“So to speak.” And with that, he smiles a very dry Herb Waite smile.
In the meantime, industry has heard about this universal superglue, and companies like Allied Signal are hovering over Waite’s work. What intrigues them is the fact that mussel glue will stick to just about anything, probably because of its elegant bifunctional chemistry that cross-links internally while also coupling to a surface.
Once Waite had described the chemistry involved in the cross-linking, Allied Signal cloned what it thought was the gene for the adhesive protein and got E. coli to start producing it. Waite also told them that the chemistry depended on a catalyst that cross-links the protein—it converts tyrosine residues into DOPA residues, and then, along with oxygen, they turn into orthoquinones, which are the basis for cross-linking. Though he knew what the catalyst did, Waite still wasn’t sure what it looked like. Instead of waiting for Waite to climb that mountain, Allied Signal scientists simply used a common, off-the-shelf catalyst—one that is extracted from mushrooms. “They missed the whole point,” says Waite. “The mussel’s catalyst is specially constructed to first help with the cross-linking and then to become a structural part of the glue. That’s why it’s packaged in a one-to-one ratio with the protein. You can’t use a nonstructural catalyst and hope to get away with it. You’re ignoring the crux of the puzzle.”
Sure enough, after years of cloning effort, Allied Signal produced an adhesive protein that wouldn’t adhere. “It converted DOPA to quinone but it didn’t lead to coating or glue. All we got was a brownish flocculent [a woolly mass at the bottom of the beaker],” says Ina Goldberg, who worked on the research. They decided they couldn’t wait for the catalyst to be identified fully, so the research folded.
In the meantime, a group in Massachusetts called Collaborative Research is simply chopping up the mussel foot and selling the purified protein as a cell-and-tissue adhesion product called Celltak. It’s not a universal glue yet, but it does work well to coat petri dishes and entice cells to settle down and grow outward in a nice sheet. Word has it that Collaborative Research is about to start marketing a product similar to Celltak that is derived from recombinant DNA. It will sell the plates itself, precoated. In the meantime, a company in Chile is chopping up large cholga mussels—they can be as large as a shoe—and separating out the protein to sell as a petri dish coating.
Using the raw precursors in the foot is one thing, but doing what the mussel does with those precursors is another. No one has yet duplicated the process by which the mussel builds its fiber, its plaque, its adhesive, or its sealant. Waite thinks we may have better luck, in the short term, looking at yet another of the mussel’s many talents. It seems that the same adhesive protein that binds so adeptly to metals in rocks or on stanchions also clamps on to heavy metals that the mussel ingests in its food. In this way, the mussel stores the toxins in its byssus rather than in its body, and when it moves to greener pastures, it jettisons the byssus and leaves the heavy metals behind.
The U.S. Environmental Protection Agency (EPA) is interested in the record of metal accumulation that’s left in that cast-off byssus. In its program called Mussel Watch, the EPA harvests byssus leftovers in the Chesapeake Bay, and analyzes them over a period of time to see if metal residues in the bay are trending up or down. Waite can envision cloning the gene for that protein (so we can make massive quantities), and then using it as a screen in a filtering system. The protein filters could be installed on ships, dragged for a time, and then analyzed for metal residues.
“It’s only one of the many practical inventions that could come from the mussel’s repertoire,” says Waite. “As we perfect our technologies, I’m sure we’ll run across other processes and designs that edulis has already worked out. The adhesive is only one patent among many.”
And edulis, of course, is just one bivalve among many, one invertebrate in the ocean among many. Suddenly I wish it were Herb Waite we were cloning, instead of just proteins.
For the reasons that Randy Lewis listed, there are not many like Waite who have decided to tackle natural materials. Although many engineers admit that there’s merit to this inquiry, the obstacles make it a long-term, hair-pulling endeavor. “You have to be sure the material is really worth it,” says Lewis. One material that has won over many researchers, including Lewis, is a 380-million-year-old fiber with a twenty-first-century future. Spider silk, says the University of Washington’s Christopher Viney, is the stuff that dreams are made of.
