Annual Review of Materials Research - Volume 41, 2011

In this article, we review critical aspects in the area of drug delivery. Specifically, delivery of siRNA, remote-controlled delivery, noninvasive delivery, and nanotechnology in drug delivery are reviewed.

A crystallization pathway describes the movement of ions from their source to the final product. Cells are intimately involved in biological crystallization pathways. In many pathways the cells utilize a unique strategy: They temporarily concentrate ions in intracellular membrane-bound vesicles in the form of a highly disordered solid phase. This phase is then transported to the final mineralization site, where it is destabilized and crystallizes. We present four case studies, each of which demonstrates specific aspects of biological crystallization pathways: seawater uptake by foraminifera, calcite spicule formation by sea urchin larvae, goethite formation in the teeth of limpets, and guanine crystal formation in fish skin and spider cuticles. Three representative crystallization pathways are described, and aspects of the different stages of crystallization are discussed. An in-depth understanding of these complex processes can lead to new ideas for synthetic crystallization processes of interest to materials science.

Bone and nacre are the most-known biological hard tissues to materials researchers. Although highly mineralized, both bone and nacre are amazingly tough and exhibit remarkable inelasticity, properties that are still beyond the reach of many modern ceramic materials. Very interestingly, the two hard tissues seem to have adopted totally different structural strategies for achieving mechanical robustness. Starting from a true nanocomposite of the mineralized collagen fibril and following up to seven levels of hierarchical organization, bone is built on a structure with extreme complexity. In contrast, nacre possesses a structure of surprising simplicity. Polygonal mineral tablets of micrometer size are carefully cemented together into a macroscopic wonder. A comparative analysis of the structure-property relations in bone and nacre helps us to unveil the underlying mechanisms of this puzzling phenomenon. In this review, we critically compare the various levels of structures and their mechanical contributions between bone and nacre, with a focus on inelasticity and the toughening process. We demonstrate that, although nacre and bone differ from each other in many aspects, they have adopted very similar deformation and toughening mechanisms.

Living cells are an active soft material with fascinating mechanical properties. Under mechanical loading, cells exhibit creep and stress relaxation behavior that follows a power-law response rather than a classical exponential response. Such a response puts cells in the context of soft colloidal glasses and other disordered metastable materials that share the same properties. In cells, however, both the power-law exponent and stiffness are related to the contractile prestress in the cytoskeleton. In addition, cells are made of a highly nonlinear material that stiffens and fluidizes under mechanical stress. They show active and adaptive mechanical behavior such as contraction and remodeling that sets them apart from any other nonliving material. Strikingly, all these observations can be linked by simple relationships with the power-law exponent as the only organizing parameter. Current theoretical models capture specific facets of cell mechanical behavior, but a comprehensive understanding is still emerging.

Mussels attach to solid surfaces in the sea. Their adhesion must be rapid, strong, and tough, or else they will be dislodged and dashed to pieces by the next incoming wave. Given the dearth of synthetic adhesives for wet polar surfaces, much effort has been directed to characterizing and mimicking essential features of the adhesive chemistry practiced by mussels. Studies of these organisms have uncovered important adaptive strategies that help to circumvent the high dielectric and solvation properties of water that typically frustrate adhesion. In a chemical vein, the adhesive proteins of mussels are heavily decorated with Dopa, a catecholic functionality. Various synthetic polymers have been functionalized with catechols to provide diverse adhesive, sealant, coating, and anchoring properties, particularly for critical biomedical applications.

Cartilage is a hydrated biomacromolecular fiber composite located at the ends of long bones that enables proper joint lubrication, articulation, loading, and energy dissipation. Degradation of extracellular matrix molecular components and changes in their nanoscale structure greatly influence the macroscale behavior of the tissue and result in dysfunction with age, injury, and diseases such as osteoarthritis. Here, the application of the field of nanomechanics to cartilage is reviewed. Nanomechanics involves the measurement and prediction of nanoscale forces and displacements, intra- and intermolecular interactions, spatially varying mechanical properties, and other mechanical phenomena existing at small length scales. Experimental nanomechanics and theoretical nanomechanics have been applied to cartilage at varying levels of material complexity, e.g., nanoscale properties of intact tissue, the matrix associated with single cells, biomimetic molecular assemblies, and individual extracellular matrix biomolecules (such as aggrecan, collagen, and hyaluronan). These studies have contributed to establishing a fundamental mechanism-based understanding of native and engineered cartilage tissue function, quality, and pathology.

