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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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ENGINEERING NOVEL STRUCTURES

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Colloidal-Scale Engineering

ERIC W. KALER

Center for Molecular and Engineering Thermodynamics

Department of Chemical Engineering

University of Delaware

Newark, Delaware

Recent advances have made it possible to apply engineering principles to the control of processes on size scales ranging from nano-to micrometers. These are now the scales where the modern technologies of biology and electronics operate, but they have long been the domain ruled by the old discipline of colloid science. In this range the interfacial properties of a fluid or substrate become important, fluid flows are always laminar, and mass and heat transfer can become controlled by surface tension forces. Here processes of mixing and chemical reaction depend on "moving nanoliters nanometers," and the successful control of these processes results in devices of revolutionary size, speed, and selectivity. In this regime grinding and milling to make small particles no longer operate reliably, and much effort must be focused instead on self-assembly to synthesize new materials with spatially ordered features on the nanometer scale. These materials have present and potential applications in optical information processing and storage, advanced coatings, and catalysis. The intellectual driving force for this process results in part from efforts to mimic hard natural materials, such as bone and shell, and soft high-strength materials like spider silk; but, current efforts fall far short of producing the structural complexity of nature, and utterly fail to mimic its full function.

A common approach to the production of nano-scale structures relies on templates that assemble into desired patterns and then can be replicated by a harder, more durable material. Surfactant molecules spontaneously assemble into a variety of useful structures and are commonly used as the template for materials with feature dimensions of a few nanometers, such as zeolites (Kresge et al., 1992). Larger templates are needed for materials with larger dimensions, and polymer beads have recently been used as templates. Surfactant molecules

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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again play an important role in this process, because they can modify and control the growth of polymeric materials in an aqueous environment, such as in the process of emulsion polymerization. The possibility of using self-assembled surfactant structures to prepare polymers with novel supramolecular architectures has long tempted researchers, and polymerization in these complex fluids offers the potential to prepare polymers with morphologies precisely controlled on the length scale of nanometers. The variety of polymer morphologies potentially accessible by this approach include spherical latex particles in size ranges inaccessible to traditional emulsion polymerization techniques (radii as low as 7 nm have been reported), as well as highly porous materials with connected open channels some tens of nanometers in diameter. A roadblock to production of designed polymer structures has been the lack of a clear mechanistic picture of the microscopic events governing the formation of polymer, but recently the basic principles have been outlined (Morgan and Kaler, 1998). Larger (50–1000 nm) polymer beads are articles of commerce, and their surfaces can be functionalized with a variety of bioactive moieties.

Once these colloidal particles are in hand, the next challenge is to assemble them into larger scale superstructures. A promising class of such materials obtained by self-assembly is the colloidal crystals, ordered arrays of particles in the nanometer-to micrometer-size range. However, because the materials obtained after colloidal crystals are dried are brittle and can be redispersed in water, replication is necessary. In addition, the size and shape of the colloidal crystal needs to be controlled on a macroscopic scale. Several ways have been developed to control the organization of colloidal assemblies by using flow, surface tension forces, or electrical fields.

In the first example colloidal crystals are used to template a unique new material, nanostructured porous gold (Velev et al., 1997, 1999). The metallic structure is assembled on the surface of a filter membrane from nanometer-sized gold particles that are templated by colloidal crystals of larger latex microspheres that have been formed by flow over the filter. When the templates are removed chemically or by calcination they leave behind a three-dimensional metallic nanostructure with long-ranged ordering of the pores. The pore size is precisely controlled in the sub-micrometer range by the diameter of the latex microspheres.

