I would strongly recommend NOT doing that unless the voltages and currents involved were so low they were not just not dangerous, also I would only use a battery powered device with sophisticated enough current limiting that you would know not just the voltage applied but also the current and voltage drop actually delivered into the body, its polarity, and for how long it was on and off, and exactly where both electrodes were. And also it likely should not be continuous, it should be pulsed with a short duty cycle on. And additionally, only applied for a short time each time its used.
Because electricity delivered in that way promotes nerves to either grow or not grow and placement and polarity and current
is still at the state of being worked out.
Its an area of study that is still in its infancy.
So it quite conceivably could have adverse effects if not done properly.
Most certainly any voltage anyone would call 'high' is likely to be WAY too high. That said it is an interesting area and even with the requirements listed a device to apply DC in this way, may be so straightforward to build for anybody with a bit of skill in basic electronics that one would likely jump to it.
Right? Not necessarily unless one has some injury and is not getting care for it and is literally desperate. Somebody shouldn't do it without quite a bit of knowledge as to what they are doing. (More than I have which is why I haven't)
There needs to be awareness that as electrical potential makes nerves more (or less) likely to grow, there are many things we don't know. Everything needs to be understood as part of a learning process and not as a 'discovery' that is an endpoint.
For example, one thing I worry about is impacting the body's repair capacity as represented by the Hayflick Limit. (look that up)
That's really interesting. It reminds me of reading years ago of 'spine induced audio'. I never tried it, but the claim was that if you take audio, modulate it onto a HV train of short spikes, and AC-couple that to skin electrode pads placed over the spine, the nervous system somehow routes the signals to the right area of the brain and you can 'hear' the audio.
Also, Rule 34 applications in 5, 4, 3, 3...