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 stretch properties allow the Expander to move with you as you sleep, vastly increasing comfort and minimising tangles. The anti-microbial treatment keeps bacteria at bay as well as prolonging the life of your liner and sleeping bag. The anti-microbial treatment won’t wash out of the liner, and does not use nano-sized particles. The treatment is approved for use by the EPA (USA) and is used in many hospitals.