Beneficial Contamination, Part II
BARBARA KANEGSBERG & MANTOSH CHAWLA, February 2002
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.
Barbara Kanegsberg at BFK Solutions, 16924 Livorno Dr., Pacific
Palisades, CA 90272, 310-459-3614; barbara@bfksolutions.com;
contact Mantosh
Chawla atPhotoemission Tech., (PET), 3255 Grande Vista Dr.,
Newbury Park,
CA, 91320; 805-499-7667
