I used to suffer chronic back pain, usually due to sleeping in weird positions. Short of strapping myself into bed, I would always end up contorted in the morning, with shooting pain in my back muscles. I had read about people using transcutaneous electronic neural stimulation (TENS) units for a long time, and decided to build one. I ended up building two: a single channel unit, and then after figuring out how to shrink the circuit, a six-channel unit. I was able to connect six sets of electrodes to my back and stimulate the nerves in various patterns, e.g., top to bottom, bottom to top, random. This significantly helped, although I was never able to figure out a current limiting problem and the main power transistors would overheat after a few hours of use. I even fabbed a smaller board with the intent of building much larger units with a dozen or more channels, but once the back pain went away I moved on to other projects.