So far, Stuckey and Morse have used only calcium carbonate (chalk), the choice of abalones. Other biomineralizers in nature (the sixty species that have been found so far) are known to work with many more exotic materials. Curious about these other materials, Peter Rieke of the Pacific Northwest Labs is going out on a ledge.
Crystal Windshields
Peter C. Rieke, mountain climber and materials scientist, takes both his recreation and his science to the edge. When I visited him at his Richland, Washington, lab, he was bundled in a three-blanket head cold that he caught while hanging against a rock face one snowy night in Yosemite National Park. The next time I saw him, half a year later at the Boston MRS meeting, he and his wheelchair were being hoisted onto a speaker’s platform that was not handicap-accessible. He had broken his neck and other bones in a climbing fall that should have killed him. When he greeted the MRS conference crowd with the customary “I’m glad to be here,” he paused a beat and then added, “believe me.”
Like Morse, Peter Rieke is also trying to grow crystals on a thin film, but instead of using L-B films, he’s trying lab-made films called SAMs, or self-assembled monolayers. Instead of being perched on the water’s surface, SAMs are films that coat glass slides at the bottom of a tray of solution. Instead of adding wallpaper to the film the way Morse and Stuckey’s method does, the charged chemical groups in SAMs are part of the film itself. That gives Rieke the ability to play with SAMs the way a mosaic artist plays with tile. “When we create the film, we can place our functional groups wherever we want them, presenting a mosaic of positive or negative charges to the ions,” he says. The ions touch down on these landing sites and crystals bloom from them. “Ultimately, we’ll be able to grow several different types of crystals on the same patterned film.”
Though Rieke’s work takes its inspiration from the organic templating of seashells like the abalone, he admits that it’s not nearly as complex. “It’s important to remember that with thin films, we’re still working in only two dimensions,” he says. “Whereas nature builds a whole apartment complex between the abalone body and the outer shell, we’re just building a crystal sheet—like the braided rugs in those apartments.”
In Rieke’s lab I see some of the first experiments, which, despite the groundbreaking work that went into them, are deceptively humble-looking. They are simply glass microscope slides that have been dipped in a coating of polystyrene substrate, the same stuff used to make squeeze bottles, bottle caps, and drinking glasses. Rieke uses polystyrene as his substrate because it’s a polymer (a repeating chain of styrene molecules), analogous to the biopolymer sheets that mollusks use. He’s “decorated” the polystyrene with sulfonate groups, similar to the acidic sulfate groups associated with nucleation in mollusks. In his spare time, Rieke has experimented with other substrates and a half dozen functional groups associated with other hard-bodied creatures. The mineral ions he’s paraded past these groups include lead iodide, calcium iodate, and iron oxide, in addition to good old calcium and carbonate.
In the real world, these humble-looking thin-film coatings could have a variety of applications. General Motors funds part of Rieke’s research because it is interested in hard, transparent coatings for the windshields of its electric cars. “One of the reasons we aren’t driving electric cars,” says Rieke, “is because we can’t find a way to seal in heat and air-conditioning, which escape through the lightweight plastic windows. Right now it takes too much energy to keep the cars comfortable and power their engines. If we could find a way to insulate the windows with a thin film, it would remove a big stumbling block from that technology.”
Car companies also need coatings for their drive gears, preferably an abrasive substance that is as thin as a second skin but will not wear down. Coatings now applied to these many-faceted gears are essentially spray-painted on in a technique called “mass transfer limited.” It is literally limited in that the spray doesn’t reach all the nooks and crannies of the gears. “What would be ideal,” says Rieke, “is if we could dunk the plastic parts in a solution of organic molecules which would adhere to every nook and cranny, and then dunk the part in a concentrated solution of precursors for an abrasive mineral. The organic molecules would act as attractors—nucleating sites for crystallization—and you’d wind up with a highly dense, perfectly oriented and ordered thin film.” The same sort of film could be used to line featherweight plastic fuel tanks and parts for electric cars.
