Futuristic Materials Inspired By Biological Counterparts

March 2000
By Robert K. Ackerman
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Scientists are finding that the Darwinian natural environment is the ultimate free market for selecting effective structures.

Researchers are tapping millions of years of biological evolutionary experience to develop the next generation of materials. This research, known as biomimetics, aims to incorporate properties unique to nature into manufactured devices.

While one key discipline of biomimetic research focuses on developing replacement parts for the human body, researchers in other areas are pursuing advances in sensors, structural materials and functional devices. This may lead to more efficient sensing systems, stronger and lighter manufactured items, and new machines that open up a host of undiscovered applications.

Biologically based sensor systems could detect invisible substances such as airborne pathogens or trace chemical particles. Fabrications based on human bone structure could offer greater strength and flexibility at less weight than the most advanced composites. Autonomous vehicles that mimic bees, butterflies or even cockroaches could provide improved versatility and mobility above or amid rugged environments.

Throughout the past 10 years, a general recognition has emerged that engineers can learn a lot from living organisms about how to make materials and devices, notes Dr. Mark Alper, head of the biomolecular materials research program at Lawrence Berkeley National Laboratory, University of California, Berkeley. Alper, who also is deputy director of the materials sciences division and an adjunct professor of molecular and cellular biology at the university, explains that this knowledge may include adapting molecules or structures from living organisms for use. It may also include altering structures or processes to improve activities or solve persistent problems.

The key to unlocking biology’s secrets is a two-step process, he says. First, researchers must understand the structure and how it provides the mechanical properties observed in nature. Then, experts must determine how to discover or synthesize materials in laboratories that effectively substitute for the living material’s functions and properties.

The potential applications for these advances could reach far beyond the imagination, Alper offers. “To limit your thinking is as foolish as to be hyperbolic in your expectations,” he maintains. “The exciting things in science are the things that you don’t even really comprehend or expect, and then somebody figures out how to do them.”

Much of this work is cross-disciplinary, explains Dr. Alan S. Rudolph of the Defense Advanced Research Projects Agency (DARPA), Arlington, Virginia. Rudolph, program manager in the agency’s Defense Sciences Office, notes that this includes basic areas such as physics, chemistry and biology as well as specific disciplines such as materials science, electrical engineering and computer science.

While biomimetic research has accelerated in recent years, some work in that arena is not new. The concept has been part of chemistry research for about 40 years, and combinatory chemistry applications already are reaching the marketplace in pharmaceuticals, biological systems and synthesized organic materials.

Approximately 15 years ago, scientists realized that this same approach could be applied to other materials. Many of these advances model the methods used by living organisms to manufacture a broad range of materials. In some cases, scientists have improved on biological manufacturing methods, and this approach likely will have an impact on the materials manufacturing industry, Alper says.

Rudolph adds that many laboratories are generating “a wealth of information on living systems” that includes knowledge on how living systems transduce information and assemble materials. Functional genomics are providing the blueprints for the genetic architecture of these biological systems. This in turn is enabling scientists to understand the genomic basis for the survival of these biological organisms in a particular environment, which lays the groundwork for using the same principles in manufacturing.

Structural material researchers are examining seashells as well as human elements such as bones, teeth and cartilage. These have properties that have thus far eluded synthetic reproduction. Learning how the body builds these inorganic/organic composite body parts soon will lead to nonbiological composites that mimic their properties of lightweight strength with flexibility. “We can make materials that are better in any one particular mechanical property, but the combination that bone has is really exceptional,” Alper warrants. The issue is understanding what aspects of bone give it these properties and determining how to use chemicals to provide the same properties to develop materials for real-world applications.

Dr. Paul Calvert, materials science and engineering professor at the University of Arizona, Tucson, explains that bone is a mineral-reinforced material, but it is not continuous fibers. Instead, it is reinforced by little ribbons of hydroxyheptide. These ribbons are so thin—only a few nanometers thick—that their ratio of length to thickness is substantial. Applying these kinds of reinforcements to synthetic composites could lead to stronger automobile components that replace far heavier steel parts, he allows.

A substance that offers high aspect ratios in small dimensions could enable production of a material that is as moldable as plastic but 10 times as stiff and five times as strong. He adds that recent developments in carbon nanotubes seem promising, but their surface chemistry may preclude getting them into suitable resins, and their technology is very expensive.

Calvert’s specialty is polymers and composite materials. One of the problems that existing ceramic materials have in fulfilling their promise is limitations in their toughness, he notes. Biological architectures, on the other hand, feature deliberately layered structures that localize the effects of any small impact, crack or other damage. Being able to reproduce this capability in ceramic materials would allow for their use in applications such as internal combustion engine blocks, for example.

Another key aspect of biological structure is that physiological systems lack screws or glued joints. Changes from one material to another tend to occur over a graded interface. Mimicking this technique of joining two dissimilar materials can avoid the problem of high stresses that emerge at the point of interface. Being able to make the gradation from one material to another—such as from bone to cartilage—by a slow, progressing change reduces the joint’s vulnerability to damage.

