Long term in vivo stability and frequency response of polyimide based flexible array probes

The frequency response of chronically implanted neuronal probes determines their usability in neural interface applications. Frequency range and noise content of the measured signal depend on the impedance of the electrode-electrolyte interface. After implantation into cerebral tissue, the impedance at the electrode-tissue interface changes. We used flexible, polyimide based multisite microprobes and measured the impedance at the probe contact sites in ionic electrolyte and brain tissue over a long time range. Adapted equivalent circuit models were used to characterize the components of the electrode-electrolyte interface and the electrode-tissue interface after implantation. The RC elements in the equivalent circuit defining the transition between electrolyte and brain tissue in the acute and chronic stage were identified. After implantation, an additional RC element was introduced. Even after implantation, characteristic 1 kHz impedances were lower than 500 k Omega.