Daystrom Institute

In VOY S1E05 "Phage", Neelix's lungs are stolen by the Vidiians, and The Doctor creates a set of holographic lungs operated by the holo emitters installed in sickbay.

The procedure is successful, but due to technical limitations, that places two very heavy restrictions on Neelix:

1. He has to have his movement completely restrained as the computer cannot compensate quickly enough to keep the holo-lungs in alignment with his internal physilogy
2. Like The Doctor, he cannot leave sickbay even if the computer could process fast enough to keep his holographic lungs aligned

Potentially, Neelix could also be transported to the holodeck with his holo-lungs intact, but given its frequent malfunctions, it may not be a wise action unless there's no other alternative. He would also still need to be restrained to keep the lungs in place.

The mobile emitter, though introduced two seasons later in Season 3's "Future Tense", might alleviate both of those restrictions and have applications in field medicine and other emergency care.

Being 29th century technology, it may very well be fast and responsive enough to keep the holographic organs aligned with the body's nervous, respiratory, and circulatory systems without requring the patient to be immobilized. Additionally, it would give the patient unrestricted mobility without confining them to a location equipped with holo emitters.

Given that by the time of PIC in 2401 the mobile emitter has been reverse engineered and reproduced, it would be logical to explore its application as an emergency organ transplant given that the procedure was successfully proven by the crew of Voyager.