Principles of Within Electrode Current Steering

tDCS is a neuromodulation technique that involves noninvasive delivery of weak direct current (1–2 mA) to the brain. Conventionally, tDCS employs rectangular saline-soaked sponge pads (25–35 cm) placed on the scalp, with an internal electrode connected to the current source. Impedance measurement across the current source output may fail to recognize nonuniform conditions at the skin interface such an uneven content or saturation. tDCS is well tolerated with minor adverse effects limited to transient skin irritation [1]. Nonetheless, technology that enhances the sophistication of electrode design would further enhance tolerability and promote broad (e.g., home) use. In order to enhance the reliability and tolerability of tDCS, we describe a novel method called WECS. This concept is distinct from (across electrode) current steering, as developed for implanted devices such as deep brain stimulation (DBS), where current is steered between electrodes that are each in contact with tissue, with the goal of changing desired brain regions that are activated [2]. WECS adjusts current between electrodes not in contact with tissue but rather embedded in an electrolyte on the body surface. The goal here is not to alter brain current flow, but rather compensate for nonideal conditions at the surface. This technology leverages our technique for independently isolating electrode impedance and overpotential during multichannel stimulation [3]. With a novel approach, the objective of this first paper was to demonstrate the principles of WECS using an exemplary electrode design typical for tDCS (four rivet-electrode sponge) and extremes of current steering (from uniform in all rivets to a single rivet). Through FEM simulation of this illustrative case, we validate the underlying assumptions of WECS: steering current within electrodes but without altering current distribution in brain target. Having presented this novel idea through an exemplary case, this report supports future studies in optimization of electrode design, automation of algorithms to control current (including using impedance measurement), and ultimately validation under experimental conditions. 2 Methods