Determination of the electric field and anomalous heating caused by exponential pulses with aluminum electrodes in electroporation experiments

Electroporation is well known to depend non-linearly on the magnitude and duration of the change Delta U(t) in transmembrane voltage. In the case of cell suspension experiments, an electric field E(e)(t) within the electrolyte causes Delta U(t), which is is governed by both the size and shape of a cell, and also by E(e)(t). It is therefore important to determine the magnitude and time dependence of the electric field to which cells are actually exposed in electroporation experiments. This can be significantly different from the nominal field E(n), which is calculated by using electrode voltages and geometries alone. Throughout we used single, nominally exponential pulses with time constants tau(pulse) ranging from about 0.6 to 5 ms and found that E(e) was always less than E(n). In order to determine the actual electric field pulse, we measured the voltage across the electrodes, the current through the cuvette, the temperature rise of the pulsing medium, and the voltage across two special electrodes placed within the cuvette. From these measurements we calculated the field strength inside the cuvette using two different methods. In addition, we compared the measured temperature rise with that expected from the electrical power dissipation. In some cases there was much larger (''anomalous'') heating, due to interfacial electrochemical heat production; for one pulsing solution T-e(t) was about 30 K larger than expected. These effects are important for experiments aimed at elucidating the electroporation mechanism, comparing results obtained under different conditions, and guiding applications.