resistant to fluctuating humidity.</p>
<p>These various specialized adhesive properties have intrigued biomaterials researchers who can dream up plenty of uses for artificial versions of spider glues. But without knowing the genes that code for these proteins, there hasn’t been a clear road map for how to produce synthetic spider glues.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/277994/original/file-20190604-69059-7uxi6o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/277994/original/file-20190604-69059-7uxi6o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/277994/original/file-20190604-69059-7uxi6o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/277994/original/file-20190604-69059-7uxi6o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/277994/original/file-20190604-69059-7uxi6o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/277994/original/file-20190604-69059-7uxi6o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/277994/original/file-20190604-69059-7uxi6o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/277994/original/file-20190604-69059-7uxi6o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Their sticky glue is part of what makes spiders’ webs so hard to escape.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/banded-garden-spider-argiope-trifasciata-grasshopper-1229048797">Robert Mutch/Shutterstock.com</a></span>
</figcaption>
</figure>
<h2>Cracking a long, repetitive code</h2>
<p>Surprisingly, researchers have only sequenced around 20 full-length spider silk genes despite the incredible diversity of spiders and decades-old interest in <a href="https://www.newyorker.com/tech/annals-of-technology/in-the-future-well-all-wear-spider-silk">silk as a useful biomaterial</a>.</p>
<p>It turns out that not only are the properties of spider silk amazing, but so is the DNA code that stores the instructions for making the protein. Spider silk genes are extremely large; in itself that’s not a problem, but the bulk of their sequence is made from repeats of the same small DNA bits.</p>
<p>Imagine that the sentence “THE QUICK BROWN FOX JUMPED OVER THE LAZY DOG” is a sequence of DNA that encodes for a protein, but whose exact order of letters is still unknown.</p>
<p>In order to discover this sequence, the main method of DNA sequencing technology available today has three main steps. Once a DNA sample is collected, many copies of the sentence are randomly broken up into small pieces. For example, you might end up with a collection of fragments like “THE QU” “QUICK B” “BROWN FO” “WN FOX J” “AZY DOG” and on and on.</p>
<p>Then a DNA sequencing machine discovers each letter of each piece. The final step is stitching all the short pieces, technically called “reads,” back together in one sequence to figure out the original sentence.</p>
<p>For the sentence above, this is an easy task. The sequence of letters is unique, and as long as there are at least five characters in each read, it’s possible to figure out where one fits relative to another.</p>
<p>Now imagine a similar sentence: “THE QUICK BROWN FOX JUMPS JUMPS JUMPS JUMPS JUMPS JUMPS JUMPS JUMPS JUMPS JUMPS JUMPS OVER THE LAZY DOG.” Given many random short reads from the middle region like “UMPS J” or “S JUMP,” no matter how you slice and dice, it’s impossible to use this method to figure out the number of “JUMPS” in the complete sentence. </p>
<h2>Sequencing a long read of DNA in one go</h2>
<p>For many years DNA sequencing has been limited to this short-read strategy: breaking a gene into bits and then reassembling into one cohesive sequence.</p>
<p>Setting aside some difficult and expensive techniques that are out of reach for standard labs, the best way to fully discover a long, repetitive gene is to sequence the repetitive part from start to finish in one go. Fortunately, emerging technology, while still in its infancy, is starting to allow this long-read sequencing by getting around the chemistry limitations of the short-read method. For those that study super-repetitive DNA this is excellent news: New types of DNA sequencers are finally resolving the “JUMPS.”</p>
<p>Now that <a href="https://doi.org/10.1534/g3.119.400065">two spider glue genes are fully sequenced</a>, the first step towards making a synthetic version is complete. Researchers can now insert the genes into other organisms, like bacteria or yeast, to make the glue in bulk.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/277981/original/file-20190604-69059-1fe6qme.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/277981/original/file-20190604-69059-1fe6qme.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/277981/original/file-20190604-69059-1fe6qme.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=402&fit=crop&dpr=1 600w, https://images.theconversation.com/files/277981/original/file-20190604-69059-1fe6qme.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=402&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/277981/original/file-20190604-69059-1fe6qme.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=402&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/277981/original/file-20190604-69059-1fe6qme.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=505&fit=crop&dpr=1 754w, https://images.theconversation.com/files/277981/original/file-20190604-69059-1fe6qme.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=505&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/277981/original/file-20190604-69059-1fe6qme.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=505&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Droplet of spider glue suspended on capture spiral silk (left) and after adhering to a glass slide (right).</span>
