This brief article describes the content of this issue of MRS Bulletin on Cellular Solids. Cork, wood, sponge, and bone are all examples of cellular solids in nature. Engineered honeycombs and foams are now made from polymers, metals, ceramics, and glasses, and their structure gives them unique properties that can be exploited in a variety of applications. The articles in this issue provide an overview of the fabrication, structure, properties, and applications of such porous solids as cellular ceramics, aluminum and other metallic foams, and scaffolds for tissue engineering, as well as discussions of techniques for understanding, modeling, and measuring their behavior and properties.

The properties of solid foams depend on their structure, which usually evolves in the fluid state as gas bubbles expand to form polyhedral cells. The characteristic feature of foam structure—randomly packed cells of different sizes and shapes—is examined in this article by considering soap froth. This material can be modeled as a network of minimal surfaces that divide space into polyhedral cells. The cell-level geometry of random soap froth is calculated with Brakke's Surface Evolver software. The distribution of cell volumes ranges from monodisperse to highly polydisperse. Topological and geometric properties, such as surface area and edge length, of the entire foam and individual cells, are discussed. The shape of struts in solid foams is related to Plateau borders in liquid foams and calculated for different volume fractions of material. The models of soap froth are used as templates to produce finite element models of open-cell foams. Three-dimensional images of open-cell foams obtained with x-ray microtomography allow virtual reconstruction of skeletal structures that compare well with the Surface Evolver simulations of soap-froth geometry.

Scale effects in cellular metals can develop when the specimen size is of the order of the cell size. Decreasing the relevant specimen dimensions—height, width, and ligament size (the region between notches in notched specimens)—leads to material strengthening in shear, in indentation, and in notched specimens and to reduced strength and stiffness in uniaxial compression. Experimental size-effect studies were reviewed, and it was concluded from discrete modeling results that scale effects are caused by two different microstructural mechanisms: boundary-layer effects and constraint effects. The first mechanism is active in shear (strong boundary layers) and uniaxial compression (weak boundary layers) and vanishes for specimens larger than two cell sizes and seven cell sizes, respectively. The second mechanism is active in indentation and in notched specimens, leading to a strengthening behavior that is inversely proportional to indenter and ligament size.

The use of microtomography to study the structure and especially the deformation modes of cellular solids is reviewed in this article. First, the technique is described in detail. Examples illustrating the power of the coupling of in situ deformation with three-dimensional (3D) imaging, drawn from the recent literature and the authors' own work, are then given. The most detailed example is the study of the deformation modes of several samples made of different aluminum foams. Four kinds of closed-cell foams were investigated, corresponding to different routes available today for their manufacture. The initial macrostructure was quantified using the 3D images combined with 3D granulometry, allowing retrieval of pertinent information about the cell size and the wall and strut thicknesses. The global behavior exhibited by the foams during the in situ compression experiments was shown to vary from one brand of material to another. Some of these variations can be explained by differences in the known microstructure and the measured macrostructure of the samples.

Metallic foams have become an attractive research field both from a scientific viewpoint and the prospect of industrial applications. Various methods for making such foams are available. Some techniques start from specially prepared molten metals with adjusted viscosities. Such melts can be foamed by injecting gases or by adding gas-releasing blowing agents that decompose in situ, causing the formation of bubbles. Another method is to prepare supersaturated metal–gas systems under high pressure and initiate bubble formation by pressure and temperature control. Yet a further class of techniques begins with solid precursors containing a blowing agent. These can be prepared by mixing metal powders with a blowing agent, compacting the mix, and then foaming the compacted mix by melting. Alternatively, casting routes can be used to make such precursors. The unique properties of foams offer promise in a variety of applications ranging from lightweight construction and impact-energy absorption to various types of acoustic damping and thermal insulation.

Cellular ceramics are a class of high-porosity materials that are used or are being considered for a wide range of technological applications. A critical aspect of this development is the materials science approach required to understand the relationships between the properties of these materials and their structure. Of particular interest are the parameters that control mechanical reliability, as ceramic materials are usually brittle. In addition, it is critical to understand the way in which processing methods can influence the cellular structure. This article emphasizes one particular group of cellular ceramics known as ceramic foams. Understanding these materials involves various interdisciplinary scientific challenges in characterizing structure, developing micromechanical models, experimentally measuring properties, developing new processing approaches, and optimizing performance.

Devices for tissue engineering comprise scaffolds with the appropriate chemistry and architecture to promote cell infiltration and colonization. The scaffold is designed with biology in mind, and thus the architecture and chemistry differ according to tissue type. In this review, we focus on scaffolds for two tissue types—bone and nervous tissue—and describe different approaches used to create them. The appropriate scaffold for a hard tissue such as bone has a high degree of interconnected macroporosity and allows the rapid invasion of cells while maintaining a rigid structure. Several approaches are described for constructing tissue-engineering scaffolds for bone. The appropriate scaffold for soft tissues like nerve fibers (e.g., axons, which conduct nerve impulses) also has a high degree of interconnected pores; however, the pores may require orientation and may be smaller. Homogeneous, high-water-content hydrogels with mechanical properties that match the soft nerve tissue are commonly used as a scaffold, and the methods used to make these are reviewed.