Living cells are formidably complex systems that perform highly coordinated tasks which relate multiple biochemical and biophysical inputs to cell activities. Cell tasks may include not only adhesion and spreading, receptor-ligand mediated signal transduction, division, growth and programmed death, but also cell-type-dependent functions such as the environmental barrier provided by skin cells. These various cellular activities, often performed simultaneously or in a hierarchical order, involve hundreds of membrane, cytoplasmic, and extracellular proteins, ions, and small molecules, which interact with one another by means of regulated forces.

For instance, cell migration requires the coordination of membrane extension and retraction, cytoskeletal gelation-contraction-dissolution, the formation of focal adhesions at the front of the cell, and detachment of these adhesions at the rear of the cell. These processes involve not only cytoskeletal polymers and motor proteins (which provide the cell with the necessary motor forces and passive mechanical resistance to sustain both cell movement and cell integrity), but also specific membrane proteins (“cell receptors”) to promote intimate contact between the cell and its extracellular milieu. Nevertheless, despite the critical function of cell migration in wound healing, immune response, cancer metastasis, and embryogenesis, the fundamental mechanisms of this phenomenon are not well understood.

One of the steps limiting our understanding of cellular activities such as cell migration has been the lack of fundamental theory, backed by experimental methods, to monitor and characterize cellular processes quantitatively, noninvasively, and in real time.

Cells are a complex topic of study for materials scientists. They are the fundamental building blocks of living organisms, able to sense their environment and act in response to it. In addition to their many biochemical functions, cells also play a mechanical role: They hold organs in place and move to the locations where they are needed in processes like wound healing, metastasis, or embryogenesis. Their mechanical behavior is mostly determined by a meshwork of three types of connected biopolymers (actin microfilaments, microtubules, and intermediate filaments) that compose a structural framework called the cytoskeleton, surrounded by a lipid membrane (Figure 1). In contrast to this simple picture, cells are very different from polymer gels or liposomes: They are active materials, powered by chemically stored energy. Their mechanical condition is closely linked to their biochemical function; for example, they may “commit suicide,” following a well-defined protocol known as apoptosis, which can be triggered by their mechanical state.

The enormous progress of modern cell biology combined with new micromanipulation techniques is leading researchers toward a more global understanding of the mechanical properties of cells and toward finding a functional link between biochemistry, chemical signaling, and cell mechanics, thus crossing the boundaries between these subjects.

The characterization of cell mechanical behavior has been the object of numerous studies. Red blood cells are a simple model system; if deprived of a nucleus while retaining a constant surface area, they have properties reminiscent of lipid vesicles.

Despite a variety of shapes and sizes, the generic mechanical structure of cells is remarkably similar from one cell type to the next. All cells are bounded by a plasma membrane, a fluid sheet that controls the passage of materials into and out of the cell. Plant cells and bacteria reinforce this membrane with a cell wall, permitting the cell to operate at an elevated osmotic pressure. Simple cells, such as the bacterium shown in Figure 1a, possess a fairly homogeneous interior containing the cell's genetic blueprint and protein workhorses, but no mechanical elements. In contrast, as can be seen in Figure 1b, plant and animal cells contain internal compartments and a filamentous cytoskeleton—a network of biological ropes, cables, and poles that helps maintain the cell's shape and organize its contents.

Four principal types of filaments are found in the cytoskeleton: spectrin, actin, microtubules, and a family of intermediate filaments. Not all filaments are present in all cells. The chemical composition of the filaments shows only limited variation from one cell to another, even in organisms as diverse as humans and yeasts. Membranes have a more variable composition, consisting of a bi-layer of dual-chain lipid molecules in which are embedded various proteins and frequently a moderate concentration of cholesterol. The similarity of the cell's mechanical elements in chemical composition and physical characteristics encourages us to search for universal strategies that have developed in nature for the engineering specifications of the cell. In this article, we concentrate on the cytoskeleton and its filaments.

