Tissue engineering offers the possibility to create completely natural tissue and replace failing or malfunctioning organs. In many cases, biocompatible, biodegradable polymers are utilized to either induce surrounding tissue and cell ingrowth or to serve as a temporary scaffold for transplanted cells to attach, grow, and maintain differentiated functions. Various processing techniques have therefore been developed to fabricate polymers with specific properties to meet the needs of a particular organ.

Polymer scaffolds must possess unique physical and chemical properties for specific applications and must satisfy some basic requirements for tissue engineering. These scaffolds may be implanted without cells, and the regeneration depends on ingrowth of surrounding tissue to such materials—a process known as tissue induction. Alternatively cells may be seeded into a porous polymer. The cell-polymer construct is then transplanted. In either case, one essential criterion for the scaffold is biocompatibility—that is, the polymer scaffolds and their degradation products should not invoke an adverse immune response or toxicity.

Because of the problems associated with long-term implants, such as infection, fibrous tissue formation, and the possible need for retrieval, the role of polymer scaffolds should only be a temporary one. The degradation rate is optimized to allow transplanted cells to proliferate and secrete their own extracellular matrix (ECM) while polymer scaffolds can vanish when necessary to leave enough space for new tissue growth. Biodegradability of a polymer is determined by its composition, molecular weight (MW), MW distribution, degree of crystallinity, and environmental conditions such as temperature and pH. Mechanical loading of the scaffold may affect its degradation.

Many naturally occurring adhesive materials have been used throughout history to bind pigments in paintings. A number of synthetic materials have been added to these during the twentieth century.

Availability and tradition can influence the choice of binders made by artists. Probably the most widely used medium throughout history, animal glue, is also the most easily obtained. Glues made from the connective tissues or skins of local animals were major media in ancient Egyptian painting and in Chinese and Japanese painting, as well as in many other cultures throughout world history. In many cases, a variety of natural binders were available, and additional factors influenced the choice of binder by a culture. Different media have highly variable properties that affect how they are used in painting. Among these properties are solubility, the transparency or depth of color that is obtainable with a given pigment, and handling properties–how the paint flows, how quickly it dries, whether it can be applied in very thick and very thin layers, etc. Knowledge of the media utilized in paintings can help us understand the intentions of artists. Medieval European painting can be used as an example.

Chemically reactive plasma discharges are widely used to process materials. A plasma is a primarily electrically neutral collection of free charged particles moving in random directions. The simplest plasma consists of electrons and one kind of positive ions. This article deals primarily with plasma discharges, which are plasmas having the following features:

(1) They are driven electrically.

(2) Charged-particle collisions with neutral-gas molecules are important.

(3) There are boundaries at which surface losses are important.

(4) Ionization of neutrals sustains the plasma in the steady state.

One simple discharge consists of a voltage source that drives current through a low-pressure gas between two conducting plates or electrodes. The gas “breaks down” to form a plasma, usually weakly ionized—that is, the plasma density is only a small fraction of the neutral-gas density.

The plasmas used in materials processing present an enormous range of charged-particle densities n and of temperatures Te, Ti, and T for electrons, ions, and processing gas, respectively. High-pressure (atmospheric) discharges are in near-thermal equilibrium (Te ~ Ti ~ T ~ 0.1–2 eV). Plasma temperatures are usually given in equivalent electron-volt units: One eV is equivalent to 11600 K through the Boltzmann constant. As discussed in the article by Boulos and Pfender in this issue of MRS Bulletin, these thermal discharges have high densities n ~ 1014-1019 particles/cm3 and are mainly used as heat sources. Low-pressure (1 mTorr–10 Torr) discharges are not in thermal equilibrium (Te ~ 2–5 eV ≫ Ti ~ T) and have low densities n ~ 109–1012 particles/cm3. As discussed in several of the following articles, these discharges are used as miniature chemical factories in which feedstock gases are broken into positive ions and chemically reactive etchants, deposition precursors, etc., which then flow to and physically or chemically react at the surface of a substrate.

