Biomedical Engineering Reference
In-Depth Information
ulation applications. In vitro pulse tests of the oxide grown using this method show
that coating maintains its electrical characteristics over at least 100 million pulses.
cult for home-based coating, but it can be used to
deposit IROX on any common metallic substrate. Robblee's patent, “Iridium Oxide Coated
Electrodes for Neural Stimulation” [1987], describes exactly how to coat platinum,
platinum-iridium, stainless steel, titanium, or tantalum with a TIROF
4
:
The TIROF method is more di
1.
An acid-alcohol solution containing dissolved iridium complexes is prepared by heat-
ing IrCl
3
3H
2
O [Ir(III)trichloride] in 5.5
M
HCl (4% wt/vol) until 75 to 80% of the solu-
tion is evaporated. Heating the Ir(III) trichloride in the acid results in conversion of the
Ir(III)trichloride to a hexachloroiridate ion, (IrCl
6
)
2−
. The solution is restored to its ini-
tial volume with the addition of alcohol. Either isopropyl alcohol or ethyl alcohol can
be used. This acid-alcohol solution is then aged for a period of time (e.g., 1 to 2 weeks),
during which time the hexachloroiridate ion is slowly converted to a chloroiridate--
alcohol complex of Ir(IV). A way of avoiding boiling iridium chloride in HCl is simply
to buy chloroiridic acid [Hydrogen hexachloroiridate(IV) hydrate] from Alfa-Aesar
(item 11031) and stir it with isopropyl alcohol to achieve the correct acid-alcohol ratio.
The solution must then be left to age for 1 to 2 weeks in a tightly sealed jar.
2.
The surface of the metallic electrode is pretreated prior to deposition of the iridium
solution to enhance adhesion of the iridium oxide
film formed. Such pretreatments
include chemical or electrochemical etching and vary depending on the metallic
electrode being coated. Common ways of preparing surfaces for coating include
etching with HCl and sandblasting using a
fi
fine abrasive powder.
3.
The metallic electrode is soaked in the aged acid-alcohol solution for a prolonged
period of time, typically 16 hours, after which it is dried for 1 hour at room temperature
(22°C) and annealed in air at 320°C for 80 to 90 minutes. Prolonged soaking of the
metallic electrode allows intimate association of chloroiridate-alcohol complexes in
solution with the surface of the metallic electrode so that the metallic surface becomes
completely covered with iridium complexes. Shorter soaking times lead to incomplete
coverage of the metallic surface so that areas of uncoated metal remain exposed.
4.
Successive layers are added by soaking the electrode in the acid-alcohol solution
of iridium again and again for 16 to 24 hours, followed by 3 to 6 hours annealing
at 320°C after each period of soaking. Typically, two to four layers are applied. The
high-temperature annealing converts the chloroiridate-alcohol complexes deposited
on the surface of the electrode to an oxide of iridium [probably a combination of
IrO
2
, IrO
2
fi
H
2
O, and Ir(OH)
4
]. The annealing temperature of 320°C was reported by
Robblee to be optimal for obtaining electrodes of the highest charge-injection
capacity. The long annealing times are required for complete conversion of the
chloroiridate-alcohol complexes to iridium oxide and the elimination of chloride
from the
film.
5.
Chloro complexes of iridium that may remain on the surface due to insu
fi
cient
annealing are susceptible to passive leaching and dissolution from the
fi
film. Moreover,
insu
films that contain a high chloride content have a very low
charge injection capacity, due to the smaller proportion of Ir(IV) oxide species, which
are responsible for the surface faradic charge injection reactions desired. As such,
ciently annealed
fi
4
Patents issued before June 5, 1995 had a 17-year term from the date of issue. Patents filed after that date have a
20-year term measured from the filing date. Patents filed before that date, and pending or issued on that date, have
the longer of the two terms. Once a patent has expired the invention is in the public domain, and you are free to
use it.
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