The strength of metals is limited by the weakest link in their microstructures. At high temperature, the weakest link is usually a grain boundary. Mobile dislocations represent another type of weak link. The strength-limitation by this two-level hierarchy of crystal defects can be minimized through microstructure design. Although the strengthening mechanisms differ for grain boundaries and dislocations, it is possible to design microstructural architectures that strengthen both. The design must take the stress state into consideration that the component will encounter during its later use. Processing strategies can then be devised for the synthesis of such components. It is imperative to stabilize the designed microstructure at high temperature because this will determine how well and how long the material will be able to perform. Design, synthesis and stabilization are discussed in the present paper. The lamp filament wire technology is the basis for a new method that enables the synthesis and stabilization of artificial microstructures in high-temperature materials.

The complex hierarchical structure of the cornea of the eye as it is known from light and electron-microscopy is reviewed. Microscopy depicts the cornea as a layered structure with each layer parallel to the cornea's surfaces. The middle 90% of the cornea is a connective tissue layer called the stroma, which in turn is comprised of 2 μm thick sheets (lamellae) of collagen fibrils embedded in a ground substance. Fibrils within a given lamella have their axes parallel to one another, while fibril axes in adjacent lamellae make large angles with one another. Predictions of light scattering theory are then used to show how light scattering measurements made on fresh tissue can test these structures for possible artifacts due to preparative procedures. With unpolarized light, the total scattering cross-section depends on the fibrillar ultrastructure, but not on the lamellar structure. Angular scattering depends on both levels of hierarchical organization. The lamellar level of organization is especially significant for the propagation and scattering of polarized light. To date the structural implications of scattering measurements are in accord with structures seen in electron micrographs.

Nonlinear optical processes provide many of the functions needed for Photonics Technology. Molecular materials are hierarchical systems in which the structure can be controlled from the angstrom level to macroscopic scale to optimize the nonlinear optical properties. In this article basic concepts of nonlinear optics and photonics are reviewed. Multifunctional molecular hierarchical systems are discussed which involves heterostructure design at molecular and bulk levels. For optimization of nonlinear response at the molecular level, guidance from theoretical modeling and study of sequentially built and systematically derivatized structures has proven to be useful. The use of Langmuir-Blodgett film approach to control the order and conformation with monolayer resolution is discussed. Examples of our approach to optimize the response at the bulk level involve the use of orientation control by electric field poling and use of composite heterostructures. An important advancement in heterostructure design is the use of sol-gel processing to make organic-inorganic composites which have successfully exhibited electro-optic modulation and femtosecond Kerr gate operations. Finally, novel photorefractive molecular materials are discussed.

Several types of biornimetic structures suitable for underseas systems are described. Composite sensors and actuators are modeled after coral, fish, sharks and bats.

Composites have been formed of Cycom, nickel-coated graphite fibers, aligned at concentrations of up to 6 weight percent in polymer matrices. Dielectric constants up to approximately 70 have been measured for these samples at x band frequencies. The results have been compared with a single non-interacting particle theory.

The components of biological signal transduction pathways have been compared with those of electronic circuits. Indeed, attempts are being made to incorporate biomolecules into electronic devices. However, man-made biomolecular devices do not yet mimic several important features of naturally occurring systems, especially those of eukaryotes. It is well known that the polypeptide components of biological signal transduction pathways include subunits of receptors, regulatory proteins, enzymes and channels. It is less well appreciated that each component often exists in many functionally related but not redundant forms. When proteins are hetero-oligomers, numerous combinations of these forms may be permitted yielding large arrays of signal transduction molecules with overlapping properties. Further, the organization of these molecules within the cell Is highly complex. Especially alien to the materials scientist are the findings that signal transductlon proteins can translocate between regions of cells in response to stimuli and change in concentration as a consequence of breakdown and resynthesis. These features, as well as the interactions between signal transduction pathways, produce far more complex and fluid signalling networks than those presently used in man-made devices. Is anything to be gained by mimicking such complexity or are we on the right track in designing relatively simple, structurally rigid devices using biomolecular components?

Electrochromic tungsten oxide thin films have been prepared by reactive dc-magnetron sputtering under different deposition conditions. Through the use of scanning electron microscopy, spectroscopic ellipsometry, and electrochromic coloration experiments, the effect of preparation conditions on film morphology and electrochromic properties has been studied. The results of this study are consistent with a previous report which found that tungsten oxide thin films are dominated by a hierarchy of dense columnar regions and less dense void regions. This morphology not only dominates the structure of tungsten oxide thin films but also strongly controls the electrochromic properties. From spectroscopic ellipsometry data and Bruggeman effective medium approximation models presented here, it seems reasonable that a cermet model of absorption can accurately describe the tungsten oxide preparationelectrochromic property relationship.

