Biomimicry

by Janine M Benyus

  One of his inventions is Ecolyte, the plastic that degrades into small pieces when the sun shines on it.

  “I have four times as many inventions as Benjamin Franklin,” he tells me, “and I’m pushing for one hundred.” Somewhere between here and one hundred, he may just invent a device that spins energy from the sun into a fuel that can run your car. When it debuts, it may just sneak up on his down-south competitors. To hear Guillet tell it, he and they are in parallel races with some very different ground rules.

  In the United States, says Guillet, the military approach is often employed to get really big things done, with the Manhattan Project being the model. But that won’t work this time, he predicts. “Buying high-tech lasers [like the ones that zip around Neal Woodbury’s train set] has always been a staple of solar energy research. But I don’t think solar energy devices are going to come via big-ticket approaches. I don’t think nature works at that scale.”

  We are strolling toward a French restaurant in the university district, and he pauses to pluck a leaf from one of the many trees lining the narrow road. “This is the solar energy device that everyone would love to mimic,” he says, handing it to me like a flower. “And this device doesn’t do chemistry under the concentrated, coherent light of lasers. Lasers are very intense, whereas sunlight is more diffuse—like a drizzle instead of a hard rain.”

  At this point he stops and squints up to the sun. “Even though a lot of sunlight falls to Earth, it is notoriously hard to collect. The trouble is timing. Green-plant photosynthesis requires that not just one but two photons hit the reaction centers of the two photosystems in rapid succession. This ‘two-photon event’ has to occur within the lifetime of the excited state, or the side reactions will fizzle—there’s just not enough energy in one photon to drive the process.” Statistically, no bookie in the world would put money on two photons hitting the same square centimeter of a leaf at almost the same time. Nature, of course, has taken these dismal odds and turned them into sure bets.

  Leaves do it, algae do it. Even photosynthesizing bacteria do it. They unfurl an antenna that photons can’t resist. Devoting a lion’s share of their chlorophyll, photosynthesizers spread out a receiving array of pigment molecules, about two hundred for each reaction center. Each lollipop-shaped antenna molecule turns its porphyrin ring, like the face of a sunflower, toward the incoming photons. When a photon hits anywhere in the array, it excites an electron in porphyrin to a higher orbital, and before the electron can decay back to its original orbital, the energy (not the electron itself, just the energy) migrates to an adjacent porphyrin ring poised to receive the energy. “The energy migration is like the sound waves that migrate from a struck tuning fork,” says Guillet. “Eventually, a tuning fork across the room will ‘catch’ the energy and start resonating with the same frequency.”

  In a leaf, the migrating energy funnels quickly down to its destination by being passed antenna to antenna. Having a whole array of these ringlike pigments on the lookout for energy is like having your whole roof collect rain instead of just the opening of the rain barrel, says Guillet. “In fact,” he says, “if you hold up an antenna of two hundred pigment molecules instead of just one, you are forty thousand times more likely to have a second infusion of photon energy hit the mark when it needs to.”

  To do anything close to photosynthesis—to split water for hydrogen fuel using sunlight, for instance—Guillet contends we will need that second photon infusion. “Once you wean yourself off lasers, you realize you’re going to need two photons arriving onstage at almost exactly the same time. No matter how good your reaction center is, it won’t have anything to work with unless you can harvest photons for it.” Once he faced that fact, says Guillet, he decided to let others perfect charge separation while he figured out how to make an artificial antenna.

  “I wanted to see if energy would migrate along a linear chain of light-sensitive pigments the way it does along a large array. I chose naphthalene, an organic chromophore used for making dyes and solvents, because it was related to the light-sensitive parts of chlorophyll molecules. I strung thousands of these naphthalenes together in a long repeating chain called a polymer [a string of like molecules]. It may help if you think of it as a long pearl necklace on a flexible string. When I put this in solution, it coiled up. When I flashed it with light, one of the naphthalene chromophores picked up the energy, which then began to travel, not just pearl to pearl along the chain but also hopping to other parts of the chain that were coiled nearby.” Guillet refers to the random hopping of energy as “the drunken sailor’s walk.”

