BENEFICIAL CONTAMINATION
Sometimes, a little contamination can be good for you--even life-saving. This may be counterintuitive to those involved in contamination control or analytical chemistry; there is, after all, the tendency to assume the fewer contaminants, the better.
To understand the value of strategically placed contamination, we need to take an historical perspective. When biologists, for example, attempted to control growth conditions by using high-purity water, they found that many plants and microbes grew poorly in deionized water (DI) because plants required trace impurities which came to be known as micronutrients. This phenomenon is not restricted to biological systems. Ultrapure, high-ohm water (UPW) can remove trace metals and produce corrosion or other undesirable surface modifications. Small amounts of oils remaining on a metal surface after solvent cleaning may prevent corrosion. If a process change, for example from a solvent to a surfactant, eliminates the trace beneficial contaminant from the surface, the component may corrode. If an anticorrosion additive is introduced, it may be necessary to redefine the appropriate surface cleanliness to ensure that the additive is not contributing to contamination.We all intuitively support the concept of contaminant-free biomedical implants, but sometimes a specific contaminant is needed to produce the appropriate surface. Back in the 1960s, researchers began to make progress in developing nonthrombogenic biomedical implants, implants that don't tend to produce blood clots, which can be life-threatening. As any contact lens wearer knows, when foreign materials are introduced into the body, there is a tendency toward protein build-up. Where implants are exposed to the bloodstream, as in heart valves, it is important to avoid the kind of protein which attracts platelets and forms a clot. If a clot were to remain in place in a heart valve, it could cause mechanical blockage; if a clot were to grow and then break free, a variety of vascular problems, including stroke, could result. After much trial and error (fortunately, predominantly in animal, rather than human experiments), commercial implants made of Stellite 21 were found to be successful.
No one knew quite why Stellite 21, an alloy primarily of cobalt, chromium, molybdenum, and tungsten, was nonthrombogenic, but a young physicist, Robert Baier, Ph. D., had became involved in the analytical detective work. It wasn't obvious why this particular alloy would be superior to some other alloy, pure metal, or metal oxide. Bob Baier obtained and examined some of the existing implants; they were highly polished (surgeons prefer a very smooth, shiny surface). Freshly made Stellite investment castings, however, were dull gray.
Surface smoothness does not in and of itself confer nonthrombogenic qualities. When devices made of the same alloy were polished metallographically and then implanted, they produced the same undesirable, dangerous blood clots as other materials. In contrast, when the commercial devices were removed and examined, in some cases years after implantation, there was the expected buildup of protein, but there was no clot formation. The reason was that the device manufacturer had fortuitously chosen a diamond-in-tallow polish. Residue of this polish, the presence of which on the surface might be considered undesirable contamination, was the key to achieving nonthrombogenic properties. Protein adhered, but clots and scar tissue didn't form.
One might reasonably ask why we don't observe spontaneous formation of clots in uninjured blood vessels. Researchers analyzed the interior surface of excised jugular veins and determined that the innermost layer behaved as if it were primarily hydrocarbon composition, a low-surface-energy layer, in spite of its hydrophilic (" water-loving") character. Not surprisingly, the surface of the commercially polished alloy was found to be comprised not of metal but of methyl groups (CH 3 ), similar to normal blood vessels. The waxes used in polishing the devices were mixtures of long chain fatty compounds (primarily stearic and palmitic acids). Simply put, coating the metal with the polishing compound (the contaminant) didn't result in a clot-resistant product; the energy associated with final polish facilitated a reaction between the chromium and the waxes to form an adherent metallic soap which is covalent-ly bound to the underlying surface.
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