Spiders make silk that is not only stronger than most manmade materials, but it is also lighter.
- Types of Spider Silk
- Spider Silk Production
- Mechanical Properties of Spider Silk
- Spiders in Captivity
- Golden Orb Spiders
- Uses for Spider Silk
- Bio-mimicry with Spider Silk
1. Types of Spider Silk
Silkworms produce only one type of silk and that is used for making cocoons. Spiders on the other hand produce a wide variety of silks. They need one type of silk for their egg cases, another type for wrapping their prey, sticky silk for catching prey in their webs, and structural silk for building the webs. When you see a spider dropping through space on a safety line, the spider is using drag-line silk, which they also use on the edges of their webs. Some species of small spider use yet another very light type of silk, called gossamer silk, for dispersal. These small spiders release several threads of gossamer into the air and let the wind carry them like a balloon.
2. Spider Silk Production
The spiders have different glands to produce the different types of silk mentioned above. Spiders produce silk by a process called ‘pull-trusion’. Unlike ‘extrusion’ in which fibre is squeezed out of a reservoir, the spider pulls the finished thread directly from the silk-making gland.
3. Mechanical Properties of Spider Silk
Spider silk, especially drag-line silk, has exceptional properties. Weight for weight spider silk is stronger than steel although not as strong as Kevlar (the material used for bulletproof vests). Let’s say that you have three strands of the same weight, one made of Kevlar, one made of steel and one made of spider silk. Now you start hanging increasingly heavy weights on them. The steel will break first then the spider silk and finally the Kevlar. The ability to bear a lot of weight before breaking is called tensile strength.
Spider silk is tougher than both steel and Kevlar. Spider silk is very stretchy (extensibility or ductility) and the silk of some spiders can stretch four or five times their relaxed length before breaking. The spider web needs to be able to withstand the impact of a potentially heavy prey insect landing on it without breaking. After all, a strand that is only one-tenth of the thickness of human hair can stop a bee travelling at 20 km per hour, without breaking. It is this combination of strength and stretchiness that makes spider silk so tough.
Spider silk is also very fine (about 2 microns in diameter, whilst silkworm silk is about 10 microns) and incredibly light (you would need less than 500 grams of it to circle the Earth).
4. Spiders in Captivity
Applications in industry and fashion for a fibre that is strong and tough as well as extremely fine and light are enormous. So why are spiders not reared in the scale that silkworms are reared?
Unfortunately, rearing enough spiders to be practical is a challenge. Spiders do not like confined spaces and prefer to be on their own; they tend to eat each other in captivity.
5. Golden Orb Spiders (Nephila clavipes)
Two entrepreneurs in Madagascar came up with a compromise. They organised workers who collected around 3,000 female Golden Silk Orb-weaver spiders a day, put each Golden Orb spider in a harness attached to a spool, gently pulled about 400 metres of the gold-coloured silk and then released the spider. More workers then hand-twisted 24 silk threads together into a single thread and wove a golden silk cape that has been exhibited at the American Museum of Natural History and the Victorian and Albert Museum in London.
6. Uses for Spider Silk
In the past spider silk was used to produce the cross hair in microscope eyepieces. It has also been made into fisherman’s nets to catch small fish. Another use was to cover wounds, the web is antiseptic and has vitamin K which helps with blood clothing.
7. Bio-mimicry with Spider Silk
An alternative to captive breeding is bio-mimicry, that is to say we study the spider silk and the way it is produced and then try to create something similar. There has been a great deal of research in this area.
One group of researchers spliced spider genes into the chromosomes of dairy goats. These genetically modified goats produce milk with a small percentage of spider silk protein. They are still researching the best way of spinning the silk protein into a thread that is a fine and as strong as the spider silk. Other groups of researchers are working with transgenic E. coli bacteria and plants to produce spider silk. The man-made spider silk could then be use to make very fine sutures, biodegradable fishing lines or even lighter weight bullet-proof vests.
In addition to being fine, light, strong and tough, spider silk is readily accepted by the human body. Research is being carried out on growing nerve cells on spider-silk filaments which could lead to nerve grafts. It may also be possible to build delicate lattices with spider silk to hold bone tissue in place.
Recently spider silk has been spun into violin strings in Japan.
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