Liquid crystals, discovered just a century ago, have wide application to electrooptic displays and thermography. Their physical properties have also made them fascinating materials for more fundamental research.

The name “liquid crystals” is actually a misnomer for what are more properly termed “mesophases,” that is, phases having symmetries intermediate between ordinary solids and liquids. There are three major classes of liquid crystals: nematics, smectics, and columnar mesophases. In nematics, although there is no correlation between positions of the rodlike molecules, the molecules tend to lie parallel along a common axis, labeled by a unit vector (or director) n. Smectics are more ordered. The molecules are also rodlike and are in layers. Different subtypes of smectics (labeled, for historical reasons, smectic A, smectic B,…) have layers that are more or less organized. In the smectic A phase, the layers are fluid and can glide easily over each other. In the smectic B phase, the layers have hexagonal ordering and strong interlayer corrélations. Indeed, the smectic B phase is more a highly anisotropic plastic crystal than it is a liquid crystal. Finally, columnar mesophases are obtained with disklike molecules. These molecules can stack up in columns which are themselves organized in a two-dimensional array. There is no positional correlation between molecules in one column and molecules in the other columns.

The influence of electric fields on the deformation and flow properties of materials has been a subject of interest for many years. Recently, there has been renewed interest in a particular branch of these electric field effects—the electrorheological (ER) effect. The ER effect is also known as the Winslow effect after its founder Willis Winslow. Winslow observed that applying strong electric fields to nonaqueous silica suspensions activated with a small amount of water caused rapid solidification of the originally fluid material. This type of behavior was seen as instrumental in the development of high-speed valves, reactive damping systems, and a host of other applications.

One hundred years after their discovery, we meet liquid crystals everywhere in our daily life. Their most widely known application is the liquid crystal displays (LCDs) in watches, pocket calculators, or gasoline pumps. Applications aside, liquid crystals show many exciting properties, making them highly interesting for fundamental research. For example, electrohydrodynamic convection (EHC) in nematic liquid crystals, which is studied in cells of a configuration similar to liquid crystal displays, serves with its characteristic properties as a model System for investigating central questions of pattern formation and chaos.

Today's liquid crystal displays work on the principle described in 1971 by Martin Schadt and Wolfgang Helfrich (Figure 1). In nematic liquid crystals, organic molecules orient on average along a macroscopic direction, described by the director field n(r), that has neither head nor tail (n = −n). Nematics are therefore anisotropic and for energetic reasons, n(r) orients parallel (perpendicular) to an electric field when the dielectric permittivity (ε∥) along n is larger (smaller) than the perpendicular (ε⊦ one. For positive εa = ε∥ − ε⊦, when an electric field is applied perpendicular to the direction of n, a reorientation of n takes place together with a corresponding change in the optical property of the cell. The controlled change by an electric field in the optic axis (orientation) in well-defined areas of the display then allows the representation of numbers, etc.

The search for biocompatible cardiovascular materials has gone on for more a century, beginning with the metallic and glass blood vessel substitutes of Alexis Carrel. The quest has become more involved as the need for flexible polymeric materials became apparent, and as knowledge has been gained about the reactions of blood with artificial surfaces. Current materials in clinical practice are, for the most part, standard polymers that have been in incardiovascular materials is thrombosis, a combination of the coagulation of the blood proteins (which traps red cells, thus obstructing vessel channels) and the activation and deposition of blood platelets, cells that rapidly aggregate with other platelets to obstruct the channel. Coagulation and platelet aggregation are interdependent, highly volatile processes, acting like two positive feedback cascade amplification processes. Thrombosis is prevented in clinical practice by administering heparin, ity. The endothelial cell releases heparin and heparan sulfate, natural anticoagulants, in response to various external stimuli. Endothelial cells produce prostacyclin, which prevents platelet activation by surface contact and is also a powerful, shortacting vasodilator. Prostacyclin release is modulated by fluid shear stresses on the endothelial membrane, among other stimuli. Endothelial cells bind the coagulation enzyme thrombin to thrombomodulin, one of the endothelial membrane proteins, removing it from circulation but also enhancing the activation of protein C, another circulating protein which inhibits blood coagulation. Creation of a material that would precisely mimic all functions of the endothelial cell is a daunting task. Fortunately, more prosaic materials exist that may be purified or modified to elicit minimal biological responses. Progress in developing these materials will be discussed in the context of typical cardiovascular devices.

We are accustomed to thinking of matter as being in one of three states: solid, liquid, or gas. Nature, however, has provided us with a marvelous intermediate state, the gel. Industrial applications are just beginning to explore the advantageous gel properties as adhesive, superabsorber, damper, membrane, toner, catalyst support, etc. Gels are good adhesives since they combine the surface wetting property of liquids with the cohesive strength of solids. Strong adhesion and damping properties suggest gels as a binder in composite materials. For example, the car of the future may operate at a much reduced noise level due to a thin layer of gel in its body. Widespread technical applications have not yet materialized because, until recently, it was difficult to measure gel behavior. This has changed, and as a consequence, gels can now be manufactured with reproducible properties.

What the many possible types of gelation processes have in common is that molecules connect into a three-dimensional network structure. Junctions between molecules form as chemical bonds (cross-links) or physical associations (such as crystalline or glassy domains, hydrogen bonds, etc.). The molecular architecture determines the gel properties in ways which have yet to be explored. Typical parameters are the monomer building block, molecular size, branching, chain stiffness, cross-link functionality, and solvent content.

Bone-interfacing surgical implants used in orthopedics and dentistry must bear the forces of normal patient activity with minimal risk of mechanical failure of the implant. This requires using appropriate materials and designs for implant fabrication. Additionally, reliable long-term implant attachment to host bone must be assured so that effective force transfer between implant and bone occurs for the patient's lifetime without the implant loosening. With recent advances in implant designs and techniques for their placement, effective implant fixation to bone can last for years (decades) either directly or through an acceptable intermediate fibrous tissue layer at the bone-implant interface. With approximately 500,000 artificial hips implanted annually worldwide and the demand for other joint replacements approaching the same order of magnitude, as well as the recent major growth in the use of dental implants (300,300 projected for insertion in North America alone in 1991), the assurance of effective implant-to-bone fixation is extremely important.

Studies of implant biocompatibility have resulted from concerns over the cumulative effects of foreign element release through implant corrosion and wear. Accumulation of this debris in tissues both local and remote to implant sites over the long term is a concern. Of equal importance, for load-bearing implants, are studies to determine the important factors for successful long-term implant fixation. Current trends in design and use of both dental and orthopedic implants reflect the trial-and-error approach that has characterized this field for decades.