text: Electrodes, especially stimulating electrodes and the high current density they discharge, can raise electrochemical issues. Electrodes will be surrounded by tissue and electrolytes; stimulation, resultant electric fields, and induced polarizations will change local ion concentrations and local pH which can then cause problems such as material corrosion and electrode fouling.
  Pourbiax diagrams will show the phases that a material will take in an aqueous environment, based on electrical potential and pH. The brain maintains a pH of around 7.2 to 7.4, and from the Pourbaix diagram of platinum it can be seen that at around 0.8 volts Pt at the surface will oxidize to PtO, and at around 1.6 volts, PtO will oxidize to PtO. These voltages do not seem to be outside of reasonable range for neural stimulation. The voltage required for stimulation may change significantly over the life of a single electrode. This change is required to maintain a consistent current output through variations in the surrounding environmental resistance. The changes in resistance may be due to: adsorption of material onto the electrode, corrosion of the electrode, encapsulation of the electrode in fibrous tissue – known as a glial scar, or changes in the chemical environment around the electrode. Ohms law V = I * R' shows the interdependence of voltage, current and resistance. When voltage change causes a crossing of equilibrium lines as seen in a Pourbaix diagram during a stimulation, the changing polarization of the electrode is no longer linear. Undesirable polarization can lead to adverse effects such as corrosion, fouling, and toxicity. Because of this equilibrium potential, pH, and required current density should be considered when making material choice since these can affect the surface chemistry and biocompatibility of the implant.