Transwell permeable supports are available in three membrane materials: polycarbonate, polyester (PET), and collagen-coated polytetrafluoroethylene (PTFE). Polycarbonate Transwell inserts feature thin, translucent membranes available in four pore sizes ranging from 0.4 µm to 8.0 µm. Most are treated for optimal cell attachment. They are supplied sterile and come preloaded in multiple well plates or dishes. The polycarbonate membrane is compatible with most organic fixatives and stains. Polyester (PET) Transwell-Clear inserts have microscopically transparent polyester membranes that are tissue culture-treated for optimal cell attachment and growth. Transwell-Clear inserts provide better cell visibility under phase contrast microscopy and allow assessment of cell viability and monolayer formation. Transwell-COL inserts have transparent (when wet), collagen-treated PTFE membranes that promote cell attachment and spreading and allow cells to be visualized during culture. The Transwell-COL membranes are coated with an equimolar mixture of types I and III collagen derived from bovine placentas. Unlike traditional coating techniques that result in occluding film layers, Corning’s coating process results in a biologically stable collagen that covers every fibril of the filter matrix, thereby retaining the porosity of the membrane.