<span class="attribution"><span class="source">Sarah Stellwagen</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Unlike solid silks, the glue proteins do not have to be transformed from a liquid to a solid fiber, something spiders do effortlessly but that scientists have trouble replicating. The glue has the potential for many unique applications and is biodegradable, water soluble and stays sticky for months or even years.</p>
<p>Imagine safer pest control or washable filters. Or frat boys wrestling in a kiddie pool of the stuff. Either way, someday soon it might be possible to reach your hand into a bucket of spider glue – the tricky part will be not sticking to whatever you touch next.</p><img src="https://counter.theconversation.com/content/107773/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sarah Stellwagen does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The glue that gives spider webs their stickiness is a form of spider silk protein. Researchers can imagine cool uses for a synthetic version – but had to wait for the tricky glue gene to be sequenced.Sarah Stellwagen, Postdoctoral Researcher in Biological Sciences, University of Maryland, Baltimore CountyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/720642017-02-01T00:02:26Z2017-02-01T00:02:26ZThe frog tongue is a high-speed adhesive<figure><img src="https://images.theconversation.com/files/154678/original/image-20170130-7894-20l6t5.jpg?ixlib=rb-1.1.0&rect=5%2C410%2C3384%2C2273&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Gotcha, five times faster than the blink of an eye. </span> <span class="attribution"><span class="source">Candler Hobbs/Georgia Tech</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>How does one get stuck studying frog tongues? Our study into the sticky, slimy world of frogs all began with a <a href="https://www.youtube.com/watch?v=LbNl3J8HXw4">humorous video</a> of a real African bullfrog lunging at fake insects in a mobile game. This frog was clearly an expert at gaming; the speed and accuracy of its tongue could rival the thumbs of texting teenagers.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/LbNl3J8HXw4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The frog that was the genesis of the authors’ tongue research.</span></figcaption>
</figure>
<p>Further YouTube research yielded amazing videos of frogs eating <a href="https://www.youtube.com/watch?v=6AILF4Spwpo">mice</a>, <a href="https://www.youtube.com/watch?v=b-K0KcbUWmI">tarantulas</a> and even other <a href="https://www.youtube.com/watch?v=2kwrWl8zWdg">frogs</a>. </p>
<p>The versatile frog tongue can grab wet, hairy and slippery surfaces with equal ease. It does a lot better than our engineered adhesives – not even household tapes can firmly stick to wet or dusty surfaces. What makes this tongue even more impressive is its speed: Over 4,000 species of frog and toad <a href="http://psycnet.apa.org/psycinfo/1990-97523-000">snag prey faster than a human can blink</a>. What makes the frog tongue so uniquely sticky? <a href="http://dx.doi.org/10.1098/rsif.2016.0764">Our group aimed to find out</a>.</p>
<h2>Baseline frog tongue knowledge</h2>
<p>Early modern scientific attention to frog tongues came in 1849, when biologist Augustus Waller published the <a href="http://dx.doi.org/10.1098/rstl.1849.0010">first documented frog tongue study</a> on nerves and papillae – the surface microstructures found on the tongue. Waller was fascinated with the soft, sticky nature of the frog tongue and what he called:</p>
<blockquote>
<p>“the peculiar advantages possessed by the tongue of the living frog…the extreme elasticity and transparency of this organ induced me to submit it to the microscope.”</p>
</blockquote>
<p>Fast-forward 165 years, when biomechanics researchers Kleinteich and Gorb were the first to <a href="http://dx.doi.org/10.1038/srep05225">measure tongue forces in the horned frog</a> <em>Ceratophrys cranwelli</em>. They found in 2014 that frog adhesion forces can reach up to 1.4 times the body weight. That means the sticky frog tongue is strong enough to lift nearly twice its own weight. They postulated that the <a href="http://dx.doi.org/10.1098/rsos.150333">tongue acts like sticky tape or a pressure-sensitive adhesive</a> – a permanently tacky surface that adheres to substrates under light pressure.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Frog tongue holding up a petri dish just with its stickiness.</span>
<span class="attribution"><span class="source">Alexis Noel/Georgia Tech</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>To begin our own study on sticky frog tongues, we filmed various frogs and toads eating insects using high-speed videography. We found that the frog’s tongue is able to capture an insect in under 0.07 seconds, five times faster than a human eye blink. In addition, insect acceleration toward the frog’s mouth during capture can reach 12 times the acceleration of gravity. For comparison, astronauts normally experience around three times the acceleration of gravity during a rocket launch.</p>
<h2>On to the materials testing</h2>
