The membrane of choice for flow-through tests is a paper cast nitrocellulose.  In paper cast materials, the membrane is cast directly onto the surface of a high-quality cellulose paper.  This simplifies manufacturing and reduces the chance of test failure due to poor contact between the components.  Small-pore unsupported membranes can be used; however, they have to be carefully encapsulated to ensure good contact between the membrane and the absorbent to give good flow.