text: One main physiological issue that current long-term implanted electrodes suffer from are fibrous glial encapsulations after implantation. This encapsulation is due to the poor biocompatibility and biostability (integration at the hard electrode and soft tissue interface) of many neural electrodes being used today. The encapsulation causes a reduced signal intensity because of the increased electrical impedance and decreased charge transfer between the electrode and the tissue. The encapsulation causes decreased efficiency, performance, and durability.
  Electrical impedance is the opposition to current flow with an applied voltage, usually represented as Z in units of ohms (Ω). The impedance of an electrode is especially important as it is directly related to its effectiveness. A high impedance causes poor charge transfer and thus poor electrode performance for either stimulating or recording the neural tissue. Electrode impedance is related to surface area at the interface between the electrode and the tissue. At electrode sites, the total impedance is controlled by the double-layer capacitance. The capacitance value is directly related to the surface area. Increasing the surface area at the electrode-tissue interface will increase the capacitance and thus decrease the impedance. The equation below describes the inverse relationship between the capacitance and impedance.
  where i is the imaginary unit, w is the frequency of the current, C is capacitance, and R is resistance.
  A desirable electrode would have a low impedance meaning a higher surface area. One method to increase this area is coating the electrode surfaces with a variety of materials. Many new materials and techniques are being researched to improve the behavior of neural electrodes. Currently, research is being done to increase the biocompatibility and integration of electrodes in neural tissue; this research is discussed in more detail below.