To illustrate the amazing properties of spider silk, Nikola Kojic offers an arresting example. Imagine a circular web with a diameter of 100 meters—about the length of a football field—spun from a silk thread about a centimeter thick. Concentric circles 4 cm apart attach to the web’s spokes, also 4 cm apart. This larger-than-life web “could stop a jumbo jet in midflight,” says Kojic.
Impressive—as would be the jumbo spider that one could imagine crawling over to the jet. But beyond the monster-movie possibilities, the scenario demonstrates what scientists covet most about spider silk: its exceptional capacity to absorb kinetic energy. Scientists would like to exploit that property in items ranging from bulletproof vests to suspension cables for bridges.
Spiders store tiny amounts of silk. Harvesting the material from the animals isn’t practical, says Kojic, a biomedical engineer at the Massachusetts Institute of Technology (MIT).
Instead, scientists have focused their efforts on making synthetic versions of spider silks, but they haven’t yet produced a material as tough as the original. The best industrial fibers, such as Kevlar, don’t absorb as much kinetic energy as spider silk does. Their manufacture, moreover, is not environmentally friendly because it requires organic solvents and high temperatures and pressures.
In contrast, natural spider silk is produced at room temperature with water as a solvent, says Chris Holland, a zoologist at the University of Oxford in England. “It’s made in the spider, and with the spider eating flies. That produces a fiber that we can’t even come close to.”
Some researchers are considering silk from the spider’s perspective. This research is providing insights into the roles that the threads play in spiders’ lives. But a focus on the spider may also offer the best chance at replicating the material. New studies examine the flow of the material during the spinning process to learn how a spider makes a thread.
At the center of these pursuits lies the winning formula. “Until we know the full system,” says Holland, “we won’t be able to make a silk as well as the spider can.”
When researchers talk about mimicking spider-silk production, they are usually referring to dragline silk. Orb-weaving spiders, which spin circular webs with characteristic wagon-wheel spokes, use dragline silk for the web’s outer rim and spokes. This thread is also the animal’s lifeline when it drops from a height.
Researchers prize dragline silk for its strength and its toughness, two distinct properties, explains Todd A. Blackledge, a behavioral ecologist at the University of Akron in Ohio. The more stationary weight a rope can support, the stronger it is.
In contrast, toughness refers to the amount of kinetic energy that a material can absorb without breaking. To lasso a running horse, for example, the rope needs to be tough. Bulletproof vests, which protect the wearer by halting an oncoming slug, are tough.
“Dragline silk is as strong as steel, but not as strong as Kevlar,” says Blackledge. “But [this silk’s] toughness is far superior to either of these.”
The orb weavers spin five fibrous silks and two adhesive silks. Among the fibrous silks is dragline, or major-ampullate silk. Orb weaving spiders build the spirals on their webs with minor-ampullate and capture-spiral silk. The spiders wrap their captured prey with aciniform silk and construct their egg sacs primarily from tubuliform silk.
When building a web, “what a spider is doing is spinning a little miniature environment,” Blackledge says. From mating to catching food to protecting the animal from the elements and predators, a web affects various aspects of a spider’s life. The properties of silk become relevant, for example, when investigating how a particular type of web catches prey. “You can’t really ask that question without understanding the material being used to spin that web,” Blackledge says.
Researchers are most familiar with the mechanics of dragline and capture-spiral silk, which is sticky, extremely stretchy, and tough. The properties of these two silks make webs effective for trapping flying insects, explains Blackledge. The dragline frame of the web absorbs the brunt of an insect’s energy. The capture-spiral silk absorbs some energy but sticks to and stretches with the insect, so that it decelerates slowly and doesn’t bounce off the web.
To learn about the lesser-known silks, Blackledge and his colleague Cheryl Y. Hayashi of the University of California, Riverside studied the five fibrous silks of the orb-weaving silver garden spider, Argiope argentata. They collected two of the silks directly from the spiders and the other silks from webs, wrapped prey, and egg sacs. They extended the fibers and measured the silks’ mechanical properties using a tensile-testing machine.
In the July 1, 2006 Journal of Experimental Biology, Blackledge and Hayashi reported that the silks make up a diverse toolkit of fibers “that seem fine-tuned for particular ecological functions.” For example, in keeping with its prey-capturing role, the capture-spiral silk is 10 times as stretchy as the other silks. Meanwhile, the tubuliform silk of the protective egg sacs is the stiffest.
Furthermore, the researchers found that aciniform silk, the threads that the spider uses to wrap and secure freshly captured—and still wriggling—prey, is two to three times as tough as the other silks, including dragline.
For a materials scientist interested in a high-performance fiber that absorbs kinetic energy, notes Blackledge, “the prey-wrapping silk may be a better model to study than the dragline.”
Blackledge is interested in the extent to which shifts in spider behavior have influenced the performance of silks. “When the silk is used in a new ecological context, what happens to the material properties?” Blackledge asks.
Silk’s mechanical properties primarily derive from two critical factors: the proteins that make up the material and the spinning process that transforms the liquid generated inside a spider into a solid fiber.
Randolph V. Lewis, a molecular biologist at the University of Wyoming in Laramie, and his coworkers have determined the amino acid sequence of several silks. They’ve found distinct amino acid motifs that contribute to different silks’ properties.
For example, the two major proteins in dragline silk contain frequently occurring stretches of the amino acid alanine. Lewis says that these alanine repeats give the fiber strength by permitting one protein chain to snap tightly to another, much as Lego blocks combine. Minor-ampullate silk, which is not as strong as dragline, has shorter stretches of alanines.
