TENS unit

Carry On Bags: Yes (Special Instructions)
Checked Bags: Yes (Special Instructions)

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

 The final decision rests with the TSA officer on whether an item is allowed through the checkpoint.