Besides abrasion-or corrosion-resistant protective coatings, whisper-thin films are also coveted by industry for electronic, magnetic, and optical devices in which precise and tiny crystals are needed to store, transport, or relay signals of light or electrons. Because they are so thin, the films could be built up into multilayered devices composed of a semiconductor layer, an oxide dielectric layer, a magnetic layer, or a ferroelectric layer for electro-optical devices. Depending on what kind of mineral you use, you could also use the crystallized coating as a sensor, a catalyst, or even an ion-exchange device.
A simple, two-bath dunking—first in template molecules, then in a bath of crystal precursors—would be a liberation from today’s slow and expensive methods of producing high-density precision films. “Nature’s idea of mineralization templated by proteins would revolutionize thin-film technology,” says Rieke. Even something as simple as an audiocassette or a computer disk could be vastly improved. Iron-oxide crystals, common in magnetic bacteria and in gastropod teeth, are what hold the zeros and ones in our magnetic media. Right now, they are essentially piled onto the surface in disarray. Lassoing and roping these crystals into alignment with protein templates would allow more crystals to fit on a disk, holding more bits and bytes.
Ultimately, Rieke’s team hopes to build a catalog of mineralizing systems, showing which crystal grows on which substrate in which concentration. “We’re learning the principles of crystallization as we go along,” he says, “but it’s still very much a black art. It took us three years of fiddling to learn the iron-oxide system, but now that we have the recipe down, no one else will have to reinvent it. In the future, materials engineers won’t have to start from scratch every time they need a two-dimensional coating. They’ll just buy a kit and read the instructions: ‘Use this SAM in this concentration of this solution for this long.’ ”
Three-Dimensional Crystal Containers
But why stop at two dimensions? Stephen Mann, a biomineralization expert in Bath, England, is re-creating three-dimensional protein sheathing, using tiny balloonlike compartments to mineralize small particles. His inspiration comes from the vesicles that living cells use to trap ions and precipitate out minerals. One-celled magnetotactic bacteria, for instance, produce incredibly tiny, defectfree crystals wrapped in organic membranes. Engineers can think of any number of uses for such small, perfectly formed, independent crystals. For instance, when you use magnetite as a catalyst to speed up chemical reactions, you would rather have a million small, separate spheres (with a lot of surface area exposed to the reaction) than a hundred large spheres. Unfortunately, without being pre-organized in balloonlike separators, most processed magnetite winds up sticking together because of the magnetic force between the particles.
To remedy this, Mann has followed the bacteria’s lead, successfully growing crystals in lab-made vesicles. He’s even built his organic balloons in various sizes and shapes, showing that curved, organic surfaces can also help us shape tiny single crystals with precision. Recently, Mann has utilized an even smaller compartment formed by a single cagelike protein called ferritin. (Ferritin is the protein that sequesters iron oxide in our bodies, thus keeping rust out of our cells.) Growing a crystal inside one protein would take templating to a new high (which, sizewise, is a new low).
Another way to “grow” a three-dimensional crystallized structure is to begin with a quivering block of jellylike polymer studded with inorganic minerals. As the jelly sets, the minerals inside crystallize, and the result is a composite—a flexible polymer stiffened by s
warms of inorganic crystals. The combination of hardness and flexibility, say the materials scientists, would come in handy in everything from aerospace to appliance design. Imagine a living-room window that is as rigid as glass, yet able to bend and bounce back when assaulted by your neighbor kid’s baseball.
Right now, we can create composites only by placing the fibers or crystals layer by layer, which is slow and expensive; crystals growing on their own inside polymer would allow us to create readily moldable composities (like car bodies) with a dramatic reduction in production costs and pollution.
Fabbers
What if you want a three-dimensional material that has an even more exacting crystalline order? What if you want a whole computer monitor, say, made of crystals in brick-wall architecture? That’s a job for 3-D templating, say the scientists, using proteins that will self-assemble into a scaffolding. In the meantime, for those who still want to put nature’s blueprints to work, there’s a halfway technology that could give us a taste of future complexity. It’s called free-form manufacturing, and with the aid of computers, it allows us to build 3-D objects from the ground up, one layer at a time.
