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Input impedance necessary to avoid attenuation of the signal
One of unique characteristics of a MEA an “electrical double layer capacitance formed between the interface of the electrodes
and the electrolyte. An electrical double layer capacitance increases impedance in the low frequency area of the electrode (see
the figure below). When the electrode impedance is higher than the input impedance of the amplifier, attenuation occurs in the
signal acquired. The platinum black electrode on the MED probe has very low impedance (at signal frequency 1 kHz, 10 kΩ for
50 µm×50 µm electrode, and 15 kΩ for 20 µm x 20 µm electrode), and impedance is low in the low frequency area. Therefore,
the MED64 System has sufficiently high input impedance of 100 MΩ and a low frequency signal can be acquired without
attenuation, such as the evoked potential of a brain slice or the pacemaker potential of a gastrointestinal tract muscle layer
tissue. When 1 Hz signal is acquired at an ITO electrode without a coating, the signal is attenuated because its electrode
impedance is higher than 100 MΩ (blue line in the figure below). On the other hand, a platinum black electrode can acquire the
signal without attenuation because its impedance is lower than 100 MΩ.
Impedance relationship between electrode and MED64 Head Amplifier.
6.2.2. Electric stimulation by the MED64 system – Applying current to the micro electrode array
The MED64 head amplifier has a 2-channel current drive stimulator that can apply electric stimulation to any of the 64 electrodes.
The figure below illustrates an equivalent circuit of electric stimulation by the MED64 system. The stimulation current flows from
the electrode selected to the reference electrode, and then it is stored in the electrical double layer capacity on the electrode
surface and changes the electrode potential. This change in potential changes the extracellular potential and induces
depolarization of the cell membrane. As shown on the right side of the figure below, the extracellular potential (Vf) changes
according to output current (Is).
Impedance of electrolyte containing specimen
Double layer capacitance
Impedance of electrode
Ref. elec
Rec. elec
Input
Amplify
ITO (□50 µm)
PtBk (□20 µm)
PtBk (□50 µm)
Input impedance of MED64 Head Amplifier (100 MΩ)
↑↑attenuation↑↑
Summary of Contents for MED64-Entry
Page 1: ...MED64 Entry Manual ...
