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Inspired by Biology: From Molecules to Materials to Machines (2008)

Chapter: 3 Advanced Functional Materials

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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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Suggested Citation:"3 Advanced Functional Materials." National Research Council. 2008. Inspired by Biology: From Molecules to Materials to Machines. Washington, DC: The National Academies Press. doi: 10.17226/12159.
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3 Advanced Functional Materials A key objective of scientists is to incorporate functional properties of biological materials into new materials and devices. The properties sought include molecular recognition, sensitivity and specificity of response, energy storage and conversion, force dynamics of elasticity, adhesive and other mechanical properties, and optical filtering and detection. In this chapter, key advances, opportunities, and challenges are reviewed in creating biomolecular materials with these specific functional attri- butes. To organize this chapter, important socioeconomic areas are described where applied biomolecular functional materials are having a strong impact: alternative and renewable energy, health and medicine, and national security. In each of these areas, exemplars of fundamental discoveries and challenges are explored. Finally, the efforts under way to exploit the functional properties of new bioderived, bio- inspired, and biomimetic materials are discussed. Advances in functionalizing biomolecular materials will have enormous impact on U.S. society. To date, there has been demonstrable progress harnessing the functional power of biomolecular materials, especially in alternative and renewable energy, health and medicine, and national security, as described in this chapter. The interest in fuels derived from plants is largely based on a greater understanding of energy conversion processes in these materials as well as a greater understanding of how to manipulate the plant genome. The significant role medical diagnostics play in everyday life (for example, the new generation of glucose testing for diabetics) is a direct result of the understanding of biomolecular recognition events and the ability to manipulate them in useful, easy-to-use devices. Approaches based on bioinformatics and synthetic biology are currently being vigorously pursued, with 31

32 Inspired by Biology the goal of making cells produce products of societal and economic value. Most of the focus today is on products of interest to the pharmaceutical industry. Related approaches may also be of value for the synthesis of new biomolecular materials. While significant progress has been made, the potential future impact of under- standing and utilizing functional biomolecular materials is enormous. Imagine that one could . . . • Engineer biological enzymes to convert organic matter to usable fuels with very high efficiency in order to substantially reduce dependence on foreign sources of fuel. • Manipulate biomolecular recognition events to create a biosensor with no false alarms that responds with sensitivity and specificity, mitigating threats before people or other key assets are exposed. • Create new arrays of medical diagnostic assays that can predict susceptibil- ity to and progression of disease. • Deploy new materials that will protect people and material assets from chemical and biological contamination. • Design and fabricate new materials that capture the superlative properties of adhesion in a gecko foot or the elegant strength in design of a diatom or mollusk shell. These seemingly futuristic touchstones represent some of the future impact that could be realized through the understanding and exploitation of functional properties of biomolecular materials. Following discussion of certain areas of research, specific challenges and opportunities are outlined at the end of this chapter. Alternative and Renewable Energy from Biomolecular Materials and Processes Life requires energy and the continual conversion of energy from one form to another. Biological systems have adopted diverse means by which to convert between different forms of energy in order to compete and survive. These unique, adaptive, energy-converting properties of biomolecules have inspired biomate- rial scientists. There have been great advances in the understanding of functional biomolecular processes that efficiently convert energy in biological systems. These include the chemical conversion of high-energy-containing materials such as poly- saccharides (one of which is cellulose) into fuels, the production of electrical energy with enzymes for fuel cell or battery applications, the conversion of light energy into chemical energy in photosynthesis, and the conversion of chemical to mechanical energy by biological motor proteins.

A d va n c e d F u n c t i o n a l M at e r i a l s 33 Biofuels and Processes Rudolph Diesel contemplated that the engine named in his honor would be powered by vegetable oils. However, the widespread availability of inexpensive petroleum during the twentieth century altered that vision. Twenty-first century realities have rekindled interest in Diesel’s original thinking. Cellulose is the world’s most abundant biological polymer. Studies demonstrate that with technological advances, biofuels, such as ethanol from cellulose, could supply a significant fraction of the world’s demand for transportation fuels in a way that is carbon dioxide neutral and does not compete with land for food production. The key to achieving these advances is research in plant genetics, biotechnology, process chemistry, and engineering that will lead to new manufacturing concepts for converting renewable biomass to valuable fuels and products. For certain practical applications, biologically based feedstocks are already having an impact. These include solvents, plastics, lubricants, and fragrances. For example, poly(lactic acid) is a hydrolytically degradable plastic that is currently manufactured on a million-kilogram scale in the United States and on a smaller scale in Europe and Japan. The route to the final product ferments corn dextrose for the production of lactic acid, which is then dimerized and polymerized. The final product is used in food packaging and the clothing industry. Substrate utilization remains a key challenge in the use of biological feedstocks for meeting energy needs. The abundant products of photosynthesis are primar- ily renewable cellulose, hemicellulose, and lignin. Although energy-rich, these abundant products are not easily transformed into usable fuels. This challenge, along with process methods that treat these substrates as chemical engineering feedstocks, will be the focus of much of the effort in this area of research. Figure 3.1 summarizes the key global biomass resources from agricultural residues, wood, and herbaceous energy crops. Polysaccharides and lignin are separated and processed to make feedstocks for materials and fuels. Current methods for separating the constituent biomolecular components rely on thermochemical processing; they are relatively harsh and energy-intensive. The development of more benign and economical processing steps is clearly a challenge. In principle, enzymes could be brought to bear, but doing so in a cost-effective manner remains problematic. The science of genomics, supported by new tools such as proteomics, metabolomics, and imaging in con- junction with the genomic databases, could give us insight into how microorgan- isms utilize untreated biomass to produce metabolic energy. The logical companion to bioinspired renewable energy production is the utilization of that energy. This is the focus of research in microbial fuel cells. Microbial fuel cells are electrochemical cells that convert chemical energy into electrical energy by using bacteria as a catalyst to convert substrate materials into

34 Inspired by Biology FIGURE 3.1  Key biomass resources: agricultural residues, wood, and herbaceous energy crops. SOURCE: A.J. Ragauskas, C.K. Williams, B.H. Davison, G. Britovsek, J. Cairney, C.A. Eckert, W.J. Frederick, Jr., J.P. Hallett, D.J. Leak, C.L. Liotta, J.R. Mielenz, R. Murphy, R. Templer, and T. Tschaplinski, “The path forward for biofuels and biomaterials,” Science 311:484-489 (2006). available electrons. The principle of operation of a microbial fuel cell is illustrated in Figure 3.2. Organic substrates (e.g., glucose) are oxidized to carbon dioxide in the anode compartment. Electrons from the substrates are transferred to the anode. The pathway by which this transfer is achieved is another topic of current research. Several options include soluble electron relays that shuttle between microbe and electrode, direct membrane contact, or nanowires that are produced by the bac- teria themselves. In principle, a broad spectrum of substrates can be utilized as fuel. The geometry of the cell is such that electrons are forced to flow through an external circuit and load resistor to the cathode compartment. Simultaneously, a proton-conducting membrane that separates the anode and cathode compartments is needed to maintain electroneutrality. Among the science and engineering chal- lenges associated with this research are the molecular mechanisms for transporting

A d va n c e d F u n c t i o n a l M at e r i a l s 35 FIGURE 3.2  The microbial fuel cell and its operating principles. SOURCE: B.E. Logan, B. Hamelers, R. Rozendal, U. Schröder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, and K. Rabaey, “Microbial fuel cells: Methodology and technology,” Environmental Science and Technology 40:5181-5192 (2006). Copyright 2006 American Chemical Society. reducing equivalents to the anode, internal resistance of the device, efficiency, the range of organic substrates, and the selection of microorganisms. These problems lie at the intersection of physics, chemistry, biology, and bioengineering. Figure 3.3 illustrates the overall concept of the fully integrated agro biofuel, material, power cycle for sustainable technologies. There is growing interest in finding alternative biological materials from which to make fuel. While biofuels still represent a small fraction (less than 15 percent) of the world’s overall energy, the yearly growth rate of 15-20 percent contrasts significantly with the rate of 1-2 percent for fossil fuels. Today, established biofuel

36 Inspired by Biology FIGURE 3.3  The fully integrated agro-biofuel-biomaterial-biopower cycle for sustainable technologies. SOURCE: A.J. Ragauskas, C.K. Williams, B.H. Davison, G. Britovsek, J. Cairney, C.A. Eckert, W.J. Frederick, Jr., J.P. Hallett, D.J. Leak, C.L. Liotta, J.R. Mielenz, R. Murphy, R. Templer, and T. Tschaplinski, “The path forward for biofuels and biomaterials,” Science 311:484-489 (2006). industries are a reality in countries like Brazil and Germany. The production and processes to efficiently produce fuels are being pursued for palm oil, cottonseed, peanut oil, castor oil, sunflower, and cattle fat. Success in this industry will require focused research to understand and utilize plant genetics, chemical processing methods to recover useful materials from the crops, and means to efficiently pro- duce carbon-rich fuels from these biomaterials. Biomimetic Photosynthesis Photosynthesis, the conversion of solar energy into stored chemical energy, is a key biological process that sustains life on Earth. The energy-rich molecules produced by photosynthesis are primarily reduced carbon compounds such as cellulose, hemicellulose, and lignin, which are difficult to convert into chemical fuels. Other photosynthetically produced carbon compounds such as glucose are directly fermentable to ethanol.

