Focalizing electrical neural stimulation with penetrating microelectrode arrays: a modeling study.

Penetrating neural probes are considered for neuroprosthetic devices to restore sensory or motor functions of the CNS using electrical neural microstimulation. These multielectrode systems require optimal electrode configurations to allow precise and focused tissue activation. Combining a finite element model of the spinal cord and compartmentalized models of both simple and complex neuron morphologies, we evaluated the use of the "ground surface" configuration, which consists in the integration of a conductive layer on the front side of electrode shanks, for the return of the stimulation current. Compared to the classical monopolar and bipolar configurations, this strategy resulted in a focalization of both the potential field and the threshold-distance curves. The improvement in focalization was highest for lowest impedance of the ground surface. Moreover, the gain in focality was highest on the side of the shank opposite to the electrode, so that only the neurons located in front of stimulation electrode were activated. This focalizing strategy will allow the design of new microstimulation paradigms aiming at precisely targeting the CNS with complex spatio-temporal stimulation patterns, which could benefit to future stimulation-based neuroprosthesis.