In the nonhuman realm, silk is generating substantial interest among researchers. The radial strands of a spider web are particularly strong and tough, Calvert notes. The U.S. Army Natick Research, Development and Engineering Center in Massachusetts is pursuing research into this area. One possibility is to use bacteria to clone spider silk in quantity. A key could lie in understanding how proteins work as structural materials, he relates.

DARPA’s Rudolph notes that his agency is working to capture the multifunctional properties of organic skin. Other programs are examining dynamic elastics such as the cross-linking of fibrous materials in sea cucumbers. These marine invertebrates can rapidly extend their lengths 400 percent, and scientists seek to understand and apply the process to materials.

One drawback, Calvert observes, in adapting biological structures to everyday applications is temperature tolerance. Many key artificial materials are developed for a wide range of temperature extremes. Biological substances, on the other hand, tend to function well only at room temperature and in wet environments. Researchers are considering several possible solutions, including layered structures or ceramics with interfaces that provide crack deflection.

Most synthetic materials are fabricated using thermal processes and molds, whereas biological processes are chemical. Calvert’s research team is exploring building a shaped host polymer into which reagents would be introduced. These reagents would precipitate growth or reinforcement inside the polymer. Combining this technique with rapid prototyping approaches allows engineers to build large objects layer by layer—just as a tree, or even the human body, assembles its structure.

This technology also has the potential for changing design and manufacturing techniques. Calvert likens it to the difference between a printing press with its mass production focus and a desktop computer with a laser printer. The mass press is designed to produce large quantities of identical objects for storage and shipping as needed. The desktop publishing suite, however, generates small quantities of customized documents. The biological process is a local distributed production that could change the way spare parts are built and inventoried. The fabrication unit might take the form of a sophisticated type of inkjet printer that “writes out” a series of layers.

Layered structures form the basis of another promising biomimetic field. Calvert relates that biological structures tend to integrate functions rather than develop them separately as is done with internal computer components. One thrust of biomimetic engineering is to develop devices that are fabricated as fully integrated systems. This would lead to manufactured items that, for example, are greatly resistant to mechanical forces such as vibration and impact.

Calvert explains that this approach is similar to that of microelectromechanical systems, or MEMS. Both feature integrated electronics and mechanics. Where MEMS are items that combine functions, however, the biomimetically layered system differs in that the electronics are an integral part of the greater structure.

Requirements for this construct call for thick layers that are built quickly. The electronics, however, must be finely scaled. This tends to indicate the need for a combination system for the two divergent processes.

A logical step in fabricating integrated systems is to embed sensors, including piezoelectric and optical technologies. Calvert foresees a system that performs electronics functions and is equipped with a large number of sensors on its surface. This object would be much more aware of its environment, he notes, in the same manner that animals are equipped with a host of different sensors that simultaneously provide information to a nerve center.

The next step would be to equip this sensate system with muscles to provide mobility, Calvert offers. “There really is, at the moment, no material in the synthetic world that does the job that a muscle does,” he declares.

This threefold goal—develop materials into which electronics can be functionally integrated, embed large numbers of sensors into the exteriors of these devices, and build artificial muscles that enable mobility—would lead to artificial objects that act like animals.

The University of California’s Alper explains that a new family of sensors may operate akin to human cells in detecting the presence of alien substances and transmitting information that prompts a response. Researchers are developing sensors that detect and report on particular biological agents, which can have applications ranging from food safety to biological warfare and terrorism defense.

This involves building membranes, similar to cell membranes, into which recognition groups are inserted. These recognition groups are similar to those found in individual cells. A signal transduction mechanism turns a target molecule on the membrane’s surface into a readable signal that alerts the system to a presence. The cell reacts even though the target substance might never enter a cell.

Demand is high for devices that can report on a variety of factors that could be present in a system. These factors can include internal machine temperatures, bacterial presence in food, airborne pathogens and chemicals in a liquid.

DARPA’s Rudolph relates that agency scientists are examining how a Melanophila beetle can sense the infrared emission of a forest fire more than 50 kilometers distant. Observers long had noted that swarms of these beetles tended to congregate at forest fires, and subsequent research determined that this was part of the adult’s reproductive cycle: The insects lay their eggs on the inner surface of tree bark.

This detection capability probably combines olfactory and heat sensing, but German researchers have defined a unique architecture in the beetle that helps the insect detect infrared emissions. Dish-like structures under the beetles’ wings contain 70 small onion-like structures that serve as transducers of infrared data. When the appropriate wavelength is impinged on the onion-like structure, its volume increases. This pressure against the structure’s skin in turn trips a natural switch of neurons that sets off the beetle in search of heat. Scientists are working to duplicate, and perhaps enhance, that natural sensor architecture using piezoelectric crystals. One significant potential advance offered by this approach is an infrared detector that requires no special cooling and operates at room temperature.