The above procedure is shown in Figure 1. The templates are assembled from monodisperse, negatively charged polystyrene latex microspheres ranging in diameter from 300 to 1000 nm, with the colloidal crystals of interest formed by concentrating the particles in the vicinity of a membrane surface by filtration. Next, 15–25 nm colloidal gold particles are deposited in the cavities of the latex crystals. The colloidal gold fills the interstices in the latex arrays, forming a gold structure that is by itself porous on a smaller mesoscopic scale. This porosity allows the solvent to flow through the deposit until the pores are completely filled by the deposited gold colloid. After removal of the latex beads the gold superstructure appears as in Figure 2. The lace-like structure shown in Figure 2

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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FIGURE 1 Schematic presentation of the method of assembly of polymer-templated gold.

is an almost exact replica of the three-dimensional wire-mesh photonic crystals described by Sievenpiper et al. (1996) but scaled down by a factor of 20,000 to the sub-micrometer region. The wire mesh photonic structure has been demonstrated to have an interesting combination of forbidden bands in the gigahertz region. As the photonic properties scale with array size, miniaturizing the structure should shift these properties to the infrared-visible region, which is of significant interest for optoelectronic devices.

Templating, therefore, guides the synthesis of new materials, but as yet

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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FIGURE 2 A representative photograph of a gold sample after dissolution of the latex.

The highly ordered, lace-like gold structure is an analogue of a 3D wire-mesh photonic crystal scaled down to the sub-micrometer region. SOURCE: Adapted from Velev et al. (1999). Reprinted with permission from Nature.

there is a lack of simple and controllable methods for extraction of colloidal crystals from their aqueous environment and mounting or shaping them into usable solid objects. A step towards producing and processing colloidal crystalline materials is assembly of spherical or globular crystalline structures (microballs) in aqueous droplets suspended on the surface of a heavier liquid immiscible with water. Here, surface tension forces can be controlled to guide the formation of the particles. The colloidal particles in the suspended template droplets are gradually concentrated by drying to the point of transition to an ordered state. The tangential mobility of the droplet surfaces provides a ''nostick" substrate, where the particles are free to move, rotate, and rearrange into defect-free lattices. After complete evaporation of the water, highly ordered and symmetric composite particles with smooth surfaces are obtained. Control of the interplay between gravity and interfacial tensions allows control of the shape of the template droplets and their assemblies.

Colloidal self-assembly also is useful in the synthesis of devices in which electronic microchips are created out of, and interface with, fluid-borne colloidal and biological systems. Lithographically patterned substrates provide a well

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×

FIGURE 3 Schematics of the main stages of the assembly and operation of the sensor. The procedure is illustrated by an immunoglobulin test. SOURCE: Reprinted with permission from Velev and Kaler (1999). Copyright 1999 American Chemical Society.

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×

defined, organized template for colloidal assembly and electrical manipulation of colloidal systems and living cells on a micron scale. Using such a substrate, we have developed a method for assembling microscopic sensor patches in situ from the same latex particles used in traditional agglutination assays (Velev and Kaler, 1999). The glass substrates for our sensors carry photo-lithographically fabricated gold patterns that form addressable electrodes of micron size with small gaps between them. To generate the active area of the sensors, the particles are collected in the gaps by dielectrophoresis and then stabilized. The assembly procedure can be repeated many times by exchanging the particle suspension and addressing different gaps so that different sensors can be assembled on the same "chip."

A schematic of the main stages involved in the preparation and read-out of the biosensors is shown in Figure 3. The limit of detection of the sensors is about 5x10-14 moles, which is comparable to the better IgG agglutination assays and immunosensors currently available. We believe this level can be reduced dramatically.

ACKNOWLEDGMENTS

This work is the result of a collaboration with O. D. Velev and A. M. Lenhoff.

REFERENCES

Kresge, C. T., M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and J. S. Roth. 1992. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 359:710–712.


Morgan, J. D., and E. W. Kaler. 1998. Particle size and monomer partitioning in microemulsion polymerization. 1. Calculation of the particle size distribution. Macromolecules 31(10):3197–3202.


Sievenpiper, D. F., M. E. Sickmiller, and E. Yablonovitch. 1996. 3D wire mesh photonic crystals. Physical Review Letters 76(14):2480–2483.


Velev, O. D., T. A. Jede, R. F. Lobo, and A. M. Lenhoff. 1997. Porous silica via colloidal crystallization. Nature 389:447–448.