Engineers have been using this technology for years to build plastic prototypes from design sketches. They take a design, digitize it in three dimensions with CAD (computer-assisted design) software, and then electronically slice the design into very fine cross-sectional layers, like those you see in magnetic resonance imaging (MRI) scans. Each slice is a complete blueprint for that layer—including its dimensions and what material it should be made of. The software sends these coordinates to the ink-jetlike heads of a rapid prototyper, or “fabber,” which will “print” the object from the ground up, layer by layer, until a three-dimensional finished product is built. Instead of ink on paper, the heads shine a laser beam onto the surface of a vat of a liquid polymer that hardens in the presence of a laser. Here’s a description from the “fabber page” on the Internet.
To print, say, a coffee cup, a fabber trains its computer-guided laser beam onto a vat of the liquid polymer. The laser first scans a solid circular region on the surface of the liquid, hardening it into a disk—the base of the cup. Then that base, which rests on a platform in the vat, is lowered about five thousandths of an inch, just enough for a thin film of liquid polymer to wash over it. The laser traces a hollow circle over this liquid, forming the bottom layer of the cup wall, which fuses with the base. Layer after layer, the laser traces the cross section of the cup, building it from the bottom up—including the handle. By printing one cross section at a time, a fabber can build objects that are much more complex than a coffee cup.
For the biomimics who study shell-and teeth-building technologies, the fabber’s moving-front technique is familiar. Nature’s twist is that instead of just one material, two or more may be used—a layer of chalk separated by a layer of proteins, for instance. Paul Calvert is now working with a company in Arizona to retrofit a fabber so that he will be able to build bio-inspired composites of more than one material.
Paul Calvert loses his normal nonchalance when he talks about the possibilities. “A layer of templating proteins may be laid down, for instance, and then along that front, a layer of mineral precursors could be laid down. We could use ink-jet heads to deliver the material. Crystals could be allowed to grow naturally, or they may be treated in some way to accelerate growth. The next layer could be composed of an entirely different mineral.” Even within a layer, a mixture of two or more materials could be used, allowing you to blend from one material to another in a gradient. “A gradient of one material to another makes for a stronger joint and eliminates the need for glues or snaps. Nature uses blurred boundaries all the time, avoiding abrupt interfaces, which are crack prone and require some kind of fastening together,” says Calvert.
This kind of layerwise growth should also give engineers the ability to vary the dimensions within a part, just as bone varies in orientation and density throughout its length, becoming thicker and thinner in places. Using the fabber, we could conceivably follow nature’s design plans much more closely than we have ever been able to do.
For now, Calvert and his company have not attempted anything more complex than some rings and cylinders made of two materials, and once, a high-tech Easter bunny figurine for an April display. Easter bunnies built layer by layer in 3-D might not constitute a materials revolution, but airplane wings or car bodies just might. Imagine being able to make light, strong composite skins for solar-powered cars without the use of high heat or chemicals. Or being able to fashion a spare part for your car when you are in a remote area, using common materials like chalk or sand. Sound like Star Trek? Stay tuned. With nature’s blueprints and Paul Calvert’s machine, science fiction might just materialize into fact.
THE SOFTER SIDE OF MATERIALS SCIENCE—
HIGH-TECH ORGANICS
Of all the materials made by biology, minerals star in only a portion. Life has also created a bounty of resilient, organic materials—skin, blood vessels, tendons, silk, adhesives, and cellulose, just to name a few. At the MRS meeting, the fans of these organic tissues gave the biomineralists a run for their money.
Not that the two groups were far apart when it came to nature’s trade secrets. Like biomineralized structures, organic materials are also hierarchically ordered. Their structure is just as faithfully coupled to function. They are templated to order, and they are self-assembled at life-loving temperatures and pressures, with no toxic aftertaste.
