Epoxy-Insulated Microelectrodes are consistently high-quality electrodes available in either Stainless Steel or Tungsten. We know that electrodes are difficult to manufacture, therefore we take extra care to monitor each step of microelectrode fabrication. Microscopic inspections are performed and microelectrode resistance and capacitance are checked to ensure insulation integrity and proper electrode impedance.
Metal electrode impedance is measured with a 2 nAp-p current at 1 kHz. The tolerance range for 5M epoxy-insulated microelectrodes is 3M to 8M, and the tolerance range for 12 M epoxy-insulated microelectrodes is 10M to 18M. We skew the values a little high because it is possible to lower the impedance at a later stage by passing current through the electrode.
Metal electrodes do not behave as resistors due to polarization. They have low series resistances for high frequency signals and high series resistance for low frequency signals. For example, for DC inputs a typical tungsten microelectrode will have a series resistance of about 200Mohms. However, for a 1kHz sine input, the same electrode will have an impedance of 1Mohm. Its impedance will further drop to only 100kohm for a 100µs pulse.