Biomedical Engineering Reference
In-Depth Information
oxidation reactions occurring within the oxide
film. The iridium oxide layer becomes the
charge transfer interface and enables charge injection densities up to 10 mC/cm
2
for either
cathodic or anodic polarities without water electrolysis or other faradic reactions involved
in corrosion of the underlying metallic electrode.
Thin
fi
films of hydrated iridium oxide have been used as low-impedance coatings for
neural stimulation and recording electrodes. The iridium oxide provides a way of inject-
ing charge into tissue while minimizing electrochemically irreversible processes at the
electrode-tissue interface, where reduction and oxidation reactions occur to mediate
between electron
fi
flow in the tissue. Electrodes coated
with iridium oxide are very good for long-term stimulation of nerves in the spinal cord
[Woodford et al., 1996], in the ear's cochlea [Anderson et al., 1989], and in the brain cor-
tex [Bak et al., 1990; Hambrecht, 1995].
The idea behind using IROX as an electrode material is that iridium can store charge by
going through valence changes that cause reversible redox reactions. The fact that these
reactions are reversible is important for biocompatibility. Reversibility means that no new
substance is formed and hence no reactants are released into tissue. The state of the irid-
ium will depend on the potential applied across the metal-electrolyte junction.
Since tissue-contact stimulation is usually accomplished with a constant-current source,
the voltage applied is dependent on the charge storage of the oxide. As the oxide absorbs
more charge (positive current), the potential across the interface will increase, which will
result in the oxide reacting with the electrolyte. Initially, with no applied voltage, the irid-
ium oxide is in the Ir(OH)
3
state. As potential is increased, the iridium oxide increases
valence by ejecting protons into the solution. The following reaction summarizes the
sequential change in the oxide as potential increases:
fl
flow in the external circuit and ion
fl
IrO
x
1
(OH)
4
x
IrO
x
(OH)
3
x
(
x
1)(H
e
)
where
x
1, 2, 3 and increases with potential. The IrO
3
state (
x
3) is unstable and its
degradation will result in oxygen evolution. This reaction de
nes the water window on the
positive voltage side. Subsequently, as charge is removed from the oxide (negative current),
the reactions reverse. Do you see where the name IrOx comes from?
Iridium oxide coatings are usually formed on electrodes by three di
fi
ff
erent processes:
1.
Activated iridium oxide
lm
(AIROF): formed from iridium metal in an aqueous
electrolyte by an activation process in which the electrochemical potential of the
metal is cycled or pulsed between negative and positive potential limits close to
those for electrolysis of water
2.
Sputtered iridium oxide
fi
lm
(SIROF): formed by reactive sputtering of iridium
medium in an oxidizing atmosphere
3.
Thermal iridium oxide
fi
lm
(TIROF): formed by the decomposition of iridium salts
to form an iridium oxide
fi
fi
film on top of a metallic substrate electrode
The AIROF method is the simplest to use to home-brew IROX-coated electrodes by
starting with electrode substrates made of pure iridium metal. Iridium grows a hydrous oxide
layer on its surface when it is activated electrochemically in an electrolyte (0.3
M
sodium
phosphate dibasic, Na
2
HPO
4
). A standard three-electrode scheme is used to perform cyclic
voltammetry using the setup shown in Figure 7.16. Cyclic voltammetry is an analytical
technique that involves application of a time-varying potential to an electrochemical cell
and simultaneous measurement of the resulting current.
3
This measurement can be used to
provide oxidation-reduction information about the system being studied. However, in this
application, the technique is used as a manufacturing process.
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