Biomedical Engineering Reference
In-Depth Information
stimulation threshold remains largely constant. Second, a high double-layer capacitance
should develop at the phase boundary between the electrode and the body
fluids so that the
polarization rise during the stimulation pulses remains under the limit of irreversible elec-
trochemical reactions.
The classical materials for invasive stimulating electrodes have been stainless steel and
platinum. These materials have a limited range for reversible charge injection by surface
faradic processes in the in vivo saline environment before the onset of water electrolysis
(approximately 0.5 mC/cm
2
for platinum and platinum alloys) or catastrophic corrosion
(approximately 0.04 mC/cm
2
for stainless steel). These charge injection limits restrict their
usefulness as stimulation electrodes to applications requiring only low charge-injection
densities. However, even low charge-injection densities are known to produce corrosion of
the metal, thereby releasing trace quantities of dissolved metal into the surrounding envi-
ronment. In the case of platinum, dissolution products may be toxic to the tissue in which
the electrodes are implanted. In the case of stainless steel electrodes, dissolution or corro-
sion of the electrode may result in electrode failure due to corrosion-induced fracture. In
fact, dissolution often results in disappearance of the entire electrode.
Another metal in use today for stimulating electrodes is iridium. Pure iridium is extremely
fl
sti
and has a much lower impedance than that of any other noble metal. It is extremely inert
and very resistant to corrosion. The de facto standard, however, is an alloy of platinum-
iridium that has a lower concomitant impedance value than that of either tungsten or stainless
steel for the same exposure, making electrodes less likely to erode during intense stimula-
tion protocols. This alloy is excellent for chronic implants because it is very biocompatible.
Table 7.2 shows the material properties of some of the most common metals used for the fab-
rication of electrodes. You can buy high-purity materials to make chronically implantable
electrodes from a number of suppliers, including Alfa Aesar (high-purity raw materials),
Noble-Met (drawn wire as thin as 0.001 in. in diameter), and Xylem Company (wires and
metallic parts).
In the 1970s, the promise of electrical stimulation as a cure for pain, paralysis, deafness,
and blindness seemed close at hand. Development in these areas prompted the search for
new electrode materials to increase safe charge-injection densities. Signi
ff
cant develop-
ments were made in the 1980s with the introduction of metallic oxide layers. Electrodes
with a large microscopic surface area were fabricated by anodizing titanium and tantalum
electrodes. The anodized electrodes operate by charging and discharging the double-layer
capacitance at the electrode-electrolyte interface. They provide an intrinsically safe means
of charge injection because they form a dc-blocking capacitor right at the electrode-tissue
interface that ensures charge balancing. Unfortunately, these anodized electrodes have lim-
ited charge densities (lower than those of platinum) and can be used for injecting anodic
charge only, unless appropriately biased, whereas the physiological preference is for
cathodic charge.
Later, coatings such as titanium nitride (TiN) [Konrad et al., 1984] and iridium oxide
(IROX) [Robblee et al., 1986] were developed to overcome the shortcomings of anodized
electrodes. IROX works by delivering charge to tissues through reversible reduction-
fi
TABLE 7.2
Desirable Material Properties for Some Metals Commonly Used in Stimulating Electrodes
MP35N
Stainless
Biocompatible
Platinum
Iridium
Gold
Silver
Tantalum
Titanium
Steel
Superalloy
Corrosion resistance
✓
✓
✓
✓
✓
✓
✓
Biocompatibility
✓
✓
✓
✓
✓
✓
✓
Electrical conductivity
✓
✓
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