BENEFICIAL CONTAMINATION: PART 2
Last month we discussed how, under certain circumstances, a little contamination can be good for you. Commercially-produced heart valves of an alloy primarily of cobalt, chromium, molybdenum, and tungsten, Stellite 21, were nonthrombogenic; they did not induce potentially deadly clots because an easy-release hydrocarbon coating had been introduced as a manufacturing artifact. Two techniques, internal reflection infrared spectroscopy (Multiple Attenuated Internal Reflection, or MAIR) and critical surface tension, were used to study and monitor essential surface characteristics and, in effect, detect beneficial contamination.
MAIR allows analysts to obtain a fingerprint of films as thin as 10 A. Internal reflection spectroscopy involves placing the surface of interest in contact with an internal reflection plate. A beam of light is directed toward the plate in such a manner that it repeatedly reflects inside the plate where it contacts the sample surface; an augmented IR scan is obtained. Some clinicians prefer IR measurements over high vacuum techniques such as Electron Spectroscopy for Chemical Analysis (ESCA), which may provide more definitive identification of molecular species at a specific location on the sample than does IR spectroscopy; but because ESCA involves placing the sample in a vacuum chamber, there is the nagging concern that certain materials on the surface may “turn away” from the surface toward the bulk or even vaporize. While MAIR does not provide complete molecular identification, the infrared scans indicate functional groups, like methyl groups. In the case of the metal heart valves, identifying part of a molecule was sufficient. Methyl groups were found on the surface of vigorously polished, non-thrombogenic metal implant material; and, while vigorous aqueous cleaning had little effect, heavy mechanical scrubbing eroded the surface.
The second technique, critical surface tension (CST), is related to contact angle measurement, which is a refinement of the water-drop test. Contact angle measurement provides an indication of non-specific organic contamination. The contact angle between a liquid droplet and the surface is determined by the nature of the gas/liquid/surface interface. Looking beyond water, the contact angle is also influenced by the quality of the liquid with solutions being less accurate than pure compounds and actual material solutions being worst of all because they attack the surface. Surface quality of a variety of materials from paper to metal have been grossly characterized by marking the surface with dyne pens, which look at bit like magic markers and contain various solvents.
In the study of surface quality of heart valves, researchers determined the critical surface tension (CST) of a solid as measured in dynes/cm. CST, the highest surface tension any liquid (actual or theoretical) can have and still completely wet the surface of the solid, involves measuring contact angles for as many as 16 liquids of known surface tension. The surface tension of each liquid in dynes/cm is plotted on the x axis and the cosine of the contact angle on the y-axis (a Zisman plot). The CST of the solid is surface tension where the cosine of the contact angle equals one.
CST characterized the desirable surface quality of the heart valve material. The CST of the properly contaminated alloy, the alloy polished with organic-based compounds, was 20 to 25 dynes/cm. Alloy polished with organic-free abrasives had a CST of over 35 dynes/cm. Alloy polished with organics but then detergent-scrubbed also showed an increased CST of 27 dynes/cm. Contact angle measurements predicted performance of the implanted device. Lower contact angles, for example, were obtained if the coating was applied with rigorous polishing but with incomplete coverage or if the coating was applied without rigorous polishing and then water-washed. In both instances, devices were thrombogenic.
What can we extrapolate from these studies? First, the mere presence of material on a surface does not necessarily mean that it is a contaminant; it may be essential to proper performance. Further, surface characterization techniques should be selected because they provide the most useful information, not necessarily complete identification. While MAIR-IR does not provide complete molecular identification, it gives enough information about the immediate surfaces of implantable clinical devices, without the concern of altering the surface during sample preparation. Similarly, the overall indication of contamination using CST was predictive of surface performance. Finally, analytical and surface testing were used not dogmatically but rather pragmatically in conjunction with actual performance studies, in this case, in living systems.
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