<p>Thoroughly intrigued, we wanted to understand how the sticky tongue holds onto prey so well at high accelerations. We first had to gather some frog tongues. Here at Georgia Tech, we tracked down an on-campus biology dissection class, who used <a href="https://en.wikipedia.org/wiki/Northern_leopard_frog">northern leopard frogs</a> on a regular basis.</p>
<p>The plan was this: Poke the tongue tissue to determine softness, and spin the frog saliva between two plates to determine viscosity. Softness and viscosity are common metrics for comparing solid and fluid materials, respectively. Softness describes tongue deformation when a stretching force is applied, and viscosity describes saliva’s resistance to movement. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1067&fit=crop&dpr=1 600w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1067&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1067&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1340&fit=crop&dpr=1 754w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1340&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1340&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">When the indentation head pulls away from the tongue, it adheres and stretches.</span>
<span class="attribution"><span class="source">Alexis Noel/Georgia Tech</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Determining the softness of frog tongue tissue was no easy task. We had to create our own indentation tools since the tongue softness was beyond the capabilities of the traditional materials-testing equipment on campus. We decided to use an indentation machine, which pokes biological materials and measures forces. The force-displacement relationship can then describe softness based on the indentation head shape, such as a cylinder or sphere.</p>
<p>However, typical heads for indentation machines can cost US$500 or more. Not wanting to spend the money or wait on shipping, we decided to make our own spherical and flat-head indenters from stainless steel earrings. After our tests, we found frog tongues are about as soft as brain tissue and 10 times softer than the human tongue. Yes, we tested brain and human tongue tissue (post mortem) in the lab for comparison.</p>
<p>For testing saliva properties, we ran into a problem: The machine that would spin frog saliva required about one-fifth of a teaspoon of fluid to run the test. Sounds small, but not in the context of collecting frog spit. Amphibians are unique in that they <a href="http://dx.doi.org/10.1111/j.1463-6395.1969.tb00527.x">secrete saliva through glands located on their tongue</a>. So, one night we spent a few hours scraping 15 dead frog tongues to get a saliva sample large enough for the testing equipment.</p>
<p>How do you get saliva off a frog tongue? Easy. First, you pull the tongue out of the mouth. Second, you rub the tongue on a plastic sheet until a (tiny) saliva globule is formed. Globules form due to the long-chain mucus proteins that exist in the frog saliva, much like human saliva; these <a href="http://dx.doi.org/10.1007/BF00305337">proteins tangle like pasta when swirled</a>. Then you quickly grab the globule using tweezers and place it in an airtight container to reduce evaporation.</p>
<p>After testing, we were surprised to find that the saliva is a two-phase viscoelastic fluid. The two phases are dependent on how quickly the saliva is sheared, when resting between parallel plates. At low shear rates, the saliva is very thick and viscous; at high shear rates, the frog saliva becomes thin and liquidy. This is similar to paint, which is easily spread by a brush, yet remains firmly adhered on the wall. Its these two phases that give the saliva its reversibility in prey capture, for adhering and releasing an insect.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/26xFAMpG2R8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A South American horned frog demonstrates capturing a cricket in this slow motion video.</span></figcaption>
</figure>
<h2>To catch a cricket</h2>
<p>How does soft tissue and a two-phase saliva help the frog tongue stick to an insect? Let’s walk through a prey-capture scenario, which begins with a frog tongue zooming out of the mouth and slamming into an insect.</p>
<p>During this impact phase, the tongue deforms and wraps around the insect, increasing contact area. The saliva becomes liquidy, penetrating the insect cracks. As the frog pulls its tongue back into the mouth, the tissue stretches like a spring, reducing forces on the insect (similar to how a bungee cord reduces forces on your ankle). The saliva returns to its thick, viscous state, maintaining high grip on the insect. Once the insect is inside the mouth, the eyeballs push the insect down the throat, causing the saliva to once again become thin and liquidy. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/fwThZXXXdTc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Frogs’ eyeballs actually help them swallow their food by physically pushing what’s in the mouth back into the throat.</span></figcaption>
</figure>
<p>It’s possible that <a href="http://dx.doi.org/10.1098/rsif.2016.0764">untangling the adhesion secrets of frog tongues</a> could have future applications for things like high-speed adhesive mechanisms for conveyor belts, or fast grabbing mechanisms in soft robotics.</p>
