An Impedance Characterisation Circuit for Implantable Research Platforms

Neuromodulation devices use stimulation to inject current through electrodes to modulate the activity of tissue for therapeutic benefit. Complex electrochemical processes happening at the electrode-tissue interface pose a challenge in ensuring long-term safety to avoid corrosion and tissue damage. Convention devices such as deep brain stimulators (DBS) providing constant, tonic stimulation patterns can be designed to operate within a safe window by controlling parameters. Recent de-vices providing closed-loop stimulation, for example, dynamically adjusting stimulation based on neural activity. Furthermore, research platforms configure devices for new neural targets, and new therapies and can use different electrodes. To ensure safe operation and to mitigate potentially hazardous states, it is now essential to monitor electrode properties. Electrochemical impedance spectroscopy (EIS) is a valuable technique for characterizing electrode properties, such as charge transfer and surface integrity. This paper describes a circuit for achieving in situ characterisation of electrode impedance using EIS. The circuit generates and injects a microamp-level sinusoidal test current, measures the voltage waveform, and sweeps frequency to obtain an impedance spectrum. This has been implemented in an integrated circuit technology for future integration within an implantable research platform. The design has been verified through circuit simulation, using a realistic model for a DBS electrode that has been experimentally characterised.