Synthetic biomaterials can be divided into metals, polymers, ceramics, and composites. Polymeric biomaterials and composites can be classified as biostable, partially biodegradable, or totally biodegradable. Biostable polymers are inert, cause minimal tissue response, and retain their properties for years. Because only a temporary supporting biomaterial is required in many clinical applications, totally or partially biodegradable polymeric materials are often better than biologically biostable materials.

Biodegradation implies the degradation of a material induced by the activities of an organism, but it is also influenced by pure physicochemical reactions. Degradable polymeric or composite materials retain their support from days to months, depending on their properties. They are biologically degraded, gradually, into tissue-compatible components that are absorbed, used in the organism, and finally secreted by the normal ways.

Biodegradable materials are the best choice for internal fixation of fractures because the biodegradable implant provides and maintains the required fixation during the healing process. It decomposes gradually and transfers the stresses increasingly to the healing bone, thus avoiding stress-shielding. Micromovement at the fracture and correction of the bone-mass decrease are possible, too. After healing, when absorbed, the device can be utilized in energy and protein metabolism. Implanted biodegradable devices do not require a removal operation, which is a financial and psychological benefit to the patient.

In spite of the success of surgical implants such as artificial hip joints, the materials used to make them are not always quite up to the job. Even stainless steel and titanium alloys can break under the enormous stress on load-bearing joints and corrode in the salty environment of the body. Deposits of inorganic salts can scratch weight-bearing surfaces, making artificial joints stiff and awkward. As a result, the lifetime of an implant is, at most, 10–15 years.

Metallurgists and engineers often treat the surfaces of metal parts to improve their properties. The use of advanced surface-treatment techniques such as glow-discharge ion implantation, plasma deposition, and plasma coating can significantly improve the strength, hardness, and corrosion resistance of metal implants. At the same time, these methods should also improve the biocompati-bility of the implanted devices.

Cobalt-based alloys are widely used for joint replacements. However, other compounds, such as titanium alloys, have excellent potential biocompatibility and interesting but imperfect mechanical properties. Stainless steel, namely 316L, has a good price-to-mechanical-properties ratio, but has the lowest corrosion resistance of the most commonly used metallic biomaterials.

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.

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.