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.

Responsive materials cover a breadth of types and many application fields. The common feature in all cases is a nonlinear change in properties or behavior as a result of a stimulus. The material response can range from a simple change in conformation or ionization state, through to phase transitions, bulk aggregation, or complete dissolution. As a consequence, sensing and actuation are the most investigated functions of these materials. In this issue, we have chosen to focus on responsive materials as exemplified by externally switchable, environmentally activated, and reversibly or controllably triggered systems. The chemistries of these materials, their physical properties, functional behavior, and activity are all linked, so we have aimed to cover the many disciplines underlying responsive materials through articles featuring areas that already span disparate research topics. These areas include drug delivery, smart surfaces, and nanotube transducers. The responsive materials field is growing in excitement as well as activity, and we hope that readers will gain an insight into this fascinating branch of materials science through this MRS Bulletin issue.

The biconcave shape and corresponding deformability of the human red blood cell (RBC) is an essential feature of its biological function. This feature of RBCs can be critically affected by genetic or acquired pathological conditions. In this review, we highlight new dynamic in vitro assays that explore various hereditary blood disorders and parasitic infectious diseases that cause disruption of RBC morphology and mechanics. In particular, recent advances in high-throughput microfluidic devices make it possible to sort/identify healthy and pathological human RBCs with different mechanobiological characteristics.

Virtually all spacecraft employ photovoltaic energy conversion for continuous power generation. Compared to their counterpart on Earth, photovoltaic modules in the space environment face a unique set of performance requirements. Among the most demanding ones are the need to have the highest possible specific power output with regard to mass and surface area while showing as little degradation as possible under intense particle and ultraviolet radiation during lifetimes of up to 15 years. In addition, the thermomechanical stresses induced by temperature fluctuations up to 200°C are not to result in additional electrical degradation. This article briefly outlines the state-of-the-art design solution to meet these requirements before it focuses on current materials issues in two core areas: On the solar cell itself, which requires new materials systems and cell concepts to surpass the efficiency of the lattice-matched triple junction solar cell technology, and on materials issues concerning the encapsulation of solar cells for space use. Closely linked to these materials challenges are testing-related issues that arise in verifying the expected material behavior during extended periods in the space environment. These are discussed in conjunction with the materials challenges.

Due to their high carrier mobilities, electromigration resistance, and tailorable optical properties, carbon nanotubes are promising candidates for high-performance electronic and optoelectronic applications. However, traditional synthetic methods have lacked control over the structure and properties of carbon nanotubes. This polydispersity problem has confounded efforts to take carbon nanotubes from the research laboratory to the marketplace, especially for electronic and optoelectronic applications, where reliable and reproducible performance is paramount. In recent years, the research community has devoted significant effort to this issue, leading to substantial advances in the preparation of monodisperse carbon nanotube materials. This article highlights the most recent and promising developments from two perspectives: post-synthetic sorting and selective growth of carbon nanotubes of predetermined physical and electronic structure. These complementary approaches have yielded improved uniformity in carbon nanotube materials, resulting in impressive advances in carbon nanotube electronic and optoelectronic technology.

Natural materials display a wealth of structures and fulfill a variety of functions. Hierarchical structuring is one of the keys to providing multifunctionality and to adapting to varying needs of an organism. As a consequence, the natural environment represents not only a direct and renewable source of useful materials, such as wood, plant fibers, or even proteins of pharmaceutical importance, but also an enormous “database” of structures with exceptional mechanical, optical, or magnetic properties. Rather than focusing on the direct use of natural materials, this article discusses the use of structures that appeared in evolution and have been implemented in artificial materials of an entirely different type and chemical composition. This may be done either by directly copying the structure (biotemplating) or by extracting the design principles encoded in them for the fabrication of novel bioinspired materials.

