HOW SPIDERS SPIN

Abstract

There are almost as many anecdotes about man trying to harvest spider silk as web styles; even Napoléon tried farming the territorial arachnids in a bid to harness the robust material. However, despite decades of intense effort and millions of research dollars, no one has successfully spun synthetic spider silk to the specifications that spiders achieve effortlessly. Gareth McKinley realised that although a great deal was known about the spun thread's composition and mechanical properties, few had studied the silk solution's behaviour as it is extruded from the animal's major ampullate gland. Knowing that polymer solutions often behave in strange and unexpected ways when squeezed thorough microscopic apertures, McKinley decided to study the material properties of natural spider silk solutions on a microscopic scale. When biological engineer Nikola Kojiç joined the MIT lab,McKinley knew he'd found the right person to begin unlocking the spinner's secrets (p. 4355).But before Kojiç could begin testing the fluid's flow properties, he had to learn how to extract the microscopic volume of silk protein solution stored in the golden orb spider's major ampullate gland. Kojiç visited Marian Goldsmith's lab at the University of Rhode Island where he learned to extract silk protein solutions from silk worms before he tried his hand on the rarer golden orb spider. Having successfully extracted a few microlitres of the scarce solution Kojiç explains that he had to work fast, keeping the gel-like solution under water to prevent evaporation, to measure the fluid's flow properties.First Kojiç measured the thick solution's viscosity using Christian Clasen's flexure-based micro-rheometer. Shearing 0.7 μl of the sample between two glass plates, the pair found that the silk solution's viscosity decreased dramatically as the plates moved faster. The silk solution was becoming increasingly slippery until it was even slipperier than silicone oil. McKinley admits that he was surprised at how slippery the material became, but adds that it explains how spiders extrude the thick gel to produce the delicate thread. According to McKinley, the shearing process aligns the long protein polymers in the silk solution until the molecules begin slipping easily past each other and the viscosity drops dramatically.Next the team tested the protein solution's stickiness by placing 1 μl of the sample between two small plates on an extensional rheometer, built by José Bico. Pulling the rheometer's plates 5 mm apart to produce a thin filament of the gooey solution, Kojiç measured the filament's diameter as it thinned and found that the silk solution was as sticky as egg white,becoming stiffer as the thread dried.McKinley explains that many of the solution's unique properties boil down to the protein's long molecules, which are relatively entangled at high concentrations making the solution highly viscous. However, as the solution is squeezed through the spider's ampullate gland the molecules become aligned,sliding past each other easily as the viscosity drops before the sticky silk is extruded into a fine filament, drying to form one of the toughest materials known to man.