Department
of Biomedical Engineering
Applied
Neural Control: Prof. J.Thomas Mortimer
Wed. May 01 2024
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Department
of Biomedical Engineering |
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Applied
Neural Control: Prof. J.Thomas Mortimer |
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Wed. May 01 2024
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Updated : August 20, 2014 |
Response of a 4 sq. mm. stainless steel electrode in 110
mM phosphate buffered saline equilibrated to 5% oxygen
to monophasic, charge balanced biphasic, and charge imbalanced
biphasic electrical protocols. Pulsing is continuous at
50 Hz. All potentials shown are after steady state is achieved.
In the monophasic protocol, the pre-pulse potential (indicative
of the potential during most of the interpulse interval)
is approximately –600mV, and the maximum negative
potential during the pulse is –1.3 V RHE. In the
balanced charge protocol, the pre-pulse potential is +500
mV, and the maximum potentials during the pulse are approximately –700
mV and +1 V. In the charge imbalanced biphasic protocol,
the pre-pulse potential is –400 mV, the maximum potentials
during the pulse are approx. –1.4 V and +200 mV.
These steady state potentials illustrate how either the
charge balanced or charge imbalanced biphasic protocols
keep the electrode potential out of the most negative ranges
during the interpulse interval, and the imbalanced charge
protocol also minimizes the positive potential excursion
during biphasic pulsing, thus preventing electrode corrosion.
The charge densities used in these examples are those used
by Scheiner, who found tissue damage with the monophasic
protocol shown above, but not with the imbalanced charge
protocol, although they have equal net charge per pulse.
This may occur, as the electrode remains relatively negative
during the entire interpulse interval with monophasic pulsing,
allowing reduction reactions such as oxygen reduction that
may be damaging to tissue. In imbalanced charge pulsing,
the unrecoverable charge appears to go into water reduction
rather than oxygen reduction.
Slow cyclic voltammogram, 4 mm2 316LVM Stainless Steel in
110 mM phosphate buffered saline, equilibrated to 0% oxygen
and 5% oxygen. Potential is swept from –660 mV to
+1.63 V RHE at 20 mV/sec. Passivation occurs in region
(a) as iron is oxidized to iron oxides and hydroxides.
The passive region (c) is observed from approximately +500mV
to +1V. Transpassivation (corrosion) occurs in region (d)
as soluble chromium oxide is formed. Region (e) represents
reduction of products from (d). Reduction of the oxide
formed in (a) occurs in region (b). Note that in the absence
of oxygen, the areas of peaks (a) and (b) are similar,
indicative of reversibility. In the presence of 5% oxygen,
peak (b) is larger than in the absence of oxygen, although
peak (a) is unchanged, indicating possible oxygen reduction
throughout the range from +500mV to –250mV.