A d va n c e d F u n c t i o n a l M at e r i a l s 37 One of the great scientific challenges of the twenty-first century for researchers inspired by naturally occurring photosynthesis is extracting and mimicking the essential solar-energy-conserving reactions. The general equation of photosynthe- sis guides the way: water + carbon dioxide + sunlight → oxygen + stored energy. Biomolecular and bioinspired energy transducers need not replicate all of photo- synthesis. Nonbiological photoreactions could in principle mimic the reactions of photosynthesis by producing small fuel molecules such as hydrogen, methane, or methanol. In all these reactions, molecular oxygen would be produced, since water is the source of electrons for the fuel molecules. For example, in the case of renewable hydrogen production by light-activated water oxidation, the reaction is 2H2O + sunlight → O2 + 2H2. Current research is focused on synthesizing efficient catalysts for molecular oxygen and fuel formation. Cutting-edge photosynthesis research is aimed at understanding the molecular mechanism of oxygen evolution, whereas cutting-edge solar fuel research is aimed at understanding the catalytic pathway for the production of the fuel molecules. Understanding light-activated oxygen evolution by plants and cyanobacteria is one of the great challenges in photosynthesis. Progress has been made in elucidating the structure of the photocatalytic oxygen-evolving center in natural photosynthe- sis. The oxygen-forming steps are mediated by an Mn4Ca-tyrosine catalytic center illustrated in Figure 3.4. The photocatalytic oxygen-evolving complex (OEC) is the starting point for bioinspired energy transduction research for biomimetic pho- tosynthetic fuel formation. Within the next 5 years the synthesis of this structure with proven oxygen-evolving capabilities should be complete. This work includes linkage to a photochemical reaction that drives the oxygen evolution reaction. Advanced computation, simulation, and modeling techniques—in addition to chemical synthesis— should be brought to bear on this problem, with the goal of understanding how this high-potential multielectron transfer reaction is achieved. Even with oxygen evolution fully understood, less than half the scientific challenge of bioinspired energy transducers for renewable energy production will have been overcome. The other part of the challenge focuses on the photocatalytic centers for fuel formation: the chemical fate of the electrons and protons in the reaction 2H2O + solar energy → O2 + 4H+ + 4e–. Biomolecular energy transducers are a rich area for basic research, with poten- tially important applications in the field of renewable fuels and chemicals pro- duction. The key challenge, following the example of natural photosynthesis, is the conversion of solar energy into stored chemical energy using electromagnetic energy contained in the visible portion of solar emission spectrum. A molecular- level understanding of the mechanisms of photosynthetic oxygen evolution must be achieved. This includes the de novo synthesis of photocatalytic oxygen-evolving complexes that are able to drive the light-activated oxidation of water. Understand- ing oxygen evolution at the molecular level, which is well under way, will permit the

38 Inspired by Biology FIGURE 3.4  Mechanistic model for the formation of photosynthetic oxygen. SOURCE: M. Haumann, P. Liebisch, C. Muller, M. Barra, M. Grabolle, and H. Dau, “Photosynthetic O2 formation tracked by time-resolved X-ray experiments,” Science 310:1019 (2005). synthesis of artificial oxygen-evolving structures without the need to synthesize or recreate an entire biological organism. The desired fuels will determine the catalysts and chemistry to be studied. For example, simple fuel molecules like hydrogen can be produced with well-known metallic catalysts or mimetic analogues of the enzyme hydrogenase. Drawing further inspiration from natural photosynthesis, light-induced charge separations of photosynthetic reaction centers occur across a membrane that separates oxidizing and reducing equivalents. Bioinspired pho- tosynthesis will build on this example by constructing artificial photosynthetic membranes that can photoproduce oxygen and fuel molecules on opposite sides of the membrane. A priority research goal should be the synthesis of artificial photosynthetic membranes that mimic the essential energy-conserving properties of natural photosynthesis with equal or greater energy conversion efficiency. Theo- retically, synthetic systems could be more efficient than natural systems for three reasons: (1) photosynthesis has evolved to maximize survivability, not efficiency; (2) in principle, a synthetic system, combined with suitable catalysts, should be able to produce small fuel molecules such as hydrogen, methane, and methanol, which are immediately useful, unlike cellulosic biomass; (3) synthetic chromophores

A d va n c e d F u n c t i o n a l M at e r i a l s 39 might be able to harvest a larger fraction of the solar emission spectrum than the chlorophyll molecules of green plants. Of course, development of a real-world system, including long-term operational stability, will be a key challenge after the basic science problems have been solved. Bacteriorhodopsin (BR) is a stable transmembrane protein of the bacterium Halobacterium halobium and thrives in salt marshes at high salt concentrations. Like photosynthesizing green plants, BR converts light energy into chemical energy. Whereas green plants convert light energy into chemical energy by photon-induced charge separation, BR is a photon-driven proton pump. In its native state, BR is a backup source of metabolic energy that is activated when available oxygen becomes too low for normal respiration, a transition that is accelerated by the poor solubil- ity of oxygen in concentrated salt solutions. Following photon absorption, protons are pumped out of the cell through the cell membrane at the rate of one proton per photon. The Gibbs energy of the hydrogen ion gradient that is formed by this process is converted into chemical energy in the form of adenosine triphosphate (ATP) molecules by ATP synthase enzymes. As illustrated in Figure 3.5, BR molecules are composed of seven α-­helical sub- domains that form a hexagonally close-packed, oriented structure that assembles into large aggregated patches in the cell’s membrane. The structure of a single BR molecule is illustrated in Figure 3.6a, along with the movement of protons from the inside to the outside of the cell. Photon absorption triggers a series of spectroscopic changes that include a trans-1,3-cis transformation, rearrangement of electronic charge within the protein, protonation, deprotonation, and con- formational changes. As illustrated in Figure 3.6b, the spectroscopic states have characteristic lifetimes and absorption spectra that collectively are known as the BR photocycle. The creation of gradients is a recurring theme in the harvesting of light and its conversion to chemical energy in biomolecular systems. The physical and chemical principles of the action of BR have inspired research in the field of light-activated proton gradient production. For example, BR processes involving light-driven proton pumping and production of ATP catalyzed by F0F1-ATP synthase have been demonstrated in artificial liposome photosynthetic membrane and other systems. In all cases energy is conserved by the creation of proton gradients following pho- ton absorption that utilizes a suitably designed chromophore. The biomolecular principles for converting light energy into chemical energy are now understood to follow a conceptually simple plan that provides guidance for bioinspired research. Design principles drawn from naturally occurring biological systems can be used to form the basis of near-term (5 years) research. Nanoscale energy sources based on light-induced formation of gradients should be mastered in a range of synthetic systems to illustrate the themes and variations of this mode of energy transduction. In addition to light-energy harvesting, research in

40 Inspired by Biology FIGURE 3.5  Illustration of the seven α-helical domains that constitute BR molecules. SOURCE: J. Baudry, E. Tajkhorshid, F. Molnar, J. Phillips, and K. Schulten, “Molecular dynam- ics study of bacteriorhodopsin and the purple membrane,” Journal of Physical Chemistry B 105:905 (2001). Copyright 2001 American Chemical Society. Reprinted with permission. BR has taken on a life of its own in fields removed from the biological origin and energy-transducing function of the protein, including protein-based field-effect transistors, artificial retinas, spatial light modulators, three-dimensional volumet- ric memories, and optical holographic processors. These applications will require standardization of materials, components, and manufacturing methods in order to be successfully developed.

A d va n c e d F u n c t i o n a l M at e r i a l s 41 FIGURE 3.6  BR structure and photocycle. (a) BR is an ion pump that results in the net transfer of a proton from the intracellular to the extracellular surface. Key residues are high- lighted, and the proton transfer channel is shown. (b) The main and branched photocycle in BR used for three-dimensional memory device applications. The branching reaction involves the O → P transition that is optimized using directed evolution. M and O state lifetimes and yields can also be optimized using directed evolution. SOURCE: Adapted from J.R. Hillebrecht, J.F. Koscielecki, K.J. Wise, D.L. Marcy, W. Tetley, R. Rangarajan, J. Sullivan, M. Brideau, M.P. Krebs, J.A. Stuart, and R.R. Birge, “Optimization of protein-based volumetric optical memories and associative processors using directed evolution,” Nanobiotechnology 1:141-152 (2005). Biomolecular Motors All active cellular movements, such as separation of the chromosomes and the two daughter cells during cell division, determination and modulation of cell shape, locomotion, and targeted transport of intracellular cargos, such as mem- brane bound vesicles containing neurotransmitters or waste products, involve the

42 Inspired by Biology transduction of chemical metabolic energy into mechanical work. Production of the cell’s main energy source, ATP, is accomplished by chemical metabolic reactions and also by mechanical-to-chemical energy transduction. The mechanoenzymes that carry out these essential roles are termed molecular motors, and they sort into several families with related structural and mechanistic features. Understanding and controlling these functional properties in detail could facilitate harnessing them or their operating mechanisms for actuators in nanoscale devices, such as molecular sorters, filters, concentrators, switches, and power sources. However, the principles of their operation are very different from those of macroscopic chemical-to-mechanical energy transducers, partly because their shapes and posi- tions undergo thermal vibrations that are comparable to the functionally relevant motions. A major challenge in biomotor research is to understand how to integrate these functional biomolecular materials into useful devices that operate efficiently. High-resolution structural biology, rapid-reaction biochemical kinetics, and sin- gle-molecule biophysical studies of these machines are improving understanding dramatically. Other challenges to using them in practical fabricated devices are to make them robust enough to retain activity for the intended life span of the device and to program them to assemble appropriately for their designed role. The com- bination of materials science with protein and nucleic acid molecular biology and biochemistry holds promise for achieving these advances. Linear Molecular Motors Biological cells are highly crowded environments in which molecular machines transport cargos to specific locations and in which metabolic enzymes interconvert energetic compounds to perform useful work. Series of molecular motors and mechanoenzymes have evolved to perform these tasks. They are remarkably effi- cient macromolecular machines used in nature to determine cellular shape and to accomplish intracellular transport, cell locomotion, muscular contraction, and cell division. The main high-energy compound that serves as the fuel for energy-requir- ing processes is ATP, which these machines cleave to the diphosphate (ADP) and orthosphospate (Pi). Splitting each ATP molecule liberates approximately 10–19 J, which can be transduced into various types of chemical, electrical, or mechani- cal energy to perform work. Of the three cytoskeletal filament systems that shape eukaryotic cells (actin, microtubules, and intermediate filaments), the first two are tracks for molecular motors. The three families of motors are called myosin, kinesin, and dynein, and all three are found in numerous intracellular locations. Bacteria also contain actin filaments and tubulin homologs, but no molecular motors have yet been discovered that actively translocate on the prokaryotic cytoskeleton. Considerable progress has been made in understanding the fundamental prop- erties of biomotors, and various attempts are under way to fabricate useful devices