Basic and applied science of electroceramic thin films is an interdisciplinary field that is rapidly evolving worldwide. A major driving force for the extensive research in many university, industrial and national laboratories around the world is the promise of a new generation of advanced microdevices that may transform various technologies, potentially creating new multibillion-dollar markets. Electroceramic-thin-film properties that are being intensively investigated include electrical conductivity, large dielectric permittivity, ferroelectricity, piezoelectricity, pyroelectricity, and electro-optic activity. Perhaps the most publicized application of electrical conductivity of electroceramics is that related to the new high-temperature-superconducting (HTS) materials, which has been extensively discussed in numerous national and international conferences and in the scientific literature. Less glamorously publicized applications, but as important as those of HTS materials, are those involving ferroelectric materials, which is the subject of this MRS Bulletin.

Practical metallurgical application of intermetallic compounds (IMCs) occurred more than 3,000 years before they were recognized as distinct entities in alloys. Pliny, The Elder (A.D. 23–79) recorded in his encyclopedia a then old practice: the use of mercury both to recover gold from sands and other dispersed sources, and to gild less-noble metal objects. In both cases, the key factor is the formation of a Au-Hg intermetallic compound (amalgam) stable at room temperature but readily decomposable on heating to produce solid gold. An illustration of the Au-recovery process, reproduced from Ercker (1574) appears in Figure 1. Use of the amalgam processes for silver apparently occurred later. Bronze mirrors were silver-coated with the amalgam process by the Chinese in the second century B.C., and silver was recovered from crushed sulfide ores using mercury in the famous Potosi process (1566, but probably known much earlier).

The key properties of intermetallics that make possible their diverse applications in chemical and metallurgical processes are their high melting points relative to one or all of their constituent elements, their often sharply defined composition, their brittleness, and their controllable reactivity/stability–that is, systems can be chosen such that a stable intermetallic forms easily at room or low temperatures that is nonetheless readily decomposable at a higher temperature. Once the intermetallic forms however, a useful physical property (e.g., hardness or conductivity) or chemical property (oxidation, sulfidation, corrosion resistance, nonsticking quality, etc.) may be that which is ultimately exploited in use.

We will review these two classes of applications using representative examples from both process metallurgy and chemistry.

Long polymer chains adsorbed onto solid surfaces are important in a wide range of applications and are relevant to many issues in biology and medicine. Adsorbed polymer layers used widely in the stabilization of colloidal suspensions, for example, are essential to the formulation of paints, coatings, printing inks, drilling needs, and ceramics processing. Adsorbed polymer layers also play a crucial role in many tribological applications such as boundary lubricants.

The interfacial behavior of biological macromolecules (such as proteins) plays an important role in biomedical applications as well as in the function of living organisms. The development of biocompatible solid materials that can be used safely and efficiently in vascular and joint prostheses, catheters, artificial-heart valves and whole hearts, cardiac-arrest devices, and hemodialysis cartridges is critically important. In some instances, the adsorption of biological macromolecules onto the artificial material can lead to deleterious effects. For example, many vascular grafts and catheters fail because of thrombotic occlusion initiated by protein adsorption. Protein adsorption is also the initial subprocess that leads to plaque formation in teeth and the fouling of contact lenses. Given the central role of protein adsorption in many physiological systems, and the great benefits that could be derived by designing materials that do not adsorb biological macromolecules, understanding the interfacial behavior of biological polymers is important.

The ultimate goal of research in polymer adsorption is to facilitate the manipulation of the properties of adsorbed polymer layers (or polymer-solid interfaces) so that materials with required properties can be fabricated. To take steps toward this goal, understanding how the nature of the polymer, the substrate, and other prevailing conditions (such as the type of solvent) affect the macroscopic properties of the interface is crucial.

