Several 40-year anniversaries of the modern area of the synthesis of diamond at high pressures and temperatures (HPHT) have come and gone. If you are a proponent of the Norton Company, this first synthesis would be somewhere between 1948 and 1950. ASEA's Feb. 15, 1953, accomplishment was noted at an international meeting in 1993 on high pressure (AIRAPT) with a small session that included papers by ASEA and General Electric (GE) representatives. The GE success in the Hall experiment of Dec. 16, 1954 (announced in February 1955), was marked only in the personal memories of the remaining members of that diamond team. DeBeers recognizes the ASEA date, but if it celebrates the 40th anniversary of its own in-house accomplishment, it will do so late in 1998; the Russians will do so on or about the year 2000. In any case, the basic, muchcopied GE HPHT process is repeated thousands of times each day at sites all over the world (Figure 1) to make a product that has replaced 90% of natural diamond as an abrasive and has given to technology new superhard products in forms unavailable from the earth (Figure 2). All manufacturers are in fierce competition for a total business that probably is in the range of $1 billion per year, and engineers spend their time primarily in finer tuning for faster growth, greater yields, lower costs, and higher quality crystals. The HPHT process seems to have survived the threat of the low-pressure chemical vapor deposition (CVD) process with respect to abrasive grain and gem-quality stones. However, the low-pressure process has some unique capabilities that obviously cannot be achieved by HPHT, and the product is finding niches in specialized applications (Figures 3 and 4).

The nervous system of the human body can be divided into two parts: the central and the peripheral nervous system. While the central nervous system (CNS) includes the brain and the spinal cord, the peripheral nervous system (PNS) consists of all the nerve branches exiting the spinal cord or brain stem that process information from our environment to the CNS and vice versa. The main extension that passes all incoming information to the next neuron or a peripheral target is called an axon. Axons can be as long as 1 meter, as in the case of axons sending electrical signals to the toes. All incoming information is received by smaller structures called dendrites. Axons are wrapped with glial cells, Schwann cells in the PNS, and oligodendrocytes in the CNS. These cells are responsible for the deposition of myelin, a powerful insulator. Each glial cell is covering an axonal segment of about 1-2 mm. An unmyelinated gap separates two glial cells. Action potentials are generated at those gaps. This electrical signal jumps from one unmyelinated gap to the next, giving rise to conduction velocities as fast as 1 m/s. In the PNS, axons are grouped together into fascicles, consisting of a layer of connective tissue surrounding bundles of axons. Several of these fascicles form a peripheral nerve surrounded by another connective tissue layer named epineurium. The epineurium is responsible for the integrity of the overall structure of a peripheral nerve. While the PNS shows regenerative capabilities, this is in general not the case for the CNS. The mechanisms explaining the poor regeneration capabilities of the CNS have started to be elucidated. The expression of inhibitory molecules in the mammalian-adult nervous system seems to be an essential component in the inherent lack of CNS regeneration. Examples for possible applications of materials systems addressing certain issues of PNS and CNS regeneration are presented in the following sections.

Ferroelectric ceramic thin films fit naturally into the burgeoning field of microelectromechanical systems (MEMS). Microelectromechanical systems combine traditional Si integrated-circuit (IC) electronics with micromechanical sensing and actuating components. The term MEMS has become synonymous with many types of microfabricated devices such as accelerometers, infrared detectors, flow meters, pumps, motors, and mechanical components. These devices have lateral dimensions in the range of 10 μm–10 mm. The ultimate goal of MEMS is a self-contained system of interrelated sensing and actuating devices together with signal processing and control electronics on a common substrate, most often Si. Since fabrication involves methods common to the IC industry, MEMS can be mass-produced. Commercial applications for MEMS already span biomedical (e.g., blood-pressure sensors), manufacturing (e.g., microflow controllers), information processing (e.g., displays), and automotive (e.g., accelerometers) industries. More applications are projected in consumer electronics, manufacturing control, communications, and aerospace. Materials for MEMS include traditional microelectronic materials (e.g., Si, SiO2, Si3N4, polyimide, Pt, Al) as well as nontraditional ones (e.g., ferroelectric ceramics, shapememory alloys, chemical-sensing materials). The superior piezoelectric and pyroelectric properties of ferroelectric ceramics make them ideal materials for microactuators and microsensors.