A d va n c e d F u n c t i o n a l M at e r i a l s 43 that incorporate these new materials. Figure 3.7 shows a typical scheme molecular motors use to generate force or move a cargo. An internal structural change, often rotation of a lever, stretches a compliant (springy) element, transferring the chemi- cal energy from the ATP into potential energy of extension of the spring module. Whether this produces force between two cellular components or causes them to move relative to each other is dependent on their mechanical properties and how they are tethered to other structures. Typical forces produced by individual molecular motors are 2-10 pN, and the distances they move per ATPase cycle are 10-40 nm. These values imply remarkable thermodynamic efficiency of energy transduction. For instance, a single actomyosin event in muscle can produce 10 pN of force with 10 nm of displacement. The energy-storing spring is nearly linear, so the mechanical work output is ½ 10 pN × 10 nm = 50 × 10–21 J, about half of the energy liberated from the ATP. Whole muscle contraction is also approximately 50 percent efficient, which should be compared to man-made chemical-to-mechani- cal energy transducers (e.g., an automobile engine) that can operate at about 20 percent efficiency. Muscles regulate their biochemical energetic utilization (the ATPase rate) according to the requirements of the mechanical load, fast shorten- ing, or heavy lifting. This mechanical feedback makes the myosin motor a smart machine. Flagellar Motors A flagellar motor is a biological rotary engine. Found in bacterial membranes, it spins a flagellum around its axis to cause the bacterium to swim (Figure 3.8). The flagellar motor assembly contains about 25 different proteins that form the motor-switch-shaft-propeller complex. The motor itself has about 12 proteins. The energy for rotation is derived from a proton gradient across the membrane in some flagellar motors such as that in E. coli or from sodium ions in others such as marine vibrio. The flagellar motor is one component in a larger biochemical system in the bacterium that governs the swimming of the cell along nutrient and chemical gradients in a process called chemotaxis. The presence or absence of “food” trig- gers a biochemical cascade that can turn the motor on and off (that is, begin the rotation) or can turn the rotation from clockwise to counterclockwise. Only 35-40 nm in size, a flagellar motor powers the motion of a cell that is nearly 100-fold larger (about 1-2 microns). E. coli has 6-8 flagella-motor complexes per cell. Each motor consumes about 10–16 W, running on ion flows of tens of femtoamps, and takes about 26 steps per revolution. While the motor can run at nearly 100 percent efficiency at low speeds, it is only a few percent efficient at high speeds. As with other biological systems, it self-assembles, requiring no external assembly machinery. The final step in mitochondrial phosphorylation of ADP to form ATP is

44 Inspired by Biology FIGURE 3.7  Typical scheme molecular motors use to generate force or move a cargo. SOURCE: T.D. Pollard, W.C. Earnshaw, and J. Lippincott-Schwartz, Cell Biology, 2nd Edition, Philadelphia, Pa.: Saunders, 2007. Copyright Elsevier 2007.

A d va n c e d F u n c t i o n a l M at e r i a l s 45 FIGURE 3.8  Schematic illustration of a flagellar motor. SOURCE: B. Alberts, A. Johnson, J. Lewis, M. Raff, K. Roberts, and P. Walter, Molecular Biology of the Cell, 4th Edition, New York, N.Y.: Garland Science, 2002. Based on data from T. Kubori, N. Shimamoto, S. Yamaguchi, K. Namba, and S. Aizawa, “Morphological pathway of flagellar assembly in Salmonella typhimurium,” Journal of Molecular Biology 226:433-446 (1992), and N.R. Francis, V.M. Irikura, S. Yamaguchi, D.J. DeRosier, and R.M. Macnab, “Localization of the Salmonella typhimurium flagellar switch protein FliG to the cytoplasmic M-ring face of the basal body,” Proceedings of the National Academy of Sciences USA 89:6304-6308 (1992). Copyright 2002. Reproduced by permission of Garland Science/Taylor & Francis LLC. accomplished by another remarkable mechanoenzyme, the F1 ATP synthase (Figure 3.9). This is a rotary motor-generator that is cranked around by another motor, F0, which in turn derives its energy from a proton gradient across the outer mitochon- drial membrane. F0 has many similarities to the bacterial flagellar rotary motor, described in the preceding section. F1 is constructed like a citrus fruit whose seg- ments are six protein subunits (α and β in Figure 3.9), with a central rotating shaft (γ, δ, and ε) in its core. The direction of the motor is readily reversible. Torque generated is approximately 50 pN·nm, and considering that the force is applied between the stator and rotor at a radius of about 1 nm, the effective linear force

46 Inspired by Biology FIGURE 3.9  F1 ATP synthase enzyme. SOURCE: A.E Senior and J. Weber, “Happy motoring with ATP synthase,” Nature Structural & Molecular Biology 11:110-112 (2004). produced is approximately 50 pN. The efficiency of energy transduction, either synthesizing or splitting ATP, can approach 100 percent! From the perspective of new materials and new nanotechnologies, these and other biological motors exemplify nanomachines that (1) convert cellular chemi-

A d va n c e d F u n c t i o n a l M at e r i a l s 47 cal or ionic gradient energy sources into work, motion, and propulsion, (2) are controllable by switching systems involving cooperative transitions within the protein subcomponents, and (3) assemble themselves from individual components, mitigating the need for expensive and complex assembly processes. In 5 years, more details of the structures and mechanisms are likely to emerge. It is likely to take 20 years before researchers can construct stable and robust molecular-scale devices that utilize biomotors or their propulsion principles. Many exciting scientific and technical challenges remain in understanding these mechano-chemical energy transducers, which offer the promise of power and propulsion systems for nanotechnology. First, while the structures of many of the individual proteins are known—such as the flagellar filaments and the motor domain of myosin—the structures of other component proteins and the full atomic structures of the assemblies have not yet been determined. How they are function- ally modulated in activity by cellular signals, how the cargos and target destinations are specified, and how the motors are integrated with other cellular activities are not known. The molecular motors appear and assemble into functionally competent supramolecular complexes when needed for various roles during the cell cycle and then disassemble to be used elsewhere for different jobs. How they cooperate in this manner is almost a complete mystery. A central problem in rotary motors is that researchers do not yet understand how torque is generated by the transfer of ions. Researchers also do not understand how motor proteins get proper anchoring in the fluid bilayer membrane to generate torque or linear force. Biophysical and structural research under way will likely resolve these questions in 5 to 10 years and reveal the design trade-offs made during evolution to tune the individual molecular motors for their specific tasks in the cell. To use molecular motors in human-designed materials and devices, the assem- bly and control mechanisms described above will have to be understood at a very sophisticated level. Proteins are perishable because they are sensitive to temperature and solvent conditions and they are food substrates for microorganisms such as bacteria. There are new efforts to exploit biomotor properties and evaluate them in new device applications. These efforts largely revolve around combining sens- ing and actuation and evaluating the ability to detect and/or amplify a signal in a biosensor or diagnostic system. Among the concepts being pursued is the use of linear motors (kinesin and microtubules, actin-myosin) to shuttle cargo consisting of antibody-coated beads through a microfluidic channel that contains an antigen or analyte of interest. In the proposed device, the cargo picks up the analyte of inter- est on the surface of the bead and shuttles it to a depot, resulting in a concentrated antigen that can be detected with a secondary antibody conjugate. The ability to create robust systems around biomotor-based devices will depend on a number of important factors, including stabilizing the motors for long-term function in an in vitro environment. Biological systems have adopted unique mech-

48 Inspired by Biology anisms to stabilize components for long-term function, and the environment in which this is accomplished in the natural system is complex and dynamic. Creating a controlled in vitro environment in which the motor can function optimally for a useful period of time in a device application is a challenge. The long-term storage of proteins has been accomplished using sugars for (1) conformational stability in the dry state and (2) storage of biomolecular components that could be used in devices. The use of sugars should extend the storage lifetime of components in devices once many of the other challenges in building them are overcome. Realizing the value of biomolecular motor devices will also require characterization tools at the scale of the device that is being engineered. We will also need to better understand how these devices work and can be used in technology or product applications. An early application of a new device using molecular motors was recently reported. The device is a molecular sorter with kinesins that direct microtubules labeled with different fluorescents into separate paths using hydrodynamic and electrical steering forces. Another group of researchers placed F1 motors on micro- fabricated inorganic pedestals and attached nickel nanopropellers to form a hybrid biotic/abiotic device. Such bioinspired actuators might serve as concentrators or nanopumps in nanoscale devices. In the future, the flagellar motor or the F0-F1 ATP synthase might serve as the model for new energy sources that convert chemical concentration gradients into chemical energy that can be stored and transported. Molecular-scale propulsion might be useful in devices that enter the bloodstream or nervous system to remove clots, to counteract toxins, to attack plaques, or to deliver pharmaceuticals or therapeutic genes. Advanced Functional Materials in Health and Medicine One obvious area for the application of advanced functional materials that incorporate or mimic biomolecular components is health and medicine. In this section, three specific examples of the application of such materials in health and medicine are explored: medical diagnostics, drug delivery, and prosthetics. These application areas take advantage of different functional capabilities of biomol- ecules, including molecular recognition, signaling, adhesion and binding, and mechanical properties. There are challenges in creating integrated systems in which working functional components are maintained. Design and fabrication of new devices or products is proceeding based on advancing knowledge of useful func- tional bio­molecular properties. Many devices have been developed into robust systems that can be used to look at clinical variability in human response. Thus a medical diagnostic or therapeutic device exploiting functional capabilities will reach its ultimate utility when the breadth of biological response variability is known. One need only look at the variability generated in a drug clinical trial or diagnostic profile of disease in a human population to appreciate this challenge.

