In vivo Impedance Characterization of a Monopolar Extra-Neural Electrode

摘要

The impedance of 4 titanium nitride (TiN) coated monopolar extra-neural electrodes has been measured in vivo for a period of 3 weeks. The objectives of the study were to quantify both the electrode-electrolyte interface, as well as the tissue resistance in vivo as a function of time after implantation. Different currents (0.05, 0.1, 0.5 and 5.0 mA) have been used at frequencies ranging from 0.1 Hz-KM) kHz for extensive measurements once a week, while the animals were under anesthesia. The tissue resistance, R_(tissue) the faradic resistance, R_f, double layer capacitance, C_(dl), and charge transfer ratio between capacitive and faradic processes, Q_c/Q_f, of the elec trode were estimated. For 3 of 4 electrodes, R_(tissue) could be reliably estimated. The tissue impedance was low in the first week, after which it increased and stabilized. Using an ampli tude of 0.1 mA, charge was transferred predominantly via a capacitive pathway. With increasing current density, the fa radic pathways became more dominant and the frequency at which the faradic pathways became more dominant decreased with increasing current density. R_f was higher at higher cur rents, whereas C_(dl) was lower at higher currents. The results indicate that when the phase angle approaches 0, R_(tissue) pro vides reliable information regarding the healing process. The Q_c/Q_f curves confirm that the charge transfer mechanism of the TiN electrode interface is mainly capacitive. The mechan ism of charge transfer changes towards faradic charge transfer for increasing current density. Both the increasing trend in R_f as well as the decreasing trend in C_(dl) reaches a plateau at the 0.5 mA. This might indicate that equilibrium is reached be tween the surface area used for faradic and capacitive charge transfer. The performance of the electrode is comparable to a Platinum/Iridium (Pt/Ir) electrode.