The liquid-phase lipid bilayer is a unique engineering material. Biologically, it holds a central position in cellular life, providing the structural basis for the membrane that surrounds every cell on the planet. From a materials perspective, it is essentially a 4-nm-thick, water-insoluble sheet of 2-poise oil. Artificial membranes were “discovered” 35 years ago. It was soon recognized that liposomes could have a range of potential uses, and investigators sought to exploit the obvious capsular and biocompatibility properties of the membrane in applications such as liposome drug delivery. Since 1966, some 18,000 papers on liposomes have appeared in the literature (listed on Medline, and see References 4–7 for reviews), and 260 patents have been issued describing the use of liposomes in the pharmaceutical industry. These patented applications have included the delivery of cancer drugs, intracellular drug delivery, inhalation, topical drugs, gene therapy, proteins, peptides, amino acids, vaccines, targeted liposomes, lipophilic drugs, and liposome production, separation, and analysis. A huge database therefore exists with which to establish boundary conditions for predicting under which circumstances the encapsulation of drugs in liposomes or other carriers may be expected to result in improved therapy. Despite this enormous effort, only a few formulations, principally for amphotericin (an antifungal drug) and anthra-cyclins (anticancer drugs), have been approved and marketed, heralding the promise and potential that these versatile lipid-bilayer materials present. The reasons for this limited success are many, not the least of which is that the cost of developing a new pharmaceutical product can be several hundred million dollars.

The following article is based on the MRS Medal talk presented by William L. Johnson at the 1998 MRS Fall Meeting on December 2, 1998. The MRS Medal is awarded for a specific outstanding recent discovery or advancement that has a major impact on the progress of a materials-related field. Johnson received the honor for his development of bulk metallic glass-forming alloys, the fundamental understanding of the thermodynamics and kinetics that control glass formation and crystallization of glass-forming liquids, and the application of these materials in engineering.

The development of bulk glass-forming metallic alloys has led to interesting advances in the science of liquid metals. This article begins with brief remarks about the history and background of the field, then follows with a discussion of multicomponent glass-forming alloys and deep eutectics, the chemical constitution of these new alloys, and how they differ from metallic glasses of a decade ago or earlier. Recent studies of deeply undercooled liquid alloys and the insights made possible by their exceptional stability with respect to crystallization will then be discussed. Advances in this area will be illustrated by several examples. The article then describes some of the physical and specific mechanical properties of bulk metallic glasses (BMGs), and concludes with some interesting potential applications.

The first liquid-metal alloy vitrified by cooling from the molten state to the glass transition was Au-Si, as reported by Duwez at Caltech in 1960. Duwez made this discovery as a result of developing rapid quenching techniques for chilling metallic liquids at very high rates of 105–106 K/s.

The vulnerabilities inherent in choosing an innovative material for a design can make for technological catastrophe. The points of weakness are not dependent on the technical nature of the innovation, but instead are intrinsic to human problem-solving and decision-making. We present a computational approach for managing and containing this fallibility using ideas from cognitive science and artificial intelligence, in particular, understanding how the boundedly rational behavior of humans and organizations leads to specific kinds of errors that affect the choice and use of materials.

Materials choice in an industrial setting is often the result of a long chain or network of circumstances whose origins lie in many domains—from traditional engineering practice to the very latest innovation, from the commercial to the technical. In turn, every materials-selection decision has a long chain of consequences that are difficult to compute. This lengthy chain of consequences of a single decision gives rise to numerous points of error. What is particularly troublesome about these errors is not that they arise because of incomplete scientific engineering knowledge, but rather that they occur in spite of the fact that all (or at least, almost all) of the technically relevant information is available and sometimes even known to the technical personnel on the project. We will illustrate this approach by using a historical example of a major materials failure. Advances in database-systems design present an opportunity for integrating the ontology of material attributes with properties data. This may enable the design of more appropriate validation procedures required in proving a material for an artifact.