A d va n c e d F u n c t i o n a l M at e r i a l s 49 The presence of regulatory review to ensure the safety and efficacy of devices that incorporate functionalized biomolecular materials is an important gatekeeper in deciding which research advances may proceed to widespread human use. Such review can account for the long lead time between a research advance and its impact on health and medicine. Medical Diagnostics The health of the U.S. population has significant implications for the economy and society as a whole. As modern medicine progresses, the ability to diagnose and ameliorate diseases has improved markedly. The cost of health care has also risen dramatically owing to the costs associated with hospitals, physicians, and diagnostic and treatment modalities. For decades now, advances in medicine and health care have been tightly linked to the development of novel technologies. Understanding biological mechanisms has improved diagnosis and treatment, so that longevity and the quality of life after diagnosis of the major illnesses have improved dramatically. One area in which the unique molecular recognition and binding properties of biomolecules have been exploited to advantage is medical diagnostics. This $6 billion industry currently has a tremendous impact on health and medicine. A number of biomolecular components are of great use in the design and fabrica- tion of diagnostics. Nucleic acids, proteins, carbohydrates, and lipids are potential sources of information insofar as they are cellular or molecular biomarkers of interest. The use of antibodies is widespread, with enzyme-linked immunosorbent assay (ELISA) testing alone being a good example of the power of high-impact products based on biomolecular functional recognition and binding properties. The application of nucleic acid testing in infectious disease blood testing as well as polymerase chain reaction (PCR)-based and other genomics-based testing is now a common practice that saves lives. Genomic and proteomic profiling is becoming more widely practiced as a means to assess a patient’s condition. Biomolecular materials have a number of advantageous capabilities that war- rant continued research and have already been proven valuable as detection ele- ments in diagnostics. These capabilities include exquisite specificity, sensitivity for binding or affinity (down to picomolar concentrations, 10–12, for antibodies and attomolar concentrations, 10–18, for DNA hybridization), and generalizability of biomolecular recognition (one can make an antibody or peptide via phage or bacterial displays or DNA/RNA via systematic evolution of ligands by exponential enrichment [SELEX] that will bind almost any water-soluble target). Amplification schemes such as those used in PCR and more recently with conformational pep- tides enhance the signal over the background. These capabilities are also sometimes augmented by the cooperativity of biomolecules and aggregation events, exploiting

50 Inspired by Biology both kinetic and thermodynamic properties of biomolecules. They can also be uti- lized in matrices or materials. For example, the antigen-antibody capping reaction on the B cell surface is dominated by cooperative biomolecular events organized in an immobilized matrix of the membrane, resulting in very high sensitivity and specificity to antigen protein. These properties have proven fruitful in biosensor research and development. Researchers have started the formidable task of designing biomolecules that undergo large physical changes in response to binding any arbitrary target mol- ecule. Two approaches have been reported to date. The first is to take one of the rare, naturally occurring biomolecules that undergoes a large change in its physical state and reengineer its normal binding site such that it binds the target of choice. The alternative has been to take a biological sensor molecule that binds a target of choice and reengineer it so that it undergoes a large-scale change in its conforma- tion upon binding. Because the scientific community’s ability to rationally modify and develop designer biomolecules has improved recently, both approaches have seen notable successes. Another promising approach is the use of molecular beacons (MBs), single- stranded stem-loop DNA molecules that adopt a linear conformation when a target oligonucleotide hybridizes to the loop region, breaking the stem. By placing optical reporter groups (e.g., a quencher/fluorophore pair) on the stem, binding is coupled to a large, readily measurable change in MB emission. More recently this approach has been expanded to the detection of nonoligonucleotide targets via the use of “aptamers” (DNA or RNA molecules selected in vitro for their ability to bind such targets) that have been reengineered to undergo binding-induced folding via any of a number of rational or semirational redesign approaches. Finally, this approach has been expanded to the use of polypeptide recognition units by (1) demonstrat- ing that it is similarly possible to engineer binding-induced folding into small proteins and (2) by developing optical reporter groups that couple folding-induced changes in biopolymer dynamics into easily measurable optical outputs. A significant challenge in creating useful diagnostics has been in the transla- tion of a binding event into a measurable output signal. These binding events often require known conformational changes, which are often hard to predict or control. Fluorescence measurements require energy transfer; this requires a conformational change and transduction into energy transfer events from fluorophores attached to biomolecular materials such as antibodies. Controlling these conformational states is challenging, because they can be affected by solvent conditions, immobilization, and background absorption, resulting in interference. The development of biosur- faces that retain specificity and sensitivity remains a challenge and is expected to be a very active area for R&D of future biosensor assays. Additional approaches have been to attach biomolecules to a surface and then monitor their binding by studying mass (quartz crystal microbalances, microcantilevers), charge (field-effect

A d va n c e d F u n c t i o n a l M at e r i a l s 51 transistors), or index of refraction (surface plasmon resonance) changes at the interface. Many of these approaches are not yet robust enough to work in real- world conditions. Cell-based biosensors offer the advantage of exploiting more complex responses and deriving signals from collaborative and adaptive cellular reactions. For exam- ple, the use of B cells to report on antigen presence relies on the sensitivity and specificity of the antibody-antigen capping reaction and associated calcium flux upon binding. The advantage of this system is the ability to engineer B cells to present different antibodies on their surface and thus engineer desired responses to known antigens. This system is being exploited for diagnostic applications because it provides the cellular machinery to amplify the signal through cellular processes. In principle, cellular response offers another advantage: the ability to respond to unknown or new threats to human health. Challenges remain in the design and implementation of detection elements for diagnostics. For example, clinically relevant HIV detection (currently achieved by PCR amplification) requires about 1,000 copies of the genome per milliliter—that is, the attomolar (10–18) level. Current detection limits are about 10 to 100 femto­ molar (10–15) with electronic and optical-based biosensors. The long-term goal of relevant DNA detection is thus attomolar to zeptomolar (10–18 to 10–21) ­levels. PCR-free detection at these levels in a convenient electronic device is also a big challenge. The picture for detection of proteins and small molecules is more favorable. Protein diagnostics of cancer markers requires about picomolar (10–12) levels. Such biosensors should become available within the next 10 years, and they will revolutionize point-of-care diagnostics and the real-time monitoring of, for example, pollutants and industrial intermediates. A number of challenges also remain in implementing detection elements in diagnostic devices. One of these is controlling functionality at an interface, because most of these applications immobilize functional biomolecular components. Other challenges include (1) the design and synthesis of new tags for fluorescence, elec- tronic, radiologic, or other contrast techniques to enhance the information gained from biomolecular components and (2) sample preparation techniques to increase signal to noise in clinical samples. The transport of sample following processing also can affect the performance of these systems. Significant progress has been made in microfluidic separation and preparation technology, while challenges in controlling optimal conditions remain. Finally, computational challenges arise from the com- plex and large datasets from multiplexed signal generation in these applications. Targeted Drug Delivery, Targeted Imaging Systems, Targeted Radiation There has been long-standing interest in understanding and utilizing different biomolecular materials for the specific delivery of therapeutic compounds. The use

52 Inspired by Biology of biopolymers for this purpose is widespread, with notable efforts in liposomes, hydrogels, and, more recently, nanoparticles. Various aspects of charge, elasticity, size, and functionalized surface biochemistry have been explored as methods by which to manipulate the pharmacodynamics of therapeutic compounds and to selectively deliver these agents to specific sites in the body. Transdermal delivery is also widespread, and it is recognized that delivery through the skin can be an effective way to use biomolecular materials in this application. New advances have been made that take advantage of nanoparticles for tag- ging, labeling, and targeted drug delivery. The coincidence of their size with that of proteins makes this possible, and the potential for nanomedicine is enormous. Many groups are developing nanoparticle targeting of anticancer drugs and testing them on animal models of human cancers. This form of drug delivery improves the therapeutic response to anticancer drugs and allows the simultaneous monitoring of drug uptake by tumors. Modified polyamidoamine (PAMAM) polymer nanoparticles (in the class of molecules known as dendrimers) smaller than 5 nm in diameter are used as carriers. One of the targets chosen for delivery is the high-affinity folate receptor for the vitamin folic acid, also known as the folate-binding protein. A therapeutic nanoparticle consists of acetylated dendrimer conjugated to folic acid as a targeting agent, later coupled to methotrexate as a drug and a fluorophore as an imaging agent. The conjugates are injected intravascularly into mice bearing human tumors that overexpress the folic acid receptor. In contrast to nontargeted dendrimers, a folate-conjugated nanoparticle concentrates in tumor tissue for over 4 days after administration. The tumor tissue localization of the targeted nanoparticle can be attenuated by prior intravascular injection of free folic acid. Internalization of the drug nanoparticle into tumor cells can be confirmed by confocal microscopy, which detects the fluorophore that is delivered with the same dendrimer platform. Targeting methotrexate increases its antitumor activity and markedly decreases its toxicity, allowing therapeutic responses not possible with a free drug. Several other targeting ligands that can be placed on the surface of the den- drimer are currently being developed for in vivo applications, such as aptamers, peptides, antibodies, and antibody fragments that interact with specific target molecules on tumor cells. Conjugation of different, clinically approved drugs such as taxol or doxorubicine is being developed for alternative nanoparticle conjugates. The lack of ligands with sufficient affinity to achieve targeted delivery in vivo could be improved by attaching multiple copies of each molecule to a dendrimer. Prior work with sialic acid-conjugated dendrimers documents the cooperative binding of dendrimers functionalized with multiple ligands to the influenza virus, while molecular modeling and in vitro experiments suggest that folate-targeted den- drimers have cooperative polyvalent binding to cells. Other potential uses of nanoparticles in medicine include the use of gold

A d va n c e d F u n c t i o n a l M at e r i a l s 53 nanoparticles as tags in the treatment of arthritis and various cancers. Unlike many semiconductor particles, gold rods are also nontoxic and can be efficiently delivered to the cells. Importantly, gold nanorods possess exceptionally strong optical absorptivity in the infrared range, where human tissues have relatively high transparency. The rods are targeted with antibodies specific to particular cell mark- ers expressed in cancer. The presence of several affinity sites of different kinds on the nanorods ensures their efficient and selective binding to the cancerous tissues. Photoacoustic imaging makes it possible to localize small clusters of cancerous cells at much greater depths than regular optical imaging can afford, which is one of its most valuable features. This technique is being developed now to apply to other diseases, such as arthritis. Targeting of drugs is also becoming increasingly sophisticated. Antibodies are being developed and identified in increasing numbers that can be used to help target either drugs or nanoparticle carriers of drugs to specific locations. In addi- tion, as genetic information about disease becomes increasingly sophisticated, additional biomarkers are being identified that can also be used to target drug delivery. This increasingly sophisticated understanding of disease is also leading to new understanding of the genetic and proteomic pathways of disease, which is providing new classes of drugs and new strategies for attacking disease. This will lead to new opportunities to create the materials that deliver these drugs and the drug molecules themselves. The increasing sophistication of the biomolecules themselves will also lead to new uses and functions. For example, in addition to using the biomolecular materials as the drug delivery vector, the biomolecules will ultimately themselves have a dual role, becoming both the delivery vehicle and the drug itself. Indeed, a major focus of much development effort in the biotech industry is the discovery of drug molecules that are much larger than the small molecules favored by the traditional pharmaceuticals industry. This dual role of biomolecules is likely to increase significantly. As knowledge of both the human genome and the function of the genes increases, specific traits of individuals will start to be identified. This will bring with it a much more personalized form of medicine, where the genetic profile of individuals will be determined, at least for specific portions of the gene, and drugs and treatments will be designed that are specific to the individual. Both the iden- tification of the specific traits and the new treatments and their delivery will again present enormous challenge and opportunity for new materials. New materials are also advancing new biomedical imaging modalities. New magnetic resonance imaging (MRI) techniques take advantage of specific spins on molecules or possibly on nanoparticles to significantly enhance sensitivity. Nanoparticles are also being developed to act as fluorophores in the use of optical techniques to probe much deeper into the body. New nonlinear optical methods