<p>Most importantly, this work provides valuable insight into the biology and function of amphibians – 40 percent of which are in <a href="http://www.iucnredlist.org">catastrophic decline or already extinct</a>. Working with conservation organization <a href="http://www.amphibianfoundation.org">The Amphibian Foundation</a>, we had access to live and preserved species of frog. The results of our research provide us with a greater understanding of this imperiled group. The knowledge gathered on unique functions of frog and toad species can inform conservation decisions for managing populations in dynamic and declining ecosystems.</p>
<p>While it’s not easy being green, a frog may find comfort in the fact that its tongue is one amazing adhesive.</p>
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<p><em>Mark Mandica of <a href="http://www.amphibianfoundation.org">The Amphibian Foundation</a> collaborated on the <a href="http://dx.doi.org/10.1098/rsif.2016.0764">research published in Journal of the Royal Society Interface</a> and coauthored this article.</em></p><img src="https://counter.theconversation.com/content/72064/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>This material is based upon work supported by the National Science Foundation Graduate Research Fellowship (DGE-1148903).</span></em></p><p class="fine-print"><em><span>David Hu does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>How do a frog’s tongue and saliva work together to be sticky enough to lift 1.4 times the animal’s body weight? Painstaking lab work found their spit switches between two distinct phases to nab prey.Alexis Noel, PhD Student in Biomechanics, Georgia Institute of TechnologyDavid Hu, Associate Professor of Mechanical Engineering and Biology, Adjunct Associate Professor of Physics, Georgia Institute of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/695052016-11-29T11:15:59Z2016-11-29T11:15:59ZGlues inspired by nature will give us faster ships, surgical adhesives and sticky car tyres<figure><img src="https://images.theconversation.com/files/147903/original/image-20161129-10969-tnaadr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Barncales stick with glue like no other.</span> <span class="attribution"><a class="source" href="http://www.geograph.org.uk/photo/4073228">Neil Theasby</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>We use adhesives all the time, from the temporary stickiness of the humble Post-It note to super-strength glues used in construction, but it is easy to overlook the many living creatures that rely on adhesion for survival in the natural world. </p>
<p>Adhesion can be used to permanently attach to a surface, as climbing plants and <a href="https://museumvictoria.com.au/discoverycentre/infosheets/how-do-barnacles-cement-themselves-to-rocks/">barnacles</a> do. It can help creatures get around, for example on the feet of <a href="http://www.livescience.com/47307-how-geckos-stick-and-unstick-feet.html">geckos</a> and various insects. Some creatures such as salamanders or <a href="http://video.nationalgeographic.com/video/weirdest-sea-cucumber">sea cucumbers</a> use glue as a defence against predators, while others such as <a href="https://asknature.org/strategy/web-glue-is-strong-adhesive/">spiders</a> and <a href="http://www.livescience.com/39047-new-velvet-worm-species.html">velvet worms</a> use it as a means of attack. Some even use adhesion during reproduction. But despite their frequent appearance in nature, most natural glues are poorly understood.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/147917/original/image-20161129-10957-19tmgm5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/147917/original/image-20161129-10957-19tmgm5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=373&fit=crop&dpr=1 600w, https://images.theconversation.com/files/147917/original/image-20161129-10957-19tmgm5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=373&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/147917/original/image-20161129-10957-19tmgm5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=373&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/147917/original/image-20161129-10957-19tmgm5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=469&fit=crop&dpr=1 754w, https://images.theconversation.com/files/147917/original/image-20161129-10957-19tmgm5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=469&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/147917/original/image-20161129-10957-19tmgm5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=469&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Gecko: sticky feet.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/24931020@N02/13468259683/in/photolist-mw9mnR-nUdA8C-41oqA-6rDYoU-5pt3qK-e9kGmF-5aevMj-4w11VL-4w11YL-nS1m9L-4w122N-6rzSiK-7uCuqr-qrCoDw-bziXqo-537DjS-9xa6xC-cj4QDs-9beSdq-9xa2fG-dJLuFP-4AKE7-4AKGd-bvLg9s-dZcCzd-9x9ZUu-98KM-dofg4F-bJF3Lz-bvLfZq-oU8Wky-4AKBN-HQMKaM-HQML4a-GUXok3-4AKJg-HMKnY1-4zqJr3-8P94yK-bvLfRN-V89rn-oBF6rY-fgBcdJ-7Q9tx-aeubi5-8iozcv-9Xja2-HQMLtD-b8jV4e-nwB3EU">Ozzy Delaney/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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<p>Nature, it seems, rarely does anything in the most straightforward of ways and adhesives are no different. Scientists must therefore be creative when trying to understand these amazing natural materials. For example, it’s not the fact that the Golden Gate Bridge is made out of steel and concrete that allows it to stand over San Francisco Bay – it is the engineering techniques that work with the properties of steel and concrete to carefully balance physical forces. In a similar way, simply knowing the chemical composition of natural adhesives is not enough. </p>