Along Came a Spider
It’s a steamy 80 degrees F. in Christopher Viney’s Seattle lab, in deference to Tiny, a six-inch-long golden orb weaver spider (Nephila clavipes), who is now flipped on her back, dining on crickets while being silked. A gossamer thread issues from her enormous abdomen at a steady clip, wound by a motor onto a revolving spindle. In this session alone, Tiny will donate about one hundred feet of “dragline,” a specialty silk designed for rappelling from drop-offs and framing the spokes and perimeter of her web.
Dragline is only one of six silks that this eight-legged factory can produce, each one mixed in its own gland, extruded through its own spinneret, and endowed with its own chemical and physical properties, all of which the spider needs to survive. As the late arachnologist Theodore H. Savory once remarked, “Silk is the warp and woof of the spider’s life.”
Many spiders begin their lives as eggs swaddled in silk and take their first trip via a thin strand that catches on air currents and “balloons” them to new, distant homes. When hunger strikes, some spiders spin a nearly invisible snare, while others spin dense sticky sheets that snag insects the way flypaper does. Still others dispense with web spinning altogether, simply extruding a single silken strand with a sticky ball attached. “The ball is hurled, gaucho-style, at insects flying by, which are then lassoed in and calf-roped,” writes entomologist May R. Berenbaum in her book Bugs in the System. Silk also figures prominently in the sex lives of spiders. In courtship, silk may be laced with pheromones (sex attractants), like a handkerchief sprayed with cologne. Once the wooing has worked, males may spin more silk to immobilize the female (who is just as likely to eat her suitor as to mate with him). Still not wanting to get too close, he deposits his sperm into a special little package of webbing, which he inserts into the female. Even in death, writes Berenbaum, spiders’ lives are tied up in silk. Certain species of spiders are known to wrap the remains of a dead compatriot in specially woven shrouds.
Lately, this mysterious material has also become central to the lives of a small cadre of materials scientists. As Christopher Viney drops another cricket Tiny’s way, he seems more surprised than I that his career has come to this. “I’m a metallurgist!” he says, feigning defensiveness. “Really! I’m a licensed physicist! I haven’t taken a biology class since high school!” I begin to pick up some of the paraphernalia festooning his room—a rubber spider, macramé spiderwebs, a can of slug chowder (“Please don’t add salt,” the label cautions), biology journals, an article that refers to him as the Spider Man. “OK.” He throws open his large hands and shrugs. “So I went astray.”
“Astray” began in high school in South Africa when Viney had a biology teacher who was also a museum curator. “He veered wildly off the syllabus, regaling us with stories about cracking the DNA code and other exciting developments going on at the moment in science. His enthusiasm was absolutely infectious. As a result, when I applied to Cambridge, I actually did better on my entrance exams in biology than I did in physics and chemistry, which was what I wanted to go into. I eventually wound up studying metallurgy in the Natural Sciences program, which was the most interdisciplinary option available. I didn’t learn a thing about welding, but I did learn about atoms and molecules.”
One of the most important classes Viney took was an elective that taught him a skill he would later use while surfing between disciplines: crystallography. Crystallography is the study of how organic and inorganic materials, under certain conditions, assume very ordered sh
apes and structures called crystals. The atoms in a crystal line up in predictable spacings and stay that way, giving you something like three-dimensional wallpaper, with a pattern that repeats itself in all directions. A liquid has a much more random arrangement of molecules. There is no pattern to help you describe or predict exactly where the molecules are.
In between the order of a crystal and the disorder of a liquid is a material called a liquid crystal, which has some qualities of both. It’s a liquid with its molecules arranged in orientational but not positional order; that is, the molecules are all aligned in some dimension—they’re facing the same way—but they aren’t positioned in a predictable pattern. Though Viney didn’t know it at the time, his early fascination with these semi-ordered crystals would lead him directly into Tiny’s web.
