Recent advances in materials science and biotechnology have given birth to the new and exciting field of tissue engineering, in which the two normally disparate fields are merging into a profitable matrimony. In particular the use of biomaterials capable of initiating new bone formation via a process called osteoinduction is leading to quantum leaps for the tissue engineering of bone.

The classic work of Marshall R. Urist and A. Hari Reddi opened the field of osteoinductive biomaterials. Urist discovered that, upon implantation of devitalized, demineralized bone matrix in the muscle of experimental animals, new bone formation occurs within two weeks, a phenomenon he described as bone formation by induction. The tissue response elicited by implantation of demineralized bone matrix in muscle or under the skin includes activation and migration of undifferentiated mesenchymal cells by chemotaxis, anchoragedependent cell attachment to the matrix, mitosis and proliferation of mesenchymal cells, differentiation of cartilage, mineralization of the cartilage, vascular invasion of the cartilage, differentiation of osteoblasts and deposition of bone matrix, and finally mineralization of bone and differentiation of marrow in the newly developed ossicle.

The osteoinductive ability of the extracellular matrix of bone is abolished by the dissociative extraction of the demineralized matrix, but is recovered when the extracted component, itself inactive, is reconstituted with the inactive residue—mainly insoluble collagenous bone matrix. This important experiment showed that the osteoinductive signal resides in the solubilized component but needs to be reconstituted with an appropriate carrier to restore the osteoinductive activity. In this case, the carrier is the insoluble collagenous bone matrix—mainly crosslinked type I collagen.

An intense worldwide research-and-development effort has been undertaken during the last seven years with the objective of developing thin-film deposition techniques and materials-integration and processing strategies capable of realizing a commercially viable solid-state nonvolatile-ferroelectric-random-access-memory (NVFRAM) technology. Many laboratories around the world have focused their work on developing strategies for integrating submicron thin-film ferroelectric capacitors with the mature silicon-based transistor technology to yield capacitor-transistor-based memory architectures as schematically illustrated in Figure 1. This figure also shows the perovskite unit cell of a capacitor, based on a ferroelectric Pb(ZrxTi 1−xO3 (PZT) layer, and the variety of structural, chemical, electronic, and ionic interfaces that arise during the fabrication and integration of these metal-oxide hetero-structure capacitors on a Si substrate. Nonvolatile ferroelectric random-access memories exploit the capacity of the ferroelectric layer to be polarized in two opposite directions, which are used as the 0 and 1 binary stable states.

Almost all stones on a lane will become glassy if they are melted and quenched. They will become transparent and quite different in appearance from before vitrification. This visible change constitutes the recording of information. We might refer to the stone as “1 bit.” If the vitrified stone is subsequently kept at a high temperature under its melting point, it will lose its transparency and turn back to the appearance it had before melting and quenching. Thus the “1 bit” is erased. This is the simple mechanism of an erasable phase-change optical memory. In practical systems, a laser beam focused into a diffraction-limited spot is used for recording. This enables the spatial size of the “1 bit” to be very small (of submicron order) so that the recording density is very high.

Figure 1 shows a transmission-electron-microscope (TEM) photograph of an actual optical disk. The elliptical smooth areas are recording marks in the amorphous state that were formed by high-power and short-duration laser irradiation. The shortest mark length is about 0.5 μm. The area surrounding the amorphous marks is in the crystalline state and consists of small grains. The two states differ from each other in optical properties such as refractive indices and optical absorption coefficients. Accordingly when the bits are illuminated with low-intensity laser light, the reflected light from the amorphous and crystalline regions is different and may be detected as information signals.

The amorphous marks are erased by heating above the glass-transition temperature by laser irradiation, but with lower power than is used in the case of recording.

The advantage that epitaxy offers the electronics and optoelectronics industries is that it allows the possibility of producing precisely controlled layers of very high crystal quality. Heteroepitaxy of different materials offers the promise of tailoring device layers in clever ways that nature did not intend. However unlike fruit juices, nature has made it difficult to epitaxially combine different materials. As the preceding articles have clearly pointed out, it is very difficult to obtain smooth epitaxial layers that are free both of defects and strain when there is a lattice mismatch between the layers and their substrates.

