What are cloth tea bags made of?!

I had some mighty leaf tea bags a couple of days ago and the material really confused me, it was clear and almost felt like plastic. The only materials that I could find that people have said work for making tea bags is cheesecloth or muslin but that looks nothing like the tea bags material.

Heres a picture, the two on the right are the cloth ones I'm curious about:
http://www.mightyleaf.com/resources/migh…

Thanks!