In vivo Impedance Characterization of a Monopolar Extra-Neural Electrode

Abstract

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-100 kHz for extensive measurements once a week, while the animals were under anesthesia. The tissue resistance, Rtissue, the faradic resistance, Rf, double layer capacitance, Cdl, and charge transfer ratio between capacitive and faradic processes, Q _c /Q _f , of the electrode were estimated. For 3 of 4 electrodes, Rtissue could be reliably estimated. The tissue impedance was low in the first week, after which it increased and stabilized. Using an amplitude of 0.1 mA, charge was transferred predominantly via a capacitive pathway. With increasing current density, the faradic 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 currents, whereas C _dl was lower at higher currents. The results indicate that when the phase angle approaches 0, R _tissue provides 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 mechanism 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 between the surface area used for faradic and capacitive charge transfer. The performance of the electrode is comparable to a Platinum/Iridium (Pt/Ir) electrode.