Velev, O. D., P. M. Tessier, A. M. Lenhoff, and E. W. Kaler. 1999. Nanostructured photonic metal synthesized via colloidal crystal templates. Nature 401:548.

Velev, O. D., and E. W. Kaler. 1999. In situ assembly of colloidal particles into miniaturized biosensors. Langmuir 15(11):3693–3698.

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Design of Biomimetic Polymeric Materials

ARUP K. CHAKRABORTY

Departments of Chemical Engineering and Chemistry

University of California

Berkeley, California

INTRODUCTION

Recent technological advances are demanding materials that can carry out functions with high specificity, but how can we design materials from the molecular scale up so that they are able to perform these functions? One class of materials that offers potential for use in such advanced applications is polymers, large macromolecules made up of a connected set of monomers. Monomers can be of different types, and the sequence in which they are connected as well as the nature of the connections (i.e., the molecular architecture) can vary. Polymers are strong candidates for use in applications where the ability of a material to perform functions with specificity is crucial because we know that nature uses polymers (e.g., proteins and nucleic acids) the same way. Evolution has allowed nature to devise schemes that allow the design of macromolecular building blocks that can self-assemble into functionally interesting structures. Thus, one way to devise synthetic systems would be to take lessons from nature. This does not mean that the detailed chemistries of nature should be copied. Rather, we should search for underlying universalities in the schemes that natural systems employ in order to carry out a class of functions, and then should explore whether the universal schemes (if they exist) can affect biornimetic behavior. The interest in generic schemes has developed because they may be easier to implement in abiotic applications. Elucidating the universal schemes also provides insight into the underlying phenomena that effect self-assembly processes, and thus allows a specific class of functions to occur.

It is clear that to engineer macromolecules to function in specific ways a synergistic effort combining synthesis, physical experimentation, and theory is required. In this presentation, we will focus on how sophisticated theoretical and

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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computational methods can be employed with experimental work to create synthetic macromolecular systems that can perform functions with biomimetic specificity. Two examples that illustrate the general ideas are: 1) biomimetic recognition between polymers and surfaces and 2) molecular architecture that can control surfactant properties of macromolecules with high sensitivity.

BIOMIMETIC RECOGNITION BETWEEN POLYMERS AND SURFACES

Many biological processes such as transmembrane signaling and pathogen-host interactions are initiated by a protein when it recognizes a specific pattern of binding sites on part of a membrane or cell surface. Recognition means that the polymer quickly finds and then adsorbs strongly on the pattern-matched region and not on others. The development of synthetic systems that can mimic such recognition between polymers and surfaces could have a significant impact on such advanced applications as the development of sensors, molecular scale separation processes, and synthetic viral inhibition agents (Sigal et al., 1996; Todd et al., 1994). Attempting to affect recognition in synthetic systems by copying nature's detailed chemistries does not seem practical for most applications. This leads to the question of whether there are any universal strategies that can affect recognition between polymers and surfaces. To deduce candidates for such strategies, some generalized observations regarding biological systems were made. Proteins are composed of many types of monomers, and have sequences that are not periodically repeating (Chan and Dill, 1991; Dewey, 1997; Irbäck et al., 1996; Pande et al., 1994). Similarly, the pattern of different binding sites on cell surfaces is not periodically repeating. These observations lead to the question of whether competing interactions (due to preferential interactions between different types of monomers and surface sites) and disorder are essential ingredients for biomimetic recognition in synthetic systems.

Disordered heteropolymers (DHPs) are synthetic macromolecules with chemically different monomer units distributed along the backbone in a disordered sequence that is described statistically.1 Like proteins, they are composed of different monomers, with sequences that are aperiodic and quenched (the sequences cannot change in response to the environment). Unlike proteins, which carry a specific pattern encoded in their sequence distribution, DHP sequence distributions correspond to statistical patterns. (The meaning of statistical patterns is discussed below.) Imagine a DHP chain interacting with a surface bearing many types of sites that are distributed in a statistically described manner.

