The Apple Watch Sport straps are all made of fluoroelastomer - here is what this means
Fluoroelastomer comes from the two words "fluorinated" and "elastomer". The former refers to a chemical process, which either treats or binds a certain material with fluorine, while the latter simply describes elastic polymers (rubber-like material). Fluoroelastomers are famous for their durability and outstanding resistance to heat, oils, solvents, fuels, and ozone – they are often used for fuel-distribution systems or for high-quality lab gloves. It is not a material that will stretch out easily, get damaged by extreme heats, or get deformed by being excessively sweat on. It is also much less prone to induce any allergic reactions than, for example, latex.
Of course, for all its properties, fluoroelastomer is a pretty expensive material, classified as a special purpose polymer, only applied where high chemical protection is required.
In other words – it is tough!
Things that are NOT allowed: