Back up parachute in case canopy bursts
Pressure sensor (in case of leaks)
But all the above sterilised by UVC from outside before flight! (That way, leaks don't jeopardise sterility - only flight time)
Ahhh - here's something interesting, a bit like the Travelling Salesman Problem:
You have one LED in the middle of a big spherical balloon
You have one LED travelling up and down a cylinder balloon
At what given combination of radius and power does one sterilise quicker than the other?