Biomedical Engineering Reference
In-Depth Information
repeated neutralization with deionized water in an ultrasonic cleaner and thorough
cleaning (e.g., with alcohol) are needed to ensure that the tissues are not poisoned by
the chemical precursors of the IROX
fi
lm.
IROX-coated electrodes acquire the electrochemical properties of the IROX and lose
those of the underlying metallic electrode. For example, the voltage change across the
IROX interface in response to a constant-current pulse is signi
cantly less than the volt-
age change across an uncoated metallic electrode interface in response to a current pulse
of the same magnitude. The corrosion resistance conferred upon the metallic electrode by
an IROX coating is also enormous. In agreement with our own experience, Robblee found
that no dissolution of the underlying metallic electrode occurred when using IROX-coated
platinum-iridium electrodes stimulated with biphasic regulated current pulses, whereas
the same electrodes with no coating lost between 2 and 8
fi
µ
g of platinum during only 24
hours of in vitro stimulation under the same conditions.
The coating process is dependent on the purity and concentration of the chloriridic acid,
the exact ratio of chloroiridate-alcohol complexes, and the preparation and cleanliness of
the base metal. You'll end up going through a lot of chloroiridic acid (and sweat) before
you achieve a repeatable coating process.
As for titanium nitride, the preferred process is to apply the porous nitride layer to the
carrier material serving as substrate by means of reactive ion plating. If you are part of a
university, you can probably ask around the material sciences department to see if they
have the setup necessary for physical vapor deposition. If you really want to home-brew
titanium nitride-coated electrodes, we recommend that you read the very practical articles
on amateur sputtering by Steve Hansen that appeared in The Bell Jar (volume 8, numbers
3-4, pages 14-16, 1999; volume 9, number 1, pages 2-4, 2000; and volume 9, number 2,
pages 10-12, 2000). 5
An alternative to doing your own coating is to contact one of the specialized vendors that
can coat electrodes for you. The two best known in the implantable devices industry are
Hittman Materials (now Wilson Greatbatch Technologies, Inc.) and W.C. Heraus. Both
companies are providers of precious metals and special material processing for implantable
devices. If you approach vendors other than Hittman or Heraeus, be aware that “desirable
characteristics” vary depending on the application. For example, one form of titanium
nitride coating (a shiny golden
finish) generates a very hard, smooth surface and is used on
metal mold surfaces and surgical tools. But the form of titanium nitride preferred for
fi
Warning! Chloroiridic acid, iridium trichloride, and hydrochloric acid are relatively
dangerous materials. Do not breathe dust or mist and do not get in eyes, on skin, or on
clothing. Sodium phosphate dibasic is an eye and skin irritant. When working with
these materials, approved safety goggles or glasses must be worn. Contact lenses are
not protective devices: Appropriate eye and face protection must be worn instead of, or
in conjunction with, contact lenses. Wear disposable protective clothing to prevent
exposure. Protective clothing to prevent skin contact includes a lab coat and apron,
fl
flame- and chemical-resistant coveralls, gloves, and boots. Follow good hygiene and
housekeeping practices when working with these materials. Do not eat, drink, or smoke
while working with them. Wash hands before eating, drinking, smoking, or applying
cosmetics.
5 The Bell Jar (ISSN 1071-4219) is a short magazine on vacuum technique and related topics for the amateur
investigator. It is edited and published by Steve Hansen, who can be contacted through the Bell Jar is Web site at
www.belljar.net , or by mail at 35 Windsor Drive, Amherst, NH 03031.
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