Does the app need to be open during a MedBed session?
This is not required. Once you have completed a MedBed course and installed the MedBed, you can close the app and, of course, put away the device on which it is stored.
However, you don't have to close the app. It doesn't matter whether the display goes into gentle mode, i.e. goes dark, and the app is still open or not. This has no influence on the function of the MedBed.