The only difference between the soft and the hard is where the precursors or building blocks originate. When a bombproof covering is required, inorganic minerals from the Earth come to the rescue. But when something more flexible is needed, life can build every bit of it from organic (carbon-based) building blocks. Here, proteins become more than directors or scaffolds; they actually are the material.
To find this softer side of materials science, I traveled to the salty tureen of life on the other coast to see how a small blue mussel uses a waterproof adhesive to tether itself to solid objects in turbulent tides. University of Delaware researcher J. Herbert Waite, tenacious in his own right, is happily stuck on Mytilus edulis. After thirty years of study, he’s begun to pry loose the secret behind the real, live superglue made from protein.
Byssus as Usual
“We have Batman and Spiderman,” yells Herb Waite at the top of his voice. He is yelling because the Atlantic breezes in December are fierce and we are out on a pier in the marsh grasses, kneeling beside a rusting fishing boat owned by the University of Delaware’s Marine Sciences lab. “But mussels are every bit as talented. I can’t believe we have no mussel superheroes.”
Waite wears a British driving cap, and a full beard and broad chest à la Hemingway. He is reeling up something heavy, pulling hand over hand on a thick and slimy rope. Finally the dark waters part and a four-foot-wide cage comes up, its sides encrusted with navy-blue bivalves called Mytilus edulis, common both to salt marshes and appetizer menus. (I am glad now that we declined to order them at the restaurant where we had lunch. We were talking too highly of them to start dipping them in drawn butter.)
“How do you suppose they are hanging on?” he yells, and I realize I don’t know bivalves well. I look closely and begin to see hundreds of small translucent threads, extending like plastic tethers from the bivalves to the cage.
“Those tethers are called byssus [pronounced biss-us], and they’re more amazing than anything you can imagine. There’s four or five patents right there that industry would love to have.” Thankfully, Waite agrees that it’s too cold to be standing here staring at gaping bivalves. We drop the cage and run back to the Cannon Hall marine lab, a building that looks for all the world like a ship gone aground. It even has porthole-shaped windows.
Once on board, we head for the tanks, where Waite has hundreds of M. edulis growing. Through the glass, we get a close-up of the translucent threadlike filaments—about two centimeters long—extending from the soft bod
y. At the end of each filament is a tiny disk, called a plaque, attached to the glass with a dab of natural adhesive.
Waite sticks his hand in the tank and dislodges a few mussels from their tethers so we can watch them create new ones. “When a bivalve wants to settle down somewhere in the tidal zone to feed, it sticks out its fleshy foot [which looks more like a tongue] and creates one of these thread-and-plaque-and-adhesive combos,” he says. The whole thing is called the byssus complex, and its manufacture is nothing short of fantastic.
The fleshy foot presses tip first against the attachment site. Specialized glands secrete collagen protein (the same protein that’s in our tendons) into a longitudinal groove in the foot that acts as a cast or mold. The thread and plaque self-assemble and harden in the groove, and then an adhesive gland near the tip of the foot squirts adhesive protein between the plaque and the surface. The entire process, including curing of the adhesive, takes only three or four minutes.
Depending on the shear of the waves, a bivalve may put out two or three more tethers, all directly opposing the stresses. Once it’s staked down, it can gape open its shell and do the filter feeding that makes turbulence a friend. Tidal flows are like a conveyor belt, sweeping in food and sweeping out wastes. Even gametes—reproductive cells—are delivered and swept away by the tides, enabling mussels to date and mate over long distances. With byssus, says Waite, mussels build themselves an anchor, a lifeline, and a niche.
It’s no different from what we do. “Nature invents and we invent. In fact, I think that humans and all other life-forms have been evolving toward similar points, but other organisms are simply farther along than we are. They have already faced and solved the problems we are grappling with. For instance, edulis, wanting to eat in the tidal zone, had to manufacture a glue that could stick to anything underwater. We know how tough that is, because our adhesive industry has been struggling for years to come up with an adhesive that can work in moist conditions and stick to anything. It’s still out of reach. Mussels are light-years ahead of us.”
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