Air flow membrane — as the name suggests, this type of material allows air to pass through. Normally its made of advective or microporous transport system where it guides the trajectory and substance of gas. The membrane material has been applied in many fields, such as industry, medicine, environmental protection and the like. Membrane material contains a vast number of tiny pores, size and dispersion of which define gas permeability. Gas molecules will move through these pores from the high pressure side to low pressure side when a pressure difference occurs over the two sides of the membrane. Membranes can be made of different types of materials, all with distinct physical and chemical properties, thus affecting the membrane permeability, selectivity and longevity.