Porous membranes commonly used in tissue barrier and co-culture models allow the exchange of soluble factors between cell populations and in some cases even cellcell physical contact . These membranes also provide mechanical support for the cells and establish a partition to define apical and basal compartments. Casillo et al. grew endothelial cells on non-porous and porous SiO2 membranes with different pore sizes and pore spacings. On porous membranes, cells showed fewer focal adhesions and less fibronectin fibrillogenesis than on nons with smaller pores compared to membranes with a bigger -porous substrate, and this effect was accentuated on membrane porosity. However, cell-cell interactions in the form of tight junctions measured by ZO-1 activity showed an opposite trend. They concluded that porosity, especially with small pore spacing, could limit cell adhesion, but weaker cell-substrate interaction might lead to stronger cell-cell interaction. Nonetheless, Wen et al. found no influence of the porosity on the differentiation of ASCs on PAA hydrogels, suggesting that the effect of porosity on cells may depend on multiple factors. Overall, chemical, and physical modifications of substrates and their effect on cell behavior are well studied and documented for cell culture on non-porous substrates, while yet limited research has been done on membrane-based models.