As already discussed in this issue, a uniform network of dislocations at the interface between a flat, uniform epitaxial layer and its substrate can completely relieve strain in the majority of the epitaxial layer. This would be a satisfactory situation for many devices so long as the active region of the device could be kept away from the interface. The problem is how to introduce the dislocations in an appropriate way. When an epitaxial layer has a larger lattice parameter than the underlying substrate, a misfit dislocation running along the interface represents a plane of atoms that has been removed from the epitaxial layer. (One would insert a plane of atoms if the epitaxial lattice parameter was smaller. For simplicity however we will continue to assume that the epitaxial layer has a larger lattice parameter.) It is not possible for a whole half plane of atoms, bounded by the dislocation at the interface and the substrate edges along the two sides, to be removed at once. The boundary between where the extra plane of atoms has been removed and where the epitaxial layer has not relaxed yet will represent a threading dislocation. This threading dislocation would continue to move as the size of the misfit dislocation along the interface grows. Ideally it moves all the way out to the substrate edge and vanishes there while the misfit dislocation along the interface would end up extending from one side of the substrate to the other. However other dislocations and other kinds of defects can effectively pin the threading dislocation resulting in an epitaxial layer with many threading dislocations. Unfortunately these threading dislocations are generally detrimental to most kinds of devices. It is precisely this high density of threading dislocations that limits applications of many heteroepitaxial layers.

Salado-polychrome ceramics, marked by distinctive black on white with red designs (Figure 1), coincided with the development of platform-mound communities and were the result of an amalgamation of technological traditions that occurred during a time of population movements and cultural changes in the prehistoric American Southwest. Saladopolychrome ceramics were the most abundant decorated ware of the Classic period (A.D. 1275–1450) and have been recovered from sites in central Arizona, southwestern New Mexico, and northern Chihuahua, including the site of Casas Grandes. Several archaeologists have interpreted Salado-polychrome designs as symbols of a regional cult (Adams, Crown, Rice) that eased the integration of diverse populations in times of migration and social stress. The color scheme and designs of Salado polychrome are distinctive compared to other contemporary and earlier ceramics, making definition of its development difficult.

Diamond has existed in the natural state for thousands of years. It was mainly used as a jewel for its optical brilliance and for its hardness. In the 1950s methods were developed to fabricate synthetic diamonds commercially. This greatly increased diamond's industrial use, mostly for grinding and lapping applications. Diamond is a crystalline form of carbon, a group-IV element in the periodic table. Silicon and germanium are also Group-IV elements and also have the same crystalline lattice structure as diamond. Hence there has been theoretical interest in diamond's electronic properties since the beginning of the semiconductor age. However the cost and poor crystalline quality of both natural and synthetic diamond have precluded any real industrial interest in diamond as an electronic material. Methods of low-temperature and low-pressure diamond-film deposition, developed initially by the Russians in the 1950s and 1960s (and thence by the Japanese, and eventually by others) has made it possible to use this exotic material as an electronic substrate.

Diamond, in single-crystalline, polycrystalline, and diamondlike carbon (DLC) forms, is a material with many unusual properties. It is the hardest naturally occurring material, the most thermally conductive, and the most transparent. It also has the slickness of Teflon. In regard to many physical properties, it is at the extreme end of the scale. One of the more unusual and important properties that it possesses is its presentation of a rather small barrier to the emission of electrons into a vacuum.

Due to the importance of ferroelectric and high-permittivity perovskite thin films for a wide range of applications, there has been extensive research devoted to understanding the mechanisms responsible for the degradation observed with time, temperature, and/or external field stress. The three most important degradation phenomena for ferroelectric materials such as Pb(Zr, Ti)O3 (PZT) and BaTiO3 are ferroelectric fatigue, ferro-electric aging, and resistance degradation. Ferroelectric fatigue is the loss of switchable polarization by repeated polarization reversals. Ferroelectric aging is characterized by a spontaneous change with time in the polarization-voltage (P-V) response. Resistance degradation is a deterioration of the insulating properties of a dielectric under direct-current (dc) bias and elevated temperature.

These degradation processes ultimately limit the lifetime and reliability of devices that use ferroelectric and high-permittivity perovskite dielectrics. Fatigue and aging lead to reliability concerns for electronic (nonvolatile memories), piezo-electric, electro-optic, and pyroelectric applications. Likewise resistance degradation typically limits the lifetime of ceramic capacitors and high-dielectric constant thin films such as (Ba, Sr)TiO3, which is the principal candidate material for very high-density dynamic random-access memories (DRAMs).