The fabrication of thin-film-transistor (TFT) devices on a transparent substrate lies at the heart of active-matrix-liquid-crystal-display (AMLCD) technology. This is both good and bad. On one hand it is a difficult task to manufacture millions of intricate semiconductor devices reliably over such large display substrates. On the positive side, AMLCD technology can aspire to become much more than a “display” technology. The idea is as follows: It is possible for one to readily fabricate additional transistors to execute various electronic functions—those that would otherwise be handled by separate large-scale-integration (LSI) and very large-scale-integration (VLSI) circuits—on the periphery of the display. Since this can be done, in principle, with no—or a minimal number of—additional processing steps, substantial cost reduction is possible and significant value can be added to the final product.

Doing so and doing it well can ultimately lead to “system-on-glass” products in which the entire electronic circuitry needed for a product is incorporated directly onto a glass substrate. This means that integrated active-matrix liquid-crystal displays (IAMLCDs) have the potential to bypass conventional Si-wafer-based products and may lead TFT technology to compete directly against Si-wafer-based monolithic integrated circuits.

Recently there has been a paradigm shift in nonvolatile computer memories from silicon-technology-based EEPROMs (electrically erasable, programmable read-only memories) to devices in which the stored information is coded into + and − polarizations in thin-film ferroelectric capacitors. Such devices have read and erase/rewrite speeds of the order of 1–35 ns, many orders of magnitude faster than the erase/rewrite speeds of the best EEPROMs (Table I). However, fundamental questions concerning their lifetimes had delayed full commercialization. Because ferroelectrics normally have extremely large dielectric constants, their use as nonswitching capacitors in dynamic random-access memories (DRAMs) is also rapidly evolving. The majority of studies to date have emphasized lead zirconate titanate (PZT)-based capacitors for nonvolatile ferroelectric random-access memories (NVFRAMs) and barium strontium titanate-based capacitor DRAMs (see Table II).

Intermetallic compounds constitute a very important class of materials for electromagnetic applications. In this article, some important materials and applications are discussed in the following areas: (1) magnetic, magnetoresistive, and magnetostrictive applications; (2) superconductor applications; (3) semiconductor and optical applications; (4) magneto-optical applications; and (5) thermoelectric applications. Emphasis is placed on materials that are important in existing devices and applications or show promise for future applications. The interested reader should consult the reviews in Westbrook and Fleischer's book, and the many references contained therein, for further information.

Paper is one of the most common supports for printed, painted, written, and drawn images. It is composed of fibers and other ingredients intertwined into a compact web, usually in the form of a thin sheet. Making paper involves combining fibers and fillers into a pulp slurry, depositing the suspension on a screen or roll, draining the water, and drying the pulp. Since the invention of paper about 2,000 years ago, the basic papermaking processes have gone unchanged, though hand papermaking has given way to a series of mechanical operations capable of manufacturing a continuous sheet of paper at phenomenal speeds. The selection of the fiber material and the method of treating the pulp during manufacture influence the qualities of the formed sheet of paper.

With significant developments in laser and semiconductor industries, optical-storage technology has successfully emerged into the consumer marketplace and more recently into the computer-based data-storage marketplace as well. For computer-based data storage and other purposes, the first generation of the International Standards Organization (ISO) standard 5-1/4-in. magneto-optic (MO) drive (325 Mbytes × 2/double-sided) was introduced in 1988 and the 3.5-in. drive (128 Mbytes) in 1991. Since then progress has been remarkable in capacity and data-transfer rate as well as cost/performance ratio, and the so-called 2X (650 Mbytes × 2 per 5-in. double-sided disk) and 3X (1 Gbyte × 2 per 5-in. double-sided disk) drives are now in the marketplace. Also a remarkable product called the MiniDisc (2.5 in.), the first recordable optical system for both the consumer and the data market, utilizes data compression and direct overwrite schemes.

The solution for increasing the capacity is by no means simple since the new technology must accommodate many factors. One of the key factors is backward compatibility. This feature is important to removable media because the customer expects new products to be compatible with their previous investments in data stored on older generation media. Another key factor is cost. The sales price ($/Mbyte) must be significantly lower than prior generations of optical-storage products and alternative storage systems (e.g., removable hard disks).

To increase the capacity in optical-storage media, several solutions are possible: short-wavelength lasers, magnetically induced super resolution (MSR), pulse width modulation (PWM), volumetric recording, land/groove recording with crosstalk cancellation, partial-response channels, banding of data, and higher numerical aperture objective lenses.

