TENS (Transcutaneous electrical nerve stimulation) units are allowed in both carry-on and checked bags. If your doctor has told you that your specific TENS unit shouldn't go through an X-ray, you should notify the TSA officer so they can conduct alternate screening. To help you get through security more efficiently, remove your TENS unit from your carry-on bag and put it in the screening bin so the TSA officer can get a clear view of it. For more information, see TSA special procedures.
Devices containing lithium metal or lithium ion batteries should be carried in carry-on baggage. Most other consumer electronic devices containing batteries are allowed in carry-on and checked baggage.
For more information to portable electronic devices, see the FAA regulations