To many people, spider webs are a nuisance, something that needs to be knocked down and cleared out of the way. They’re reminders that you haven’t cleaned lately. To Jeffery Yarger, spider webs, and more specifically the spider silk that makes up the webs, are structures of beauty, especially when you look at them at the microscopic level.
Structurally, spider silk is five times stronger than steel and twice as strong as Kevlar. On a per unit weight basis, it is one of the toughest fibers to break, says Yarger, a professor in Arizona State University’s Department of Chemistry and Biochemistry.
Because of those properties, there is keen interest in mimicking the spider and making silk on a large scale. Anything from greener processes for making proven materials like Kevlar to finding a possible material that can be used in replacement tendons, spider silk is being considered.
The problem is, humans don’t make the silk as well as the spiders do, and spiders don’t make the material in sufficient quantities for human use.
“Ten years ago everyone thought the problem was we couldn’t make a lot of the spider silk,” Yarger says. So a good deal of effort was put into taking the newly decoded gene sequence of spider silk and encoding it into other animals or bacteria to produce the silk. That effort led to transgenic silk worms that produced spider silk, transgenic goats that produced spider silk in their milk, even a Canadian company began making the synthetic silk in bulk.
“But the mass produced silk didn’t have nearly the same properties of spider silk,” Yarger explains. “The silk had the exact same primary amino acid sequence as spider silk but they weren’t folding into the same structures. They didn’t form into three-dimensional templated structures. The stuff they made wasn’t even close to natural silk.”
So Yarger and his group at ASU set out to find out why. Yarger’s expertise is determining on a molecular level the structural dynamic characterization of materials. In order to understand the materials he was about to study, he needed to understand spiders.
Spiders produce a range of different silks.
“A single orb weaving spider or cob weaving spider, like a black widow, produces six different types of silk,” Yarger says. Take a classic web with a point in the center with spokes going out from center and a spiral that goes around the spokes.
“The spokes are a completely different biopolymer from that spiral,” he says. “They have completely different amino acid sequences, the only thing they have that is similar is they both are made of proteins. How those proteins fold, what their properties are, for example, the one that is the spiral is very elastic (the elasticity of a rubber band), the one the spokes are made of are a very high tensile strength. They usually use three to four silks to make their web.”
In their latest work, Yarger’s group used a modified Brillouin spectroscopy technique to obtain a wide variety of elastic properties of the silk of several intact spider webs. Four different spider webs were studied, including Nephila clavipes (golden orb-web spider), A. aurantia (gilded silver-faced spider), L. Hesperus (western black widow) and P. viridans (green lynx spider).
The Brillouin technique uses an extremely low power laser, less than 3.6 milliwatts and the researchers record what happens as the laser beam is passed through the intact spider webs. It allows them to spatially map the elastic stiffnesses of each web without deforming or disrupting it. This noninvasive, non-contact measurement produced findings showing variations among discrete fibers, junctions and glue spots.
“Brillouin spectroscopy can tell you, in incredible detail, the elastic modulus or elastic tensor in the material,” Yarger explains. “It gets at a very fundamental level of why materials have specific mechanical properties available. Whether it is its ability to handle shear, its ability to handle longitudinal stress, what its stress-strain curve looks like, what its young’s modulus is, its toughness, all of the mechanical properties and physical properties of the material are almost all dictated by the elastic moduli.
“What we obtained is its complete elastic moduli,” he adds. “So we know how it will handle any stress or strain in any direction. That is something no one knew before,” Yarger explains.
“When we figure out what structure allows the silk to have certain mechanical properties, then we really have everything we need for producing artificial fibers,” he adds. “It will tell us that with this structure, it will produce this strength, this toughness, this elasticity.”
With 40,000 spider species, each producing numerous different types of silk, Yarger sees no end in sight to his work.
“We’ll never run out of interesting things to look at,” he says.