  Guillet also recognized that in the leaf, nature manages to gently direct this random walk—like putting the drunken sailor on a sloping drainfield so he eventually heads toward the bottom. In the plant’s case, the “bottom” is the reaction-center chlorophylls, where the action really begins. Each step along the way, each antenna, is at a slightly lower level in the energy landscape. Heading from high energy to lower energy is like going down a slippery slope; the energy can’t travel the other way, so it gets trapped at the central chlorophylls.

  Guillet wanted to mimic nature’s trick with his single chain. “After fishing the photons out of the drizzle, I wanted to have all the energy report to a single location at the end of the chain—a basin in the energy landscape.” Once it was trapped in a central spot, he could devise a way to use the energy to make and break chemical bonds, to split water, to make pharmaceuticals, to do all sorts of chemistry.

  Anthracene proved to be a perfect basin—Guillet put it at the end of the chain, and after the naphthalene necklace was flashed with light, the spectral signature changed, signaling that the energy had moved. “That signal was a heartwarming sight. I knew immediately that most of the light energy had left the naphthalene and landed in the anthracene. To top it off, the process was also efficient—ninety-five out of every one hundred photons of light cause anthracene to light up. This ninety-five percent conversion rate rivals photosynthesis, which told us we can build antennas which are just as good as nature’s antennas are at collecting photons.”

  Now that you can trap the energy, I asked, what can you do with it? Guillet brightens here, and I sense that moment when a scientist wears his heart on his sleeve—for Tom Moore it was membrane potential, and for Jim Guillet, I think it’s chemistry in water. “Life has some very universal, common strategies—tricks that it uses across the board because they work so well. One of these is doing chemistry in water—whether it’s in a tree, a corn plant, or a brain cell—the solvent of choice is water.” We, of course, have been pursuing a different tack. When we make plastics, synthetic fibers, coatings, pharmaceuticals, agricultural chemicals, and other products out of petroleum products, we use organic solvents, which can give off toxic emissions and are hard to store and dispose of safely. Once Guillet got his energy necklace to work, he started fantasizing about making these organic solvents obsolete. “I thought, why not mimic nature and use the benign fluid of life as the medium for chemistry?”

  In his wildest dreams, Guillet began to see his polymer antenna ushering in a new era in which chemical-manufacturing plants would be truly plantlike. “There was only one problem,” he tells me. “Naphthalenes hate water.” Like membrane proteins, naphthalenes are water-fearing and won’t stay suspended for very long. His solution was to attach some water-loving molecules to the chain, giving the polymer a Jekyll and Hyde personality. The hydrophillic groups would happily mingle with water, while the naphthalenes would cluster at the center, forming a cozy hydrophobic pocket.

  This Jekyll and Hyde personality is a repeating theme in chemistry, even when the chemistry concerns laundry. It’s because of the soap molecule’s split personality, in fact, that we can get our clothes clean. Think of the soap molecule as a “magnetic” bar with a north and south pole—one pole is water-loving and one is water-fearing. Drop the tiny bars in water, and the water-fearing ends will find each other and huddle together while the water-
loving ends point out toward water. Essentially you have a spiny sphere of soap molecules, called a micelle, suspended in your washing machine. At the center of each sphere is a water-fearing “pocket” that actually attracts other water-fearing molecules floating by, including greasy-stain molecules. Once these water-fearers escape from the fibers of your jeans, it’s only a matter of time before they run across one of the spiny spheres of detergent. Sensing refuge, they dive into the center of the micelle and are washed away with the dirty laundry water.

  The same sort of drama occurs with Guillet’s new polymers. Each long, necklacelike polymer creates its own coiled mob, called a pseudomicelle, with a hydrophobic pocket in the middle. What Guillet has created is a globular antenna with a sweet spot—the energy goes to the center, and so do any water-fearing molecules in the neighborhood. When the water-fearing molecules happen to be the precursors in a chemical reaction, says Guillet, you’re ready to do chemistry in water: The precursors head for the center, where they’re zapped by the energy coming from sunlight, which either forms or breaks a bond.