54 Inspired by Biology are also being developed to overcome the strong scattering of light typical of tis- sues. These also enhance the use of optical methods for imaging. Even ultrasonic imaging is increasingly benefiting from the development of contrast agents to enhance its sensitivity. Neural Prosthetics Medical prosthetics are a good example of integrating a number of fundamen- tal challenges in the design, fabrication, and integration of functional biomaterials and will require interdisciplinary advances in the physical, chemical, biological, and engineering sciences. Next-generation devices will need to integrate microelec- trodes, power, and communications in a biocompatible material. Artificial materi- als that will replace limbs, for example, will have to be impedance-matched to the signals that drive them. For some applications, like artificial limbs, force dynamics and elastic materials that can be seamlessly integrated into a neural-controlled device will be required. Signals generated from neural sources will advance by incorporating closed feedback loops in which sensing and actuating components are integrated to control a limb. One example is in the area of devices for neuro- prosthetics. There has been considerable research activity and early application in health and medicine of neuroprosthetics based on advances in the R&D for these systems. A good example is the more than 35,000 cochlear implants for the hear- ing impaired that stimulate the cochlea and make auditory processing possible for them. Another more recent example is the deep brain stimulators that have con- siderable therapeutic utility in Parkinson’s disease and are being more widely used as a therapeutic option for mitigating symptoms of the disease. Many new areas are being explored for these next-generation devices, including their application as replacement limbs and sensory organs. For neuroprosthetics, the use of recording and stimulating electrodes in n ­ eural devices presents the challenge of creating materials that function well at the interface of the device and the tissue. This interface needs to be bio­compatible and stable over many years of use. There is a growing trend toward measuring signals over ensembles of neurons to capture more signals over wider arrays of electrodes. These ensembles are thought to contain patterns of information encoded by neurons. They have also been used to explore dynamics and plasticity events in the brain that will be important for using these devices in controlling neural prosthetics. Dramatic new demonstrations of the power of recording or stimulating neuronal ensembles (10-1,000 neurons) include the recent demon- stration by nonhuman primates of utilizing an implanted neural interface and “thought” to control a prosthetic arm in a controlled cursor computer game. This has led to a surge of new activity funded by many agencies in the area of neural prosthetics, given the severe social impact of losing a limb in an accident or in

A d va n c e d F u n c t i o n a l M at e r i a l s 55 combat. These activities seek to integrate a working neural prosthetic interface in both the central nervous system (brain) as well as peripheral sites (Figure 3.10). Another example is the not-so-recent effort to create an artificial visual system by stimulating populations of retinal ganglia through an implanted neural device in the retina (Figure 3.11). The development of these new devices presents a host of challenges in research on and application of advanced functional materials. In addition to the molecular and neural physics of cell stimulation, materials science and biocompatibility are important issues in these prosthetics. Fundamental research in biocompatibility, biofouling, and loss of electrical connectivity in these implanted devices is leading to a greater understanding of the dynamic interface between an electrode and neu- ral tissue. There have been considerable efforts to identify biocompatible materials for many of these applications, including both coatings (polyethylene glycol, nerve FIGURE 3.10  Conceptual model of DARPA’s artificial arm that would be wired to the periph- eral nervous system. SOURCE: DEKA Research and Development Corporation.

56 Inspired by Biology FIGURE 3.11  Retinal prosthesis for artificial sight. The microelectrode array stimulates sur- viving retinal neurons based on patterned signals that are received from the video camera. SOURCE: U.S. Department of Energy Artificial Retina Project. growth factor) and the incorporation of biomolecular components that can reduce inflammation and stimulate growth into the surrounding tissue. These challenges, even if overcome, will also require significant advances in the understanding of neural code. There are also efforts to explore alternative “electrodes” for these applications, such as radiofrequency, magnetic, and optical energy to record and stimulate neurons. Many of these efforts are still in the early stages, and the focus is on understanding the signals harvested or induced by these methods. The field of neuroprosthetics is enormously challenging. It comprises neurosci- ence, materials science, tissue engineering physics, chemistry, robotics, and more. In the next 5 years, research should focus on decoding the signals from neuronal

A d va n c e d F u n c t i o n a l M at e r i a l s 57 ensembles, fabrication and stability of electrodes, the physics and chemistry of elec- trode interactions with neural cells, and the opportunities for molecular structures and other materials to influence electrode design. Research in the 20-year time frame will build on these advances to construct integrated systems that restore neural function. A few examples of the focus areas of this research are artificial sight, weak electric field nanosystems for the study and control of cells and neurons, virtual-reality-based rehabilitation, rehabilitation robotics, deep brain stimulation, and next-generation neural interfaces, including molecular prosthetics. Advanced Functional Materials and National Security The unique functional properties of biological molecules offer new opportuni- ties to create useful devices and systems for national security. Materials that have properties similar to those of biological materials may be used in applications such as environmental surveillance, protective clothing, and decontamination. These are discussed in the following sections. Environmental Surveillance and Biosensing Understanding changes in the environment has taken on ever-increasing importance for the United States and has implications for both global and national security. One focus of this report is the long-standing R&D in biosensors for the detection of harmful agents in the environment (pollutants and chemical or biological agents). The design and fabrication of useful biosensor devices for this purpose require the understanding and exploitation of a number of biomolecular functional properties. The unique ability of biomolecules to be sensitive, specific, and sometimes adaptive makes them very attractive for use in biosensor devices. Many of these same properties and challenges in the efforts to create working devices are similar to those faced by efforts in medical diagnostics, given that both applications exploit similar biomolecular functional properties. There has been tremendous progress in engineering and using new engineered biomolecules in biosensors, many of which share the same advantages as the biosensors used in medical diagnostics. These biomolecules include antibodies, nucleic acids, proteins, and cells. They offer exquisite specificity to a target, and in some cases signal amplification can make them very sensitive detecting elements. One important consideration in the functional properties of these components in biosensors is that the signal transduction event is often based on key biomolecular interactions on immobilized surfaces. Most biomolecular interactions in biology take place at two-dimensional constructs (e.g., a lipid membrane), but the ability to mimic this environment in an engineered system is limited. In spite of these

58 Inspired by Biology limitations, the ability to detect the presence of pathogens in the environment is improving dramatically. However, significant challenges remain for the real-world deployment of envi- ronmental biosensors, including both fundamental research challenges and the interpretation of responses in order to make effective decisions. For example, most of the biosensors deployed to detect the release of a biological agent in the environ- ment have too slow a response time to limit public exposure to an agent (detect to warn). They are, however, informative about what exposure has already taken place and are useful in determining who should receive palliative care (detect to treat). While there has been progress in moving biosensors to the field, their success in the field is so far somewhat limited. In part, this is due to the very low tolerance for false alarms from these devices. Biosensors can raise false alarms for a number of reasons, including fouling and loss of functional activity. In order to optimize the utility of biosensors, a large community is addressing other reasons for false alarms, including manufacturability, sample processing, data collection, analysis, and information processing. The advent of multiplexing techniques to dramati- cally increase the quantity of signals generated by biosensors also contributes to solving the bioinformatic challenge, which is to derive useful knowledge and make decisions based on signals from these devices. The lifetime of biomolecular components (and thus the shelf life of biosensors) is also a limiting feature of these systems. When they are external to an organism, functionalized biomolecular materials have a limited lifetime during which they maintain the biological conformational integrity of working components. There are tight tolerances for such system conditions as temperature and ionic strength, to name just a few. Some of the interfering components in real-world samples (e.g., proteases and heavy metals) can actually degrade properties and performance and limit the application of these systems. Some progress has been made in increasing the shelf stability of these systems through the use of protective agents and con- trolled storage, but this is an area that has not been carefully explored. Efforts to address stability should be based on clear demonstrations of utility in the field. Functional Biomaterials for Decontamination and Protection The use of biomaterials in the design and fabrication of materials that can neutralize environmental pollutants or chemical or biological threats has been an area of active research and development. These technologies have progressed to field testing with various polymers that contain a number of biomolecular com- ponents. Enzymes that degrade environmental contaminants have been engineered into polymeric materials with moderate success. One success is the incorporation of organic phosphatases into polyurethane materials for the decontamination of media exposed to nerve agents. Other nanoparticular detergent-like materials have

A d va n c e d F u n c t i o n a l M at e r i a l s 59 also been modified to degrade biological contaminants. These efforts continue to show promise. More recent efforts have integrated these advances into protective clothing, masks, and wipes, air filters, protective wear, paint and coatings. The question often posed in the development of these materials is, How clean is clean? with the answer often requiring some follow-up detection method. Recent cleanup activities to address the anthrax contamination of congressional office buildings and a nearby post office reveal the challenges of developing and deploying these materials and using them to determine a safe end point in decontamination. Fundamental challenges remain in designing materials that maintain functionality for the desired properties and in ensuring that the media that are to be decontami- nated (soil, water, material assets) are directly exposed to the functional agents. Next-Generation Bioinspired Materials The diversity of materials that nature offers as inspiration for the design of new materials is enormous. The flora and fauna on our planet exhibit a variety of sophisticated structures that are perfected to perform multiple biological functions. Understanding the underlying design principles of these unique biological mate- rials drives much of the research aimed at developing new bioinspired materials. Societal interests in areas such as energy, nutrition, and health will also motivate the exploration and development of new bioinspired materials. Functionality can be defined in many ways for these materials, one of them being the biological ability to create supermaterials with exceptional physical and chemical properties. Supermaterials from Biology In the last decade, there has been an explosion of information about unusual natural structures that are superstrong, superadhesive, superhydrophobic, super- hydrophilic, superefficient, self-cleaning, self-healing, and self-replicating, with superior designs and intricate shapes. Biological materials are also often multifunc- tional, a characteristic highly desirable in artificial materials and processes. The various “super” properties of biological materials come from a sophisti- cated structural design exerted by self-assembled biomolecules, but the details of how this is achieved are still largely unknown, so that now more than ever is the time to study the underlying biological control mechanisms using advances in the physical sciences and applying this knowledge to bioinspired engineering. Scientists are beginning to answer important questions on how to use biological strategies to make materials that build themselves, repair themselves, and evolve. Nature keeps surprising researchers by revealing new and ever-more-interest- ing biomaterials with unexpected properties. The gecko’s foot, the lotus leaf, mussel byssus threads, mollusk shells, bones, spider silk, Venus’s flower-basket, butterfly

