Beneficial Contamination, Part I
BARBARA KANEGSBERG and MANTOSH CHAWLA, January 2002
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
primarilyhydrocarbon
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.
IN and OUTof the cleanroom Contact Barbara Kanegsberg at BFK Solutions, 16924
Livorno Dr., Pacific Palisades, CA 90272, 310-459-3614; barbara@bfksolutions.com;
contact Mantosh Chawla at Photoemission Tech., (PET), 3255 Grande Vista Dr.,
Newbury Park, CA, 91320; 805-499-7667
