The Strength of Spider Silk

Four domesticated silk moths.

The weekend when we saw the last episode of Silk air on Public TV coincided with a riot of cobwebs in our back yard. The silk of the show refers to the fabric worn by Queen’s Counsels (barristers) in Britain; it comes from silk-moth larvae. The moths are quite lovely—and butterflies are another of our perennial outdoor topics. The same silk is also made by our spiders. These coincidences reminded Brigitte of an attempt, announced some years ago already, of inserting silk-producing genes into…let’s see…was it sheep? was it goats?—the harvesting of which, somehow, would give us access to this magic fiber in a form more easily processed? That opened the topic on the supposedly astounding the strength of spider silk—and produced a note to look it up.

We remembered correctly. Spider silk, the strands of which are exceedingly thin, around one to four one-thousandth of a millimeter, approach the tensile strength of steel, measured in Giga-Pascals; silk is 1.3 and steel is 1.65 GPa. Silk, however, is less dense; it is more ductile, meaning stretchable. It is also, and for this reason, much tougher—so that a strand of silk is three times stronger than a strand of steel of the same thickness. Two engineers, in fact (Ed Nieuwenhuys, Leo de Cooman), have even calcuculated, not entirely tongue-in-cheek, how thick the strands of a cobweb would have to be to capture of Boeing-747 in full flight (link): the thickness of a pencil.

Tensile strength is resistance to pulling; ultimate tensile strength is pulling to a break—the measurement used in the paragraph above. A Pascal is a measure of force on a defined area, thus a square meter. It is the same measure as pounds per square inch. 1000 Pa equal 0.145037738 psi.

Wikipedia’s article on Spider Silk ends with a listing of projects, some quite successful but none as yet commercialized, that produce spider silk by genetic engineering. The project we were vaguely remembering is the work of Nexia, a Canadian company, which put spider genes in goats. The silk appears in the goats’ milk.

What must surely be the latest news on this subject—dated Septerber 11, 2013—is shown here. It tells of work at the National High Magnetic Field Laboratory, Tallahassee, Florida, where a team headed by Eden Steven coated spider silk with nanotubes of carbon; they are superstrong and also conduct electricity.

Where will it all end?

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My images are from Wikipedia here and here. An earlier post on silk production appears here.

Some Silk Statistics

Having just celebrated the successful raising of seven of nine caterpillars into butterflies this summer, a little perspective might be in order. Silk, that most prized of all fabrics, derives from the silkmoth, Bombyx mori. The caterpillars undergo five transformations. In the fifth of these, they cover their bodies with a dense coating of raw silk. When the larvae have developed into moths inside these silky covers, they release an enzyme that produces a hole in the silk—and incidentally also causes silk fibers to break down. For this reason, the making of silk begins before the moth is ready to emerge. The pupae are dipped into boiling water to kill them.

Each chrysalis or cocoon is formed of a single strand of silk. Yes. Not a typo. The  cocoon is about an inch in length. The thread of silk that forms it ranges in length from a fifth to more than half a mile. After the cocoons are boiled, a human must find the end of the thread by brushing the cocoon by hand. That end is then threaded into a needle affixed to a machine or a wheel. Then the unraveling begins.

Now for the statistics. It takes 3,000 to 5,000 larvae to make one pound of silk. The world produces 70 million pounds of raw silk every year—and to do so, 210 to 350 billion larvae must die so that we can dress our ladies decoratively. And we were proud to have helped seven butterflies to flutter away freely this summer. Hhmm.

To raise such large numbers of silk-covered larvae makes the white mulberry tree (Morus alba) happy; the caterpillars feed on its leaves until they go into the silk-chrysalis state. To be sure, being eaten in order to be cultivated is not necessarily an ideal sort of existence. In 2002, I learn from Wikipedia, 3,890 square miles were under white mulberry cultivation in China to support the silk trade, which is more than the areas of Delaware and Rhode Island combined or 80 percent of Connecticut’s surface.

How did I get here? We were discussing the fine threads that hold the Swallowtail chrysalides suspended from a plant stem—while the bottom is glued to the stem so well that the chrysalis gives way before the seal does. Brigitte got to talking about silk, which those suspending filaments surely are—and we realized that we knew nothing about the subject except that silk came from worms. We soon learned that these worms are caterpillars…

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My image of the white silkmoth cocoons is from Offset Warehouse (link). A very instructive site on how the actual silk extraction takes places comes from wormspit.com (link).