Layered electrochromic glass was investigated in order to test the feasibility of using hierarchical designs for new electrochromic devices. Matrix-type synthetic electrochromic structures were designed, and two types of matrix devices, polarization and plating, were produced and evaluated. The application and device requirements of electrochromic glasses are discussed.

We describe techniques used to characterize hierarchically structured synthetic and biological materials. These techniques decipher the structures through the dimensional spectrum from the molecular and atomic scales (10.−10 m) to the macro scales (10−3 m) where the overall shape of the material emerges. Techniques used to image surfaces and internal structures can be categorized according to their wavelength and thus spatial resolution as light, x-ray, and electron microscopy. We also discuss newly emerging microscopy techniques that image surfaces at the atomic level without a focusing lens, such as scanning tunneling and atomic force microscopies, with the aid of field ion microscope and scanned probe microscopes. Transmission electron microscopy (TEM), a unique tool for multipurpose imaging, provides structural information through direct imaging, diffraction, and spectroscopic analysis. We illustrate the major TEM techniques used to analyze structural hierarchy with examples of synthetic and biological materials. We also describe light optical microscopy and scanning probe microscopy techniques, which cover the opposite ranges of the dimensional spectrum at the micrometer and subangstrom levels.

Rescaling (renormalization-group) [1] and restructuring [2] transformations are used in the statistical mechanics of systems with different complexities at different length scales, starting with the microscopic scale. These transformations are effected by a partial trace of the partition function, yielding effective coupling constants that incorporate the free energy contribution of the structures and length scales that are summed out of the problem. An example of this approach is a quantitative treatment [3] of the phase diagram of krypton adsorbed onto graphite. At the smallest length scales, adatom adsorption (first and second layer) and vibrations at adsorption sites are considered. At the next scale, sublattice formation and vacancy occurrence on the hexagonal graphite surface are considered. At a larger scale, heavy and superheavy domain wall formation, wall crossings and annihilations (dislocations) [4] are considered. This problem is treated by a sequence of 5 different transformations. Starting with the microscopic potentials, phase diagrams are obtained in temperature versus pressure or coverage, featuring disordered, commensurate and incommensurate solid phases, in good agreement with experiments [5]. The mechanism for reentrant phase transitions is divulged. Such solutions can be considered approximate treatments of physical systems or, alternatively, exact solutions of “hierarchical models” [6]. This “realizability” on hierarchical lattices insures the robustness of the approximation.

Tiling problems arise in many areas of science, and they raise fascinating but difficult mathematical questions. Nonperiodic hierarchical tilings are particularly interesting and important. We discuss several examples of such tilings and indicate some of the unsolved problems related to them.

We first summarize the scaling behavior of breakdown strengths and transport and elastic moduli of random two phase composites. We then consider the effect of disorder on two hierarchical structures. The first, a tree-like structure illustrates the fact that disordered trees, such as occur in many natural circulatory systems, show large fluctuations in their conductance and current flow. The second, a continuous fiber composite (CFC), leads us to suggest that, due to its greater flaw tolerance, a hierarchical microstructural design may improve the longitudinal and transverse toughness of CFC's.

A working definition of the term hierarchy that is consistent with past usage is that a hierarchy is an assemblage of assemblages. Self-assembling systems are essential to those hierarchical structures that contain structure on micro and nano distance scales since these tiny structures being so large in number can not be manipulated individually as is possible for larger structures. It is shown how self-assembling structures are nothing more than materials that have undergone a structure inducing phase transition. The induced structures can be either equilibrium structures (example lamellar block copolymers) or kinetic patterns (example snowflakes). Polymer systems provide a rich source of phase transitions. There are at least ten classes of polymer phase transitions. [1] helixrandom coil [2) polymerization or one-dimensional crystallization [3] adsorption [4] collapse transition [5] coilfstretch transition [6] liquidliquid transitions [7] crystallization [8] liquid crystals-plastic crystals [9] glass-amorphous and sol-gel transitions [10] soaps-membranes-micellesvesicles-block copolymers. These phase transitions can interact with each other creating an even richer overlay of self-assembled systems. We discuss three aspects of the building of Hierarchical structures from selfassembling systems.

