Methods for extra-low voltage transcranial direct current stimulation: Current and time dependent impedance decreases
Highlights
► Transcranial direct current stimulation is accompanied by a characteristic drop in skin impedance which significantly reduces compliance voltage. ► Skin impedance changes have been investigated experimentally and approximated by a 4th order linear model. ► Reduced-voltage and Limited Total Energy tDCS are viable approaches towards more protective and robust brain stimulation protocols.
Introduction
Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique that is being evaluated for the treatment of depression, epilepsy, pain, facilitating stroke rehabilitation and further neurological conditions (Hummel et al., 2005, Fregni et al., 2006, Nitsche et al., 2008, Nitsche et al., 2009, Nitsche and Paulus, 2009). During tDCS, a weak constant current is passed across the brain using electrodes placed on the scalp; prolonged passage of current (e.g. >10 min) can lead to lasting changes in neuronal excitability (Nitsche and Paulus, 2000, Nitsche and Paulus, 2001). Most commonly, conductive rubber pads wrapped in saline-soaked sponge pockets are used as tDCS electrodes.
Stimulation protocols for tDCS consist of a fade-in phase in which current is ramped up to the desired intensity (typically <30 s), the main stimulation phase at target intensity (typically 1–2.5 mA, for 10–20 min), and a fade-out phase. The voltage needed to ramp up current and maintain stimulation depends on the impedance across the body and the electrodes. Skin (scalp), skull, CSF, and brain tissue contribute to body impedance, with skin impedance known to change depending on current intensity and density and stimulation duration (Kalia and Guy, 1995, Prausnitz, 1996). Electrode impedance is a function of dynamic electrochemical processes and is also a complex function of stimulation waveform and time (Prausnitz, 1996, Merrill et al., 2005, Minhas et al., 2010). It is precisely because tissue and electrode impedance across subjects and time is highly variable, that current controlled stimulation is used to ensure reproducible delivery of stimulation dose to the brain.
During tDCS, the voltage is therefore adjusted to maintain the desired current level across variable impedances. Poor electrode design and preparation can thus lead to higher voltages being applied. Although current/charge density and total delivered charge are considered the main parameters causing tissue damage and painful sensation (McCreery et al., 1990, Nitsche et al., 2003, Liebetanz et al., 2009), unnecessarily high voltages are also undesirable for several reasons. For example, as electrode voltage increases, additional electrochemical reactions are triggered (Merrill et al., 2005) leading to potentially undesired chemical products and pH changes (Minhas et al., 2010). In addition, with improperly designed stimulation electronics (e.g. using off-label iontophoresis devices), a sudden drop in impedance can lead to a surge in current. Joule heating leading to temperature increases is a function of both current and voltage, though is likely not significant during conventional tDCS or High-Definition tDCS (Datta et al., 2009).
As it may play a role in painful skin sensation and irritation, it seems worthwhile to limit the electrical potential applied to subject’s scalps. Current tDCS devices are limited to output voltages of 20–43 V, independent of current applied (Iontophoresis devices can reach >90 V). Anecdotal evidence suggests that in some subjects the maximum voltage of the stimulator is reached (either software or hardware limited “compliance voltage”) leading to stimulation being aborted. The goal of this short study was to determine lowest possible voltage limits for tDCS devices for 1.5 mA and 2.5 mA target currents using a common electrode design and montage. Experimentally and with a 4th order linear circuit model, we demonstrate that the decrease in impedance produced by the passage of current itself significantly reduces the compliance voltage required. Furthermore, recognizing that reaching and maintaining the target current intensity does not require a strictly controlled (linear) ramp, we develop a “Limited Total Energy” (LTE) approach to tDCS which allows robust extra-low voltage stimulation.
Section snippets
Methods
This study was approved by the IRB of the City College of New York. All seven subjects gave written informed consent.
tDCS was administered for 6 min (30 s ramp-up, 5 min stimulation, 30 s ramp-down) to seven healthy adult subjects, using a custom-designed battery-run circuit (Soterix Medical Inc., NY) which allows free adjustment of maximum output voltage and current magnitude. Current was ramped up and down linearly over 30 s. Two rubber electrodes wrapped in saline soaked sponges, one over the
Results
Impedance across electrodes was monitored before, during, and after tDCS. Pre- and post-tDCS impedance values differed significantly (Fig. 1c). Prior to stimulation the average impedance was 39.4 kΩ ± 8.9 (I = 50 μA), 32.1 kΩ ± 5.1 (I = 100 μA), and 23.7 kΩ ± 4.6 (I = 150–200 μA). As tDCS was initiated, impedance decreased significantly, approaching a minimum value after ∼30 s (corresponding to the ramp up duration where target current is achieved). Average impedance continued to decrease incrementally over
Discussion
Our results indicate that using conventional electrode montages (e.g. C3–SO, C3–C4), and appropriate electrode types and preparation (e.g. impedance checks), the voltage limits of clinical tDCS devices can be decreased to values <20 V. Specifically, in a current target specific manner, for 1.5 mA stimulation, output voltage of the device can be limited to 14.5 V, whereas 18.5 V is sufficient for successful stimulation at 2.5 mA target current intensity. Using smaller electrodes (Nitsche et al., 2007
Conflict of Interest
The City University of New York has patent applications on brain stimulation with Christoph Hahn, Justin Rice, Preet Minhas and Marom Bikson as inventors. Marom Bikson is co-founder of Soterix Medical Inc.
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2020, Medical Engineering and PhysicsCitation Excerpt :When using HD-tDCS electrodes, electrode resistance was reduced to 10 kΩ by adding more electrolyte [77]. Very high impedance of the electrode/body circuit will require sufficiently high voltages to provide the required current but this is undesirable because high voltages can cause other electrochemical complications such as increase in pH, temperature and unwanted chemical by-products [75]. Recent experimental studies showed that pre-stimulation impedance, below 50 kΩ (when tested with a current of 50 µA) could ensure delivery of 2.5 mA of constant current with a driving voltage of 20 V [75].
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2019, NeuroImageCitation Excerpt :The model-derived skin DC voltage predicted the distribution of stimulation voltages across the scalp and head, where the largest voltages were directly under the stimulation electrodes (Fig. 1E and G). Across protocols, the DC offset changed incrementally (“drift”) while stimulation was sustained (Hahn et al., 2013), this fractional change (up to ∼ 3 mV or 2% of the DC offset over 50 s with 2 mA of current) was still larger than neurogenic EEG signals. Across protocols, there was a residual DC offset present post stimulation, and was evident for up to approximately 1 min after the end of the ramp-down (Fig. 2C).
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These authors contributed equally to this work.