Mother Nature can sometimes be peculiarly amazing. She hides some of her greatest of creations in the simplest of creatures. A gecko seemingly defies the gravity with its special fingertips. Many simple insects including honey bees, navigate long distances using special nanoscale structures in their eyes. Marble berries produce the most intense blue color known to man, without a single blue pigment molecule in its shell. Nature is filled with examples like these. However, today we will discuss about something even more extraordinary. It’s a material so strong that you can stop a fully loaded jumbo jet in the midflight, just with a strand with the thickness of two centimeters. It’s so lightweight that only one kilogram of this material is long enough to stretch a fiber around the equator three times over. It can easily beat man made materials like steel or aramid fibers like Kevlar in terms of toughness and can stretch one and half times of its original length. The most remarkable thing is this material is made by most humble of the creature that you can imagine; garden spider. The material is none other than the spider silk.
Spider silk fibers
Now you may wonder, if spider silk is so strong, why we can break it with almost no effort. Well this is because, individual spider silk fiber is very thin, typically 1-5 micrometers in diameter ( micrometer = one thousandth of millimeter). If you can make materials like Kevlar or steel this thin, even a small gush of wind could break it off. However, spider silk can, not only survive many environmental torments, but also catch speeding insects that may reach to speeds more than 40 km/h. In fact one type of spider in the name of golden silk orb weaver, spins a silk so strong that it can even catch speeding birds or small snakes.
There are over 40,000 identified species of spiders all over the world, which is almost 400 times the number of primates. All these spiders will make spider silk at some point in their life. Spiders use silk for many purposes, including reproduction, moving, defense and catching pray. Contrary to popular belief, there are number of different types of spider silks. Some are stretchy than others, some have more strength and some others are particularly sticky. Depending on the requirement, an individual spider may spin any one of these silk types. The highly researched spider silk type is Ampullate (Major) silk which is mostly used as the dragline or to shape the outer rim of the web by the spider.
How strong is spider silk?
Spider silk is a smooth fiber which is completely made of proteins. Secret to its strength is in the special nanoscale arrangement of these protein molecules and the nanostructures they make inside the silk fibers. So just how strong is spider silk? If you just compare the tensile strength of high grade steel (1.65 GPa) to spider dragline silk (1.3 GPa), it may appear that silk is less stronger than steel. However, things get very interesting when you compare the densities of these materials. Spider silk is much less denser (1.35 g/cm3) compared to steel (8.05 g/cm3), which means that if you compare the strength of given weight of these two materials, spider silk is around five times strong as steel. Another amazing property of spider silk is its ability to stretch while maintaining a high tensile modulus. Many other conventional high modulus materials like steel or aramid fibers, have comparatively low breaking strain which make it less tough. However, spider silk is tougher than both and takes more energy to break. Or in other words, if you want to break a string made of steel, you may have to pull it very hard for a short distance (because steel is less ductile and breaks after small strain) to break it. However, to break a spider silk of the same size, you may not need to pull as hard as you would in steel, but you may need to continue pulling for a very long distance, which ultimately takes more energy than in the case of steel.
Let’s look at how nanotechnology operates in spider silk to produce these amazing properties. Every strand of spider silk is made up of even smaller fibers. These, so called silk fibrils are around 100 nm in width and made of long chains of proteins. What special about these proteins are the way they are bound to each other. One type of these silk proteins named as alanine have a structure similar to a corrugated sheet which allows them to pack on top of each other in to a very rigid block. Each of these block is in the size range from 20-30 nm in width and 30 -40 nm in length. Glycine, the other major silk protein in spider silk, resembles a structure similar to a flexible spring and attached to blocks made of alanine in both ends. When a force is applied, glycine stretches up like a spring giving elasticity to silk fibers while rigid blocks made of alanine work as molecular anchors, providing the firmness. This gives silk fibers, best of the both worlds. On the one hand, spider silk fiber is firm like corrugated sheeting and in the other, flexible as a spring. This allows silk fiber to be simultaneously strong and stretchable, making it an extremely tough material.
There are many applications for spider silk fiber if we can harvest spider silk from spiders in commercial scale or synthetically make it in large quantities. Due to high toughness, it would immediately have applications such as spider silk armor in bullet proof vests, high strength ropes, nets, seat belts, parachute cords, etc. Smooth surface and elasticity of the spider silk would make great textile fibers with high strength and beautiful luster. Like any other natural material, spider silk fibers are biodegradable. Coupled with its strength, we would be able to make new breed of biodegradable materials to replace many types of plastics. Spider silk fibers are also biocompatible. Researchers are now investigating their applicability in artificial tendons or ligaments.
One would think that, because we know the structure of spider silk and the proteins involved in the material, finding of a synthetic route to manufacture them in a lab is easy. Unfortunately, millions of years of spider evolution is not easy to match even with the advance techniques available to us. However, there are some interesting developments in the field, but no one still can match the quality of the product that spiders can make. Lot of people are waiting for the day that scientists can spin the perfect spider silk at the lab that is as good as those humble eight legged creatures can make. However, it looks like it’s going to take few more years, until a scientist can figure it out.