In biology, organic-inorganic hybrid materials are used for several purposes, in particular, for protection and mechanical support. These materials are generally optimized for their function through precise control over the structure, size, shape, and assembly of the component parts and can be superior to many synthetic materials. The shapes and forms of minerals encountered in nature strongly contrast with those that are generally formed in a synthetic environment. According to current understanding, this is achieved through different modes of control: their shape can be controlled by restricting their growth to a confined space or by influencing their preferred direction of growth; in addition, for crystalline materials, polymorph selection and oriented nucleation are achieved through specific interactions between a template or additive and the developing nucleus. Also, controlled arrangement of nanoparticles into superstructures can lead to a complex structure. The understanding and, ultimately, the mimicking of these processes will provide new synthetic routes to specialized organic-inorganic hybrid materials. On the other hand, transformation of existing complex hierarchical natural structures such as wood or diatom frustules into other materials using shape-preserving chemistry is another approach toward minerals with complex biomimetic structure. The theme topic in this issue will focus on recent biomimetic and bioinspired approaches used to achieve control over the shape and organization of mineral and organic-inorganic hybrid materials. The different contributions will also highlight the advantages of these methods for advanced materials synthesis, and possible applications will be discussed.

Amorphous silica is (next to CaCO3) the second most abundant biologically produced inorganic material. A certain group of photosynthetic microalgae, called diatoms, forms complex 3D silica architectures (frustules) containing regularly arranged nanoscale features (pores, channels, protuberances). Recently, biomolecules involved in diatom silica formation have been characterized, and first insights into their structure-function correlations have been obtained. This has spurred the development of synthetic (bio)polymers capable of directing the in vitro formation of silica and other inorganic materials from aqueous precursor solutions under mild conditions. Here we present a summary of current insight into the mechanism of silica formation by diatom biomolecules and provide examples of synthetic (bio)polymers for the formation of silica and titania materials with complex structures.

Biominerals such as the nacre of shells, spicules of sea urchins, teeth, and bones are inorganic-organic hybrids that have highly controlled hierarchical and complex structures. These structures are formed in mild conditions, and the processes are controlled by macromolecular templates of proteins, peptides, and polysaccharides. Materials scientists can obtain ideas from the structures, properties, and formation processes of biominerals for use in creating synthetic, biomimetic materials. This article highlights bioinspired synthetic approaches to the development of organic/CaCO3 hybrids using macromolecular templates. These hybrids have oriented, patterned, and 3D complex structures, as well as thin films with smooth surfaces. The structures are formed by templating synthetic and semisynthetic macromolecules. These materials have great potential for new functional materials.

Oriented aggregation is a nonclassical crystal growth mechanism resulting in new secondary particles composed of crystallographically aligned primary crystallites. These secondary crystals often have unique and symmetry-defying morphologies, can be twinned, and can contain stacking faults and other significant defects. A wide range of materials, such as titanium dioxide, iron oxides, selenides and sulfides, and metal oxyhydroxides, are known to grow by oriented aggregation under certain conditions. Evidence for oriented aggregation also has been observed in natural materials. Over the last decade, reports of this crystal growth mechanism have appeared with increasing frequency in the scientific literature. The development of kinetic models aimed at improving our fundamental understanding as well as facilitating purposeful control over size, size distribution, and shape has ranged from simple dimer formation models to polymeric models and population balance models. These models have enabled detection and characterization of crystal growth by oriented aggregation using methods such as small-angle x-ray scattering, among others, in addition to transmission electron microscopy. As our fundamental understanding of oriented aggregation improves, novel and complex functional materials are expected to emerge. This article presents a summary of some recent results, methods, and models for characterizing crystal growth by oriented aggregation.

In the sophisticated architectures of various biominerals, a multilevel hierarchy of lengths from nanometers to millimeters is commonly observed. In this article, superstructures composed of inorganic crystals associated with organic molecules from carbonate-based biominerals were identified and categorized as mesocrystals, which are composed of crystallographically oriented crystals. For the production of hierarchically structured mesocrystals, two strategies using self-organized growth with organic agents in aqueous solution systems were proposed. Arranged structures of micrometric and nanometric units were induced with an insoluble gel matrix and soluble adsorbable organic molecules, respectively. Consequently, bioinspired hierarchical crystals were successfully achieved by combining the matrix and soluble species.

Biological preforms such as plant tissue offer a novel approach for manufacturing biomorphous ceramics with an anisotropic cellular micro- and macrostructure pseudomorphous to the natural template structure. Mimicking the hierarchical microstructure of the native template at different length scales from large vessels (mm) down to a cell wall microstructure (μm to nm) offers the possibility to tailor the local strut microstructure in biomorphous ceramics in order to improve mechanical properties at low density. Mineralization may be achieved by intercalation of the cell walls with an inorganic, metal organic, or organometallic sol. Heating above the pyrolysis temperature of the hydrocarbons forming the cell wall material in an inert atmosphere finally results in a positive replica of the cellular structure with a metal oxide/carbon composite forming the cell walls. Amorphous, nano- or microcrystalline C/Si-O-C(-N) composite materials are obtained by infiltration with a low viscosity preceramic polymeric precursor, such as polycarbosilane, -silazane, -siloxane, or a copolymer or mixture thereof. Pyrolysis into a biocarbon template and subsequent metal alloy melt or vapor infiltration and reaction at high temperatures above 1000°C is an alternate way to produce single and multiphase carbides and composites.

Stardust, a NASA sample return mission, safely landed in the Utah desert in January 2006 after a seven-year mission, bringing with it the first cometary material from a known parent source, Comet 81P/Wild 2. One of the mission goals is to determine the starting material of the solar system. By sampling a comet, which has spent most of the past 4.6 Gyr beyond the orbit of Neptune, we expect to measure material presumed to be unaffected by the ignition of the sun. The Stardust spacecraft swept through the tail of the comet, collecting hundreds of micron-sized particles from that stream into aerogel, a low-density silica foam. An international team of materials scientists have studied the mineralogy, petrology, and elemental and isotopic abundance of these materials. Our group has studied elemental abundance using an x-ray microprobe; the morphology of the particles was examined using an x-ray microscope, which enables nanotomography of the particles while encased in aerogel. The unexpected conclusions are that much of the material from this comet was formed near the sun, after its ignition, and soon thereafter transported to the outer reaches of the solar system. These results have changed the way astrophysicists think about solar system formation.