Experimentally determining the iR drop in solution at carbon fiber microelectrodes with current interruption and application to single-cell electroporation.

Single-cell electroporation uses microelectrodes, capillaries, or micropipets positioned near single, adherent cells to increase transiently the membrane permeability of the cell. The increased permeability permits, for example, transfection without chemical reagents. When using microelectrodes to apply an electric field to the cell, there is a question of how much voltage to apply. Unlike in bulk electroporation, where hundreds of volts may be applied between electrodes, a rather small voltage is applied to a microelectrode in single-cell electroporation. In the single-cell experiment with microelectrodes, a substantial fraction of the voltage is lost at the interface and does not therefore exist in solution. This problem is the same as the classical electrochemist's problem of knowing the "iR" drop in solution and correcting for it to obtain true interfacial potential differences. Therefore, we have used current interruption to determine the iR drop in solution near microcylinder electrodes. As the field is inhomogeneous, calculations are required to understand the field distribution. Results of the current interruption are validated by comparing two independent measurements of the resistance in solution: one value results from the measured iR drop in conjunction with the known applied current. The other value results from a measured solution conductivity and a computed cell constant. We find substantial agreement in the range of resistances from about 2 to 50 kOmega, but not at higher resistances. We propose a simple, four-step plan that takes a few minutes to calculate the approximate current required to electroporate a cell with an electrode of a particular size, shape, and distance from the cell. We validate the approach with electroporation of single A549 cells.