Methods for extra-low voltage transcranial direct current stimulation: current and time dependent impedance decreases.

OBJECTIVE Though tDCS is well tolerated, it is desirable to further limit the voltage applied for additional safety factors and optimized device design. We investigated the minimum voltage required for tDCS using 1.5 and 2.5mA. METHODS Impedance data has been collected prior to, during and after 18 tDCS sessions, using 1.5mA and 2.5mA tDCS currents and three different test current magnitudes. Data was pooled and tested for differences using t-tests, corrected for multiple comparisons. Average impedance data was fitted into a RLC circuit model with additional double integrator. RESULTS We report that the impedance drop during tDCS initiation significantly reduces the voltage compliance required to achieve the target current (14.5V for 1.5mA, 18.5V for 2.5mA). Data was well approximated by a 4th order linear impedance model. CONCLUSION In addition to indicating the feasibility of reduced voltage tDCS, we propose an extra-low voltage "Limited Total Energy" approach where stimulation is continued at voltage compliance allowing time for impedance to decrease and target current to be reached. SIGNIFICANCE Reduced-voltage and Limited Total Energy tDCS are viable approaches towards more protective and robust tDCS protocols.