Recently much interest has been devoted to Si-based heteroepitaxy, and in particular, to the SiGe/Si system. This is mostly for economical reasons: Si-based technology is much more advanced, is widely available, and is cheaper than GaAs-based technology. SiGe opens the door to the exciting (and lucrative) area of Si-based high-performance devices, although optical applications are still limited to GaAs-based technology. Strained SiGe layers form the base of heterojunction bipolar transistors (HBTs), which are currently used in commercial high-speed analogue applications. They promise to be low-cost compared to their GaAs counterparts and give comparable performance in the 2-20-GHz regime. More recently we have started to investigate the use of relaxed SiGe layers, which opens the door to a wider range of application and to the use of SiGe in complementary metal oxide semiconductor (CMOS) devices, which comprise strained Si and SiGe layers. Some recent successes include record-breaking low-temperature electron mobility in modulation-doped layers where the mobility was found to be up to 50 times better than standard Si-based metal-oxide-semiconductor field-effect transistors (MOSFETs). Even more recently, SiGe-based p-type MOSFETS were built with oscillation frequency of up to 50 GHz, which is a new record, in any p-type material for the same design rule.

Ferroelectric films can display a wide range of dielectric, ferroelectric, piezoelectric, electrostrictive, and pyroelectric properties. The potential utilization of these properties in a new generation of devices has driven the intensive studies on the synthesis, characterization, and determination of processing-microstructure-property relationships of ferroelectric thin films during the last five years. In addition there has been an increased drive for integrating ferroelectric film-based heterostructures with different substrate materials to demonstrate numerous devices that exploit the dielectric, ferroelectric, piezoelectric, electrostrictive, and pyroelectric properties of these materials. For example the high dielectric permittivities of perovskite-type materials can be advantageously used in dynamic random-access memories (DRAMs), while the large values of switchable remanent polarization of ferroelectric materials are suitable for nonvolatile ferroelectric random-access memories (NVFRAMs).

Various vapor-phase deposition techniques such as plasma and ion-beam sputter deposition (PSD and IBSD, respectively), pulsed laser-ablation deposition (PLAD), electron-beam or oven-induced evaporation for molecular-beam epitaxy (MBE), and chemical vapor deposition (CVD) have been applied to produce ferroelectric films and layered heterostructures. See References 4–7 for recent reviews. However, work is still necessary to optimize the techniques to produce device-quality films on large semiconductor substrates in a way that is fully compatible with existing semiconductor process technology. Therefore research efforts should be focused on the optimization of suitable process methods and on the investigation of processing-composition-microstructure property relationships. These efforts are the focus of this article with emphasis on PSD and IBSD techniques.

As an imaging method of elastic properties and subsurface features on the microscopic scale, the scanning acoustic microscope (SAM) provides spatial resolution comparable or superior to that of optical microscopes. Nondestructive evaluation methods of defects and elastic properties on the microscopic scale were developed by using the SAM, and they have been widely applied to various fields in science and technology. One major problem in acoustic microscopy is resolution. The best resolution of SAM with water as the coupling fluid has been 240 nm at a frequency of 4.4 GHz. At a more conventional frequency of 1 GHz, the resolution is about 1 μm. Therefore the resolution of SAM is not always sufficient for examining nanoscale defects and advanced micro/nanodevices.

For materials characterization on the nanometer scale, atomic force microscopy (AFM) was developed and extended in order to observe elastic properties in force-modulation mode. In the force-modulation mode, the sample is vibrated, and the resultant cantilever-deflection vibration is measured and used to produce elasticity images of objects. The lateral force-modulation AFM can evaluate the friction force or the shear elasticity in real time. However in the force-modulation mode, it is difficult to analyze stiff objects such as metals and ceramics.

