--an internally planted device, but merely an external unit, with pads connected to a battery operated central device that sends varied pulses.

There are internal devices that are used to help mask the sensations of intractable pain, but this is invasive and usually done only after all other avenues have been exhausted.

You might want to look at the Chronic Pain/Spinal
forums here--particularly the area on implanted medication pumps and implanted devices:

Spinal Cord Stimulator/Pain Pump Information