[Editors] MIT engineers probe spiders' polymer art

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT engineers probe spiders' polymer art

--Manufactured silk could be used for
artificial tendons, parachutes, more

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For Immediate Release
MONDAY, OCT. 30, 2006
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTOS AVAILABLE

CAMBRIDGE, Mass.--A team of MIT engineers has identified two key 
physical processes that lend spider silk its unrivaled strength and 
durability, bringing closer to reality the long-sought goal of 
spinning artificial spider silk.

Manufactured spider silk could be used for artificial tendons and 
ligaments, sutures, parachutes and bulletproof vests. But engineers 
have not managed to do what spiders do effortlessly.

In a study published in the November issue of the Journal of 
Experimental Biology, Gareth H. McKinley, professor of mechanical 
engineering, and colleagues examined how spiders spin their native 
silk fibers, with hopes of ultimately reproducing the process 
artificially.

McKinley heads the Non-Newtonian Fluid Dynamics research group in 
MIT's Department of Mechanical Engineering. Non-Newtonian fluids 
behave in strange and unexpected ways because their viscosity, or 
consistency, changes with both the rate and the total amount of 
strain applied to them.

Spider silk is a protein solution that undergoes pronounced changes 
as part of the spinning process. Egg whites, another non-Newtonian 
fluid, change from a watery gel to a rubbery solid when heated. 
Spider silk, it turns out, undergoes similar irreversible physical 
changes.

Stickiness and Flow

McKinley and Nikola Kojic, a graduate student in the Harvard-MIT 
Division of Health Sciences and Technology, studied the silk of 
Nephila clavipes, the golden silk orb-weaving spider. One species of 
golden orb spider creates a web so strong it can catch small birds. 
In the South Pacific, people make fishing nets out of this web silk.

The researchers chose the golden silk spider because of the 
formidable strength of its web. But Kojic was taken aback when the 
first palm-sized spider crawled out of the box he received in the 
mail from an accommodating employee of Miami's MetroZoo. (She simply 
gathered some up from the grounds; the zoo does not exhibit golden 
orb spiders.)

"This is pretty scary," he said. "I'd never seen a spider this big. I 
never grew up around anything with furry knuckles." But he quickly 
settled into dissecting the peanut-sized and -shaped protuberance on 
the spiders' backs containing their silk-producing glands and 
spinnerets.

Spiders don't actually spin ("spinning" refers to the age-old art of 
drawing out and twisting fibers to form thread); instead, they squirt 
out a thick gel of silk solution. (One teaspoonful can make 10,000 
webs.) They then use their hind legs as well as their body weight and 
gravity to elongate the gel into a fine thread.

Kojic, who first practiced on silkworms, learned how to extract a 
microscopic amount of the gel-like solution from the spider's 
silk-producing major ampullate gland.

The researchers used devices called micro-rheometers-custom-made to 
handle the tiny drops of silk solution-to test the material's 
behavior when subjected to forces. The team tested the thick 
solution's viscosity, or how it flowed, by "shearing" it, or placing 
it between two rapidly moving glass plates. They tested its 
stickiness by pulling it apart, like taffy, between two metal plates.

The magic that makes silk so strong, the researchers discovered, 
happens while it flows out of the spider's gland, lengthens into a 
filament and dries.

Engineering Nature

The key to spider silk is polymers.

Plastics, Kevlar (used in bulletproof vests) and parts of the 
International Space Station are some of the many items made from 
polymers. The proteins in our bodies are polymers made from amino 
acids. From the Greek for "many" and "units," polymers are long 
linked chains of small molecules. They can be flexible or stiff, 
water-soluble or insoluble, resistant to heat and chemicals and very 
strong.

Silk protein solution consists of 30-40 percent polymers; the rest is 
water. The spider's silk-producing glands are capable of synthesizing 
large fibrous proteins and processing those proteins into an 
insoluble fiber.

"The amazing thing nature has found is how to spin a material out of 
an aqueous solution and produce a fiber that doesn't re-dissolve," 
McKinley said. Like a cooked egg white, dry spider silk doesn't 
revert to its former liquid state. What started out as a water-based 
solution becomes impervious to water.

The silk protein's long molecules are like tangled spaghetti. They 
form a viscous solution but are slippery enough to slide past each 
other easily and squeeze through the spider's ampullate gland. As the 
silk gel flows from the gland through an S-shaped, tapered canal to 
the outside of the spider's body, the long protein molecules become 
aligned and the viscosity (or resistance to flow) drops by a factor 
of 500 or more.

As the resulting liquid exits the abdomen through the spinneret, it 
has the characteristics of a liquid crystal. It's the exquisite 
alignment of the protein fibers, Kojic said, that gives silk threads 
their amazing strength.

While the silk stretches and dries, it forms miniscule crystalline 
structures that act as reinforcing agents. Engineered 
nanoparticles-tiny materials suspended in artificial silk-may be able 
to serve the same purpose.

In conjunction with the polymer synthesis and analysis work of Paula 
T. Hammond, an MIT professor of chemical engineering, McKinley's 
laboratory will use the new insights about spider silk to team up 
with MIT's Institute for Soldier Nanotechnologies to emulate the 
properties of silk through polymer processing.

"We're interested in artificial materials that emulate silk," 
McKinley said. Tailoring the properties of the liquid artificial 
spinning material to match the properties of the real thing "may 
prove essential in enabling us to successfully process novel 
synthetic materials with mechanical properties comparable to, or 
better than, those of natural spider silk," the authors wrote.

This work was supported by the NASA Biologically Inspired Technology 
Program, the DuPont-MIT Alliance and the MIT Institute for Soldier 
Nanotechnologies.

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Elizabeth A. Thomson
Senior Science and Engineering Editor
Massachusetts Institute of Technology
News Office, Room 11-400
77 Massachusetts Ave.
Cambridge, MA  02139-4307
617-258-5402 (ph); 617-258-8762 (fax)
<thomson at mit.edu>

<http://web.mit.edu/newsoffice/www>
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