  In many ways, this is what Neal Woodbury envisions when he talks about catalysts with a power pack. Guillet has created micro-reactors that float in water and act like catalysts or enzymes—“grabbing” substrates in their hydrophobic hot pocket, and using the energy from sunlight to make and break bonds. He calls it a photozyme.

  The photozyme that most of Guillet’s studies have focused on has a nickname that numbs the tongue: PSSS-VN. It’s made up of two compounds: sodium styrenesulfonate and 2-vinylnaphthalene. His first test run of PSSS-VN was in a beaker filled with water and pyrene, which is a carcinogen. As soon as he sprinkled the water-fearing pyrene into the beaker, it dove into the central pocket of the polymer coil, where it would be at the receiving end of photoenergy. When sun rays bathed the polymer, energy zipped to the center of the coil and performed extremely rapid photochemical reactions, breaking the pyrene down into less dangerous molecules.

  To demonstrate his idea to a wider audience, Guillet chose something that people care about: polychlorinated biphenyls, or PCBs. These common industrial chemicals (found in 40 percent of all electrical equipment) are now being found everywhere, even in arctic waters. The reason PCBs are so ubiquitous is that they are resistant to breakdown in sunlight. Conventional cleanup of PCBs and other pollutants is often stymied by the fact that the pollutants are present in trace amounts, spread over large bodies of water.

  Photozymes offer an ideal solution because they can scavenge out PCBs, even when they are present in concentrations of only a few parts per million, and then, with the help of light energy, they can chew off the offending chlorines from PCBs, rendering them harmless. It would work like this, Guillet explains: A PCB molecule would be attracted to the center of the micelle, and once there, a shot of light energy would cause one of its chlorine bonds to sever. The micelle would then release the crippled PCB, and another would enter. In a week or two, after half a dozen dechlorinating trips to the center, every PCB molecule would be chewed to a nonchlorinated, biodegradable state.

  Instead of tearing something down, I ask him, will we also be able to make something using photozymes? “Yes! You’d be surprised how many reactions can be carried out with light instead of heat or pressure or harsh chemicals. We’ve shown, for instance, that you can mix photozymes with the precursors of vitamin D and make it in one step instead of the several it now takes—energy courtesy of the sun. Which means, of course, a lot of energy. We figured we could make the entire annual Canadian consumption of vitamin D in a backyard swimming pool with the existing efficiency of our process.”

  With the photozyme, photochemistry becomes very specific—you get the product you want without the side or ancillary reactions that produce products you don’t want. The process can also be calibrated. You can adjust the molecular weight of the photozyme, engineer the pocket so that only certain hydrophobic compounds can get in, or match the energy levels of the antenna to particular substrate molecules so that the antenna “finds” and excites just the right substrate in a stew of molecules. Besides being efficient and using the boundless energy of the sun, the photozyme is a durable workhorse. Once you extract the vitamin D or whatever product you are making from the solution, the polymer can be used again.

  Not that “chemistry au naturel” is a complete panacea, says Guillet. It brings problems of its own, the same problems that natural organisms face. “Anytime you do chemistry in water using natural sunlight, you have to work in layers—the best light is at the top, and becomes less saturated as you work your way down. My problem—an engineering problem, really—became, how do I make sure my reactions will always be on top and exposed to maximum sunlight? I pondered and pondered this until, one day, out at my weekend place at Stony Lake, I took a walk and wound up sitting by a quiet cove. There, right in front of my eyes, was the world’s best strategy for collecting light for solar-driven chemistry. I’ll show it to you.”

  He reaches behind him for a small plastic container, then shakes its contents into my hand. Translucent plastic disks the size of hole-punches pile up in my palm. “These are Solaron beads. They’re made of a cross-linked polymer called polyethylene. Right now they’re dry, but if you put them in a liquid, they would quickly absorb it the way absorbent diapers do. To do solar chemistry with them, you first let them soak up a liquid starting material for a product such as vitamin D. You throw the loaded beads onto a pond, where they spread out in a uniform layer, soak up the sun, and do serious chemistry on the precursors. To remove the beads, you either screen them all at once or push them bit by bit across the surface with a slow-moving boom, letting new ones fill in behind to take their place. To ‘harvest’ the vitamin D, you flush the beads, then soak them in starter material again and toss them back out onto the pond.

