Over the last two decades, our ability to create networks of fluidic channels of submillimeter or even sub-micrometer diameters has led to a wide range of microchemical applications. Whereas early efforts were directed toward the development of microanalysis systems, in more recent times the development of microreactors and tools for biotechnology and basic biological studies has emerged. This issue of MRS Bulletin highlights the many different ways in which material properties are crucial in the fabrication, assembly, and operation of micro- and nanofluidic systems. Choice-of-material considerations range from an assessment of whether a desired channel design can be microfabricated in a certain material to whether the material is compatible with the operating conditions (i.e., pressure, temperature) and the chemical composition (solvent, solutes) of the fluid used. Moreover, in certain cases, specific surface or bulk material properties can be used to the benefit of the application of the device. In the development of today's wide range of integrated micro- and nanofluidic applications, a common challenge emerges: meeting the often contradictory set of constraints imposed on the physical and chemical properties of materials by the envisioned applications. This issue reviews these challenges and their solutions and provides an outlook on how the ingenious use of existing and new materials can spur the development of ever more sophisticated micro- and nanofluidic systems.

As more micro- and nanofluidic methodologies are developed for a growing number of diverse applications, it becomes increasingly apparent that the choice of substrate material can have a profound effect on the eventual performance of a device. This is due mostly to the high surface-to-volume ratio that exists within such small structures. In addition to the obvious limitations related to the choice of solvent, operating temperatures, and pressure, the method of fluidic pumping—in particular, an electrokinetics-based methodology using a combination of electro-osmotic and electrophoresis flows—can further complicate material choice. These factors, however, are only part of the problem; once chemicals or biological materials (e.g., proteins or cells) are introduced into a microfluidic system, surface characteristics will have a profound influence on the activity of such components, which will subsequently influence their performance. This article reviews the common types of materials that are currently used to fabricate microfluidic devices and considers how these materials may influence the overall performance associated with chemical and biological processing. Consideration will also be given to the selection of materials and surface modifications that can aid in exploiting the high surface properties to enhance process performance.

Microfabrication techniques and scale-up by replication promise to transform classical batch-wise chemical laboratory procedures into integrated systems capable of providing new understanding and control of fundamental processes. Such integrated microchemical systems would enable rapid, continuous discovery and development of new products with the use of fewer resources and the generation of less waste. Additional opportunities exist for on-demand and on-site synthesis, with perhaps the first applications emerging in portable energy sources based on the conversion of hydrocarbons to hydrogen for miniaturized fuel cells.

Microchemical systems can be realized in a wide range of materials including stainless steel, glass, ceramics, silicon, and polymers. The high mechanical strength, excellent temperature characteristics, and good chemical compatibility of silicon combined with the existing fabrication infrastructure for microelectromechanical systems (MEMS) offer advantages in fabricating chemical microsystems that are compatible with strong solvents and operate at elevated temperatures and pressures. Furthermore, silicon-based microsensors for flow, pressure, and temperature can readily be integrated into the systems.

Microsystems for broad chemical applications should be discovery tools that can easily be applied by chemists and materials scientists while also having a convincing “scale-out” to at least small production levels. The interplay of both these capabilities is important in making microreaction technology successful. Perhaps the largest impact of microchemical systems will ultimately be the ability to explore reaction conditions and chemistry at conditions that are otherwise difficult to establish in the laboratory. Case studies are selected to illustrate microfluidic applications in which silicon adds advantages, specifically, integration of physical sensors and infrared spectroscopy, highthroughput experimentation in moisture-sensitive organic synthesis, controlled synthesis of nanoparticles (quantum dots), multiphase and heterogeneous catalytic reactions at elevated temperatures and pressures, and thermal management in the conversion of hydrocarbons to hydrogen.

One of the key problems in microfabrication and especially nanofabrication applied to biology is materials selection. Proper materials must have mechanical stability and the ability to hermetically bond to other surfaces, yet not bind biological molecules. They must also be wettable by water and have good optical properties. In this article, we review some of the attempts to find materials for micro- and nanofluidic systems in biological applications that satisfy these rather conflicting constraints.We discuss the materials properties that make poly (dimethylsiloxane) or non-elastomeric materials more or less suitable for particular applications in biology. We also explore the effects and the importance of surface treatments for integrating biological objects into microfabricated and nanofabricated fluidic devices.

Biomedical applications—prostheses, tissue engineering, drug delivery, and wound healing—demand increasingly sophisticated characteristics from the materials that come into contact with living systems in the laboratory and the clinic. With the development of microfluidics, there is an opportunity to create active biomaterials based on embedded microfluidic structures. These structures allow for control of the concentrations of soluble chemicals and hydrodynamic stresses within the material and at its interfaces, and thus allow one to tailor the environment experienced by the living tissue. In this article, we review initial efforts to develop these microfluidic biomaterials and present considerations regarding the required characteristics of the materials and of the microfluidic-mediated mass transfer. As specific examples, we present work toward microfluidic control of mass transfer in scaffolds for tissue engineering and in wound dressings.

Efficient and reliable particle separators and concentrators are needed to support a wide range of analytical functions including pathogen detection, sample preparation, high-throughput particle sorting, and biomedical diagnostics. The advent of lab-on-a-chip devices based on the phenomenon of dielectrophoresis offers advantages that can meet several of the challenges associated with cell sorting and detection. The majority of the devices presented in the scientific literature have used glass-based devices for these applications, but there has been recent activity that indicates that polymer-based devices can operate as effectively as their glass progenitors. Processing and operational advantages motivate the transition from glass and silicon to polymer microdevices: mechanical robustness, economy of scale, ease of thermoforming and mass manufacturing, and the availability of numerous innate chemical polymer compositions for tailoring performance. We present here a summary of the developments toward, and results obtained from, these polymeric dielectrophoretic devices in the selective trapping, concentration, and gated release of a range of biological organisms and particles.