2008 2ND PLACE

Felicity Muth
Edinburgh University

What can be stronger than steel, tougher than a bullet-proof vest, and more elastic than nylon? Spider silk. It snares prey, encases offspring, and provides a relentlessly reliable safety line. Silkworm silk has been used in fabrics for more than 5,000 years. However, spider silk is mechanically far superior, being both strong and stretchy. It is also produced in nine different varieties per spider, each for a different function.

Unlike the passive silkworm, spiders are not ideal for domestication, due to their tendency to fight or even eat each other. But if spider silk could be produced synthetically, it could help make lightweight bullet-proof vests, cable for suspension bridges, and even be used in orthopaedics.

For us humble humans, spinning silk requires extremely high temperatures and strong acid. The spider, however, carries out this synthesis in its – and our – own backyard. Since the main source of fuel powering this manufacture is flies, this procedure really is as energy-efficient as it gets.

Fritz Vollrath and colleagues from Oxford University have been exploring how the Golden Orb-weaving spiders, genus Nephila, make their silk. Orb-weavers are spiders that make circular webs, and Nephila is the largest of them all. Although her size may be alarming to many, it is an advantage for scientists, as she contains ample amounts of silk to extract and study.

Before it emerges as a solid fibre, silk is stored in a specific gland as a liquid “dope”. In order for spider silk to be artificially produced, we need to unravel both what this dope consists of and how it transforms from liquid to solid.

Nephila’s “dragline”, the major type of silk studied, constitutes the main spokes of her circular webs, as well as being dragged behind her at all times, providing an emergency “bungee cord”. Dragline silk dope is a liquid crystal: the silk protein molecules in the dope flow like a liquid, but are aligned like a crystal. Like a Scottish ceilidh, the participants all face in one direction, but have the freedom to move, weaving in and out of one another. The dope travels from the gland down a narrow duct, squeezing out water and forcing the molecules to become tightly bonded in zigzag layers. Finally, it emerges from the spider’s abdomen as a strong solid thread.

Converting liquid dope to solid fibre requires energy, which is supplied by forces exerted on the liquid as it squeezes down the spinning duct. In order to determine how Nephila “uses the force” to control the properties of her silk, Vollrath and his team have used a rheometer. a machine that mimics the forces inside the spider’s duct.

To understand the results, think of Silly Putty. It was an enchanting toy as a child, as it could magically be both liquid and solid. Holding the putty between your fingers and pulling it apart slowly gave a gooey, stretchy material. But jerking it apart abruptly gave a material that was extremely brittle and would snap. Rheometry found that spider silk followed a similar principle: if Nephila chooses to make a flexible and stretchy fibre, perhaps to swathe her eggs, she spins the silk slowly. However, reeling quickly produces a stiffer and stronger fibre, ideal for a sturdy web spoke. As Nephila increases her reeling speed, the silk molecules are forced into a more highly oriented position, resulting in a closer stacking of the zigzag sheets and a stronger, but stiffer, fibre.

While the spinning process fine-tunes the silk’s properties, it is its constituents that determine the range of properties available. Vollrath has recently found that the key linking all silks’ properties may lie in the concentration of an amino acid called proline. He demonstrated that the more proline present in the silk, the more it shrinks in water. This quality translates into the silk’s mechanical properties, since quickly spun, strong and stiff silk shrinks much more in water than slowly spun silk. Proline is thought to break up the stiff crystalline structure of silk, allowing water to flood in and resulting in a softer, flexible silk. 

This has applications beyond biology: in fact, findings such as these are paving the way to creating the perfect fibre. Scientists are now looking into creating spider silk using the same methods employed in synthesising polymers like nylon. Another approach is to regenerate more common silks, modifying them to have more properties of spiders’. This would open up a wide range of applications in a multitude of fields. In medicine alone, silk may be used in sutures, bandages, and even as a substrate to grow stem cells on. There are still hurdles to overcome – but considering it took spiders 400 million years to perfect their techniques, we are not doing a bad job by comparison.