Biomedical Engineering Reference
In-Depth Information
Effects from intensity and frequency of stimulation pulses were also examined.
The pH changes with stimulation time were monitored at different pulse currents
and frequencies. To avoid gas evolution reactions, very low charge densities
(30 to 120C/cm
2
) were used for monophasic stimulations. It was noticed that
pH changes increased with the increased pulse current and pulse frequencies.
Figure 12.4 shows both cathodic-induced pH increase and anodic-induced pH
decrease under different pulse current amplitudes. It is clear that the higher the
pulse current, the more the pH changes are detected at any given time. This
indicates that an electrode with a lower threshold in clinical applications, which
requires less pulse current, will minimize possible damage due to the pH shift.
The frequency effects on the pH changes during pulse stimulation were measured
at 60C/cm
2
and the results showed similar effects (data not shown). Higher
frequency pulse (60-120 Hz) induced more pH shift for the electrodes pulsed
with lower frequency (10-30 Hz) at the same current amplitudes. For neural
stimulation, pulsing should use low frequencies to improve the pH balance at
the electrode-electrolyte interface.
The pH changes in the test solution were confirmed using a pH indicator,
phenol red. Time for color change of phenol red indicator was recorded. For
cathodic-only pulses, phenol red color change was noted around the stimulating
electrode to change from yellow to pink in about 10 minutes, which indicates an
increased pH, shown in Figure 12.5a. For anodic-only stimulation, phenol red
color change could not be easily detected due to small contrast of yellow to light
yellow color change; however, color change could be seen around the Pt counter
Stimulating Current (uA)
10
-32.3 uA
8
-16.1 uA
6
4
-8.1 uA
2
0
+8.1 uA
-2
-4
+16.1 uA
-6
-32.3 uA
-8
Figure 12.4. Induced pH shifts with various stimulation current intensities. Cathodic
pulse current is shown to cause an increased pH while anodic pulse current caused a
decreased pH, correlating with amplitude of current.
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