An Electronically Configurable Multi-Channel tACS Device

Introduction: Transcranial alternating current stimulation (tACS) is an emerging tool for modulating brain oscillations. Recent research theoretically proved the possibility of focal stimulation using multi-channel tACS setups. However, the step from simulation to reality is still missing. Existing multi-channel systems feature a low number of channels. Electrode arrays are configured manually on a per-patient basis. A clinical application of these systems is practically challenging and time-consuming and thus not cost-effective. Aims: We focus on the technical realization of an affordable multi-channel tACS device for clinical use. Besides the integration of multiple current sources for highly focused stimulation, we develop an analog switching matrix to connect each stimulation channel dynamically to a freely selectable stimulation and reference electrode. In this way, stimulation channels can be positioned on a multi-electrode grid as needed and channel count can be preserved for focusing purposes. Methods: We designed and built a unified solution comprising 16 independent tACS channels. The design of the current sources is based on earlier results [1]. Each channel features full output supervision including current and voltage measurements, extensive safety circuitry and electrode impedance determination. Up to 128 electrodes can be connected dynamically to the 16 stimulation channels via an analog 16x128 high voltage switching matrix. Results: The architecture offers the possibility for the use of standard electrode caps with fixed wiring and a selection of stimulation paradigms exclusively in software. Automatic reconfiguration becomes possible for dynamic focus adjustments. The switching matrix was evaluated for its total harmonic distortion and charge injection through switching. Both were found to be less than 0.1 percent of the input signal and therefore uncritical. Conclusion: We have shown the feasibility of an affordable multi-channel tACS device. One drawback of the system is the relatively low stimulation voltage of +/- 12V. Chosen for safety and cost-saving reasons, this might represent a limitation for high impedance regions and may have to be increased in future revisions. [1] Wilde C, Bruder R, Binder S, Marshall L, Schweikard A: Towards Closed-Loop Transcranial Alternating Current Stimulation, 87. Kongress der Deutschen Gesellschaft für Neurologie, Munich, 2014.