“Actually, it all started one Saturday night while I was on the couch reading dirty physics magazines,” he laughs. “I came across an article by Robert Greenler [physics professor, University of Wisconsin-Milwaukee, and president of the American Optical Society] on why you can see rainbows in spiderwebs at dawn and dusk. It combined optics, which I love, with silk, which I knew very little about. As it turns out, no one else did either. We’d been cultivating silkworm silk for four thousand years, but when Greenler needed the refractive index (a very common measurement) for spider silk, he had to guess at it.
“This made me curious about the refractive index of spider silk. I did a test and realized that it was very high. Usually, a high refractive index points to some sort of crystallinity, and that’s just what we found in spider silk—small crystallites embedded in a rubbery matrix of organic polymer. Somehow the spider had learned to manufacture a composite [two types of material in one], three hundred eighty million years before we decided composites would be all the rage!”
As a metallurgist, Viney knew that this unusual structure must impart an equally unusual function. Sure enough, the stellar properties of spider silk are enough to make materials scientists suspect typos. Compared ounce to ounce with steel, dragline silk is five times stronger, and compared to Kevlar (found in bulletproof vests), it’s much tougher—able to absorb five times the impact force without breaking. Besides being very strong and very tough, it also manages to be highly elastic, a hat trick that is rare in any one material. If you suspend increasingly heavy weights from a steel wire and a silk fiber of the same diameter, their breaking point is about the same. But if a gale force wind blows, the strand of silk (five times lighter in weight) will do something the steel never could—it will stretch 40 percent longer than its original length and bounce back good as new. Up against our stretchiest nylon, spider silk bungees 30 percent farther.
This energy-absorbing elasticity comes in handy when moths and other “meals on wings” come hurtling into the web at top speed. Instead of breaking, the gossamer strands stretch, giving off most of their impact energy as heat. Fully spent, the web recoils so gently that it doesn’t trampoline the moth back out. “None of our metals or high-strength fibers can come even close to this combination of strength and energy-absorbing elasticity,” says Viney. According to Science News reporter Richard Lipkin, in a January 21, 1995, article, spider silk is so strong and resilient that on the human scale, a web resembling a fishing net could catch a passenger plane in flight!
Another characteristic in silk’s favor is its unusually low glass-transition temperature. This simply means that silk has to get very, very cold before it becomes brittle enough to break easily. In the frigid temperatures that parachutes encounter, for instance, spider silk would make ideal lightweight lines. Other uses for a fiber as strong as spider silk would be bulletproof fabrics, cable for suspension bridges, artificial ligaments, and sutures, to name just a few. The question is, how would we go about packing so much function into such a small package?
Spider silk begins as a pool of raw liquid protein sloshing around in a gland that Viney says looks like “the business end of a bagpipe.” The raw silk (a liquid protein) travels from the gland to a narrow duct before being squeezed through one of the six spinnerets—minute groups of nozzles at the spider’s back end. The miracle is that what goes into the spinneret as soluble liquid protein (easily dissolved in water), somehow emerges as an insoluble, nearly waterproof, highly ordered fiber. “It’s enough to make a fiber manufacturer very jealous.”
Viney guessed that the raw silk somehow went through a liquid crystal phase just before squeezing through the spinneret. This would align the molecules and give them a jump on their ordering. To be capable of achieving the liquid crystal state, Viney figured, the subunits—proteins—would have to be “anisotropic” in structure. “An anisotropic substance is one that has a definite directional order,” says Viney. “The uncooked strands of spaghetti in a box are anisotropic. They look different depending on whether you are viewing them end on or from the side. The opposite of anisotropic would be an isotropic tangle of cooked spaghetti, which looks the same in all directions. Although most people thought soluble spider protein was isotropic, I was expecting to see anisotropic rods of some sort.”
