The Expander liner is made from a premium stretch knit poly cotton, making it super stretchy and even warmer than a standard cotton weave liner. The stretchiness allows the Expander to move with you as you sleep, vastly increasing comfort and minimizing tangles. The anti microbial treatment keeps microbial life, thus bad odors, at bay which also helps prolong the life of your liner and sleeping bag. The anti microbial treatment wont wash out of the liner and does not use nano sized particles. The treatment is registered with the EPA.