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Polymers for biomedical applications.

This area is part of the biomaterials field which encompasses metals, ceramics and re-processed biological materials, as well as polymers. Applications of biolog-materials continue to expand at a fairly rapid rate. Among the better known are membranes for hemodialysis, materials for hip-joint prostheses; vascular grafts, heart valves, dental prostheses and controlled drug release. The medical devices market, which depends heavily on suitable biomaterials, is currently estimated to be worth $2-billion in Canada. Most of the problems associated with using biomaterials stem from a lack of compatibility between the material and the tissue with which it is in contact. Durability can also be a problem in the harsh' environment of the body. One can readily appreciate the demands on, say, a heart valve which must'cycle' 70 times a minute over many years in contact with blood.

Biomaterials divide roughly into dental, orthopedic and cardiovascular. Our group at McMaster is working on cardiovascular biomaterials, ie. materials that can be used to replace parts of the system of the heart and blood vessels. Valve prostheses and vascular grafts are the best known of these replacements but heart-assist devices, both permanent (total heart) and temporary (intra-aortic balloon), have also been developed with varying success. As indicated previously, biocompatibility is of primary concern and often limits the usefulness of the device. In cardiovascular implants, the major incompatibility is blood coagulation and thrombosis caused simply by contact of the blood with the foreign' surface. Such reactions can lead directly to blockage of a flow conduit or embolization of the clot to a downstream location where blockage occurs. This reaction is severe enough that grafting of blood vessels less than about six millimeters in diameter (eg. all vessels below the knee) is not possible with present technology. Large vessels, such as the aorta and femoro-popliteal arteries can be grafted using porous Teflon and woven Dacron. At McMaster, we are pursuing two lines of research in relation to this problem. In the first, we are investigating blood-material interactions on the basis that the mechanisms of clotting and thrombosis on surfaces are imperfectly understood. Design of better materials is not likely to advance in the face of an information vacuum. We are particularly interested in the very first events following blood contact. These consist mainly of rapid adsorption and exchange among the 200 or so proteins in blood. In this work, we make extensive use of radiolabeled proteins, methods to study adsorption in real time, and surface analysis methods such as contour angle and x-ray photoelectron spectroscopy. Proteins, being macromolecules, have much in common with synthetic polymers with respect to behaviour at interfaces. An important difference is that proteins have a definite shape and biological activity, which can be profoundly altered by adsorption. Our second major area of activity is materials development per se. In this area, we are working on the segmented polyether polyurethanes. These materials can be tailored to provide a range of mechanical properties suitable for vascular prostheses. A commercial polyurethane known as Biomer, which is a medical grade of the spandex fiber material Lycra has been used extensively in construction of experimental devices. However, this material is unacceptably 'thrombogenic'. Much of our work in this area consists of modification of the hard segment to achieve specific effects such as anticoagulant or fibrinolytic (clot-dissolving) properties. This involves attachment of specific chemical functions, various amino acids and peptides to the hard segment chains. An example is the use of lysine which specifically binds plasminogen, the main blood protein involved in fibrinolysis. In this work, we must be familiar with (or develop) suitable biological evaluation methods, as well as the methods of polymer synthesis and evaluation.

Our activities have benefitted from interactions with other parts of the MIPPT, and analytical facilities, such as differential scanning calorimetry and size exclusion chromatography, have been extremely useful. The excellent thrombosis research groups in Health Sciences have also been a considerable resource. The advent of the Ontario Centre for Materials Research (OCMR) with one of the five programme areas in biomaterials has provided sophisticated surface analysis facilities (eg. XPS installed at the University of Toronto) and has stimulated excellent interactions with groups at the University of Toronto, where a Centre for Biomaterials has been formed. Our group at McMaster currently numbers nine, including technicians, students and postdoctoral fellows. Operating funding comes from the Medical Research Council, NSERC, the Heart and Stroke Foundation of Ontario, OCMR and industrial sources.
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Author:Brash, John L.
Publication:Canadian Chemical News
Date:Apr 1, 1991
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