One of the best tests for anisotropy would be to look at the raw silk under a polarizing light microscope, an instrument invented over one hundred years ago, which fewer and fewer people know how to use. Not only did Viney know the instrument, he had become somewhat of an expert, even writing a modern-day manual on its use. “The polarizing light microscope uses the same principle as polarized sunglasses. Only instead of one filter, it has two—one cuts out everything except light vibrating vertically, while the other cuts out everything except light vibrating horizontally. For most objects, this accounts for all the light passing through, so you see only darkness in the scope. An anisotropic material, however, plays with the polarization state of light.” When Viney looked at liquid spider silk, especially at the edges of a slide where it was drying, he clearly saw light coming through the filters, a sure sign of anisotropy. “In fact, according to the patterns we saw under the scope, it looked to be a rod that was thirty times longer than it was wide.”
To check his hunch, Viney consulted the protein sequencing data published by Randy Lewis of the University of Wyoming and Dave Kaplan of the U.S. Army, only to meet with more frustration.
Pop Beads and Slinkys
The amino acid sequences of raw liquid silk didn’t seem to correspond to any protein that would fold up into a rod. In fact, the repetitious sequences pointed to a protein that, while it was in the gland, was most likely to be tangled and globular, “like a ball of wool the cat’s gotten hold of.” The water-fearing amino acids in the chain were probably hiding in the middle of the ball while the water-loving amino acids hung on the periphery. This arrangement wouldn’t change until the ball was physically sheared by the spinneret.
In a way, this made sense. Globular molecules floating in water would be a good way to store the protein in the gland. When the spider twisted and scurried through its days, the globules would simply roll with the punches, and the spider didn’t have to fear “becoming constipated with its own silk” if the liquid protein somehow sheared into fiber form. But if there were only globular molecules, thought Viney, why was the polarizing light microscope showing undeniable evidence of rodlike structures?
“The mystery unraveled for me when I attended a lecture by one of my colleagues in the bioengineering department,” he says. The speaker was talking about actin, a protein that self-assembles to help form our muscles. Actin is essentially a globular protein, but the balls hook up to one another—like the baubles in a kid’s pop-bead necklace—to form a chain. As Viney looked at the cartoon graphic, something breached and leaped from his subconscious.
“There was my rod!” he said.
Viney turns on his computer and we look at cartoon depictions of his evolving theory of spider silk formation. He now hypothesizes that the raw liquid silk leaves the gland and travels through a thin duct just before entering the spinneret. As it squeezes through the duct, water is wr
ung out of the protein and calcium is added. (Calcium is what allows actin globules to hook up, so Viney thinks it may also be at work here.) The globules hook up in a pop-bead necklace, making the solution one thousand times less viscous, because the rodlike assemblies can now slide past one another. It’s analogous to putting lanes of traffic on a highway sliding past one another, versus the mess that is a laneless, lawless Manhattan jam.
Connected, aligned molecules are not only easier to push through the spinneret, they are also more susceptible to the shearing action that turns liquid protein into fiber. Because the globes are unable to roll out of the way, the squeeze through the spinneret disrupts the water-loving residues on their periphery, exposing their water-fearing parts.
“These hydrophobic parts go ‘ARRRGG!’ and cluster together as tightly as they can,” says Viney. They assume a zigzag shape, folded accordion-style into pleats. One pleated sheet stacks on top of another, as close as they can get to lock out the water. The water-loving portions of the proteins remain loose and curly at the edges, forming the springy matrix that the accordion crystal parts are embedded in.
Viney’s model has a pleasing simplicity and completeness: The globular proteins line up into a pop-bead necklace, which squeezes through the spinneret to become a silk fiber. The final product is partly flexible and partly rigid, like a reinforced Slinky. The amorphous part gives, but the stiff crystalline domains don’t give. When the fiber becomes notched, a crack or tear gets interrupted by the crystalline regions and can’t propagate. The model also explains why the material goes from being a soluble liquid to an insoluble fiber. Once the water-fearing portions of the proteins crowd together, they resist water, ensuring that the silk won’t fall apart.
Biomimicry Page 17
