The oxygen permeability coefficient is usually expressed by the DK value, D is the diffusion coefficient, which indicates the diffusion ability of the gas through the material, and K is the solubility coefficient, indicating the degree of oxygen dissolved in the material. Oxygen permeability is to maintain the physiological needs of the cornea. The higher the oxygen permeability coefficient, the better the wearing feelings. Materials that DK values over 90 are high oxygen permeability materials.