When the sample is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance frequency, the tip cannot vibrate due to the inertia of the cantilever. However by modulating the amplitude of the ultrasonic vibration, deflection vibration of the cantilever at the modulation frequency is excited due to the rectifier effect of the nonlinear force curves. Based on the tip-sample indentation during ultrasonic vibration, we developed ultrasonic force microscopy (UFM) for contact elasticity and subsurface imaging of rigid objects using a soft cantilever with a stiffness of the order of 0.1 N/m.

A major surge of activity is underway to understand the dynamics of polymer chains at interfaces. This stands in contrast to the situation a generation ago when much of polymer-materials research revolved around understanding dynamics in the bulk (isotropic) state. Building in part on earlier studies that had been somewhat neglected, striking new findings have been obtained. The new methods and equipment include surface-specific spectroscopies; advanced, in situ time-resolved methods to determine surface structure and composition; and the surface-forces apparatus for measuring adhesion and interfacial rheology. Also, older methods (such as contact angle) have been revitalizated and applied to new problems. Theoretical calculations and molecular-dynamics simulations are also emerging.

Appreciation is growing that scientific understanding is possible of these systems that are so complex and, often, so far from equilibrium. Polymer surfaces are becoming recognized as an area with many opportunities to do exciting and useful surface science, particularly regarding kinetics, diffusion, surface chemistry, and other rate-dependent processes.

The engineering significance is that while polymers and plastics-based applications are rooted in our economic life, too often the technologies and formulations are empirically derived. One tends to take plastics and their communication with adjoining materials for granted. A molecular understanding is needed so that better design can emerge by rational extension.

During the course of these new activities, the community of polymer science has rubbed shoulders with and has thereby become increasingly integrated with other disciplines-colloid science, surface science, biomedical science, and microelectronics, to cite a few examples. When dealing with interfaces, one's parochial materials interests quickly become generalized.

The flat-panel-display (FPD) market is experiencing rapid growth due to increased demand for portable computers, communication equipment, and consumer electronic products. In all of these applications, the display is the primary human interface that conveys information. The size of the flat-panel-display market is presently estimated to be $10 billion/year and is projected to grow to over $18 billion/year by 1998. Although most current FPDs utilize either passive- or active-matrix liquid-crystal-display (LCD) technology, electroluminescent (EL) displays and light sources, because of their solid-state construction and self-emissive characteristics, can provide improved performance for many demanding display applications. Thin-film electroluminescent (TFEL) technology has been demonstrated over a broad range of display sizes from 1-in. to 18-in. diagonal with resolutions from 50 to 1,000 lines per inch. Also, because of its unique solid-state characteristic, TFEL technology is well-suited to provide a fully integrated display with the light-emitting element and electronics fabricated on the same substrate. An example of a full-color TFEL display is shown in Figure 1.

Thin-film electroluminescent display panels are finding increasing applications in the FPD marketplace due to several fundamental performance advantages over LCDs. These include wide viewing angle, high contrast, wide operating-temperature range, ruggedness, and long lifetime. Alternating-current (ac)-driven monochrome TFEL displays (ACTFEL displays) have become the most reliable, longest running devices on the market. Commercial ACTFEL display panels have operated for more than 50,000 hours with less than 10% luminance change, the equivalent of 25 working years.

In a classification of solids according to their bonding character (into metals, ceramics and glasses, polymers, and semiconductors), the ceramic class includes an enormous range of industrially important materials. From the archetypal ionic solids through oxides to silicates, and to covalently bonded materials such as SiC, they exhibit a rich variety of structures and properties. They occur as structural materials, either on their own or as composites such as SiC/Al2O3. They are important functional materials, such as fast-ion conductors as electrolytes in fuel cells (for example ZrO2/Y2O3 for hydrogen combustion) or batteries (β-alumina in the sodium-sulfur battery), ferroelectric materials such as BaTiO3 and piezoelectrics such as PZT—a solid solution of PbTiO3 and PbZrO3. The high-temperature superconductors (for example, YBa2Cu3O7) are ceramics above the superconducting transition temperature. The products of corrosion and oxidation are ionic materials, and the properties of oxide coatings are vital to the survival of high-temperature alloys in gas turbines or fuel-element claddings in nuclear reactors.