<p>The <a href="http://www.americanscientist.org/issues/feature/2006/2/how-gecko-toes-stick">toe pads of geckos</a> and <a href="https://asknature.org/strategy/byssus-threads-resist-forces/">byssus threads</a> of marine mussels (their “beards”), for example, are formidably complex feats of engineering, of which the chemical composition is only one element. </p>
<p>When it comes to mussels, each byssus thread is injection moulded in place along the full length of the mussel’s foot, which temporarily connects the body of the mussel to the surface. Two types of collagen produce an elastic gradient between the hard rock and soft mussel tissue, which distributes the physical stresses of wave action or attack from predators. More than half a dozen specialised proteins and enzymes generate the thread’s sticky properties and form the permanent attachment to the surface. On top of that, the mussel can also choose where and when to attach. </p>
<p>To understand the adhesion process properly we need to understand the physics, the chemistry and the biology. It is what we call a multidisciplinary problem – in other words, it’s complicated. The challenge for bio-inspired technologies in general, and adhesives specifically, is to identify the important elements of the system and simplify them so that they can be translated, artificially recreated, and turned into products that we can all benefit from.</p>
<h2>More uses for nature’s products</h2>
<p>Why are biological glues so interesting to us? Because there are numerous technical applications where progress has been hampered by a lack of appropriate adhesives, and where natural adhesives have already solved the problems. Bioadhesives can do things that most synthetic glues cannot – joining surfaces underwater, for example. Human surgery is a clear application for such a glue. </p>
<p>One of the most common procedures during pregnancy is <a href="http://www.nhs.uk/conditions/amniocentesis/Pages/Introduction.aspx">amniocentesis</a>, used to test a foetus for Down syndrome. This carries a risk because it perforates the foetal membrane which cannot subsequently heal. Bio-inspired adhesives and sealants that can heal these perforations could help to significantly reduce instances of <a href="https://www.acs.org/content/acs/en/pressroom/newsreleases/2014/august/solving-a-sticky-problem-with-fetal-surgery-using-a-glue-inspired-by-the-sandcastle-worm.html">pre-term delivery</a>. Based on natural molecules like proteins and carbohydrates, bio-inspired glues could also reduce the need for unpleasant chemicals such as formaldehyde to manufacture adhesives, reducing the environmental impact of the industry.</p>
<p>In other instances, knowledge of adhesion could help us to design non-stick surfaces – anti-fouling coatings for the hulls of ships that prevent creatures like barnacles from attaching, causing drag, slowing ships, using more fuel and increasing the emission of greenhouse gases. Or other materials that prevent bacterial biofilms, or slimes, from accumulating in food processing plants, on medical implants, environmental sensors or in your washing machine.</p>
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<p>Wouldn’t it be nice to have streets free from chewing gum, clothes that resist the onslaught of messy children and windows that shed dust and dirt? All of these have potential solutions in nature. You don’t need to take my word for it: take a cabbage leaf (Savoy is best) and look how droplets of water roll off, collecting dirt as they go. This is the so-called <a href="https://asknature.org/strategy/surface-allows-self-cleaning/">lotus effect</a> which works through a combination of the surface chemistry and texture of the leaves. A natural, self-cleaning material.</p>
<p>Adhesion is everywhere and the benefits of understanding it are clear. But it can be a challenge to link up the necessary expertise to tackle the biology, chemistry and physics of such complex systems in a coordinated way. This has led to the establishment of a four year-long, EU-funded <a href="http://www.cost.eu/COST_Actions/ca/CA15216">European Network of Bioadhesion Expertise</a>, which will help bring together researchers from across Europe to address the biological aspects of adhesion in nature, from frogs to fungi, as well as the fundamental physical and chemical processes that underpin it.</p>
<p>What will come of research into natural glues? It might be car tyres based on the toe pads of tree frogs that grip the road better in wet conditions, climbing robots based on geckos, or specialist adhesives for hi-tech applications. It was recently suggested by researchers that <a href="http://www.redorbit.com/news/science/1113412094/sorry-marvel-fans-spider-man-is-physically-impossible-and-heres-why-011915/">Spiderman’s powers of attachment are impossible</a>. I, for one, remain hopeful.</p><img src="https://counter.theconversation.com/content/69505/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nick Aldred's research is supported by Office of Naval Research awards N00014-13-1-0633/4, COST Action 15216 and a Newcastle University SAgE Faculty Fellowship.</span></em></p>Nature has created some of the strongest glues with properties we could use – if we understood how they worked.Nick Aldred, SAgE Research Fellow, Newcastle UniversityLicensed as Creative Commons – attribution, no derivatives.