I'm searching for a term for a hospital ward where patients who are particularly interesting for research are kept. I believe that such a term exists and that I've heard it, but forgot.
While I suspect the wording is not intended to be problematic, given the long history of research abuse, I believe it is important to mention that "interesting patients" are not "kept" for research. Participants are invited to volunteer for research studies while providing all of the necessary information about the benefits and risks of the study. They are free to end their participation at any time.
That said, perhaps the prototypical research hospital is the US National Institutes of Health (NIH) Clinical Center:
Masm2016 via Wikipedia.
The NIH website notes:
The Clinical Center is where NIH conducts its intramural clinical research; and that research is done in a hospital setting with patient/participants who receive not only experimental treatments but also the best in hospital care.
Expanding on the NIH's Clinical Center, Wikipedia also indicates that:
The term "Clinical research center" ... refers to any designated medical facility used to conduct clinical research, such as at a hospital or medical clinic.
It goes on to list some high profile institutions with clinical research centers. In my experience, the exact name is slightly different at each institution and is referred to by its proper name and not a generic term.