1  

If a number of DHPs made up of the same monomers are synthesized in a reactor, each DHP chain will have a different sequence. However, every such sequence belongs to the same statistical distribution determined by the reactivity ratios (Odian, 1991)

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×

Such surfaces resemble cell surfaces wherein the pattern of binding sites is not periodically ordered. They are different from cell surfaces in that they bear statistical rather than specific patterns. Let the DHP segments exhibit preferential interactions with the surface sites (i.e., a particular type of segment prefers to interact with a certain type of surface site, and the other types of segments exhibit a different preference). Studying such a system allows an exploration of whether competing interactions and quenched disorder are sufficient ingredients for biomimetic recognition.

Our recent theoretical and computational studies (Bratko et al., 1997; Golumbfskie et al., 1999; Srebnik et al., 1996) suggest the occurrence of a phenomenon akin to recognition when the statistics characterizing the DHP sequence and that of the surface site distribution are related in a special way (i.e., matched). Specifically, these studies suggest that frustration (due to the competing segment-surface interactions and quenched disorder) and statistical pattern matching lead to one of the hallmarks of recognition, a sharp discrimination between regions of a surface to which a given type of DHP sequence binds strongly and those onto which it does not adsorb. In other words, statistical pattern matching is sufficient for biomimetic recognition to occur, provided the statistical patterns are designed properly. Our results show how a mixture of DHPs bearing different statistical patterns can be separated on a surface by discriminatory adsorption onto regions that bear complementary statistical patterns. The results also show dramatic differences in chain dynamics in the "wrong" and pattern-matched regions of the surface; these differences shed light on the kinetic behavior of frustrated systems, or systems that include biological macromolecules.

Some specific results can be illustrated by considering a surface comprised of four quarters, each of which bears a distribution of sites of two types (red and yellow) on a neutral background. Such an arrangement is shown in Figure 1. The top right quarter is such that in a certain correlation length there is a high probability of finding sites of opposite types adjacent to each other (statistically alternating). The bottom left quarter illustrates that in a certain correlation length there is a high probability of finding sites of the same type opposite to each other (statistically blocky). All quarters of the surface are characterized by an average total loading of 20 percent, and the correlation length is ~ 1.4 for the statistically patterned regions.

Consider a mixture of DHPs with statistically blocky and alternating type sequences in solution interacting with such a surface. DHP segments of type A prefer to interact with red sites on the surface, and those of type B prefer the yellow surface sites (Figure 1). Statistically blocky (alternating) DHPs are statistically better pattern matched with the statistically patchy (alternating) part of the surface. If the surface shown in Figure 1 is exposed to a fluid solution containing a mixture of statistically blocky and statistically alternating DHPs, will the chain molecules selectively adsorb on those regions of the surface with which they are statistically pattern matched? Such recognition due to statistical

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×

FIGURE 1 Surface bearing different statistical patterns. The two different types of sites are colored red and yellow, with the neutral background being blue. The top right (bottom left) quarter is statistically alternating (blocky); the other quarters are statistically random. The black and white trajectories are 2-D projections of the center of mass positions of statistically alternating and blocky DHPs, respectively. 1 (2) and 1' (2') are starting and ending positions of the statistically blocky (alternating) DHP. Figure can be viewed in color online at <www.nap.edu>. SOURCE: Reprinted with permission from Golumbfskie et al. (1999). Copyright (1999) National Academy of Sciences, U.S.A.

pattern matching requires not only that the "correct" patch be strongly favored for binding thermodynamically but also that the "wrong" regions of the surface not serve as kinetic traps.

Figure 1 depicts typical trajectories at T/Tref = 0.6. All points on these trajectories do not correspond to adsorbed states. Both the statistically alternating and blocky DHPs begin on randomly patterned parts of the surface and ultimately find their way to the region of the surface that is statistically pattern

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×

matched with its sequence statistics. These results show that biornimetic recognition between polymers and surfaces is possible due to statistical pattern matching. It seems possible to exploit this notion to design inexpensive devices that can separate a large library of macromolecules into groups of statistically similar sequences. Sensor applications are also suggested.

