A chemist friend of mine thought of some very interesting things to make balloons out of. I'm still waiting to hear from him for some more details, but here are the highlights:
1) A superhydrophobic material, so that microbes can't stick to it
2) A superhydrophilic material, so that they are stuck fast and broken up
3) A photocatalytic material, which would catalyse anything on it to break down, ultimately into CO2 and water
I think (2) is out because the canopy would pick up too much moisture in the troposphere and would presumably be weighed down, reducing the height reached.
1 and 3 might well work, and 3 seems most elegant since it is an active process of sterilisation. Having said that, the same colleague still thought UVC might be best.
Our ultimate task is to kill spores - if we can kill spores then we can kill anything! Hence a massive UVC dose from LEDs etc might be necessary - if we can carry that many LEDs and enough power...
As always in "balloon material fantasies", it comes down to if we can find a suitable compound, if it is gastight for helium, robust enough for flight (without tearing etc), if someone will make balloons out of it for us, and for how much!
Cyclo-olefins is at least a name we can bandy around for the chosen property of UVC transparency... Any ideas who to order that from and any ideas about the other sorts of compounds?
If we can't fly a full size balloon made of our dream compound, then perhaps we can fall back on a small scale model, and then fly other hardware on a "dirty" normal balloon for a high altitude proof of concept?