60 Inspired by Biology wings, and brittle stars, described below, are just several inspirational examples that have escalated interest in smart biological materials on the part of physical scientists and engineers. These attributes lie beyond conventional engineering. If these biological systems can be reformulated in a synthetic context, it might one day be possible to design nano- and microscale materials and composites for various applications. Multidisciplinary research in this exciting area will become a critical theme of the biomolecular and bioinspired physical sciences. Superhydrophobic The lotus flower grows out of the mud, repels dirt when it unfolds, and then remains pristine. In 1975, two botanists discovered that its self-cleaning ability comes from the presence of microstructures coated with a nanoparticulate hydro- phobic layer on the leaf surface (Figure 3.12). These structures render the surfaces superhydrophobic, so that water droplets are unstable and pick up dirt, insects, and contaminants as they roll off. Interestingly, a general theory that accounts for the lotus effect had been developed by physicists Wentzel, Cassie, and Baxter 50 years earlier, but its importance was appreciated only once the biological effect was described. Analogous rough surfaces with water-repelling properties were then reported to exist in a variety of organisms. The carapace of the desert beetle, for example, has another ingenious feature—it controls the direction in which the droplets roll, thus capturing and using the sparse water in the extremely arid environment. Biological discoveries have opened up for human use a new field of self-cleaning surfaces, and numerous technical inventions in the spirit of bioinspi- ration empowered by advances in nanotechnology keep appearing. FIGURE 3.12  Left: Microstructures on the surface of a lotus leaf are covered by a nano- coating of wax. Right: A water droplet on the leaf adsorbs particles of dirt as it rolls. In this picture, one can also see the microstructured papillae on the cuticula. SOURCE: Copyright Wilhelm Barthlott, University of Bonn.

A d va n c e d F u n c t i o n a l M at e r i a l s 61 These initial successes are gratifying, but a real scientific triumph would be to go beyond the simple imitation of the biological structures and use nature’s principles to create materials with properties beyond those found in nature. Can a surface nanostructure or surface chemistry (or a combination thereof) be designed that will repel any liquid, including organic solvents? Can a strategy be invented for the fabrication of reversible nanostructured surfaces that repel water in wet conditions and collect moisture in dry conditions in response to the environmental changes? Superstrong Nature fascinates scientists and engineers with numerous examples of excep- tionally strong building materials. It has been shown that whether the structures are fully organic (for example, wood), hybrid organic/inorganic (for example, bone), or nearly fully inorganic (for example, mollusk shells and Venus’s flower-basket), their supermechanics usually arises from the successive hierarchical assembly of the constituent structural units from the nanometer to the macroscopic scale (Figure 3.13). Nature’s ability to improve inherently poor and brittle building materials, such as glass, calcium phosphate, or calcium carbonate, by introducing a molecular-level control of the structure implemented by biomolecules (proteins, polysaccharides) is unmatched in technology. The biomolecular components and cells improve the mechanical performance by dissipating the energy of advancing cracks and by creating a responsive living interface that provides a flux of ions and macromolecules for the localized material deposition or dissolution. As a result, the final products often confer a remarkable capacity for recovery, self-repair, and fault resistance or tolerance. The fracture energy of spider silk, for example, is two orders of magnitude greater than that of high-tensile-strength steel, and shells are three orders of mag- nitude more fracture-resistant than a single crystal of the CaCO3 that constitutes their structure. Spider silk and mollusk shells are regarded by many as the holy grail of materials science: Despite a concerted effort over the decades to explain their underlying mechanical principles, to determine the role and the structure of the proteins that make up the biomaterial, and, ultimately, to replicate it, there has not been much success yet. The same is unfortunately true for the extensive studies of the biomolecular mechanisms controlling the formation and structure of bone and teeth. Artificial bioinspired composites and elastic polymers are significantly inferior to their natural analogues. While this challenge is not new, it remains critical to study the structural complexity of unique biological materials and the underlying biomolecular mecha- nisms of their synthesis and organization. Will it be possible to then disengage from nature and, by only using the concept of hierarchy, suggest new materials

62 Inspired by Biology FIGURE 3.13  Skeleton of Venus’s flower-basket, a glass sponge. This complex, tough glass architecture hierarchically designed from the nanometer to the centimeter scale houses a pair of mating shrimp fully protected from the environment. Scale bars from top down: 5 mm, 100 μm, 25 µm, 5 µm, 500 nm. Glass fibers forming a crown at the base of the sponge house possess wave-guiding properties similar to those of commercial fibers. The tips of the optical fibers are outlined in red. SOURCE: J. Aizenberg, Harvard University.

A d va n c e d F u n c t i o n a l M at e r i a l s 63 and design solutions not necessarily seen in nature, in which the properties of each structural level are not perfect but the successive levels compensate for the defects and together contribute to the mechanical stability and toughness of the resulting design? Superadhesive Mussels are adapted to survive in the wave-swept environments of tidal zones. Their competitive dominance and success in these extreme conditions are in part due to a unique anchorage system that reliably attaches them to any substratum (rocks, wooden piers, metal bridges, and so forth). The attachment of a mussel to a substratum is achieved by the mussel byssus, which is composed of a bundle of extracellularly secreted collagenous threads that are glued to the substratum by an adhesive plaque at their distal ends. The threads show unusual mechanical properties: a stiff tether at one end and a shock absorber with 160 percent exten- sibility at the other end (compared with a typical ~10 percent extensibility of other ­collagenous materials). The mussel byssus thus has two super­characteristics that scientists dream of understanding and mimicking—high elasticity and super­ adhesion in a wet environment. The latter makes them very attractive for medical and dental applications in particular, as a wet, biocompatible glue that has no analogues so far. A previously unknown natural block copolymer was recently identified as being responsible for the elastic properties of the threads. In earlier work, the byssal’s superadhesion was attributed to a new protein rich in the amino acid dihydroxyphenylalanine (DOPA). This protein is extremely sticky from the moment it is formed, and getting it to the place where you want it is nearly impos- sible. Nature solves this problem by using an ingenious just-in-time technique, whereby the protein is first formed without its sticky domain and the adhesive moieties are synthesized at the later attachment stage. A gecko can climb a perfectly smooth wall or cling to the ceiling, supporting its entire body weight with only a single toe! Their supergripping has been shown to arise from highly developed hydrophobic nanomicrostructures: densely packed keratin microcolumns (setae) further split into bundles of nanospatulas cover a gecko’s toes, providing an extraordinarily high adhesion to different surfaces (Figure 3.14). The gecko adhesive works in a vacuum and under water, leaves no residue, and is self-cleaning. Most importantly, the adhesion is reversible, and geckos alternatively stick and unstick themselves 15 times per second as they run up walls. In an attempt to mimic the properties of webbed gecko’s feet, dense arrays of high-aspect-ratio polymeric microcolumns have been fabricated and shown to provide a significant adhesive capability. Their flexibility, however, which is neces- sary for conformal surface contacts, leads to an undesired outcome: The polymer fibers tend to stick to each other, entangle, and irreversibly collapse.

64 Inspired by Biology FIGURE 3.14  A Tokay gecko with its toe outlined (a). Scanning electron micrographs of rows of setae from a toe (b), a single seta (c), and the finest terminal branches of a seta, called spatulae (d). A single seta attached to a MEMS cantilever capable of measuring force production during attachment parallel and perpendicular to the surface (e). A single seta attached to an aluminum bonding wire capable of measuring force production during detach- ment perpendicular to the surface (f). Angle between setal stalk and wire represented by α. SOURCE: K. Autumn, Y.A. Liang, S.T. Hsieh, W. Zesch, W.P. Chan, T.W. Kenny, R. Fearing, and R.J. Full, “Adhesive force of a single gecko foot-hair,” Nature 405:681-685 (2000). Despite long-standing admiration for the superior properties of mussel threads and gecko toes, all attempts to mimic their design or to synthesize artificial poly- mers that are analogous to the bioadhesives in structure or function have been largely unsuccessful, and the mystery of a mussel’s unique elastic, highly adhesive fibers and the magic of a gecko’s “dry” glue with its reversible attachments remain unsolved, unmatched, and more challenging than ever. Biooptics Manipulation of light is a basic feature of many living organisms. Well-known examples of biological optical structures and processes are the eyes of higher organisms and the photosynthesis mechanism in plants. Over millions of years,

A d va n c e d F u n c t i o n a l M at e r i a l s 65 biological optical structures and processes have evolved that can compete with or even outperform the cutting-edge optical technology being developed by today’s scientists: multilayer reflectors, diffraction gratings, optical fibers, liquid crystals, and structures that scatter light—all of them devices that can be described using optical theory—are found in animals as well, based on a diversity of designs. Iri- descent sparkle and the blue color of a Morpho butterfly, which can be seen for hundreds of meters, are caused by a periodic photonic structure in scales cover- ing the wings (Figure 3.15). No dye is involved. Structural color appears to be a common strategy in nature: “Living opals” are reported to be present in peacocks, beetles, and marine organisms. FIGURE 3.15  Optical microscope photograph of scales on a Morpho wing. SOURCE: P. Vukusic, University of Exeter.