The reason why statistical pattern matching allows biornimetic recognition to occur is made clear by the free-energy landscape shown in Figure 2. The "wrong" regions of the surface correspond to local free-energy minima that are

FIGURE 2 Free energy (in arbitrary units) versus center of mass position for a statistically alternating DHP interacting with the surface in Figure 1. Blue corresponds to deep free-energy minima, red patches are shallower free-energy minima, and yellow regions are free-energy barriers. The two deep free-energy minima along the right edge of the surface (wrong region) are due to periodic boundary conditions. Figure can be viewed in color online at <www.nap.edu>. SOURCE: Reprinted with permission from Golumbfskie et al. (1999). Copyright (1999) National Academy of Sciences, U.S.A.

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×

separated by relatively small barriers from each other. In contrast, in the statistically pattern matched region there exist a few deep global minima; each of these minima, in turn, is very rugged.

The nature of this free-energy landscape leads to many interesting kinetic phenomena. One that is particularly stimulating is the shape of the macromolecule that evolves with time. When recognition occurs due to statistical pattern matching, the macromolecules adopt a small class of shapes.

Adsorbed macromolecules are characterized by loops. A loop is a length of contiguous unadsorbed chain segments between two consecutive adsorbed segments. The distribution of these loops fluctuates in time, and hence loop fluctuations are dynamic modes. Prior to considering the dynamics of these modes, let us examine the distribution of these loops as trajectories evolve, starting from wrong parts of the surface. All trajectories show the qualitative features depicted in Figure 3 where we plot the probability distribution Pn for an arbitrary segment to be part of a loop of length n for a statistically alternating DHP. Each panel shows Pn averaged over different time (Monte-Carlo step) windows. The first panel shows that, when the center of mass of the chain is in the wrong part of the surface, Pn is essentially structureless. This implies that all possible macromolecular shapes are being adopted as the chain samples the wrong parts of the surface. Remarkably, the other panels in Figure 3 demonstrate that, as the chain center of mass enters the statistically pattern-matched region, Pn begins to show structure. Ultimately, as shown in the panel labeled b, it exhibits a spectrum of peaks that corresponds to preferred loop lengths and hence macromolecular shape in the adsorbed state. This observation of adsorption in preferred shapes due to statistical pattern matching is very similar to recognition in biology. Results also suggest an intriguing connection between the experimentally realizable system studied and some provocative ideas concerning the competition between self-organization and selection in evolution (Kauffman, 1993).

MOLECULAR ARCHITECTURE CAN CONTROL SURFACTANT PROPERTIES OF MACROMOLECULES

The ability of amphiphilic molecules to form organized assemblies in solution has important commercial and biological consequences (Discher et al., 1999; Gompper and Schick, 1994; Larson, 1999; Lasic, 1993; Safran and Clark, 1987; Won et al., 1999; Zana, 1986; Zhao et al., 1998). A variety of products, such as detergents, emulsifiers, catalysts, and vehicles for drug delivery, rely on this ability. Membranes in plant and animal cells are composed of self-assembled phospholipid bilayers. Self-assembly into these organized motifs is driven by the amphiphilic character of the molecular building blocks (i.e., different chemical groups in the molecules exhibit different solvent affinities). The simplest organized assembly is a micelle, which is formed to minimize unfavorable interactions between the medium and the poorly solvated moieties of the amphiphile.

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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a) 3.0 108 to 3.2 108 MC Steps

b) 3.2 108 to 3.4 108 MC Steps

c) 3.4 108 to 3.6 108 MC Steps

FIGURE 3 The probability distribution of loops (Pn). a) center of mass in the wrong parts of the surface; b) center of mass has entered the statistically pattern matched region; c) shape selective adsorption has occurred.

SOURCE: Reprinted with permission from Golumbfskie et al. (1999). Copyright (1999) National Academy of Sciences, U.S.A.