Cellular materials are made up of an interconnecting network of strut- or plate-like members. Natural cellular materials include wood, cancellous bone, cork, palm and leaves. Engineering cellular materials are either in the form of honeycombs, with two-dimensional, prismatic cells, or foams with three-dimensional, polyhedral cells. Natural cellular materials usually have multiple phases at multiple scales while engineering cellular materials use a single phase at a single scale.

In this paper, the efficiencies of various natural cellular materials are estimated by analyzing their behavior at various microstructural scales. The results of the analysis suggest microstructural design of hierarchically structured engineering cellular materials.

The hierarchical evolution of the different mimetic systems and the concomitant level of control in the biomimetic mineralization will be critically discussed.

An in-vitro method of mineralizing an aligned, self-assembled collagenous matrix is presented. Reconstituted collagen fibers were mineralized by exposure to saturated solutions of calcium and phosphate of varying pH in a double diffusion chamber for seven days at room temperature. Microscopic investigation of the mineral precipitate within the fibers indicate the formation of hydroxyapatite crystals with features comparable to mineral observed in bone and avian tendon. Mechanical test results indicate that tensile strength and tangent modulus increase after mineralization in comparison to unmineralized control fibers. These results suggest that mineralization of collagen fiber in-vitro may parallel some of the events seen in mineralization of bone and turkey tendon. In addition, mineralized collagen fibers may be useful in the design of composites for the replacement or augmentation of hard tissue

Mechanisms for the production of oriented precipitates are reviewed in the context of the orientation in mineralized biological tissues. Organic crystals have been oriented in polymer matrices by crystallization in a temperature gradient and by crystallization within a drawn crystalline polymer. Elongated oxide particles can be formed in a polymer by drawing of a material which contains phase-separated droplets of alkoxide, followed by hydrolysis of the alkoxide. Highly elongated rod-like or disc-like particles can be formed. Alternatively drawn films of a polymer blend can be swollen with alkoxide and hydrolysed to produce elongated chains of particles.

The structural polysaccharides are a family of polymers of hexoses and pentoses that occur in all plant cell walls. The distinguishing characteristic of these polymers is a β-1,4-linked backbone. The most common among these is cellulose, which is the linear homopolymer of anhydroglucose. These polysaccharides are capable of aggregating into highly ordered structures that are the primary determinants of the mechanical and physical properties of cell walls. An overview of the variations in patterns. of structural-polysaccharide aggregation within cell walls is presented here. Among the majority of the algae cellulose is the dominant structural polysaccharide; thus the habit of aggregation is dominated by the patterns of cellulose. Among primitive plants, other structural polysaccharides represent a larger fraction of cell-wall mass and cellulose is less dominant. In woody tissues of higher plants, structural polysaccharides are the major components of the cell wall, and the patterns of aggregation are again dominated by the characteristic habits of cellulose. Within tile phylogenetic framework, higher levels of morphological development apparently involve greater complexity in the molecular architecture of the cell walls and a finer level of blending of the components of aggregates at the molecular level.

Natural cellulosic fibers have in common the fact that cellulose is the key polymeric component in the structure, although the chemical composition varies widely depending on the specific source of the fibers. Cellulose is a long-chain linear condensation polymer of β-D-glucose with three free hydroxyl groups on each monomeric unit, resulting in strong inter- and intramolecular hydrogen bonds. Because of the hydrogen bond network, and also due to restricted rotation around the polymeric 1,4- β-linkage, cellulose is a rigid and stiff chain with a Tg well above the chemical decomposition temperature.

Despite the high Tg native cellulose is invariably highly crystalline as a result of the biosynthetic process. In naturally occurring cellulosic fibers, the cellulose crystallites are aggregated into fibrils which constitute the underlying building blocks of the fiber. In cotton, the fibrils are laid down during the development or growth of the fiber in the form of concentric layers. The fibrils are disposed at an angle of 23° with respect to the fiber axis and thus they describe a helical pattern. The sense of the helix reverses frequently along the length of the fiber. This morphology is unique to cotton; other cellulosic fibers such as ramie and jute have similar fibrillar structures, but fibrillar angles in the 5 to 10 degree range, with no reversals. Wood fibers, on the other hand, are structurally more heterogeneous and may be considered as composites.

Poly(γ-benzyl α,L-glutamate) (PBLG) has been widely used in studies of the physics of rod-like polymer chains [1]. The helical structure of PBLG gives rise to considerable chain stiffness, such that the persistence length of the chain is on the order of 70 nm in helicogenic solvents [2]. This feature, coupled with the ease of synthesis and good solubility of the polymer has made PBLG the system of choice for the study of both isotropic [2,3] and liquid crystalline [4] solutions of rod-like macromolecules.