Monophasic waveforms are generally considered less safe than biphasic waveforms. That's why many TENS units have an output stage based around a pulse transformer, so they can deliver two pulses, one in each direction, shortly after each other. Leaving this out seems to me to be a false economy when you've gone to all the effort of building the rest of the system.

Coming in a V2, semester just started again, so I have a little less time for it. Given that I also have some other projects on the way :)