In spite of being rare, actinide elements provide the building blocks for many fascinating condensed-matter systems, both from an experimental and theoretical perspective. Experimental observations of actinide materials are difficult because of rarity, toxicity, radioactivity, and even safety and security. Theory, on the other hand, has its own challenges. Complex crystal and electronic structures are often encountered in actinide materials, as well as pronounced electron correlation effects. Consequently, theoretical modeling of actinide materials and their 5f electronic states is very difficult. Here, we review recent theoretical efforts to describe and sometimes predict the behavior of actinide materials and complexes, such as phase stability including density functional theory (DFT), DFT in conjunction with an additional Coulomb repulsion U (DFT+U), and DFT in combination with dynamical mean-field theory (DFT+DMFT).

We review microstructures and properties of metal matrix composites produced by severe plastic deformation of multiphase alloys. Typical processings are wire drawing, ball milling, roll bonding, equal-channel angular extrusion, and high-pressure torsion of multiphase materials. Similar phenomena occur between solids in frictional contact such as in tribology, friction stir welding, and explosive joining. The resulting compounds are characterized by very high interface and dislocation density, chemical mixing, and atomic-scale structural transitions at heterointerfaces. Upon straining, the phases form into nanoscaled filaments. This leads to enormous strengthening combined with good ductility, as in damascene steels or pearlitic wires, which are among the strongest nanostructured bulk materials available today (tensile strength above 6 GPa). Similar materials are Cu-Nb and Cu-Ag composites, which also have good electrical conductivity that qualifies them for use in high-field magnets. Beyond the engineering opportunities, there are also exciting fundamental questions. They relate to the nature of the complex dislocation, amorphization, and mechanical alloying mechanisms upon straining and their relationship to the enormous strength. Studying these mechanisms is enabled by mature atomic-scale characterization and simulation methods. A better understanding of the extreme strength in these materials also provides insight into modern alloy design based on complex solid solution phenomena.

Polymer-coated and polymer-based cardiovascular implants are essential constituents of modern medicine and will proceed to gain importance with the demographic changes toward a society of increasing age-related morbidity. Based on the experiences with implants such as coronary or peripheral stents, which are presently widely used in clinical medicine, several properties of the next generation of cardiovascular implants have been envisioned that could be fulfilled by multifunctional polymers. The challenge is to combine tailored mechanical properties and rapid endothelialization with controlled drug release in order to modulate environmental cells and tissue. Additionally, degradability and sensitivity to external stimuli are useful in several applications. A critical function in terms of clinical complications is the hemocompatibility. The design of devices with improved hemocompatibility requires advanced in vitro test setups as discussed in depth in this article. Finally, degradable, multifunctional shape-memory polymers are introduced as a promising family of functional polymers that fulfill several requirements of modern implants and are of high relevance for cardiovascular application (e.g., stent technology). Such multifunctional polymers are a technology platform for future cardiovascular implants enabling induced autoregeneration in regenerative therapies.

The emergence of organic electronics represents one of the most dramatic technological developments of the past two decades. Perhaps the most important frontier of this field involves the interface with biology. The “soft” nature of organics offers better mechanical compatibility with tissue than traditional electronic materials, while their natural compatibility with mechanically flexible substrates suits the nonplanar form factors often required for implants. More importantly, the ability of organics to conduct ions in addition to electrons and holes opens up a new communication channel with biology. In this article, we consider a few examples that illustrate the coupling between organic electronics and biology and highlight new directions of research.

This article reviews polymer behavior at the air-water interface with a focus on the Langmuir technique and spreading processes to gain insight into this interface. The influence of the solvent and the water subphase on some polymer monolayers is discussed. The surface pressure–area isotherms (π–A) are described in terms of the chemical structure and the hydrophilic-hydrophobic balance of selected polymeric systems, with an emphasis on the behavior of the specific diblock copolymer films at the air-water interface. In some cases, molecular dynamic simulation studies can be used to visualize the organization and orientation of polymers at the air-water interface and to identify the main molecular interactions involved in such processes.

Many polymers, paints, and organic-based materials exposed to the space environment undergo dramatic changes and irreversible degradation of physical and functional characteristics. While many protective approaches, including protective coatings and mechanical metal foil wrapping or cladding—especially for synthesized bulk materials, are used to reduce the effects of the space environment, the protection of such materials in space remains a major challenge, especially for future long-duration exploration missions or permanent space stations. In addition to the traditional approaches, surface modification processes are used increasingly to protect or to impart new properties to materials used in the space environment. This article presents a brief overview of the present situation in the field of surface modification of space materials. A number of surface modification solutions that differ from the traditional protective coating approaches are discussed that change the surface properties of treated materials, thus protecting them from the hazards of low Earth orbit and geostationary orbit environments or imparting new functional properties. Examples of their testing, characterization, and applications are provided.

