Flask venting, breathable plastics?

[Djarum Black]

I was doing some research on replate flask venting because I'm having trouble finding a good way to do it.

I've seen people use little round bandaids to vent a flask jar with a hole drilled in the top. I have also seen people who use tightly woven cotten and some other type of material.
However I swear I heard somewhere that there is a plastic that will allow air exchange on it's own without contamination of the flask. Is this true? Is there such a plastic container of some kind thats being used for this purpose?
For those of you out there who flask, what method do you use to vent your replate jars/flasks?

A big thank you in advance to anyone who can help me with any information.