66 Inspired by Biology In addition to the beauty and structural complexity of Venus’s flower-basket, there is now a scientific understanding of the efficient fiberoptical lamp in the base of its latticework (Figure 3.13). The optical features of these biological fibers are remarkably similar to those of their commercial counterparts, and their mechani- cal properties are better still. Another example of a cutting-edge technology found in nature is the array of optically optimized, single-crystalline calcitic microlenses formed by a brittle star. In this amazing, tunable optical structure, the lenses are compensated for birefringence and spherical aberration and are combined with a microfluidic system that transports pigment to regulate the intensity of light reaching the receptors. Recent studies mark the beginning of our appreciation for the complex optical structures found in biological specimens and open this field for future discoveries. Only now, when the physical and biological sciences are beginning to come together and new methods of nanoscale fabrication and characterization are developing, do these studies have a good chance of reaching fruition. It is noteworthy that only one optical device in animals has been taken through the manufacture stage: the fly-eye antireflector. Currently there are attempts to fabricate a functional microlens array combined with a microfluidic system, which mimics the features of brittle star optics. However, natural photonic and optical structures are often highly complex at the nanoscale, and mimicking their design may be beyond the ability of today’s engineers. This might justify the study of cellular and biomolecular mechanisms and processes. Can researchers also exploit the flawless processes of optical manu- facture employed by animals? Biological Inorganic Supermaterials Although polymers and organic materials have found their way into modern technology, inorganic materials will remain the basic elements in engineering. There is, however, a growing realization that the traditional methods of “heat and beat” will not be able to fulfill the requirements for future advanced materials, in particular the construction of higher-order architectures from the nanometer to the macroscale. New approaches that use molecular design and assembly should be developed. The ability to take inorganic building blocks and organize them into nanoscale, microscopic, or bulk materials is of importance in electronics, catalysis, magnetism, optics, sensors, and mechanical design. Again, the best guidance in the search for new inorganic materials and fabrica- tion strategies should come from the study of biological processes. The formation of inorganic materials in organisms, so-called biomineralization, results in exqui- site, finely tuned hybrid superstructures that possess exceptional mechanical, opti- cal, and magnetic properties. In addition to the well-known structural biominerals, such as calcium carbonates, calcium phosphates, and silica, a variety of unexpected

A d va n c e d F u n c t i o n a l M at e r i a l s 67 inorganic materials (Fe, Sr, Ba, Cu, Ag, Au, sulfides, oxides, sulfates, and hydroxides) have recently been found in organisms. The organisms control the inorganic poly- morph, the location of nucleation, the size of the inorganic particles, their crystal- lographic orientation, and their intricate shapes and assembly, from the nanoscale to the macroscale. The importance of the study of the (bio)molecular processes that lead to these supermaterials and the relevance of biomineralization processes to modern technology were wonderfully depicted by Stephen Mann, one of the pioneers of biomimetic inorganic chemistry. He generalized that three biological principles are critical: (1) preorganization/assembly of (bio)organic molecules into structural and chemical scaffolds at the nanometer scale prior to mineral deposi- tion; (2) templating and molecular recognition at organic-inorganic interfaces that result in controlled, site-directed nucleation of oriented inorganic nanoclusters within preformed supramolecular assemblies; and (3) larger-scale ­cellular process- ing of the nanostructures into higher-order, functional architectures. The molecular details of these processes are largely unknown and require thorough investigation by cell biologists, structural chemists, materials scientists, molecular biologists, physicists, and engineers working in tandem. Recently it has been shown that even the most primitive use of these principles, when complex bio- logical matrices were replaced by synthetic self-assembled molecules that mimicked biological supramolecular architectures and surface micropatterning was used to template site-specific oriented nucleation, could lead to significant successes in controlling the orientation, polymorph, shapes, positioning, and nano/micropat- terns of synthetic inorganic crystals. “Currently we are only at the beginning of a biomimetic approach to inorganic materials, and there is a long way to go, but it is feasible that one day, the dusty, dirty world of minerals could be transformed by biological insights. A quiet revolution is underway.” Materials That Mimic Proteins and Membranes There are considerable efforts to mimic many of the functional properties of biomolecules and their assemblies. Most man-made polymeric materials used today serve structural purposes in plastics, fibers, paints, and rubbers. These polymers lack precise sequence specificity, and do not approach the functional sophistication of biomolecular materials. For example, proteins can catalyze chemical reactions, transport ions, convert energy to motion, repair other biopolymer molecules, and transduce signals and energy. Biomolecular functions can be linked together into complex, interacting and responsive systems that exhibit emergent, higher-level  S. Mann, D.D. Archibald, J.M. Didymus, T. Douglas, B.R. Heywood, F.C. Meldrum, and N.J. Reeves�� , “Crystallization at inorganic-organic interfaces: Biominerals and biomimetic synthesis,” Science 261:1286-1292 (1993).

68 Inspired by Biology functionalities. These powerful functionalities are possible because, unlike current man-made polymers, biopolymers are informational as well as structural in nature: Their functions are encoded within distinct sequences of diverse monomer sets. An exciting research area that has emerged in the past decade involves the mimicry of these functions in new man-made polymeric materials that are informational and designed to have a wide range of sequence-structure-function relationships. For example, microbes are transfected with artificial genes with de novo- designed monomer sequences, allowing these biological cells to be harnessed for the production of nonnatural proteins. Researchers are synthesizing nonnatural amino acids that can still be polymerized by a microbe’s protein synthesis machin- ery, leading to proteins with novel properties. In other cases, simple oligopeptide sequences are patterned after Alzheimer’s disease peptides, both to study the aggre- gation process that underpins the disease and also to create new self-assembling materials. Other researchers are creating minimalist proteins—molecules that are simple enough that the sequence patterns can be designed without computers, often to test folding principles and create simple proteinlike functionalities. Folded peptides are being designed as cages, with gates that can be triggered to synchro- nize the release of dyes that allow complex biological and chemical events to be followed in real time. An important recent development in creating useful and functional new pro- teins that can mimic particular functions is directed evolution. In this approach, a gene is first randomly mutated to create a library of variants. The library is then screened to select mutant proteins having the desired properties. This method is now widely practiced in academia and being industrialized to produce new proteins that are more stable or enzymatically more active or that have different substrate specificities. Efforts are also under way to mimic the conformal activity of proteins in con- structs termed foldamers, which are chain molecules composed of non­biological monomers that, in principle, can fold like proteins and possibly function like proteins. One example is a class of polymers called peptoids, which are sequence- specific heteropolymers based on chains of N-substituted glycines. Peptoid-based helical bundles have been made that fold cooperatively, have hydrophobic cores, and can bind zinc tightly and cooperatively, serving as a proof of principle that such polymers are proteinlike in important ways. A family of peptoid 9-mers with hydrophobic side chains was discovered to form a uniquely folded, highly stable threaded-loop structure in acetonitrile, held together by hydrogen bonding, which surprisingly has a polar core and a hydrophobic outer surface. Like mirror-image proteins, peptoids are not degraded by protease enzymes and so may be useful as therapeutics and biomimetics. Peptoids have been used to create novel lung sur- factant protein mimics, as well as bioavailable mimics of antimicrobial peptides. Excellent progress is being made as well in the design of protein-mimetic structures

A d va n c e d F u n c t i o n a l M at e r i a l s 69 based on β-peptides and chimeric α/β-peptides, including peptidomimetics that kill bacteria in the same manner as antimicrobial peptides and inhibit protein- protein binding interactions involved in viral fusion and the cell cycle. Mimicry and patterning of lipid bilayer membranes is an active area of research to create spatial domains at the micro- and/or nanoscale, potentially leading to nanowires, nanonetworks, and to the formation of nanoscale “corrals” in which different membrane proteins may be localized. These efforts have obvious appli- cations because many of the functional activities of biomolecules (for example, drugs) are manifest in lipid membranelike environments. Since 40 percent of the blockbuster drugs currently on the market act on membrane proteins, any new method that can successfully isolate, characterize, or utilize membrane proteins is an important advance. To advance the field of mimicry of functional materials, there are several chal- lenges that remain to be addressed: in synthetic chemistry, in the creation of new monomer types and new ligation strategies for linking them together with high efficiency, and to improve the yields of functional polymers. There are challenges in characterizing structural and physical properties of bioinspired or mimicked polymers that are created. And, there are substantial challenges in computational chemistry, to be able to design the sequences having targeted properties. Integration of Functional Biomolecular Materials Exciting progress is now being made, but a great deal of work remains, toward seamlessly integrating functional, nonbiological materials and devices with living, mobile biological systems, including cells and tissues. Such nonbiological materials must be stable for a tunable period of time and remain uninfected and ideally non- encapsulated when implanted in vivo. While certainly functional, today’s metallic replacements for missing bones and joints remain at a primitive state of integration with the recipients’ living tissue. An ideal compatibility of medical implants such as pacemakers and artificial hearts with the host site is clearly very important since these devices remain in place for 5-10 years. Outside the medical realm but in the area of biotechnology, a host of biomol- ecules and their assemblies (cells, enzymes, antibodies, light-harvesting complexes) could provide useful functional system elements if they could be stably interfaced with man-made systems, many of which tend to be metallic or plastic (electrodes, batteries, computers, etc.). Many of nature’s most useful proteins are embedded in lipid membranes, and learning to work with these types of proteins in artificial membranes will allow many important advances in biotechnology. Finally, a variety of machines could be designed to interface more intimately and effectively with their operators (for example, fighter jets, passenger vehicles, cranes and other heavy

70 Inspired by Biology equipment, surgical robots and lasers) if effective interfacing from the human mind and nerves to the machine could be created. A variety of polymeric materials have been used in the human body, but so far, none perform as fully required or desired. Most of the presently used materi- als were “found” or “discovered” rather than “designed” materials. For example, polymers and plastics not originally created for use as biomaterials were found to be serviceable in artificial hearts, valves, and so on. Clearly, these found materi- als have very limited biocompatibilities. All of the presently used biomaterials eventually fail in vivo, in part because of problems with uncontrolled nonspecific cell/protein binding or fouling and a propensity for infections to occur at the site. Virtually all of today’s biomaterials cause undesired clotting of blood; all patients implanted with cardiovascular materials require systemic anticoagulant therapy. All endotracheal tubes become infected after 5 days; 99 percent of systemically delivered nanoparticles or nanogels end up uselessly in the liver or spleen; 100 percent of small-diameter vascular grafts fail within 21 days in vivo; and virtually all implanted biosensors fail in vivo within 20 days. This highlights the tremendous work that remains to be done by engineers and scientists in understanding and controlling the interactions between abiotic and biotic interfaces. Today’s approach to integrating elastomeric or inorganic materials with living cells or tissue is to functionalize the surfaces with biocompatible and/or bioac- tive materials. Such tailored interfaces are now starting to make their way into clinical trials but still represent technology that is primarily academic and not yet commercialized. A significant amount of fundamental research has been done to discover the best ways of forming strong bonds between inorganic surfaces (such as gold, copper, silicon, silica, titania, and carbon surfaces) and tethered or bound polymers or biomolecules. Thiol moieties bind strongly to gold or copper, while reactive organosilanes can be used to modify silica or oxidized silicon. Some of the strongest bonding moieties yet discovered are inspired by natural systems, such as the modified L-DOPA amino acid that evolved to serve in the tethering elements of shellfish to allow them to adhere to shoreline rocks. When chemical approaches are taken, this is typically done using hydrophilic, uncharged oligomers or polymers, most commonly based on ethylene oxide. Polyethyleneglycol seems, at this time, to be the best polymer to reduce nonspecific binding of proteins and cellular material. On the other hand, some very physical approaches to modifying surfaces, including laser treatments, glow discharge plasma treatments, and supercritical fluid process- ing are showing significant promise as well in decreasing the nonspecific binding or fouling of abiotic interfaces with biomolecules. Second, for the best biological compatibility, the appropriate molecular signals, typically but not always proteins or their assemblies, must be presented on the engineered surfaces. Micropattern- ing of these surfaces appears to be a powerful way to control cellular behavior, as

