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Bioengineering Spider Silk

One look at a spider web and you know that spider silk is fairly strong stuff. Each strand is thin, ranging from 1 micron - 0.15 mm in diameter, and yet it can capture a bee flying at full speeds while maintaining the web’s integrity.

Spider silk, particularly the dragline silk of the golden orb weaver Nephila clavipes, is one of the strongest natural substances known, being able to hold 400 000 pounds per square inch without snapping.

Nephila clavipes.
Click to enlarge

It has been calculated that a stretch of spider silk about as thick as a pencil held across a runway could stop a Boeing 747 right in its tracks (1). Spider silk is more bullet-proof than Kevlar (the strongest synthesized polymer). There have been stories of people who have had silk handkerchiefs in their pockets and who were shot in the same region as their handkerchief. Although they died, the bullet did not penetrate the silk, but rather pulled the handkerchief through the body with it (9). Spider silk can handle more pressure without snapping than steel of the same diameter. Unlike steel, spider silk is also elastic, being able to stretch to 30-40% of its original length. For these very reasons scientists have been trying desperately to successfully – and cost-efficiently – market spider silk to be used in such applications as stronger bullet-proof vests, cable for bridges, parachute cords, clothing, army fatigues, car bumpers, aerospace products, supports for damaged cells, and even artificial tendons (1,5,7,9). However, it hasn’t been easy.

Perhaps one of the most obvious ways of obtaining spider silk is to get it in its already-made form: in cocoons. In the early 1700's some spider silk socks and gloves were produced. However, this method was not practical, as over 1 million cocoons were needed to produce a mere kg of silk (2). Also, cocoon silk is not as strong as the greatly-desired dragline silk.

Another obvious way of obtaining spider silk is to milk it from the spiders themselves. This was attempted in Madagascar; a spider would be captured, the silk would be pulled from the spinners by hand, and then, when the spider "got tired", it would be released back into the wild (2). However, this proved to be quite impractical, as thousands of spiders would need to be “milked” to produce a single piece of clothing, and ultimately the entire practice was banned.

Later, spider farms were set up, but the overall yield was too small, the setup was too expensive, and spiders have the tendency to become cannibalistic when placed in large groups. So, this, too, was dropped (2).

And that’s where biomimetics came in. This is a relatively new branch of science that combines the fields of biochemistry, zoology, engineering, and physics, and has the sole purposed of figuring out how natural materials, such as spider silk, horse hooves, slug slime, mollusc shells, etc, work. The scientitsts then find ways to copy it (“mimic” it) so that it can be mass-produced (1).

Scientists in this field have yet to actually produce top-grade spider silk, but they have been extensively studying it, and have learned a great deal about its structure:

Dragline silk (left)
Pyriform silk (right)
Tiny, oriented crystals
in dragline silk
gives it its strength
Click to enlarge

1. Spider silk consists of several proteins including fibroin(2), which in turn are composed of seven amino acids: alanine, glycine, glutamine, leucine, arginine, tyrosine and serine (4). These protein chains are repeating polymers and are 1500 times smaller than a micron (which is one-thousandth of a mm) (1).

2. Spider silk is thought to be a liquid while in the body, and then polymerizes as it leaves the spinnerets (3).

3. Spider silk is highly complex. As it leaves the spider, some of the protein chains bend back (1), causing them to align with others and forming crystals. There are two types of crystals now known to exist in spider silk: a highly ordered variety, similar to that in Kevlar, and a less ordered type. These crystals are rich in arginine. Also within the spider silk is an amorphous, or jellylike, glycine-rich polymer (5,9). Thus, the silk is thought to be composed of alternating highly-ordered crystals and amorphous regions, with the less-ordered crystals acting as fingers, holding the two regions together (5,9). This produces the strong properties in the silk.

4. The elasticity of spider silk is obtained by there not being too many of these crystals, thereby limiting the silk’s density and maximizing elasticity (1).