Plant stems are one of nature's most impressive mechanical constructs. Their sophisticated hierarchical structure and multifunctionality allow trees to grow more than 100 m tall. This review highlights the advanced mechanical design of plant stems from the integral level of stem structures down to the fiber-reinforced-composite character of the cell walls. Thereby we intend not only to provide insight into structure-function relationships at the individual levels of hierarchy but to further discuss how growth forms and habits of plant stems are closely interrelated with the peculiarities of their tissue and cell structure and mechanics. This concept is extended to a further key feature of plants, namely, adaptive growth as a reaction to mechanical perturbation and/or changing environmental conditions. These mechanical design principles of plant stems can serve as concept generators for advanced biomimetic materials and may inspire materials and engineering sciences research.

The more than 60 ternary carbides and nitrides, with the general formula Mn+1AXn—where n = 1, 2, or 3; M is an early transition metal; A is an A-group element (a subset of groups 13–16); and X is C and/or N—represent a new class of layered solids, where Mn+1Xn layers are interleaved with pure A-group element layers. The growing interest in the Mn+1AXn phases lies in their unusual, and sometimes unique, set of properties that can be traced back to their layered nature and the fact that basal dislocations multiply and are mobile at room temperature. Because of their chemical and structural similarities, the MAX phases and their corresponding MX phases share many physical and chemical properties. In this paper we review our current understanding of the elastic and mechanical properties of bulk MAX phases where they differ significantly from their MX counterparts. Elastically the MAX phases are in general quite stiff and elastically isotropic. The MAX phases are relatively soft (2–8 GPa), are most readily machinable, and are damage tolerant. Some of them are also lightweight and resistant to thermal shock, oxidation, fatigue, and creep. In addition, they behave as nonlinear elastic solids, dissipating 25% of the mechanical energy during compressive cycling loading of up to 1 GPa at room temperature. At higher temperatures, they undergo a brittle-to-plastic transition, and their mechanical behavior is a strong function of deformation rate.

Electrocaloric materials are reviewed through a discussion of ferroelectric crystals used to achieve refrigeration via adiabatic depolarization. Related thermodynamics is briefly summarized, but emphasis is on experiments and materials.

Electrochromic materials have the property of a change, evocation, or bleaching of color as effected either by an electron-transfer (redox) process or by a sufficient electrochemical potential. The main classes of electrochromic materials are surveyed here, with descriptions of representative examples from the metal oxides, viologens (in solution and as adsorbed or polymeric films), conjugated conducting polymers, metal coordination complexes (as polymeric, evaporated, or sublimed films), and metal hexacyanometallates. Examples of the applications of such electrochromic materials are included. Other materials aspects important for the construction of electrochromic devices include optically transparent electrodes, electrolyte layers, and device encapsulation. Commercial successes, current trends, and future challenges in electrochromic materials research and development are summarized.

The nanowire geometry provides potential advantages over planar wafer-based or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, improved band gap tuning, facile strain relaxation, and increased defect tolerance. These benefits are not expected to increase the maximum efficiency above standard limits; instead, they reduce the quantity and quality of material necessary to approach those limits, allowing for substantial cost reductions. Additionally, nanowires provide opportunities to fabricate complex single-crystalline semiconductor devices directly on low-cost substrates and electrodes such as aluminum foil, stainless steel, and conductive glass, addressing another major cost in current photovoltaic technology. This review describes nanowire solar cell synthesis and fabrication, important characterization techniques unique to nanowire systems, and advantages of the nanowire geometry.

Although photovoltaic devices and modules made from crystalline silicon currently dominate the market, many efforts in developing photovoltaics involve the use of alternative materials. Binary and multinary materials with direct band gaps and therefore high absorption coefficients allow for the fabrication of thin-film photovoltaic modules with minimized material use and the possibility for depositing on large areas and alternative substrates such as glass, stainless steel, or polyimide foils. With the great diversity of optoelectronic properties of binary and multinary materials, highly efficient photovoltaic devices fabricated at very low cost are in principle possible. Requirements for efficient photovoltaic devices using nonconventional materials are discussed, and results obtained for photovoltaic devices based on selected binary and multinary materials obtained during the past few decades are summarized.

