While it is possible to carry out fracture experiments in single crystals, in everyday experience fracture occurs in heterogeneous and often disordered materials. Tear a piece of paper, and the resulting ragged edge shows evidence of the local variation in the paper's mechanical properties. One might expect the same behavior in fracture of other disordered materials such as polycrystalline solids, fiber composites, and concrete.

The use of artificial materials in medicine is increasing. The most important and common reason for their use is to replace tissue that has become damaged or destroyed through some pathological process, for example, destruction or degeneration of bones, joints, ocular lenses, heart valves, and arteries. Artificial materials are also indicated for medical applications that support or monitor more complex body functions, for example, cardiac pacemakers, dialysis membranes for extracorporeal gas or fluid exchange, and biosensors. The research for appropriate materials is determined by the desired function of the medical device (biofunctionality) and by the biological response to these materials o (biocompatibility).

According to different functional and biological requirements, a huge variety of different materials are currently in use. Metals and alloys-particularly stainless steel, titanium, and cobalt chromium alloys—and different ceramics, such as alumina and calcium phosphates, are used for bone and joint replacement. Different polymers, including polypropylene, poly(tetrafluoroethylene) (PTFE), polyesters, and polyurethanes, are applied for sutures, soft tissue augmentation, and vascular prostheses. Silicones and poly(methyl methacrylate) (PMMA) are used for intraocular lenses. Membranes of regenerated cellulose were the most widely used hemodialysis membranes, but in the recent decades, several attempts were made to replace cellulose with alternative polymers in order to improve blood compatibility-particularly polyacrylonitrile, polycarbonate, PMMA, and polysulfone.

However, today there remain problems of both biocompatibility and biofunctionality. Clinically, the main problems emerge from interactions between materials and the local tissue environment, as summarized in Table I. To ensure a maximum of biocompatibility and functionality, the material in vivo should not evoke any of the reactions outlined in Table I. Today, none of the classical materials fulfills this high demand.

As the areas of the major applications of transparent conducting oxides (TCOs) increase, demand will grow for materials having lower sheet resistance while retaining good optical properties. Simply increasing the film thickness is not acceptable because this would increase the optical absorptance. New materials must be developed with lower resistivities than previously achieved and with optical properties superior to those of the present generation of TCOs. This has now been recognized internationally, and novel materials are being investigated in Japan and the United States.

Thermal-spraying of polymers can be traced back to the 1940s, when polyethylene (PE) was first produced by E.I. du Pont de Nemours & Company. Early work with flame-spraying guns was unsuccessful because the equipment, designed for spraying metals, produced a flame that was both too hot and too short to melt the PE without degradation. The technology has advanced considerably in the past 20 years. Flame-spray equipment and hardware accessories have been designed by numerous companies (PFS Thermoplastics, Alamo, Eutectic + Castolin, etc.) specifically for polymeric materials. The new equipment provides either a lower flame temperature or uses special cooling shrouds to cool the center of the flame, providing a longer residence time at a lower temperature. Many plastics can now be completely melted in-flight and allow heat-sensitive components to be coated with plastic.

This article describes the production of different types of microstructures using high-energy heavy-ion beams, a process called single-ion micromechanics. The formation of high-resolution “radiation shadows” using unfocused, high-energy heavy ions is first described, and the rest of the article discusses the production of microstructures using precisely aimed single ions and the technology underlying this process using a microprobe.

The process of using heavy ions for microlithography was initially developed at GSI (Gesellschaft für Schwerionen-forschung, Darmstadt, Germany) because of their property of traveling in straight lines within matter. Owing to their very high momentum, they tend to undergo very little lateral scattering. This is an even more pronounced characteristic for heavy ions than for light ions, such as MeV protons (as were discussed for lithography in the article by Watt et al.). Figure 1 shows an example of this behavior for 7.5-MeV per nucleon xenon ions in mica. To make the tracks visible, the irradiated mica was broken along the tracks. The length of the individual tracks from top to bottom is about 25 μm.

Such heavy, high-energy ions cause greater radiation damage than lighter ions. Their large rate of nuclear energy loss results in their displacing lattice nuclei with enough imparted energy that these nuclei create more displacements in subsequent collisions of their own. Therefore, high-energy heavy ions are very efficient for creating microstructures, compared with other types of radiation, because each individual ion can produce an observable irradiation effect, while it always takes more than one light ion, electron, x-ray, or light photon to render developable one volume element of the sample. Figure 2 shows the lattice disorder created by a single high-energy uranium ion in germanium sulfide.

Over the past decade, orthopedic surgery and related research have made increasing use of the opportunities offered by computers. Many clinical and research procedures rely on medical images (e.g., x-ray films and histology) and their numerical evaluation (e.g., geometric analysis and surface metrology). This has led to the development of computer applications in orthopedics, with computer graphics playing a fundamental role. The image-processing field has also been studied in greater depth, with a view toward enhancing systems automation.

Image processing relies on capturing measurable variations in some image parameters (e.g., light intensity, color, and signal-to-noise ratio). The extracted numerical values then can be used to modify the source image. From this definition, image processing can be considered a measurement technique, and in this light it has been introduced into histomorpho-metrical practice.

Image processing speeds up routine analysis procedures by evaluating, in a less tedious and quicker way, a greater number of images, parameters, or both. Semiautomated computerized image-processing systems have been in use for a long time, based on computer-assisted analysis of objects whose contours were tracked manually by a digitizing cursor. Histological images were, generally, projected from the microscope over a digitizing tablet, with the operator visually recognizing the structure of interest and picking up its contours. The critical advance brought to histomorphometry by semiautomated techniques has been the practical measurement of surface-related parameters.

The following article is an edited transcript of the presentation given by Chad A. Mirkin (Northwestern University), recipient of the 1999 Outstanding Young Investigator award, at the 1999 Materials Research Society Spring Meeting on April 6 in San Francisco. Some examples of new work have been added to the transcript.

Our group has been developing a couple of projects over the past few years, both of which deal with the general area of nanotechnology. We are very excited about this work because we think it will lead to a general methodology for preparing nanostructured materials from common inorganic building blocks and readily available DNA-interconnect molecules. The intellectual payoff from this work will be a greater understanding of the collective interactions between nanoscale building blocks in the context of organized materials, while the technological payoffs range from the development of new and useful types of DNA detection strategies, to highperformance catalysts, to the realization of bioelectronic nanocircuitry.

The field of nanotechnology faces three main challenges. The first is to develop a combination of tools and materials that allows us to make small structures and control the architecture of large structures on the nanometer-length scale. Of course, we must be able to do this routinely before we can really explore this field in detail. The second important challenge is to determine the chemical and physical consequences of miniaturization, which is where the real science comes into play in nanotechnology.