Because of the importance of these degradation processes, it is critical to understand them and to develop methods of eliminating or mitigating their effects. By combining results from studies on thin films with ones on ceramics and single crystals, a consistent picture of the mechanisms involved in these degradation processes is emerging. In this article, we discuss these degradation mechanisms with particular emphasis on the interaction between ferroelectric domains and charge trapping and the role of oxygen vacancies and associated defect dipoles.

Tremendous advances in computing speed, increasingly clever algorithms, and more accurate interatomic-force models have made atomistic simulation a powerful tool in many areas of materials science and engineering. Indeed, from traditional areas such as the study of grain boundaries, fracture, and point defects to somewhat less traditional areas such as nanometer-scale engineering and device fabrication, atomistic simulations are providing exciting new data and insights that cannot be obtained in any other way. Central to the success of an atomistic simulation is the use of an appropriate force model. For simulations requiring large systems and/or long times, the computational efficiency offered by classical potential-energy functions is necessary. However, obtaining quantitative results requires a model that can both accurately describe an appropriate database of physical properties and that is transferable to structures and dynamics beyond those to which it is fit. The latter property is especially critical if an atomistic simulation is to have useful predictive capabilities. While an extensive and well-chosen database from which parameters are determined is important, transferability ultimately depends on the chosen functional form. The definitive mathematical form, however, has yet to be developed.

This article covers my attempts to develop classical potential-energy functions based on an empirical bond-order formalism that put qualitative and semiquantitative features of chemistry into large-scale condensed-phase simulations. To help explain this work, the next section gives a brief tutorial of the “chemistry” of few-body potentialenergy surfaces. Although familiar to chemists, the concepts discussed in this section may not be as familiar to mate rials scientists and engineers. A discussion of the bond-order formalism and how it can be related to both solid-state structure and chemical dynamics follows. The article ends with a discussion of two applications of this formalism, modeling chemical dynamics near a shock front and the reactive chemistry of diamond deposition.

As adaptable as polymeric materials are in their many applications to our daily lives, the need exists to tailor the polymer surfaces to provide even more flexibility in regard to their uses. Plasma treatments offer an unprecedented spectrum of possible surface modifications to enhance polymers, ranging from simple topographical changes to creation of surface chemistries and coatings that are radically different from the bulk polymer. Furthermore plasma treatments are environmentally friendly and economical in regard to their use of materials.

Plasma processing can be classified into at least four categories that often overlap. These are the following: (1) surface preparation by breakdown of surface oils and loose contaminates, (2) etching of new topographies, (3) surface activation by creation or grafting of new functional groups or chemically reactive, excited metastable species on the surface, and (4) deposition of monolithic, adherent surface coatings by polymerization of monomeric species on the surface. Key features of these processes will be briefly discussed, with a rudimentary introduction to the chemistries involved, as well as examples. Focus is placed on capacitively coupled radio-frequency (rf) plasmas (see Figure 1 in the article by Lieberman et al. in this issue of MRS Bulletin) since they are most commonly used in polymer treatment.

So ubiquitous are intermetallic compounds (IMCs) in all areas of materials application that examples of their use in industrial, medical, consumer, and military products are almost limitless. We will in this brief discussion attempt to identify some of the more important of these applications, which we categorize as miscellaneous. These include shape-memory alloys (SMAs), gold alloys used in jewelry, dental amalgams, tribology applications, diffusion barriers in electronic devices, elevated-heat storage systems, nuclear applications, metal-matrix composites, and high damping alloys. In some cases, the IMC is present as a precipitate or dispersed particle that provides strengthening or other property modification while in other examples, the IMC is employed in bulk form.

Characterization of materials usually requires microscopy techniques. Some of the most useful are based on a scanning microscope and involve scanning the sample surface with a focused beam (e.g., photons, electrons, ions, etc.). For example, photoacoustic microscopy uses a laser beam, acoustic microscopy uses an ultrasound beam, and scanning electron microscopy uses an electron beam. The interaction between the material and the beam produces a signal that can be used to generate a two-dimensional image.