A d va n c e d F u n c t i o n a l M at e r i a l s 71 well as the integration of microscale technologies with the use of biocompatible polymers and hydrogels. Understanding the mechanisms of cell adhesion at the molecular level will be very important. Reengineered enzymes, more stable than natural proteins, may be critical for the tasks of interface engineering. Strategies for the synthesis and derivatization of nanoparticles such as cross-linked copolymer micelles and dendrimers are maturing rapidly, and these types of particles may soon find their way into clinical use. In general, the primary challenges faced today involve fool- ing biological molecules, which are very potent sensors of their environment, into believing that abiotic, human-made functional elements are not there. Using engi- neered biomaterials, artfully applied, researchers may succeed in putting realistic sheep’s clothing on inorganic materials that are normally seen by living systems as the wolf. Opportunities and Challenges The wide range of biological functions and advances in the understanding of their properties present enormous opportunities for further science and techno- logical advances. In this chapter, selected topics were reviewed in the discovery and application of these functional properties, including alternative and renewable energy, health and medicine, and biomaterials for national security needs. Particu- lar functional properties of biological systems were also reviewed, and superior functional properties that have not yet been fully appreciated or exploited were described. Finally, issues in mimicry, synthesis, and integration of functional bio- molecular materials were discussed as important underpinnings to the science and technology of functional biomolecular materials. Some challenges to the scientific understanding of advanced functional materials are enumerated below where new understanding is beginning to emerge (first-level bullets). They are followed by opportunities that might arise if researchers develop enough of an understanding to address the challenges. Alternative and Renewable Energy There have been considerable advances in the understanding of energy stor- age and conversion properties of biomolecular materials and greater interest in engineering new properties that extend these important functional attributes. Efforts to create new functional biomaterials in plants, based on engineering the plant genome, that can be efficiently converted to usable, high-energy-containing materials are largely motivated by the promise of new sources of biofuel. Biomi- metic photosynthesis is gaining momentum as researchers unlock the mechanisms involved when light is used to fix carbon.

72 Inspired by Biology • Challenges to scientific understanding: Expanded use of biofuels from plant and animal biomaterials —Opportunity: Improved conversion efficiency and utilization of substrates —Opportunity: Robust and controlled genetic engineering of new syn- thesis in plants • Challenges to scientific understanding: Research and development of arti- ficial photosynthetic and other proton gradient systems —Opportunity: Efficient conversion of light to useful products —Opportunity: Expanded selection of new biomolecules for energy con- version and storage —Opportunity: Integration of photosynthetic complexes into artificial matrices • Challenges to scientific understanding: Discovery of the structural and functional dynamics of biological motors in situ —Opportunity: Assembly and integration of linear and rotary biologi- cal motors into useful nanomachines or devices that realize biomotor performance —Opportunity: Stability and robustness of biological motor complexes Health and Medicine The use of biomolecular materials in health and medical devices is widespread and has already had a significant socioeconomic impact on society. The engineering of new biomolecules for increased functions such as sensitivity or specificity is an active area of research and has shown early payoff in the design of new tools such as diagnostic assays. Nanotechnology efforts have found new ways to manipulate the delivery of drugs to specific sites in the body and to enhance the delivery of new therapeutic compounds. Finally, the area of neuroprosthetics, which combines innovation across a number of disciplines, including neuroscience, materials sci- ence, microelectronics, and engineering, is moving rapidly. • Challenges to scientific understanding: New functional biomolecular mate- rials for diagnostic array detection with desired sensitivity and specificity —Opportunity: Sample preparative and other methods to enhance signal detection and reduce background noise in a diagnostic platform —Opportunity: Computational analysis of multiplexed, large diagnostic and profiling datasets for prognostic medicine • Challenges to scientific understanding: Improved delivery of drugs using functionalized and controlled-size biomolecular materials

A d va n c e d F u n c t i o n a l M at e r i a l s 73 —Opportunity: Prediction and control of in vivo response to implanted materials —Opportunity: Synthesis and characterization of controlled nanoparticles, polymers, and dendrimers with functionalized properties —Opportunity: Targeted delivery to specific sites • Challenges to scientific understanding: New neuroprosthetics with dynamic function for artificial limbs and sensors —Opportunity: Development of reliable interfaces with long-term function —Opportunity: Integration of recorded signals with response in closed- loop function —Opportunity: Understanding information coding from neuronal ensembles National Security Global threats from the environment and in the context of military and home- land defense present unique opportunities for the application of functional materi- als. The most mature application may be biosensors, where the hope of developing detect-to-warn systems continues to drive this field across a variety of interested government customers (Department of Energy, Department of Defense, Depart- ment of Homeland Security, and Environmental Protection Agency). Biosensor performance has improved, but needed advances in reducing false alarm rates and integrating samples from different sources are still a challenge. Decontamination materials that can detect and protect, degrade, and perhaps regenerate are under investigation by a number of agencies and provide good application platforms to integrate advanced functional materials. • Challenges to scientific understanding: Biosensors that reliably detect threats in time to prevent exposure and consequences of environmental threats (detect to warn) —Opportunity: Reduction of false-alarm rates —Opportunity: Sample processing and handling for optimal performance • Challenges to scientific understanding: New decontamination and protec- tive materials that incorporate functional biomolecules —Opportunity: Methods of incorporation that optimize biomolecular function —Opportunity: Cooperative functions that derive from multifunctional materials —Opportunity: Demonstration in real-world conditions with reliable measures of determining safety after cleanup

74 Inspired by Biology Next-Generation Bioinspired Materials • Challenges to scientific understanding: Wide selection of biological mul- tifunctional systems to drive inspiration of new designs with advanced materials properties (e.g., strength, adhesion, optics, hierarchy, assembly) —Opportunity: Enriching and harvesting underlying principles and pro- cesses to drive engineering activities in a productive way —Opportunity: Fabrication and assembly of inspired materials • Challenges to scientific understanding: Biomimicry of biomolecules with directed conformation and information content —Opportunity: Defining desired properties and designing controlled synthesis —Opportunity: Characterization and implementation of new materials • Challenges to scientific understanding: Seamless integration of functional- ized biomolecular materials in the body and in devices —Opportunity: Preventing fouling and foreign-body reaction from func- tionalized biomaterials —Opportunity: Controlling reactions at the surface to optimize desired integration of material —Opportunity: Longevity and robustness Suggested Reading Department of Energy, Basic Research Needs for Solar Energy Utilization, Washington, D.C., 2005. Available online at http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf. Last accessed March 26, 2008. Gellman, S.H. “Foldamers: A manifesto,” Accounts of Chemical Research 31:173-180 (1998). ����������������� Humayun, M.S., J.D. Weiland, G. Chader, and E. Greenbaum (editors), Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances, New York, N.Y., Springer, 2007. Jung, C.C., E.W. Saaski, D.A. McCrae, B.M. Lingerfelt, and G.P. Anderson, “RAPTOR: A fluoroimmunoassay-based fiber optic sensor for detection of biological threats,” Sensors Journal, IEEE 3(4):352-360 (2003). Langer, R. “Drug delivery: Drugs on target,” Science 293:58-59 (2001). Langer, R., and N.A. Peppas, “Advances in biomaterials, drug delivery, and bionanotechnology,” American Institute of Chemical Engineers Journal 49(12):2990-3006 (2003). Mann, S., Biomineralization: Principles and Concepts, Oxford University Press, 2001. National Research Council, The New Science of Metagenomics: Revealing the Secrets of Our Microbial Planet, Wash- ington, D.C., The National Academies Press, 2007. National Research Council, Polymer Science and Engineering: The Shifting Research Frontiers, Washington, D.C., National Academy Press, 1994. Pollard, T.D., W.C. Earnshaw, and J. Lippincott-Schwartz, Cell Biology, 2nd Ed., Saunders, 2007, Chapters 36-39. Raber, E., A. Jin, K. Noonan, R. McGuire, and R.D. Kirvel, “Decontamination issues for chemical and bio- logical warfare agents: How clean is clean enough?” International Journal of Environmental Health Research 11(2):128-148 (2001). Ragauskas, A.J., C.K. Williams, B.H. Davison, G. Britovsek, J. Cairney, C.A. Eckert, W.J. Frederick, Jr., J.P. Hal- lett, D.J. Leak, C.L. Liotta, J.R. Mielenz, R. Murphy, R. Templer, and T. Tschaplinski, “The path forward for biofuels and biomaterials,” Science 311:484-489 (2006).

A d va n c e d F u n c t i o n a l M at e r i a l s 75 Russell, A.J., J.A. Berberich, G.F. Drevon, and R.R. Koepsel, “Biomaterials for mediation of chemical and biological warfare agents,” Annual Review of Biomedical Engineering 5:1-27 (2003). Vogel, P.D., “Nature’s design of nanomotors,” European Journal of Pharmaceuticals and Biopharmaceuticals 60(2):267-277 (2005). Vukusic, P., and J.R. Sambles, “Photonic structures in biology,” Nature 424:852-855 (2003). Wainwright, S.A., W.D. Biggs, J.D. Currey, and J.M. Gosline, Mechanical Design in Organisms, New York, N.Y., John Wiley and Sons, 1976. Wang, L., J. Xie, and P.G. Schultz, “Expanding the genetic code,” Annual Review of Biophysics and Biomolecular Structure 35:225-249 (2006). Weiland, J.D., W. Liu, and M.S. Humayun, “Retinal prosthesis,” Annual Review of Biomedical Engineering 7:361- 401 (2005).

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Scientists have long desired to create synthetic systems that function with the precision and efficiency of biological systems. Using new techniques, researchers are now uncovering principles that could allow the creation of synthetic materials that can perform tasks as precise as biological systems. To assess the current work and future promise of the biology-materials science intersection, the Department of Energy and the National Science Foundation asked the NRC to identify the most compelling questions and opportunities at this interface, suggest strategies to address them, and consider connections with national priorities such as healthcare and economic growth. This book presents a discussion of principles governing biomaterial design, a description of advanced materials for selected functions such as energy and national security, an assessment of biomolecular materials research tools, and an examination of infrastructure and resources for bridging biological and materials science.

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