I am preparing a co-culture experiment in which transwell inserts will be used. According to the references, 0.4 micrometer PET membrane was selected. And different drugs were added to the upper and the lower chamber, respectively. I am confused about the reason why the insert membrane (in addition to a monolyer of cells, perhaps) can cut off small molecular drugs and prevent them to influence with each other. I guess the hydrophobicity of membrane and the small pore size were the main reasons.