It looks like the pads with a salt that can be boiled and stored and later when you click the thing inside the pack the salt will crystallize an release the hea

It looks like the pads with a salt that can be boiled and stored and later when you click the thing inside the pack the salt will crystallize an release the heat. I think the salt packs are quite stable. (Sodium acetate?) Anyway, they are using another molecules, that looks very tasty for bacteria and fungus, so I hope they keep it perfectly sealed, otherwise in winter they will only have a nasty green goo.