I think in general, an unapproved medical device can be used on only human subjects when the device is under some clinical investigation and when it is used by some investigators participating in a clinical trial. (1) However, there may be circumstances under which a health care provider may wish to use an unapproved device to save the life of a patient or to prevent something when the alternative is not available. I read something in the regulatory information document about that a physician can treat a patient using an unapproved medical device in an emergency situation is this patient concludes that: the patient has a life-threatening condition that needs immediate treatment, there is no generally acceptable alternative treatment for this specific condition, and making sure that there is no time to use existing procedures to get the FDA approval for the use. (1) If the patient’s circumstances meet the above criteria then the FDA expects the physician to make the decision to use an unapproved device.

(1) https://www.fda.gov/downloads/RegulatoryInformation/Guidances/UCM127067.pdf