The Science of Spider Silk
Spider webs are everywhere. From the ornate death trap spanning two tree branches in the woods to the cobweb in the dusty corner of an old attic, we sometimes overlook the amazing properties of spider silk in our daily lives. Fortunately, the scientific community has not overlooked Mother Nature’s excellent materials science acumen. Many researchers have focused their efforts on the material properties of spider silk and how they can be harnessed in a variety of applications.
Spider silk is a rather general term — spiders actually make many different types of silk, all produced in different types of silk glands (Li 2003). Picture a spider dangling from its web. The spokes of the spider web are spun from the strongest type of silk, dragline silk. The spider uses viscid silk to connect the spokes, forming the spiraling web structure that entraps unwitting prey (Li 2003; Römer and Scheibel 2008). Once the spider’s delicious insect snack gets stuck on the web, the spider uses swathing silk to enfold the prey. Spider babies use yet another type of silk, ballooning silk, to travel after hatching (Li 2003; Römer and Scheibel 2008).
Dragline silk is made up of a type of protein known as fibroin (Li 2003). Proteins are biological polymers, long chain molecules made up of repeating units called amino acids (Li 2003). These repeat units affect the mechanical properties of the silk (Römer and Scheibel 2008). Being a composite material, spider silk has properties of all it’s different components (Kang 2014). Scientists hypothesize that some repeat units contribute to the silk’s mechanical strength, while others contribute to its elasticity (Römer and Scheibel 2008).
The variation in the structure of the different types of spider silk lends them their mechanical properties and this affects their function.
The variation in the structure of the different types of spider silk lends them their mechanical properties and this affects their function. Since dragline silk has to support the weight of the spider (Li 2003), its tensile strength — the maximum stress it can support before it breaks — is high (Gosline et al. 1986). For comparison, high tensile steel, per unit mass, isn’t even as strong as dragline silk (Gosline et al. 1986). Kevlar, the polymer fiber used in bulletproof vests, is, on the other hand, stronger than dragline silk (Gosline et al. 1986). However, the most important material property in web-building is toughness, as this measures the resistance to breakage, or the amount of energy absorbed before fracture (Gosline et al. 1986). A spider web must be able to withstand the energy and force of an incoming flying insect, which is why both dragline silk and viscid silk are tougher than steel or Kevlar. The viscid and dragline silks are able to dissipate the energy of the flying insect (Römer and Scheibel 2008) due to a phenomenon known as hysteresis (Gosline et al. 1986). During hysteresis, mechanical energy (i.e., from moving insects) gets converted into heat and is lost (Gosline et al. 1986). The lost heat energy cannot be used to cause fracture of the web or elastic recoil, when the insect bounces back from the web (Gosline et al. 1986). In addition, viscid silk is highly elastic and will not break even if a big insect flies into the web and begins to struggle (Gosline et al. 1986). These properties of spider silk make it a highly desirable material that we can even apply to our own technology.
Spider silk, being a biological (and thus biodegradable) material, would be a good material for use in a variety of areas (Li 2003). For example, bulletproof spider silk vests would be more ecologically friendly to produce than Kevlar, which generates pollution during production (Li 2003). Its use has also been suggested for bandages, airbags, artificial ligaments, surgical sutures, durable but lightweight clothing, anti-rust covers for vehicles and artificial skin scaffolds (Fecht 2012; Li 2003; Kluge et al. 2008). Spider silk even has applications in electronics and sensor technology (Stevens et al. 2013).
Though the possible applications for spider silk are endless, one challenge is finding a way to produce it in mass quantities for our usage. Spiders cannot produce enough silk for inexpensive mass production. Recent efforts to decode spider silk genes have led to transgenic approaches to producing spider silk, including recent production of spider silk from the milk of genetically engineered goats (Zyga 2010). These are just a few steps forward, however. In the coming years, we may unravel more mysteries of the super-material spider silk.
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Gosline, John M., M. Edwin DeMont, and Mark W. Denny. “The Structure and Properties of Spider Silk.” Endeavour 10.1 (1986): 37-43. Print.
Kang, Soyoung. “Biomimetics: Engineering Spider Silk.” Illumin. University of Southern California, Fall 2013. Web. 1 Jan. 2014.
Kluge, J., O. Rabotyagova, G. Leisk, and D. Kaplan. “Spider Silks and Their Applications.” Trends in Biotechnology 26.5 (2008): 244-51. Print.
Li, Vivienne. “Spider Silk and Venom.” Spider Silk and Venom. University of Bristol, n.d. Web. 05 Jan. 2014.
Römer, Lin, and Thomas Scheibel. “The Elaborate Structure of Spider Silk.” Prion 2.4 (2008): 154-61. Print.
Steven, Eden, Wasan R. Saleh, Victor Lebedev, Steve F.A. Acquah, Vladimir Laukhin, Rufina G. Alamo, and James S. Brooks. “Carbon Nanotubes on a Spider Silk Scaffold.” Nature Communications 4.2435 (2013): 1-8. Print.
Zyga, Lisa. “Scientists Breed Goats That Produce Spider Silk.” Scientists Breed Goats That Produce Spider Silk. Science X Network, n.d. Web. 05 Jan. 2014.