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09.12 Smooth as Silk
Smooth as Silk    
 

Over 4,000 years ago in China, an insect was discovered that would change the course of history.

The larvae of the moth Bombyx mandarina, now known as the Chinese silkmoth, made cocoons out of a fine white material that was—literally—as smooth as silk. It wasn´t long before this wild silk was harvested to clothe the royal family and adorn the royal palace. Eventually the silkmoths were domesticated and the process of producing silk from silkworms, known as sericulture, reached all corners of the globe.


The mythic origin of silk making: “Leizu was a legendary Chinese empress. According to tradition, she discovered silk and invented the silk loom in the 27th century BC. She found silkworms eating the mulberry leaves and spinning cocoons. She collected some cocoons, and then sat down to have some tea. While she was sipping a cup of tea, she dropped a cocoon into the steaming water. A fine thread started to separate itself from the cocoon. Leizu found that she could unwind this soft and lovely thread around her finger.” http://history.cultural-china.com/en/48History5683.html

In modern times, silk has been recognized not only for the incredible fabrics it produces but for its medical applications. For example, silkworms are the biological source for many of the sutures used by medical professionals to seal wounds and incisions.

And researchers see great potential for using silk in a variety of other applications including tissue scaffolds and artificial ligaments and tendons.

Silk from other sources

Silkworms are not the only animals that produce silk, however. Recently researchers have turned to spider silk as a finer, tougher, alternative for many of these applications. The problem has always been how to harvest spider silk.

Given their territorial and cannibalistic nature, it isn´t realistic to farm spiders.

That´s why a team of researchers, led by Dr. Randy Lewis at Utah State University, set out to create a system to harvest spider silk without spiders. They´ve tried and succeeded at genetically engineering alfalfa plants and goats to produce spider silk, but in the end these models were not feasible for large-scale production. Now Dr. Lewis and his team have turned to genetically-modified silkworms that, after many years of research, can produce spider-silk that may one day be used in operating rooms around the world.

Silk worm larva

To begin this process, Dr. Lewis first had to understand the structure of natural spider silk. He studied the silk made by the Golden Silk Orb-Weaver spider Nephila clavipes, which is commonly found in North and South America near the Caribbean and makes characteristic circular spider webs.

Golden Orb spider in its web

N. clavipes uses several types of silk in the production of its web that provide differing levels of strength, elasticity and stickiness. For example, the extremely strong dragline silk is used to form the frame of the web, while the super-elastic flagelliform silk acts as a landing pad to absorb the shock of prey flying into the net. Swathing silk is used to wrap and immobilize prey once it has been caught in the web, and yet another type of sticky silk is used to hold everything together.

Silk Proteins

Each of these different types of silk is made from a combination of one or more of the six silk proteins Dr. Lewis identified. Dragline silk is made of two proteins, Major Ampullate Spidroins 1 and 2 (MaSp1 and MaSp2), which form interlocking sheets similar to the way Legos stick together. The interlocking sheets of protein cannot be pulled apart, giving dragline silk its extreme strength. Flagelliform silk, on the other hand, is made of only one protein, Flag, repeated over and over again in alternating shorter and longer sequences. The alternating pattern of the Flag protein resembles a Slinky, making the flagelliform silk extremely elastic.

In-depth analysis of the six proteins produced by N. clavipes revealed the exact amino acid sequences of the proteins and the way the amino acids fit together to form the proteins. With this information, Dr. Lewis and his team were able to create their own silks from various combinations of the natural silk proteins. The goal was to create a silk with optimal strength and elasticity for specific biomedical applications. For example, silk to replace ligaments requires more stretch and not as much strength, which can be achieved by combining the silk proteins in just the right way. “It´s definitely a lot of trial and error,” remarked Dr. Lewis. “But we are extremely hopeful given our results so far.” Each combination created by the Lewis Lab is tested using a special machine that can stretch and break the silk to measure its elasticity and strength, respectively.

To actually produce the silk, Dr. Lewis has used a variety of systems, most recently silkworms. He and his collaborators used a virus to insert the genes for spider silk proteins into the DNA of the silkworms, which then produce spider silk combined with their own silkworm silk. Currently about 5% of the silk fiber produced by the spider-silk worms is spider silk protein, the other 95% is silkworm silk proteins.

Dr. Lewis believes his technique has the potential to produce enough spider silk for commercial use.

His next experiments will focus on substituting spider-silk proteins for the silk proteins in silkworms, in order to produce silk that contains more of the properties of spider silk.

In addition to its biomedical applications, Dr. Lewis has received requests to use spider silk in climbing ropes, athletic clothing, airbags, and parachutes, as well as the inner tube of bicycle tires. “The possibilities are almost endless,” remarks Dr. Lewis.

Dr. Randy Lewis is a Professor at the USTAR Synthetic Bioproducts Center of Utah State University. He has spent the last 20 years studying spider silk and its properties for use in biomedical applications. When not in the laboratory, Dr. Lewis enjoys hiking, cross country skiing and flyfishing.

 

To Learn More:

Spider Silk Applications:

  1. ¨Spider Silk Could be the Secret Ingredient in Tomorrow´s Electronics.¨ http://io9.com/5891331/spider-silk-could-be-the-secret-ingredient-in-tomorrows-electronics

  2. ¨Japanese Researcher Creates Spider Silk Violin Strings.¨
    http://abcnews.go.com/blogs/technology/2012/03/japanese-researcher-creates-spider-silk-violin-strings/.
  3. ¨Silk Could Help Repair Nerves.¨
    http://news.bbc.co.uk/2/hi/science/nature/5172422.stm.


For More Information:

  1. Tuele, F. et al. 2012. ¨Silkworms transformed with chimeric silkworm/spider silk genes spin composite silk fibers with improved mechanical properties.¨ Proceedings of the National Academy of Sciences, 109(3): 923-928.

  2. Tuele, F. et al. 2011. ¨Combining flagelliform and dragline spider silk motifs to produce tunable synthetic biopolymer fibers.¨ Biopolymers.

  3. Tuele, F. et al. 2009. ¨A protocol for the production of recombinant spider silk-like proteins for artificial fiber spinning.¨ Nature Protocols, 4(3): 341-352.

Rebecca Kranz with Andrea Gwosdow, PhD www.gwosdow.com


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USTAR BioInnovations Group

Dr. Lewis explains the potential uses for spider silk.

Dr. Lewis shows how to get spider silk from goat’s milk and how Legos, Slinkys and zippers explain the wonderful properties of dragline silk.

Spider Silk: An Ancient Biomaterial for the Future


To Think About
September 2012


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