Hierarchical Composite Materials: Achieving What a Single-Phase Material Cannot

In recent years, goal-oriented composite processing techniques, making use of the Advanced Materials by Design 1 concept, have been investigated with the goal of creating materials with a desired behavior, response, or performance. When no single homogeneous material is sufficient to fulfill design requirements, fibers, particulates, precipitates, or other phases are introduced, creating composites with complex structures. In hierarchical composites, the various components in the composites are also themselves composites; 2 hence, hierarchical composites have components that span different length scales, exhibit levels of critical material responses, and in some cases have functional structural geometry. Such natural materials as bone, nacre, and wood have hierarchical structures that benefit from levels of component functionality and mechanical responses. 3 Corresponding to these natural hierarchical structures, new composite systems (e.g., bio-inspired ceramic composites and advanced metal-matrix composites) are designed to achieve what a single-phase material cannot. There will be many opportunities in the future to design composite systems with hierarchical structure impacting material properties from the nano- to the macroscale. The importance of length scales and the effect of salient properties of the different components at characteristic lengths 4 is addressed in the following papers. In the article by Tarakanova and Buehler, the structure–function relationship of spider silk is described. They consider the mechanical response of the silk at three different levels. At the lowest length scales (below 20 nm), features of the protein, such as hydrogen bonds, beta sheet crystals, and the interaction between amorphous and crystalline domains, are considered. At the next level (100 nm), a coarse-grain model of the protein networks constituting individual fibrils is subjected to four idealized loading conditions for a variety of fibril diameters. Using parameters obtained from the lower-scale simulations in the coarse-grain model, the authors demonstrate that the optimal fibril diameter is 20 nm to 80 nm, which is consistent with fibril diameters found in nature. The final level is the spider web, with the radial and spiral silk threads modeled using another coarse-grain scheme. Results from the lowest-level simulations were directly applied to model the web’s response to relevant loads at the scale of the web. The end results are two distinct multiscale models (proteinto-fibril and protein-to-web) incorporating relevant mechanical responses from the lowest length scale, linked in a bottom-up approach. The article by Dabbs and Aksay describes their recent research on processing nanoporous silica structures by liquid-crystalline L3 phase templating. The L3 phase templating technique can be used to create hierarchical structures of silica monoliths, coatings, and fibers. The silica monoliths (which have structures that resemble cancellous bone) can be tuned to have densities between 0.5 g/cm 3 and 0.8 g/cm 3 and channel diameters between 1 nm and 10 nm, simply by varying the nonpolar constituent concentration. The high void volume, void connectivity, and large surface area allow for adsorption sites, fluid infiltration, and drug incorporation while maintaining the structure and strength of the silica framework. The authors describe infiltrating photopolymers into L3 templated silica disks for optical data storage, using templated silica coating for heightened sensitivity to adsorption of targeted molecules, integrating the drug indomethacin into a thermoresponsive polymer for controlled drug release, and creating low-density, high-strength silica fibers. The L3 phase templating technique can be used to generate a variety of silica composites, thus producing application-specific hierarchical structures.