In scanning photoacoustic microscopy (SPAM), an intensity-modulated light beam is used to produce oscillations in the surface temperature of the sample. These oscillations induce changes in the pressure of a fluid in the photoacoustic cell as a consequence of the periodic heat conduction from the surface to the cell fluid. Subsequently many material-characterization methods have employed the same philosophy as SPAM, using a modulated beam as an excitation probe. The breadth of such techniques is due to the large number of possible excitation sources and signal detectors that have been proposed to probe the specimen response. In particular, scanning electron-acoustic microscopy (SEAM), also referred to as thermal wave microscopy, is a technique based on the utilization of a scanning electron microscope developed in 1980 and applied in recent years to material characterization. It can be considered an additional mode of scanning electron microscopy (SEM), which uses the generation of acoustic waves in the sample. Most reviews have concentrated on the application of SEAM to metals and semiconductors. However many other possibilities exist.

Ion-beam analysis of chemical composition as a function of depth is by now well-established for inorganic materials and is an important method of investigating growth of thin films. It has been applied to polymers much more recently, perhaps because fairly obvious problems with radiation damage discouraged workers in this field initially. Ion-beam analysis has developed, however, into a analytical tool that complements other methods, such as x-ray photoelectron spectroscopy and neutron reflection, very well. The purpose of this short article is to give the reader an introduction to its current uses in polymers.

The ion beams of ion-beam analysis are typically highly energetic (1–5 MeV) beams of 4He++. While other beams are used, for example, 3He and 15N, alpha particle beams are used in the vast majority of experiments reported in the literature. Two major categories of experiments are carried out with such beams. Rutherford backscattering (RBS) spectrometry to detect heavy elements in the polymer and forward recoil spectrometry (FRES) (also known as elastic recoil detection) to detect the isotopes hydrogen and deuterium. The basic principles for each method are similar.

A nonconventional way of producing nonvolatile memories is to use ferroelectrics, a class of electroceramic materials. These materials have a remanent polarization. The direction of this polarization can be changed by an electric field. Ferroelectric materials possess a “natural memory,” so to speak. Ferroelectrics have been known for a long time, and the idea to use them for binary data storage originates in the 1950s. The basic element of this type of memory is formed by a ferroelectric capacitor—a ferroelectric layer sandwiched between electrodes. Early prototypes were unsuccessful because rather high voltages were needed to switch the ferroelectric capacitor (200–300 V) and the memories suffered from crosstalk. (Programming one particular cell influenced neighboring cells.) The revival of ferroelectric memories was driven by the development of thin-film deposition techniques that allowed the formation of capacitors with ferroelectric thin films of submicron thicknesses. These capacitors can be switched with normal intergrated-circuit (IC) voltages. The crosstalk problem is circumvented by isolating each memory cell by a transistor (similar to a dynamic random-access memory [DRAM]). Compared to “standard” nonvolatile memories, ferroelectric memories offer the advantage of very fast access times (both for reading and writing), low-voltage operation, and good write/read endurance. A ferroelectric material that is already being used in commercially available memories is lead zirconate titanate, PbZrxTi 1−xO3. To combine a ferroelectric material with IC technology is a challenge, and many problems have been (and will be) encountered.

Recent demonstrations of color-plasma displays with exceptional image quality and performance that meet and in some cases exceed that of cathode-ray tubes (CRTs) have resulted in a renewed interest in this 25-year-old technology. The tremendous market potential for large-area plasma displays for both home-entertainment and computer applications now appears to be an attainable goal. This new generation of plasma displays features full color capability, high brightness and contrast ratio, wide viewing angles, and rapid refresh rate. The success of today's plasma displays is the result of the integration of advancements in driver electronics and the materials and processes used in the fabrication of the plasma-display structures.

Using color-plasma technology, two-inch-thick direct-view displays can be made that are up to 60 inches in diagonal. During the past several months, numerous articles have appeared in the media announcing major investments in facilities and technology for largescale manufacturing of these large-area plasma displays. Table I lists the companies committed to plasma-display research and development. Additional advancements in materials, processes, and electronics will be required to achieve high-volume, low-cost manufacturing.

