The Steel Strength of Featherweight Spider Silk
Spider silk. It’s lighter than cotton but stronger than steel. Thinner than a human hair, but handles loads hundreds of times its size. It can be woven into elaborate structures by minute arthropods. Legions of chemists, material scientists, and engineers study and try to mimic silken threads that spiders create, dismantle, and re-create every day to trap prey.
A wonder of the creature world for over 300 million years, natural silk is produced by invertebrate animals in the phylum Arthropoda, also known as arthropods. Common silk producers include silkworms, lacewings, and spiders. Of the 35,000 types of spiders in the world, only half of them spin webs, but all of them produce a strong, sticky substance that continues to fascinate the STEM community.
Science magazine recently published a special feature on the evolution of spider genomes and the properties and uses of spider silk. The Scientist featured a story about transgenic silkworms that are spinning “spider silk.” Sporting wear companies such as Adidas, The North Face, and Patagonia are all looking for ways to produce shoes and jackets using a synthetic spider silk. One company, Bolt Threads, has recently developed a necktie made from synthetic spider silk. And the latest version of synthetic spider silk is 98% water and environmentally friendly.
Why the fascination with spider silk?
For something that you may not notice until it gets stuck in your hair, spider silk has been attracting a lot of attention.
One reason spider silk fascinates the scientific world is the variance in structure and properties of a single web. You can see differences in the silks, just by looking at a garden spider’s orb web, a radial netting with cross fibers. You will notice that the silk that comprises the frame is different from that of the silk that is connected to the frame, those strands that make up the classic expanding “spirals” radiating out from the center. There are at least two distinctive types the frame silk, or rigid type, and the spiral silk.
There is a reason for the difference in strength and elasticity. When a spider starts construction, it uses major-ampullate (or dragline) silk as a dragline for itself and minor-ampullate silk as a temporary scaffolding. The major-ampullate silk forms the spokes and outer support for the web, while the stickier, more elastic flagelliform (flag) silk is used for the radial capture threads. The threads are held together by disks of piriform silk. Finally, aciniform silk is used to wrap prey, and tubiliform silk creates protective egg sacs.
To get an idea of the types of properties that up make spider silk, compare the strength and toughness of the strong silk with that of Kevlar fibers. Spider silk fibers have a strength rating of 1.1 gigapascals, which is not as strong as Kevlar’s 3.6 gigapascals. But, they are tougher than Kevlar. Rigid spider silk has a toughness factor of 180 megajoules/meter compared to Kelvar’s toughness factor of 50 megajoules/meter. The spiral silk has different properties, which the spider uses for its flexibility and elasticity. The combination of the two types makes for an effective and durable trap, allowing the spider to catch its next meal.
The properties of all these types of silk varies. The tensile strength (the amount of stress a substance can withstand before it starts to fracture) of spider silk ranges from 0.45 – 2.0 GPa. Dragline silk is around 1.1 GPa. Depending on the specific alloy, this can be stronger than steel, which lies in the same range. But it is still beaten by Kevlar, which has a tensile strength of 3.0 – 3.6 GPa.
Still, spider silk outperforms the synthetic aramid, Kevlar, in fracture toughness, the amount of energy needed to complete fracture the material after it starts to crack. The fracture toughness of dragline silk is around 180 MJ∙m1/2 compared to Kelvar’s 50 MJ∙m1/2. Unlike Kevlar or steel, spider silk is highly elastic. Dragline silk can increase its length by 27%, while flag silk can increase its length by 270%, which allows the silk to absorbe the impact of an insect hitting and struggling in the web.
Spider silk is also biocompatible with humans, biodegradable, and lacks immunogenicity and allergenicity, attracting interest in biomedical applications.
What is spider silk made of?
Spider silk is comprised of proteins, called spidroins, that are produced in special glands of the spider. The amino acids, proline, alanine, and glycine, are the main components of these proteins. Different glands produce different types of silk; dragline silk, for example, is produced in the major ampullate glands. Dragline silk has two types of spidroin that are classified by their proline content. The MA spidroins can contain a repetitive core of amino acid blocks repeated up to 100 times. Non-repetitive amino- and carboxy-terminated domains flank these cores.
Subsequent research has indicated that the alanine and glycine content seems to help determine strength or elasticity. These amino acids are arranged in blocks with the alanine sections forming crystalline beta sheets and the glycine sections forming a more amorphous network. Hence, the glycine seems to play a role in the silk’s elasticity while the alanine plays a role in its strength.
But, contrary to what you might think, the silk itself is not sticky. Different types of spiders apply different methods to achieve the adhesive properties of silk. Some spiders hackle—or comb—the silk to increase the surface area, which, in turn, allows attractive van-der-Waals forces to hang on to the prey. This is the same principle that allows a gecko’s foot to adhere to a smooth surface. Other spiders apply a sticky aqueous layer to the flag silk. The glue—a complex mixture of organic molecules, salts, fatty acids and glycoproteins—is manufactured in the aggregate gland of the spider.
It is the elasticity, toughness, and strength of the silks that make them attractive as potential textiles. Other properties, such as the biocompatibility, make the silk attractive for biomedical purposes including using the fibers as scaffold materials for tissue engineering. Spider silk has already entered the marketplace in some cosmetics and medical devices. And, with the potential promises of other applications, work to synthetically recreate the silk is on-going.
While the fibers produced by the Amazing Spiderman may be a product of science fiction, the fibers in the milk produced by Freckles, a goat at Utah State University, or the modified silkworms are not. Other scientists are working with microbes and yeasts to produce the required proteins. Although most of the work is still in the prototype phase, fashion designer Stella McCartney is set to produce a line of spider silk fashions that will be available next year. Who knows what web will be weaved next?
From the Editor
Every evening just as it gets dark, the barn spider in this video weaves its prey catcher with incredible speed, finesse, and ingenuity. It is a marvel that I usually only see after all the work is done (see photo). Finally, I had a chance to watch the orb master pin silk together row by row. See for yourself!
—Natasha Bruce, Editor, inChemistry