Alper’s group at Lawrence Berkeley National Laboratory is creating colorometric sensors that change from blue to red in the presence of a target substance. Based on the model of a cell membrane, they can be designed for a specific biological target. The molecules self-assemble by aligning themselves to create the membrane, after which they are polymerized by light to link them together in a blue cast. Scientists can insert proteins, carbohydrates or other molecules into the membrane to bind to a specific target substance. This binding event caused by molecular recognition brings about changes in the membrane that turn its blue color to red. This tips off the observer to the presence of the target substance.

This type of sensor can be fabricated to detect everyday items ranging from influenza viruses to deadlier or more exotic pathogens such as E. coli or cholera toxins. The future holds more sensitive, better, faster, more selective and less expensive sensors, Alper maintains. Work underway at other facilities focuses on using parts or entire living cells in sensor systems, for example.

Calvert notes that work on artificial noses involves polymers that undergo conductivity changes as they are exposed to different substances. Arraying a variety of polymers in parallel that react differently to substances can produce a fingerprint of a detected vapor, for example. He characterizes this approach as a simple technology when linked to the processing power of computers. How to integrate these sensors remains a vital area of research.

Alper describes an ideal goal of a 1-cent-per-unit sensor that can detect a single molecule of a particular target substance. Arrays of different sensors could betray numerous substances in the environment by their varying response patterns.

In military and security applications, chemical and biological warfare are potential prime beneficiaries of this technology. These types of sensors could provide immediate identification of suspect pathogens in the event of a biological attack. Not only would this alert people to the early presence of these microbes, their rapid identification would speed presymptomatic treatment, which could spell the difference between survival in a contained attack or a high fatality rate in an epidemic.

In the consumer arena, the food industry could establish sensor system safeguards in processing plants to alert monitors to food spoilage or contamination. An inspector could rub a sensor across a slab of meat, for example, to detect the presence of hazardous pathogens such as E. coli. Even shrink-wrapped food could include an internal sensor strip visible to shoppers to alert them to spoilage with a color change.

Teaming these structural and sensate advances with biomimetic functions is leading to new devices that duplicate animal behavior. DARPA’s Rudolph notes that force dynamics studies on legged locomotion and winged flight will be key to fabricating versatile land-roving robots and small airborne drones akin to flying insects.

Only recently have researchers been able to establish a foundational understanding of insect flight, he observes. Science long maintained that the bumblebee was aerodynamically incapable of the rotational lift necessary for its observed flight, for example. Significant research advances have taken place, and now more than a half dozen laboratories worldwide are working to engineer micromechanical flying insects.

Advances in understanding biological locomotion could enable the development of robots that can traverse rough terrain in the same manner as a cockroach. These mechaniroaches offer a wide range of applications, from commercial uses to battlefield applications and exploration of extraterrestrial bodies such as planets, moons and asteroids. An activity that involves jumping, maneuverability or mobility over fractal surfaces poses an obstacle to a wheeled vehicle. An autonomous or semiautonomous mechaniroach, on the other hand, could traverse these obstacles effectively.

Autonomous biomimetic systems have considerable potential for the military marketplace, Rudolph offers. Capturing materials structure and mechanical performance could lead to new defense capabilities. These could range from synthetic microbutterflies carrying advanced sensors to small battlefield robots scouting enemy forces or serving as mine hunters.

One of the biggest hurdles facing successful biomimetic application is communication among its researchers, Rudolph says. Many scientists in the diverse disciplines “are learning amazing things,” but are challenged in describing them in terms that are understandable by experts in the other vital fields. A related problem is that physicists and engineers are used to building systems based on fairly established laws and principles, and they are challenged by the variability and “bugginess” of biological systems.

Calvert notes that many materials are developed with a specific application in mind, after which other uses emerge that may ultimately predominate. Actual applications tend to lie 20 years upstream from the materials’ development. “Exactly what anyone can do with these technologies is very hard to predict,” he states. “You’re talking about materials that don’t exist for machines that don’t exist.”

“We’re only at the beginning in terms of creating artificial [biological] structures,” Alper adds.

Neural Circuitry Research Matures, Spawns Algorithm Equivalents

Researchers long have recognized that brain cells can be grown to form a substrate for parallel processing (SIGNAL, February 1996, page 17). Now, some Defense Advanced Research Projects Agency (DARPA) programs explore growing slices of living neurons integrated with microelectronics. And, a related effort is exploring bio-algorithms for understanding how living things take action based on sensory input.

In addition to circuitry advances, the neuron growth experiments seek to discover how the neurons process information and communicate about patterns. Being able to harness their bio-algorithms could have broad applications in computational pattern recognition, with uses ranging from automatic target recognition to speech recognition.

Research into how the Melanophila beetle can sense the infrared emission of forest fires also seeks to determine the organism’s search strategy. Understanding this process could lead to new algorithms for robots. Other search strategies examine how moss navigates.

DARPA Program Manager Dr. Alan S. Rudolph offers that understanding how information is processed in biological systems is a major growth area. This largely is being driven by neuroscience research. Computation and algorithmic tools have potentially broad applications. One processor that is based on brain pattern recognition now can recognize words against a noisy background.

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