Electrode assembly design for transcranial Direct Current Stimulation: A FEM modeling study

Despite accelerating progress in transcranial Direct Current Stimulation clinical and cognitive research, there remains remarkably little consistency in the control of electrode design and preparation. Electrode assembly design determines skin sensation and failure at the electrode can lead to skin burns. Though tDCS is generally well tolerated, the desire for rigor in electrode design is motivated by applications in increasingly diverse environments and populations. Generally the tDCS electrode assembly consists of a flat rubber or metal electrode and a saline/water saturated sponge. Here we show using FEM simulations, that each of these factors should be controlled to regulate current flow density across the skin: 1) sponge thickness 2) solution salinity 3) electrode size, 4) electrode placement in the sponge (including surface or pocket configuration) 5) control of excess fluid at the skin surface 6) use of rivets. Two general patterns of current distribution emerge as a result of integrated design: edge concentration or center concentration. Poor control over any of these electrode assembly parameters will result in unpredictable current density at the skin during tDCS.