Experimental methods for studying membrane permeability and partitioning typically rely on measuring the concentration change of a single permeant in two macroscopic reservoirs separated by a planar membrane model. The permeation barrier can be formed synthetically from lipidic materials as in the PAMPA assay , assembled from living cells as in the Caco-2 permeability method 2 , or collected from real tissues such as skin in the Franz diffusion cells .