We are all familiar with spiders, and whenever we think of these little creatures we probably also think of their most familiar accessory, the spider's web. Spiders make extensive use of the silk they spin, and the web is the commonest use they make of the silken threads. Spider silk has remarkable physical properties and could be put to many uses in our modern world if a reliable and economical way could be devised to produce it in quantity.
Many unsuccessful attempts have been made over the years to mass-produce spider silk. Efforts are ongoing because of the alluring properties of the product. A strand of spider silk is stronger than a steel strand of the same dimensions, but it can also stretch 40 per cent beyond its normal length and thereby accommodate to a gale of wind. The silk contracts in the cool morning dew, thereby reducing the surface area on which moisture can condense and enabling the web to avoid collapsing under the weight of the water. Also, the elastic properties of the silk threads are such that when an unfortunate fly (the prey) bumps into the trap, the web neither breaks nor bounces the fly off into the air like a trampoline.
Spider silk is a composite material made up of protein crystals dispersed in a background matrix. The background matrix is responsible for the "give" in the silk thread referred to earlier. Partly because of this the temperature at which it becomes brittle and liable to break (the glass transition temperature) is low. Spider silk remains flexible at sub-zero temperatures and is an ideal material from which to make special items like parachute cords which encounter great fluctuations in temperature.
Another practical use for spider silk, if it became possible to produce it economically in some quantity, would be to weave it into protective clothing, for example bulletproof vests. It could also be very useful medically in the manufacture of synthetic tendons, sutures, implants and synthetic skin.
Human use of spider silk for practical purposes has a long, if narrow, history. The ancient Greeks used cobweb wads to inhibit the flow of blood from wounds. The webs also discouraged infection. Even today some South Seas islanders induce spiders to spin silk webs between bamboo poles and use them as fishing nets.
Probably the most important practical use of spider silk has been in the manufacture of astronomical and other optical instruments. Most telescopes and many other optical instruments are fitted with two fine lines at right angles to each other in the field of view. Some telescopes are also equipped with several parallel lines in order to observe the movement of heavenly bodies. Since these various lines are viewed under magnification, it is important that they be extremely fine and straight. Strands of spider silk have been used for this since 1880.
The second World War saw a dramatic leap in demand for spider silk, particularly for telescopic gunsights and bombsights. Spider silk is particularly good for bombsight crosshairs because its low glass transition temperature ensures that no warping occurs at high altitude. Silk from the deadly black widow spider is particularly popular because it is so extremely fine. Human hair has a diameter of approximately 1/250 of an inch. By contrast immature black widow spiders can spin silk threads as thin as 1/20,000 of an inch in diameter.
Traditional procedures for raising spiders and harvesting silk from them are difficult and labour-intensive. The spiders must to be fed with a continuous supply of live insects and housed separately in order to prevent them from cannibalising each other. When it is time to collect silk the spider must be kept still on a pedestal while the operator carefully grabs a silk fibre and reels it gently on to a special frame. Over 100 feet of silk fibre can be taken from a spider in this manner, but the spider must then rest for several days in order to replenish its silk supply.
Spiders use silk to perform a wide variety of jobs. Many species of spiders lay eggs wrapped in silk. Spiders take their first ride from the nest on a thin strand of silk anchored to home-base. As we all know, they trap food using silk. Some species spin aerial webs to catch flying insects. Others spin sticky sheets that trap insects like fly-paper. Others don't use webs but spin a single strand attached to a sticky ball. They hurl the ball at passing insects and reel them in like a cowboy ropes a steer.
SILK is an essential part of the sex life of spiders. The male ejaculates his sperm into a special little web of silk and deposits this in the female. A male spider with amorous intentions has to be very careful when approaching the female, as she would just as soon eat him as mate with him. He often finds it convenient therefore to use silk to temporarily immobilise the female while they are doing the business.
The silk protein is carried in solution (i.e. dissolved) in the spider's abdomen. When the spider wants to spin fibres he/she passes this solution through many narrow passages and eventually extrudes it through spigots called spinnerets as water-insoluble and dry fibres. The spider can alter the diameter of the thread it spins and can coat the threads with a variety of oily liquids useful for trapping insects.
There is much current interest in developing convenient methods for making spider silk in quantity, and the favourite approach is through biotechnology and genetic engineering. Attempts are under way to identify the gene sequences that code for fibre silk protein. These will be inserted into a bacterium, which will produce the silk in quantity. The silk will be collected and spun into threads. Much progress is being made in producing the silk protein in quantity, but it remains to be seen how successfully it can be spun into fibres.
The physical properties of spider silk fibres clearly promise exciting technological applications. It must be admitted that practical applications to date have been less than exhilarating - bombsights for example. The spider has found uses for silk in almost every facet of its life. Perhaps we should study the spider in order to learn how we might use this wonderful material. Who knows, we may yet drive over suspension bridges that hang from cords of spider silk.
William Reville is a senior lecturer in biochemistry at UCC