Hi Nula
Tens machines are often used by women in labour (in fact I used one myself 12 years ago ) so I would think them gentle and safe. They act to 'irritate' the nerve endings and so to deaden them a little. Such as when you scratch the same part of the back of your hand, eventually the feeling goes. Did it work for labour? I can't tell because I don't know how it would be without it. - and I'm not about to go through it again to experiment
The advantage of using it for kidney problems is you can try with and without to see if it makes a difference.
They are commonly available from chemist shops (even from mail order parts & manufacturers supplies) and a local supplier to me lets you test one in their offices before buying.
I am a great believer in alternative therapies, accupuncture is another thought to give great relief for pain, and I have heard marvelous stories. I tried but hate needles and felt uncomfortable; husband found it helped his muscular problems. How about accupressure which might be less painful or reflexology, all these work on the chinese medicine lines of energy through the body.
Hope something works for you
regards
BrendaM in UK