In low air loss systems, the patient lies on a loose-fitting, waterproof cover that is placed over the cushions. The waterproof covers are designed to have air pass through the pores of the fabric and are usually made of a special nylon or polytetrafluorethylene fabric with high moisture vapor permeability. Manufacturers have addressed the problem of dehydration of the skin by altering the number, size and configuration of the pores in the covers . The material is very smooth with a low coefficient of friction, bacteria impermeable, and easy to clean .