To understand the behavior of ceramic materials, and to optimize their production, processing, and application, it is often necessary to model their behavior at an atomic level. In some cases this is obvious. Ionic diffusion in a solid electrolyte is a self-evidently atomic process. In other cases the need for atomistic simulation is less clear. Oxidation, for example, is a subtle blend of atomic diffusion (often along grain boundaries), metal-ceramic bonding, stress relief, and grain growth. The course of oxidation can be spectacularly affected by impurities and alloying, and this can only be understood by considering the atomicscale processes involved.

The ever-shrinking dimensions of microelectronic devices has mandated the use of plasma processing in integrated circuit (IC) factories worldwide. Today the plasma-processing industry has grown to over $3 billion in revenues per year, well in excess of predictions made only a few years ago. Plasma etching and deposition systems are also found throughout flat-panel-display (FPD) factories despite the much larger dimensions of thin-film transistors (TFTs) that are used to switch picture elements (pixels) on and off. Besides the use of plasma in etching and depositing thin films, other processes include the following: removal of photoresist remnants after development (descumming), stripping developed photoresist after pattern transfer (ashing), and passivating defects in polycrystalline material. Why are plasma processes so prevalent?

In etching, plasmas are used for high-fidelity transfer of the photolithographically defined pattern that defines the device or circuit. More generally, plasma provides the means to taper sidewalls. In Si processing, the sidewalls must be nearly vertical to obtain high density integration and faster performance. However in making FPDs, sidewalls are tapered to obtain uniform step coverage and reduce shorting. In deposition, plasmas are used to enable processing at low temperature. For both etching and deposition, only plasma processing provides an economically viable means for processing large area substrates: 300 mm for Si and more than 550 × 650 mm for FPDs. It is the ability to scale uniform reactant generation to larger areas that sets plasma apart from beam-based processes that might otherwise offer the desired materials modifications. The nonequilibrium characteristics of plasma further distinguish this processing method. Energetic electrons break apart reactant precursors while ions bombard the surface anisotropically.

Novel applications exist for biomedical materials that can undergo transitions in material properties in situ—that is, at the site of implantation in the body. Such transitions in polymeric biomaterials can be accomplished by crosslinking a material in situ, by heating or cooling to induce thermal transitions, and by precipitating polymer from solution in situ. This article will point out the need for materials that can be induced to undergo such transitions in situ and will describe selected tissue-engineering approaches that have been employed for this purpose.

Delivering materials to the body in one form and utilizing them in another form after a transition at the site of implantation has, generally speaking, two potential advantages: (1) the ability to match the morphology of a material implant to a complex tissue shape and (2) the ability to deliver a large device through a small hole in the body. With respect to the former motivation, tissue shapes in the body have an enormous range of complexity: a region of a blood vessel—for example with a curved central axis, a diameter that varies along the length, an eccentric diseased plaque, and numerous side branches. One could develop a variety of implant shapes—arterial stents in this example (for use in structurally supporting arteries after balloon angioplasty, thereby creating a larger cross section for blood flow above the diseased plaque)—and then select the most appropriate implant shape after detailed imaging of the tissue site. One can alternatively attempt to employ some material transformation to deliver a precursor to the final shape of the implant, utilizing the tissue shape to obtain the proper final implant morphology. With regard to the second motivation, it may be desirable to deliver a large object through a small hole, utilizing material transformations. Advances in surgery have focused on manipulating (cutting, coagulating, suturing, stapling) large tissue sites through small holes in the body via minimally invasive surgery. Using such approaches, it has become possible to perform many complex surgical procedures in the joints, abdominopelvic cavity, thoracic cavity, and nasal sinuses, for example, using surgical instruments that are manipulated through surgical access holes less than 1 cm in diameter. Even procedures as complex as coronary-artery bypass surgery have been performed in this way. It still remains generally impossible however to implant devices in the body through such holes unless these implants are very small. If such devices were for example able to be delivered as liquids and then shaped into devices at the implant site, such minimally invasive surgical-device placement could be envisioned.