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×

Although a large majority of studies have been conducted using water as the solvent, the amphiphilic character of molecules can be expressed in a variety of media such as organic solvents and polymers (e.g., Balsara, 1998; DeSimone et al., 1994; Discher et al., 1999; Gompper and Schick, 1994; Johnston et al., 1996; Larson, 1999; Lasic, 1993; Safran and Clark, 1987; Won et al., 1999; Zana, 1986; Zhao et al., 1998). Extensive theoretical and experimental studies of micelles formed by single-tailed and double-tailed amphiphiles (Figure 4a), including those of biological importance, have been conducted (e.g., Balsara, 1998; DeSimone et al., 1994; Discher et al., 1999; Gawrisch et al., 1992; Gompper and Schick, 1994; Johnston et al., 1996; Karaborni and Smit, 1996; Karbaroni et al., 1994; Larson, 1999; Lasic, 1993; Lewis et al., 1994; Pochan et al., 1996; Safran and Clark, 1987; Tate et al., 1991; Won et al., 1999; Zana, 1986; Zhao et al., 1998). Recent studies have focused on synthetic double-tailed surfactants (Gemini surfactants) due to their superior properties (e.g., Danino et al., 1995; Zana, 1996). However, most efforts to control surfactant properties focus on proper choice of the chemical structure of the amphiphilic molecule.

The surfactant properties of macromolecules (and hence their ability to self-assemble into functionally interesting motifs) can be controlled with high sensitivity by manipulating molecular architecture without changing the chemical identity of the amphiphilic moieties. In addition to differences in surfactant properties between macromolecules in different architectural classes, subtle variations in an architectural class also lead to significant effects. This is due to the importance of conformational entropy for self-assembly processes of polymers. This notion of choosing the nature of the connections between the amphiphilic moieties to control surfactant properties may prove useful in applications where the choice of chemical structure is restricted (e.g., for concerns related to biocompatibility or toxicity).

We use light scattering experiments and a field-theoretic model to show that a class of branched macromolecules (see Figure 4c) is an extremely efficient surfactant. It is worth remarking that agrecans, one of the most effective biological surfactants, have a similar architecture (Alberts et al., 1994). For the branched macromolecules shown in Figure 4c, a slight affinity of the medium toward the backbone relative to the branches is sufficient for micelle formation. Such a small selectivity is insufficient to induce micelle formation in linearly connected polymeric amphiphiles such as diblock copolymers (Figure 4b). In addition, an intermediate branching density optimizes surfactant properties due to the interplay between molecular architecture and conformational entropy (i.e., a particular architecture in an architectural class works best).

ACKNOWLEDGMENTS

Financial support for the work described here was provided by the National Science Foundation, the U.S. Department of Energy (Basic Energy Sciences),

Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×

FIGURE 4 Amphiphilic molecules. (a) small molecule amphiphiles: single-tailed and double-tailed surfactants; (b) linear polymeric amphiphiles: diblock and triblock co-polymers; (c) branched amphiphiles with a well-solvated backbone and poorly solvated branches.

and the Camille-Dreyfus Foundation. I would like to acknowledge collaborations with Prof. E. I. Shakhnovich, Prof. N. Balsara, Prof. V. Pande, Dr. S. Y. Qi, and Mr. A. Golumbfskie.

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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×
Page 44
Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×
Page 45
Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
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Suggested Citation:"3 Engineering Novel Structures." National Academy of Engineering. 2000. Frontiers of Engineering: Reports on Leading Edge Engineering from the 1999 NAE Symposium on Frontiers of Engineering. Washington, DC: The National Academies Press. doi: 10.17226/9774.
×
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Frontiers of Engineering is the fifth book highlighting the presentations of the National Academy of Engineering's (NAE) annual symposium series, Frontiers of Engineering. The 1999 NAE Symposium on Frontiers of Engineering was held October 14-16, at the Academies' Beckman Center in Irvine, California. The 101 emerging engineering leaders (ages 30-45) from industry, academia, and federal laboratories who attended the meeting heard presentations and discussed cutting-edge research and technical work in four engineering fields. Symposium speakers were asked to prepare extended summaries of their presentations, and it is those papers that are contained here. The intent of this book, and of the four that precede it in the series, is to describe the content and underpinning philosophy of this unique meeting and to highlight some of the exciting developments in engineering today.

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