Meanwhile, capture-spiral silk has a motif, based on a sequence of five amino acids that’s repeated up to 68 times in a row (SN: 2/21/98, p. 119: http://www.sciencenews.org/pages/sn_arc98/2_21_98/fob2.htm). Lewis speculates that this sequence introduces a series of spiraling molecular springs into the protein, which may explain the silk’s extreme stretchiness.
Lewis’ group has built artificial silk genes and inserted them into the common bacterium Escherichia coli to make proteins that are shorter than the natural versions. The researchers add the resulting proteins to organic solvents, spin this material into fibers with a commercial spinning machine, and test its mechanical properties. If the researchers increase the number of capture-spiral-silk motifs, for example, the elasticity of the fiber grows, although not in direct proportion to the number of motifs.
While scientists know a lot about the sequence of individual chains and a bit about the chains’ interactions with each other, higher levels of structure are “basically completely unknown,” Lewis notes. A silk thread contains hundreds of thousands of protein chains, each of which folds on its own and also arranges itself among other chains in the fiber, he says. He and his colleagues have begun nuclear magnetic resonance studies to explore these structural details.
“The spider hasn’t given us all the secrets,” Lewis says.
Go with the flow
Silk’s transformation to a solid fiber from a thick liquid containing primarily protein and water begins in specialized glands, one for each type of silk. In each gland, a structure called the tail secretes the starting solution, or spinning dope, into a storage sac. When the spider is ready to spin, the dope moves into a duct. The diameter of the duct narrows as it reaches a nozzle from which the thread exits the spider.
To understand the characteristics of the spinning dope, some scientists have turned to rheology, the study of how materials deform and flow. Silk dope has properties intermediate between those of typical liquids and solids, explains Gareth H. McKinley, a mechanical engineer at MIT. Such viscoelastic materials are thick rather than runny. They’re also elastic: After being stretched, they return to their original states. Silly putty and uncooked egg white are two familiar examples of viscoelastic materials, McKinley says.
The handful of previous rheology studies of dope used samples that had been diluted to make their volumes large enough to be tested. But machines that can work with small samples of material are now available, notes Holland. Scientists can test tiny amounts of silk dope that have been extracted from a spider. Reports on freshly obtained dragline-dope samples were published last fall by an Oxford team, led by zoologist Fritz Vollrath and including Holland, and by McKinley’s team, which includes Kojic.
An important concept in rheology is shear, the sliding motion of adjacent layers of material. Silk dope experiences shear forces as it moves through the spinning duct. McKinley’s group built a microrheometric device that measures how the viscosity of the dope changes in response to shear forces. The researchers place the sample—a drop of dope the size of a pen tip—between two plates. The lower plate remains stationary as the upper plate moves back and forth. The machine’s action is much like rubbing a drop of lotion between thumb and forefinger to gauge its slipperiness, says McKinley.
The researchers found that the faster the upper plate moves, the more readily the dope flows. Shear forces align the proteins in the dope, Kojic says, “and as the proteins align, it becomes easier for them to move relative to one another.” Adds McKinley, “Take a big bucket of spaghetti. If you keep stirring it clockwise, it gets easier because the spaghetti strands are lining up.”
This effect explains how the thick dope can progress through the narrowing duct in an energy-efficient manner, Kojic notes. The team calculated that overall, the viscosity of the dope decreases 10-fold as it flows through the duct.
Vollrath, Holland, and their colleagues examined shear viscosity by using a commercial rheometer. Like McKinley’s team, they found that the dope flows more easily as the shear rate increases.
To learn how a spider spins a continuous fiber, McKinley’s group developed another microrheometric device that measures the dope’s resistance to stretching, its extensional viscosity. The device’s operation is akin to a saliva-dabbed thumb and forefinger pulling apart rather than sliding. A laser determines the diameter of the resulting silk.
The researchers found that the dope’s extensional viscosity increases 100-fold as the dope is pulled into a thread. This change prevents the thread from breaking before it solidifies. The researchers reported their results in the Nov. 1, 2006 Journal of Experimental Biology.
“Hopefully, [the findings] give you guidelines on how you would want to formulate a synthetic equivalent,” says McKinley.
By testing an artificial silk in a similar way, “you can see whether you match these properties,” adds Kojic.
Vollrath’s team also compared the rheology of the dragline dope with that of silkworm dope. Spiders and silkworms evolved the capacity to spin silk independently of each other, Holland says.
The dopes contain different proteins, and the resulting fibers have distinct properties. Yet “what we see is that the flow properties are very similar,” Holland says. Despite their differences, the spider and silkworm “use similar tricks,” he continues. “This gives fantastic insight into how silk production has evolved and how the production of an energy-efficient, high-performance fiber is made by nature.”
Moreover, Holland and his colleagues reported in the November 2006 Nature Materials, both the spider and silkworm dopes behave like melted polymers. This is “a most welcome observation,” they say, because well-developed theories of polymers can be used in studies of silk dope.
With rheology proving to be “a valuable tool in showing how silk is physically processed,” says Holland, scientists can now move forward in an area that was largely absent from previous attempts to replicate silk.
As scientists working to make artificial silks apply new information about dope and how spiders spin it, perhaps they should take a longer-range view of success. As Blackledge notes, “Spiders have been spinning these silks for almost 400 million years.”