5. There are several genes responsible for the production of spider silk, consisting of over 22000 base pairs (9).

General overview
of recombinant DNA
techniques.
Click to enlarge

Now that so much is known about the structure of spider silk, it is up to the geneticists to find a way to produce it cheaply. One of the most obvious ways to do this is to use bacteria (8,9) using the techniques of recombinant DNA; that is, taking the genes responsible for spider silk production from the cell of a spider, cutting out some genes from the plasmid of a bacterial cell by way of special enzymes, placing the spider genes where the bacteria genes used to be within the plasmid, and then re-inserting the new recombinant plasmid back into the bacteria (10). When the bacteria reproduce, the daughter cells also have the spider genes, and they all act as silk-producing factories, making large quantities of raw, liquid silk which can then be spun out into fibres in a factory (9). This has been carried out by several companies, but none are yet ready to actually manufacture spider silk. One reason for this is cost: bacteria cannot produce their own supply of arginine and glycine, two of the main constituents of spider silk, and as a result these amino acids have to be added. And the amino acids are expensive (8) Also, no top-grade spider silk has actually been produced during this method. It seems that bacteria actually truncate the spider genes as the genes consist of so many repeating units. The result is that the silk proteins are shorter than those found in nature and thus have different properties (11).

Other organisms besides bacteria are also being looked upon as potential silk-producers. For example, some companies are using potatoes or even tobacco plants. Using recombinant DNA techniques, they have inserted spider genes into these plants, and the plants have produced spider silk proteins in their tissues. In fact, researchers have found that, of the total protein mass produced by the plants, 2% of it was silk protein (8). Now all that has to be developed is a way to isolate those proteins and spin them into fibres. If this can be successfully done, using plants will be 50% cheaper than using bacteria, as plants can provide their own amino acids.

A third, highly innovative way to make spider silk has been developed by Nexia Biotechnologies in Montreal. They use goats. Their idea is that, since goats have mammary glands that produce water-soluble proteins in large amounts, then goats should also be able to produce spider silk proteins in their milk, just as spiders produce these water-soluble proteins in their silk glands (7). Researchers first took eggs from one goat and removed their nuclei, replacing them with the nuclei of a source goat bred to mature quickly. These transgenic goats, called BELE goats (breed early, lactate early) have been produced quite successfully. The first goat of this kind was named Willow.

Webster and Pete
The "Web Kids"
Click to enlarge

The second step was to inject spider silk genes into the one-celled unfertilized egg of a BELE goat, and then fertilize the egg and allow the goat to mature. In 2000 the first two of these goats were produced, named Webster and Pete. Because these two so-called "web kids" are also BELE goats, they produce milk earlier than other goats, and within their milk is the spider silk proteins (12). Nexia is also beginning to mate transgenic males with regular females (6), in the hopes that the kids will also produce spider silk.

Although scientists are still a few years from developing top-grade spider silk cheaply and efficiently, they are closer to their goal then ever before.

1. Coleman, Bruce et al. “Stealing Nature’s Secrets” Equinox Magazine April 1996
2. http://www.xs4all.nl/~ednieuw/Spiders/InfoNed/body.html
3.
Levi, Herbert W. and Lorna R. Spiders and Their Kin. Golden
Press, NY USA, 1968
4.
http://www.washington.edu/research/pathbreakers/1990g.html
5.
Segelken, Roger. Spider Silk is Model for Super Fibres http://www.news.cornell.edu/Chronicle/96/1.18.96/spider.html
6.
http://www.purefood.org/Patent/xgenicanimals.cfm#Goats
7. http://www.howstuffworks.com/news-item38.htm
8.
http://www.nature.com/nsu/010531/010531-11.html
9.
http://www.sciencenews.org/sn_edpik/ps_5.htm
10.
"Recombinant DNA" Understanding Biology 2nd Ed. Mosby-Year Book Inc, 1991
11.
Biosteel (R) Extreme Performance Fibres. http://nexiabiotech.com/HTML/technology/biosteel.shtml
12. GM Goat Spins Web Based Future. http://news.bbc.co.uk/hi/english/sci/tech/newsid_889000/889951.stm

Picture taken from:
1. http://creatures.ifas.ufl.edu/misc/golden_silk.htm
2. http://www.fofweb.com/Subscription/Science/Science-Detail.asp?SID=1&iPin=G0641
3.
http://news.bbc.co.uk/hi/english/sci/tech/newsid_889000/889951.stm