Multichannel Systems MEA2100-System Manual - Manual (Page 65)

Appendix
61
7.6
Stimulation
Please read also paragraph “Setting up the Internal Stimulus Generator” in chapter “Software”.
It is possible to stimulate in voltage and current mode. Each electrode of the MEA layout can be
used as stimulation electrode.
In current stimulation mode, if you select two or more stimulation electrodes for the same
stimulation pulse, please be aware that in some configurations (when more than one electrode
is used within a block of 30 electrodes which are powered by the same current source) the current
level of the stimulation pulse will be divided among these electrodes. That means the current level
is lower than expected.
The “blocks” of electrodes which are powered by the same power source are summarized by
the hardware channels of the electrodes: HW ID 1 to 30 builds the first block belonging to
the first current source, HW ID 31 to 60 is the second block with the second current source in
MEA2100-HS60-Systems. In MEA2100-HS120-Systems additionally to the first and second block,
the third block HW ID 61 to 90 is powered by the third current source and the fourth block
HW ID 91 to 120 by the fourth.
Please see the datasheet of the respective MEA Layout to check the position of the electrodes.
7.7
Safe Charge Injection Limits
Titanium nitrite (TiN) electrodes are generally more robust than electrodes from other
materials, for example platinum (Pt). In the Appendix, you find safe charge injection limit
curves that document maximum current and stimulus durations for standard TiN electrodes.
Please note that these curves document the limits. Stimulus pulses should be kept safely below
these limits. The safe charge injection limit of platinum (0.4 mC/cm
2 ) is much smaller than for
TiN (23 mC/cm 2 ). This fact results in a considerably lower charge that you can inject into the
electrode before faradic reactions occur that will lead to electrolysis of the electrode.
Please note that, when using voltage driven stimulation, the current flow to the electrode
depends on the electrode impedance. The lower the impedance, the higher is the current.
Please make sure to obey the safe charge injection limits always. Generally, TiN electrodes have
lower impedances than Pt electrodes, and larger electrodes also have lower impedances than
smaller.
When using TiN electrodes, it is extremely important to not charge the electrodes positively,
as this will lead to electrolysis. This is not an issue for Pt electrodes. Therefore, when using voltage
driven stimulation, it is important to apply negative voltages only. Positive voltages will shortly
charge the electrodes positively, even though the electrode is discharged at the end of the pulse.
As a consequence, biphasic voltage driven stimulation is not recommended. When using current
stimulation, it is required to use biphasic stimulation, and to apply the negative phase first to
avoid a positive net charge on the electrode.