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From 2012.igem.org

The spores of Bacillus subtilis will be contained in a compartment which is impermeable to bacteria and liquid, but can still let through volatiles. Metabolites, water, and alanine should be separated from the spores by a breakable membrane. Mixing these compounds will cause germination.

The material we use for our sticker is TPX®, or polymethylpentene. This polymer is available as thin, transparent sheets. The advantage is that it is relatively cheap, strong, and capable of letting through volatiles. The radius of the pores in TPX® is 0.5 µm, which is 500x larger than the average badmeat-volatile, but still small enough to keep liquid and bacteria in (see the figure below).

Comparison of the size of the TPX pores, volatiles and Bacillus subtilis.