Spider Silk: Sronger than Steel? Nature's Supermaterial

Abstract

B S J A train full of passengers is about to plummet into a river. The brakes are stuck. Who will save the day? Who else but Spider-man! He slings thick strands of spider silk onto adjacent buildings, bracing himself on the front of the train until it comes to a grinding halt at the last moment of safety. Spider-Man might be a fictional superhero, but the incredible properties of his spider webs are not so far-fetched. In a recent study published in The Journal of Physics Special Topics, graduate students at the University of Leicester decided to myth-bust the above scene from Spider- Man 2. To their surprise, theoretical calculations showed that spider silk is, in fact, strong enough to stop a runaway train (Bryan, 2012). They began by estimating the force needed to stop the train: about 300,000 newtons. After analyzing the web, the geometry, and the anchor points, they calculated the tensile strength required of the silk fibers (the maximum stress they can withstand while being stretched before breaking). This type of strength is reflected in a value called Young’s Modulus which in this case worked out to be 3.12 gigapascals. As it turns out, spiders produce silk with Young’s Moduli ranging from 1.5 to 12 gigapascals — meaning that Spider-Man could indeed have stopped a fast moving train with spider silk (Bryan, 2012). In fact, biologist William K. Purves (2003) writes that, “The movie Spider-Man drastically underestimates the strength of silk - real dragline silk would not need to be nearly as thick as the strands deployed by our web-swinging hero in the movie”. O ver the past few decades, spider silks have attracted the attention of the scientific community for their amazing mechanical properties and endurance under stress. Of course, silk and its relationship to humanity is nothing new. According to Confucius, it was in 2600 B.C.E. that a silkworm cocoon fell into the tea cup of Chinese princess Leizu. Attempting to remove it from her beverage, she began to unroll the silken thread of the cocoon. By the 3rd Century B.C.E., Chinese silk fabrics were traded throughout Asia and the West by way of the famous Silk Road. However, silk production remained a closely guarded secret. Most Romans, who highly prized the cloth, were convinced that the fabric came from trees. The Chinese monopoly was defended by an imperial decree, condemning to death anyone attempting to export silkworms or their eggs. In 552 C.E., the Roman Emperor Justinian sent two monks on a mission to Asia, and they returned with silkworm eggs hidden inside their bamboo walking sticks. Soon, sericulture (silk farming) spread across the world (Silk Association, 2012). During the 19th and 20th centuries, modernization and industrialization of sericulture in Japan made it the world’s foremost silk producer. During World War II, western countries were forced to find substitutes as supplies were cut off. Recently invented synthetic fibers such as nylon became widely used. Now silk has largely been replaced by these artificial polymers which are far more cost effective. However, silk polymers (of which scientists have only recently understood the full potential) are poised for a possible comeback if they can be mass produced cheaply and efficiently (Silk Association, 2012). pider silk may seem weak and flimsy, useful for nothing better than haunted house decor. Yet for its miniscule weight and size it can absorb a surprising amount of energy. It’s also stretchy - it can stretch 30% farther than the most pliable nylon. If spider silk were as thick as a steel beam, it would be very difficult to S 46 • B erkeley S cientific J ournal • S tress • F all 2013 • V olume 18 • I ssue 1