  “In many ways it resembles farming more than industrial chemical processing,” says Guillet. “In fact, we can envision using grain handling equipment—airveyers, high-speed blowers, and silos—to store and transport the beads. In the company I’ve formed, called Solarchem, we’re already making a number of products this way. It costs us about fifty cents to cover a square meter with these tiny solar-chemistry labs, in contrast to something like photovoltaic cells which cost fifty to two hundred dollars per square meter.”

  All this time I am rolling the disks together in my hand. Finally, I take a good look at them. They are oval and slightly concave. “Can you guess where I got my inspiration?” he asks.

  I envision the tiny disks floating on the surface of water, and I can imagine how well they would pack, one next to the other, covering the entire surface. Suddenly, in a rush, I know. My quest has come full circle, and the lessons I must learn—what is a weed, what is a nuisance, what is a brilliant model of efficiency and elegance—all float to shore at once. Guillet’s inspiration is the Cheshire cat that I can’t catch, part of the ineluctable genius that surrounds us.

  “I know exactly what this is,” I tell him.

  “Incredible, isn’t it?” he says, and we smile at one another as he pours a small mountain of it into my waiting hands.

  Artificial duckweed. Patent number 84.

  CHAPTER 4

  HOW WILL WE MAKE THINGS?

  FITTING FORM TO FUNCTION: WEAVING FIBERS LIKE A SPIDER

  Though environmental policy makers have focused on the growing glut of garbage and pollution, most of the environmental damage is done before materials ever reach the consumer. Just four primary materials industries—paper, plastics, chemicals, and metals—account for 71 percent of the toxic emissions from manufacturing in the United States, according to the researchers. Five materials—paper, steel, aluminum, plastics, and container glass—account for 31 percent of U.S. manufacturing energy use.

  —JOHN E. YOUNG and AARON SACHS, authors of The Next

  Efficiency Revolution: Creating a Sustainable

  Materials Economy

  We are on the brink of a materials revolution tha
t will be on a par with the Iron Age and the Industrial Revolution. We are leaping forward into a new era of materials. Within the next century, I think biomimetics will significantly alter the way in which we live.

  —MEHMET SARIKAYA, materials science and engineering professor,

  University of Washington

  “That’s why babies’ heads are soft,” said the man riding down the escalator as I was heading up. “They haven’t completely mineralized yet.” Babies’ heads? I ran up my escalator and joined him on the down ride. He was going where I was going.

  The Materials Research Society (MRS) meeting is held every year in downtown Boston, filling three of the major hotels to capacity. Everywhere you look there are scientists—3,500 strong—carrying their two-inch-thick book of seminar abstracts in materials science, a field most of us have never even heard of. Strange, because materials science literally touches everything we touch; every object we walk on, ride in, pick up, put on, or pour from is made of a material or several different materials. Yet the people who worry about shatter resistance, tensile strength, and surface chemistry—the ceramists and glass engineers, the metallurgists and polymer scientists—are soundly unsung. I don’t know any kids who want to be materials scientists when they grow up.

  Maybe the field is just too new. Materials used to be manufactured solely by nature, and we took what we were given—wood, hide, silk, wool, bone, and stone. Eventually people learned to fire slurried sand into pots and hammer iron from the Earth. Throughout history, our progress as a people has been date-stamped by the types of materials we used—the Stone Age, the Bronze Age, the Iron Age, the Plastic Age, and now, some would say, the Age of Silicon. With each epoch of civilization, we seem to have distanced ourselves further from life-derived materials and from the lessons they teach us.

  In the vivid glow from the slide shows featured at Symposium S (the bio-inspired-materials segment of the meeting), I began to see that nature has at least four tricks of the trade when it comes to manufacturing materials: