10
Stimulus Current to be Applied to Electrodes
As described in the previous section, the applied current charges the electrical double-layer capacitor and causes
the voltage (Vc) to change (Fig.7). When the voltage (Vc) rises beyond 1V, the electrode will undergo electrolysis
and release H2 gas. This causes the stimulation efficiency to drop dramatically, introduces extremely large
stimulus artifacts, harms cells, and damages the electrodes. Thus, the stimulus current (Is) should not go
beyond this level.
The (Vc) is defined by
Vc = Is x t / Ce
Vc: Voltage of Ce
Is: Stimulator output current
T: Pulse duration
Ce: Electrode capacitance
(electrical double layer capacitor’s capacitance)
The capacitance (Ce) of the MED64 microelectrode is as
high as 50,000 pF for 50 x 50
m electrodes (22,000 pF
for 20 x 20
m electrodes), allowing stimulation with
current amplitudes up to 200
A for 0.2 msec (duration
for each phase is 0.1 msec).
DO NOT use stimulus current/duration parameters
outside the shaded area in the Fig.7.
CAUTION:
•
DO NOT use combinations of stimulus current & duration outside the shaded area in the Fig.8. Higher values
might damage the electrodes.
•
The numerical value of this graph is based on “Single pulse stimulation”. If multi-paired stimulation with
pulse interval shorter than 1 second is used, please decrease the stimulus current and duration values to
80% of the graph.
Vc with 100
A
stimulation
Vc with 500
A
stimulation
Fig.7.
Changes for Voltage of Ce (Vc) with different
0.2V
Electrolysis
1V
0.1 ms
stimulus current amplitude.
(electrolysis)
(T)
2 mA
200
A
20
A
2
A
S
tim
u
lus Current
0.01 ms 0.1 ms 1 ms
10 ms
Duration for a phase
Fig.8.
Suggested stimulation parameters.
