POLARIZATION PHENOMENA RELATING TO PHYSIOLOGICAL ELECTRODES

Electrochemical polarization of physiological electrodes is an undesirable but seemingly unavoidable phenomenon that detracts from the performance of implanted electronic prosthetic devices. In the case of noble metals, polarization causes a significant waste of stimulation energy at the electrode surface. With non-noble metals, the energy waste is even greater and may involve electrolytic corrosion reactions. Such corrosion may destroy the electrode and may possibly leave toxic residues in body tissues. The electrode-electrolyte interface presents to a cardiac pacemaker a highly capacitive load having multiple time constants of the same order of magnitude as the 1or 2-millisecond (msec) duration of a pacemaking impulse. Thus, an applied square wave of current on the electrodes does not obey Ohm’s law and does not elicit a square wave of voltage, nor is the voltage waveform a constant slope (ramp), as would be expected from a single lumped capacitor. Rather, the voltage rises in less than a microsecond to an initial value and then more slowly, in at least two different time constants, until the end of the pulse. Polarization of physiological electrodes was noted many years ago by Cole ( 1934) and later by Schwan and co-workers (1954). Previous publications have reported electrode data to confirm the surface aspect of this phenomenon (Schneider, 1964; Greatbatch, 1966; Mansfield, 1967). It has also been shown that the phenomenon is strongly dependent on the specific electrode material as well as on current density (Weinman & Mahler, 1964; Briller ef al., 1966; Greatbatch, 1966, 1967c), leading to the inescapable conclusion that the phenomenon must be electrochemical polarization of the electrode surface. The fundamental electrochemistry of electrode processes is very complex and beyond the scope of this presentation, but some experimental observations and considerations can be cited relating to important properties of physiological electrodes. When a voltage is applied to a pair of electrodes in a saline bath, charged ions in the fluid drift along the resulting potential gradient, chloride ions toward the anode and sodium ions toward the cathode. If the electrodes are nonpolarizable types such as silver coated with silver chloride, the accumulating ions will discharge at the electrodes, and current will flow into the external circuit, even at small applied voltages. The electrode-electrolyte system will then present a relatively linear resistive load to the voltage source and will approximately obey Ohm’s law if the current density is not too high. If, however, the electrodes are polarizable, and this includes all purely metallic conductors, ions and monatoms will accumulate within a micron of the surface of the metal, but very little steady-state current will flow across the electrode-electrolyte interface until a