Dialysis modules (Type LunDia Alpha 600) originally designed for medical dialysis purposes were obtained from Gambro Medizintechnik, Germany. This flat membrane module was specially optimised for flow of highly viscous fluids, making it also appropriate for very high cell densities. This module enables high mass transfer rates because of large effective membrane area (1.3 m2) and very thin membrane (8 m). The membrane is made from cellulose and has a molecular cut-off of 4500 Da. The module is a cheap disposable one-way product only to be used once. The body of the module is made of polycarbonate and does not resist steam autoclavation. It is supplied sterile after EtO-sterilisation.