Photoacoustic spectroscopy is a technique in which the absorption of periodically intensity-modulated light is detected by the sound that it produces at the (acoustic) modulation frequency in a closed volume of gas or liquid in thermal contact with the material absorbing the light. The spectroscopic aspect of the technique relies on the ability to scan the wavelength of the light that stimulates the sound. Thus one can determine the absorption as a function of wavelength through the conversion of absorbed energy to heat and thence to sound. The acoustic detection is carried out synchronously with respect to the imposed intensity-modulation frequency on the light, making it possible to use narrow-band noise reduction. The existence of this technique for detecting sound generated by the absorption of light led Wong and co-workers to investigate the possibility of using the same techniques for microscopy.

Recent technological and materials advances in semiconductors have brought about the possibility of producing heterostructures within which carriers are confined to an ultrasmall region of space (a few thousand atoms) by a potential barrier. When the dimensions of the confining potential are smaller than the electron wavelength (a few tens of nanometers), the semiconductor electronic and optical properties are drastically altered. In these so-called quantum structures, carrier energy levels are quantized and their energy depends on the confining-potential dimensions and magnitude.

Some of these quantum structures have already found technological applications. For example the quantum-well (QW) semiconductor laser is part of every CD player. It is also widely used as the light source for intercontinental optical communications. The carrier confining potential in this case is provided by two wider bandgap semiconductor layers sandwiching a thin (3–20 nm) smaller bandgap semiconductor film. The carriers have two degrees of freedom within the QW. The QWs are grown by epitaxial deposition on a crystalline substrate. The substrate may or may not be lattice-matched with the epitaxial film. In some instances, a small lattice mismatch may be required to obtain the desired band-gap value for the QW material. These are the so-called pseudomorphically strained QW structures and devices.

The majority of musculoskeletal injuries involve the soft connective tissues including cartilage, meniscus, tendon, and ligament. In contrast to bone tissue, which regenerates after injury, these soft tissues tend to heal poorly through formation of weak, disorganized scar tissue. In fact certain soft-tissue injuries cannot heal on their own so surgical repair or complete reconstruction is required to restore function. Biological grafts (autografts and allografts) and “permanent” synthetic prostheses are currently used to reconstruct large soft-tissue defects. Autografts (a patient's own tissue transferred to another site) can be successful, but as you “rob Peter to pay Paul,” complications at the donor site can be significant. Allografts (transplants from another human) are in short supply and carry the risk of rejection or disease transmission. In general, permanent synthetic prostheses fail to withstand long-term mechanical loading associated with the musculoskeletal system. New strategies are clearly needed to manage musculoskeletal soft-tissue reconstruction.

Several tissue-engineering strategies are potentially useful to enhance repair of these troublesome injuries or to induce regeneration. One interesting approach (developed for skin and cartilage regeneration) combines scaffolds, cells, and cell signals to grow living tissue analogues in the laboratory. Scaffolds are natural or synthetic biomaterials upon which the cells attach and grow. The cells may be fibroblasts, chondrocytes, etc., depending on the specific application. Influential cell signals can include attachment factors, growth factors, and mechanical loads.

Our laboratory is developing a similar approach for ligament reconstruction. Ligaments are the strong, flexible bands of collagenous tissue that connect bone-to-bone to provide a delicate balance of stability and flexibility to the joints of the body. The anterior cruciate ligament (ACL) is the primary stabilizer of the knee and is frequently injured in sporting activities and accidents. Serious ACL injuries can lead to disability and progressive degeneration of cartilage and meniscus that normally serve as shock absorbers in the joint. Anterior-cruciate-ligament ruptures do not heal without surgical intervention, due to a poor intrinsic healing response and poor vascularity. To restore normal joint function, ruptured ACLs can be surgically reconstructed using a patellar-tendon autograft or allograft. While significant progress has been made in understanding ACL anatomy, structure, biomechanics, and healing, there is still no biological graft or biomaterial ideally suited for ACL reconstruction.

An important application of ferroelectric films is their incorporation into dynamic random-access memories (DRAMs) as the storage node capacitor dielectric. Dynamic random-access memories represent a large market that is experiencing strong growth, but they are particularly significant as the technology leader for semiconductor devices. As products move to higher and higher integration density, new developments are first introduced in DRAMs. The steady trend toward higher density has placed severe demands on the device designs, particularly with respect to “squeezing” the capacitor into the available space.