With monopolar electroporation pulses, the electric current often decreases over successive pulses, but not with voltage-equivalent bipolar pulses. In this work, we experimentally separate the possible causes: heating, bubbles from electrolysis, electroosmosis and pH changes.
Experiments are done in a cell-free agarose phantom, so there is no electroporation. We use a printed electrode with two radial poles and trains of 500 pulses at 250 and 500~V. Bipolar pulses are compared with monopolar pulses with an open via in the electrode center, through which gas escapes and a closed one, which retains it. The bubble coverage of the electrode is determined from video recordings and used as input to an axisymmetric COMSOL model with heating and temperature-dependent conductivity.
Conductance decreases only when bubbles remain on the electrode. At 500~V it drops markedly in three of five runs with closed vias and in none with open vias. The conductance with monopolar pulses and open vias increases faster than with bipolar pulses, although the energy is about the same. The difference grows with the net charge, which points to pH changes near the electrodes. The model with measured coverage captures the differences between runs, but not this increase. Two opposing mechanisms thus act with monopolar pulses: bubbles decrease the conductance, while a contribution linked to the net charge increases it. The current therefore depends mainly on whether gas can escape from the electrode.
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