Wednesday, September 11, 2013

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.

3 comments:

  1. My admiration for spiders has only grown by these descriptions of the characteristics of their delicate products.
    For 2 or 3 years I have been carefully catching and moving these insects from the inside of our house to the out-of-doors, bidding them build their gossamer webs under the eaves, along clothes lines and our many tomato plants. They must have heard me; there are so many of them this year.

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  2. I was just admiring a rather elaborate web that spans one of the transom windows next to our front door. It is full of tiny little bugs, glistening in the recently fallen rain. They will make a splendid feast, no doubt. And yes, spiders are thick in our yard this year too.

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  3. Evidently Mother Nature -- who is harsher than we think she is, Brigitte, as you observed yesterday -- had had enough of all our spider webs. Why else that torrent of broken branches that litter our yard this morning thickly after last night's big thunder storm.

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