Solution deposition has been used by almost every electroceramic research-and-development organization throughout the world to evaluate thin films. Ferrite, high-temperature-superconductor, dielectric, and antireflection coatings are among the electroceramics for which solution deposition has had a significant impact. Lithium niobate, lithium tantalate, potassium niobate, lead scandium tantalate, lead magnesium niobate, and bismuth strontium tantalate are among the ferroelectric thin films processed by solution deposition. However, lead zir-conate titanate (PZT) thin films have received the most intensive study and will be emphasized in this article.

Solution deposition facilitates stoichiometric control of complex mixed oxides better than other techniques such as sputter deposition and metalorganic chemical vapor deposition (MOCVD). Solution deposition is a fast, cost-efficient method to survey extensive ranges of film composition. Further it is a process compatible with many semiconductor-fabrication technologies, and it may be the deposition method of choice for applications that do not require conformal depositions and that have device dimensions of 2 μm or greater. Specific applications for which solution deposition is commercially viable include decoupling capacitors, uncooled pyroelectric infrared detectors, piezoelectric micromotors, and chemical microsensors based on surface-acoustic-wave technology. Reviews of some of the more fundamental aspects of solution-deposition processing may be found in the scientific literature.

The remanent-polarization states of ferroelectric capacitors have long been of interest for nonvolatile storage of digital data. A simple memory array can be formed by perpendicular rows and columns of conductors with a ferroelectric capacitor at each crosspoint. In the simplest geometry, these conductors also are the capacitor electrodes on either side of a ferroelectric layer. Each capacitor represents one bit of digital data. If a voltage of V0 is required to write a capacitor, then data can be written to a specific bit by applying V0/2 and –V0/2 to the appropriate row and column. Such ferroelectric memory arrays were constructed as early as the 1950s. However, due to the breath of the ferroelectric transition, they were not successful because during the write of a specific capacitor, the V0/2 voltage signals disturb the polarization in other capacitors. This is in contrast to the successful use of the analogous ferromagnetic crosspoint arrays, which are familiar as the core memories of early computer technology.

Soluble signaling proteins called growth factors execute critical functions during the formation of specialized tissues throughout the developing embryo. When growth factors are provided to adult animals, they often encourage regeneration or repair of organs damaged by disease or trauma: Basic fibroblast growth factor (bFGF) and transforming growth factor ß1 (TGF-ß1) encourage wound healing hematopoetic growth factors stimulate the production of blood cells, bone morphogenetic proteins (BMPs) induce bone formation, nerve growth factor (NGF) enhances the survival of degenerating cholinergic neurons, and angiogenic growth factors activate new blood-vessel growth. Our understanding of the role of growth factors in development and regeneration should continue to expand dramatically over the next decade, inasmuch as new molecules (and new activities for known molecules) are appearing at a rapid rate.

Protein growth factors may be useful in augmenting the new approaches for tissue engineering. Modern biotechnology permits the large-scale manufacture of highly purified proteins so that large quantities can be produced for use in humans. However proteins are often exceedingly difficult to administer, particularly if sustained levels are required. Most protein growth factors have short half-lives after intravenous injection, with their biological activity lasting only a few minutes in the circulation, so that injection must be repeated frequently to obtain sustained blood levels (Table I). Since these molecules are large, they penetrate tissue barriers, such as the capillary wall, very slowly. In addition, growth factors are extremely potent, often possessing biological activity at a number of tissue sites throughout the body. Therefore systemic administration can lead to toxicity. In view of these difficulties, new methods for growth-factor delivery are needed. The most promising new methods involve polymers, which can be engineered to provide precisely controlled, prolonged growth-factor delivery at a localized site.