Polytetrafluoroethylene (PTFE) is a robust fluorocarbon polymer (plastic) material that’s commonly used to insulate cables to improve performance for demanding applications.
It’s also a very stable material – meaning that if you put it in a vacuum it has an extremely low loss of residue and won’t give off any gasses that could potentially transfer to other instruments. It’s also great for things like medical implants – as even if it ends up inside your body somehow, it won’t decompose or give off any nasty chemicals.