Surface treatments and storage conditions of PDMS membranes also influence their gas permeability. A common treatment to convert PDMS surface from a hydrophobic to a hydrophilic state is the exposure to oxygen or air plasma. As part of such a transformation, surface methyl groups are removed and replaced with hydroxyl groups (SiO2). Studies from Markov et al. and revealed that the SiO2 layer created by plasma treatment on the exposed surface significantly impedes the diffusion of oxygen through PDMS. Moreover, Markov and colleagues also investigated the effect of storage conditions after treatment. They found that the PDMS completely recovered its original permeability after a 3-day storage in air, while the gas diffusion barrier remained significant for up to 3 weeks if PDMS was maintained in contact with water.