LONG BEACH, California -- Spider webs may be a nuisance in your home, but it turns out the silk used to form some webs is comparable to Kevlar in strength and may protect soldiers on the battlefield one day, says UC Riverside biology professor Cheryl Hayashi.
Hayashi received a John D. and Catherine T. MacArthur “genius” grant in 2007 for research into the properties of spider silk and methods for replicating it. Those properties, Hayashi says, could make spider silk perfect for use in body armor, surgical sutures and field bandages, or even aerodynamic body suits.
Spiders weave their webs from proteins secreted from silk glands in their abdomen. What starts as liquid becomes a solid fiber as it's extruded from spinnerets. Some spiders make only one kind of silk. But many of the 40,000 categorized species of spiders possess a toolkit of different glands to make silk for various functions -- such as constructing egg sacks, trapping and wrapping prey or creating a dragline to hang from the eave of a roof.
Each type of silk is composed of a unique set of proteins that when combined make spider silks some of the toughest natural fibers known. Though less than a tenth the diameter of human hair, certain types are several times stronger than steel.
In advance of her talk at the Technology, Entertainment and Design (TED) conference on Wednesday, Hayashi spoke with Wired.com about the killer strength of the Black Widow's silk and the role that tobacco plants and goats are playing in replicating silk.
Wired.com: Just how strong is spider silk and what makes it so strong?
Cheryl Hayashi: Some spider silks are really strong, but not all of them are. The ones that are really strong can actually rival steel and approach the tensile strength of Kevlar. Thus far, the dragline silk seems to be the strongest. That's the silk that a spider uses as their trailing safety dragline. It's also the same silk that's used as the framing and radii of an orb web, that classic wagon-wheel shaped web. Black widow dragline silk is one of the strongest ones.
Wired.com: Scientists have tried to mimic this in a lab but when they squeeze the same liquid proteins out of a syringe it doesn't work. Why doesn't it work?
Hayashi: When you watch the spider spin a web they pull it out with their leg -- they touch a leg to the correct spinnerets and then they yank -- it's sort of like a painter's palette you dab your brush into whatever color. So the idea of shooting out silk proteins through a syringe was that perhaps it was the narrow aperture and the force of pulling it was maybe all it took. You can get a fiber that way, but unfortunately it's thin and kind of brittle. So there's something about this whole machinery that the spider has that makes it into the fiber.
Wired.com: How many varieties of spider silk have been categorized at this point and what distinguishes them?
Hayashi: Twenty years ago there was just one. The very first paper on a spider-silk genetic sequence appeared in 1990. Where we are now, we have dozens and dozens of silks from different species and different kinds of silks that have been characterized. For the most part it appears that ... our ancestral [spiders] probably had one silk gene ... we've had this whole series of gene duplications and specializations that lead to the wonderful molecular diversity we have today.
Wired.com: Why would we want to switch to spider silk as opposed to silk from silkworms?
Hayashi: Silkworms just make one kind of silk. They have one pair of silk glands.... For the most part ... the spider silk just seems to be stronger than silkworm silk. There are great applications for silkworm silk but there's this whole variety and new combinations of stretchy, stiff and strong that we can get from looking at spiders.
Wired.com: Why are most spider webs shaped like a wheel?
Hayashi: The orb web is actually a shape that evolved a long time ago and since it's evolved many descendants of the orb-weaving ancestor have modified it and even lost it. Black widow spiders have orb-weaving ancestors. But clearly a black widow makes a cobweb. So since that wagon wheel design evolved over 100 million years ago, some species have maintained it, others have altered it and some have completely lost it. Most orb webs are spun in midair and capture flying prey. A black widow cobweb is usually near the ground so they're going to catch a lot of pedestrian or walking prey.
Wired.com: What is the biggest prey that you've seen a web capture?
Hayashi: Some orbed webs spun by a spider called Nephila have been documented as being able to capture small birds. With black widow webs, there's lots of documentation of them catching small reptiles, little snakes, little lizards can get stuck in them and the spider will wrap them up in silk and will feed on them.
Wired.com: What potential uses are there for spider silk and what's the advantage to using it over other materials?
Hayashi: The kinds of products that are possible are bulletproof vests or other kinds of body armor or equipment armor. Another one would be new varieties of high-performance ropes, where you could have a rope that's thinner but might be just as strong as ones we have today.... You could use them for sutures, implants -- anywhere where this kind of toughness and flexibility could be an advantage.
Other materials might be very strong, but ... tend to be very stiff. Spider silks turn out to be very strong but ... they have a fair amount of stretch to them. Spider silk also is biodegradable. Many orb-web spinning spiders actually recycle their silk. They eat it. So silk could make for a very green product. Spider silk is also spun under benign ambient room-temperature conditions. That's really different from something like nylon, which is a petroleum-based product that's produced under high temperature, high-pressure conditions. Also, Kevlar has great attributes but it's essentially inert -- so if you want to dispose of it you pretty much have to incinerate it.
Wired.com: How far away are we from any of these uses and what obstacles are we facing?
Hayashi: We're several years away. The obstacle is how to economically produce large amounts of spider silk. We can't count on spiders themselves to do it. If you want to sell spider silk, even the biggest spiders start looking pretty small as a factory. Also this issue of spiders making more than one kind of silk creates a quality-control type problem. If you want to buy dragline silk, it's always possible that the dragline silk could be mixed with other kinds of silks they make.
We could purposely breed spiders for their silk, but ... spiders are very cannibalistic. There's just a handful of spiders that are social spiders.... All the others live solitary lives and they just get together during mating time and even that is precarious for the parties involved. They don't play well with others.
So the approach that people are turning to is to take the spider silk gene and move it to some more benign host, such as bacteria, yeast and even plants and farm animals.
Wired.com: How do you harvest the silk from a tobacco or tomato plant?
Hayashi: The plants look like any other normal plant.... Basically you put the tobacco leaves into a blender and [start] making a tobacco-leaf smoothie. You're just getting the protein. [Then] it needs to be artificially spun. That's the other hurdle. Bioengineers have been working on this artificial spinning ... progress is being made on both fronts simultaneously.
Wired.com: Where is the silk protein being harvested in the goat?
Hayashi: This is research done by a Canadian company called Nexium.... There's a gene called beta casein that's one of the milk proteins. Nexium took the on-off switch for the milk protein gene and fused that with the spider silk gene. So only a female goat that is producing milk will make [the silk protein]. Basically every morning you go out there with your bucket and you milk your goat. The silk protein will come out, but so will the other milk proteins, and then they have to be separated.... They engineer a special tag on the silk protein so you can have something [that identifies and separates] the silk proteins from the milk proteins.
Wired.com: What brought you to study spider silk?
Hayashi: When I was an undergraduate there was a professor who offered me the opportunity to do research in her lab and earn part-time money feeding her spider colony.... Picture something like an industrial walk-in refrigerator, except it ... was hot and humid inside like the Panamanian rain forest. The spiders weren't kept in cages, ... so I would have to go in and put fruit flies or crickets into their webs. That was just so fascinating to look up close and personal at a spider, which was something I'd never done before. Then she gave me the opportunity to be her field assistant to work on a project in the summer in Panama and that involved waking up really early in the morning and looking at spider webs, because we looked at a spider that spun a web at dawn. It was just so cool. You don’t work with spiders very long before you start noticing how important silk is to their life and just how special that is for spiders.
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