Advanced spectroscopic techniques provide new and unique tools for unraveling the nature of the electronic structure of actinide materials. Inelastic neutron scattering experiments, which address temporal aspects of lattice and magnetic fluctuations, probe electromagnetic multipole interactions and the coupling between electronic and vibrational degrees of freedom. Nuclear magnetic resonance clearly demonstrates different magnetic ground states at low temperature. Photoemission spectroscopy provides information on the occupied part of the electronic density of states and has been used to investigate the momentum-resolved electronic structure and the topology of the Fermi surface in a variety of actinide compounds. Furthermore, x-ray absorption and electron energy-loss spectroscopy have been used to probe the relativistic nature, occupation number, and degree of localization of 5f electrons across the actinide series. More recently, element- and edge-specific resonant and non-resonant inelastic x-ray scattering experiments have provided the opportunity of measuring elementary electronic excitations with higher resolution than traditional absorption techniques. Here, we will discuss results from these spectroscopic techniques and what they tell us of the electronic and magnetic properties of selected actinide materials.

Ferroelectric oxides underwent a renaissance in the 1980s and 1990s, driven by the success in commercializing thin-film ferroelectric random-access memory devices (FRAMs) for applications such as the SONY PlayStation 2 memory board. Materials scientists gravitated into this new field from magnetic oxides and from high-Tc superconductivity. But as the FRAM prospects wane and neither dynamic random-access memory devices nor FLASH memory has been replaced, we now require new directions for materials research on oxides. In this article, I outline briefly four new directions for ferroelectric oxide research: something old—ferroelectrically induced ferromagnetism and multiferroic switching; something new—THz emission from oxide ferroelectrics; something borrowed—Heisenberg-like switching of domains in nanoferroelectrics; and something blue—ZnO light-emitting devices. Magnetoelectricity—the linear coupling of polarization and magnetization—was theoretically predicted by Igor Dzyaloshinskii in 1957 and measured experimentally by Astrov two years later. It did not produce commercial devices. Although a flurry of new work occurred in the 1970s, emphasizing boracites—mostly by Hans Schmid in Geneva, no materials were found that exhibited large effects at room temperature. In the past decade, the search has been renewed, emphasizing rare earth systems such as Tb manganites.

Nature produces a wide variety of exquisite mineralized tissues, fulfilling diverse functions. Organisms exercise a level of molecular control over the detailed nano- and microstructure of the biomaterials that is unparalleled in today's technology. Our understanding of the underlying design principles of biomaterials provides ample opportunities for developing new approaches to materials fabrication at the nanometer and micrometer scale. It is clear that valuable materials lessons can be taught by any organism. I will exemplify this point by describing new nano- and microfabrication strategies and devices that have been inspired by the studies of biomineralization in echinoderms. The topics will include self-assembly, control of crystallization, synthesis of adaptive optical structures, hybrid materials, and novel actuation systems at the nanoscale level.

Extensive research activity is currently devoted to controlled drug delivery systems, mainly as nano-sized particles. Although biocompatible and (bio)degradable polymers play a key role in this field, their shaping into colloidal particles (e.g., polymeric micelles and nanoparticles) usually requires the proper design of amphiphilic copolymers as effective stabilizers. Strategies for synthesizing these copolymers that preserve the intrinsic properties of the constitutive polymers are discussed in this article. Synthesis of amphiphilic copolymers with a more complex structure and endowed with functionality is also considered, with the purpose of enhancing the performance of the nanocarriers. The focus is increasingly on nanocarriers of the third generation, which resist coalescence and elimination by the immune system, and which are readily incorporated into chosen target cells. The more recent quest is for smart nanocarriers that exhibit the additional capacity of being stimuli-responsive.