Safe direct current stimulator 2: concept and design.

Essentially all neuroprostheses use alternating biphasic current pulses to stimulate neural tissue. While this method can effectively excite neurons, it is not very effective for inhibiting them. In contrast, direct current (DC) can excite, inhibit, and modulate sensitivity of neurons. However, DC stimulation is biologically unsafe because it violates safe charge injection criteria. We have previously described the concept of a safe direct current stimulator (SDCS) that overcomes this constraint. The SDCS drives DC ionic current into the tissue by switching fluid valves in phase with biphasic current pulses delivered to the metal electrodes within the device. The original prototype of this device, SDCS1, could both suppress and excite the vestibular nerve with DC stimulation delivered by the device. In the process of building the SDCS1 we identified several problems that must be addressed to further develop this technology. Consequently, we designed the SDCS2, which eliminates periodic interruptions in stimulation current flow observed in the original SDCS1 design and is small enough for head-mounted use in chronic animal studies.