Comformable gumsheild material

Hi All , I noticed that modern gumshields are made from a conformable material , which when boiled, moulds itself exactly around the teeth. Can anyone tell me what material this is and who makes it? It is reversible or does it take the shape permanently? Is it the same as good 'ol heat shrink material used in wiring? Many thanks. P.s. I have had trouble replying to answers so not being rude if I don't reply. Opticsman