The routine availability of metals, long assumed by materials scientists to be unworthy of concern, is now a topic of wide interest but one with little clear guidance. This review discusses availability issues from the perspective of the metals utilized in the energy industry. Although the availability of metals is a dynamic characteristic, availability of the widely used base metals appears assured in the immediate future. The same cannot be said for by-product (daughter) metals, which are increasingly vital for many carbon-free energy technologies but are produced only if recovered as part of parent metal processing. Additionally, the direct substitution of one metal for another in short supply is often difficult because the best substitutes tend to have the same availability constraints as did the original metal. Gallium, indium, and neodymium are singled out as elements of particular concern from a long-term-supply standpoint.

Although phase transitions have long been a centerpiece of condensed matter materials science studies, a number of recent efforts focus on potentially exploiting the resulting functional property changes in novel electronics and photonics as well as understanding emergent phenomena. This is quite timely, given a grand challenge in twenty-first-century physical sciences is related to enabling continued advances in information processing and storage beyond conventional CMOS scaling. In this brief review, we discuss synthesis of strongly correlated oxides, mechanisms of metal-insulator transitions, and exploratory electron devices that are being studied. Particular emphasis is placed on vanadium dioxide, which undergoes a sharp metal-insulator transition near room temperature at ultrafast timescales. The article begins with an introduction to metal-insulator transition in oxides, followed by a brief discussion on the mechanisms leading to the phase transition. The role of materials synthesis in influencing functional properties is discussed briefly. Recent efforts on realizing novel devices such as field effect switches, optical detectors, nonlinear circuit components, and solid-state sensors are reviewed. The article concludes with a brief discussion on future research directions that may be worth consideration.

Optimization of electrical, optical, mechanical, and other properties of many advanced, functional materials today relies on precise control of point defects. This article illustrates the progress that has been made in elucidating the often complex equilibria exhibited by many materials by examining two recently well-characterized model systems, TlBr for radiation detection and PrxCe1−xO2−δ, of potential interest in solid-oxide fuel cells. The interplay between material composition, electrical conductivity, and mechanical properties (electrochemomechanics) is discussed, and implications in these relations, for example, enhancing electrical properties through large mechanical strains, are described. The impact of space charge and strain fields at interfaces, particularly important in nanostructure materials, is also emphasized. Key experimental techniques useful in characterizing bulk and surface defects are summarized and reviewed.

Recent advances in semiconductor thermoelectric physics and materials are reviewed. A key requirement to improve the energy conversion efficiency is to increase the Seebeck coefficient (S) and the electrical conductivity (σ) while reducing the electronic and lattice contributions to thermal conductivity (κe + κL). Some new physical concepts and nanostructures make it possible to modify the trade-offs between the bulk material properties through changes in the density of states, scattering rates, and interface effects on electron and phonon transport. We review recent experimental and theoretical results on nanostructured materials of various dimensions: superlattices, nanowires, nanodots, and solid-state thermionic power generation devices. Most of the recent success has been in the reduction of lattice thermal conductivity with the concurrent maintenance of good electrical conductivity. Several theoretical and experimental results to improve the thermoelectric power factor (S²σ) and to reduce the Lorenz number (σ/κe) are presented. We briefly describe recent developments in nonlinear thermoelectrics, as well as the generalization of the Bergman theorem for composite materials. Although the material thermoelectric figure of merit Z [=S²σ/(κe + κL)] is a key parameter to optimize, one has to consider the whole system in an energy conversion application. A rarely discussed but important efficiency/cost trade-off for thermoelectric power generation is briefly reviewed, and research directions for the development of low-cost thermoelectric materials are identified. Finally, we highlight the importance of the figure of merit, Z, beyond macroscale energy conversion applications in describing the microscopic coupling between charge and energy transport in materials.

Over the past 10–15 years, there have been significant advances in the scientific understanding as well as in the performance of thermoelectric (TE) materials. TE materials can be incorporated into power generation devices that are designed to convert waste heat into useful electrical energy. These TE materials can also be used in solid-state refrigeration devices for cooling applications. The conversion of waste heat into electrical energy will certainly play a role in our current challenge for alternative energy technologies to reduce our dependence on fossil fuels and to reduce greenhouse gas emissions. This article provides an overview of the various TE phenomena and discusses some of the primary TE materials that are currently being investigated. Several of the key parameters and terminology are defined and discussed along with an overview of some of the current and emerging technologies. The phonon glass–electron crystal approach to new TE materials for developing new materials is presented along with the role of solid-state crystal chemistry and the criteria for higher-performance TE materials. This article discusses TE phenomena, the selection criteria for higher-performance materials, and a few key materials.