Critical Issues for the Use of Tangential Flow Filtration Systems for Cell Harvesting Applications

Abstract

The development of techniques for the separation of biological molecules, such as proteins, has been an important prerequisite for the many advancements made in biotechnology. Substantial worldwide markets exist for highly purified therapeutic proteins, enzymes, vaccines and hormones. The advent of molecular biology techniques has enabled scientists to manipulate cells to over-express recombinant products during fermentation to produce maximum amounts of product which is located either in the cell or secreted into the fermentation medium. The next challenge is to lyse the cells to release product or to separate the cells from the product in the fermentation medium. A number of options are available: centrifugation, direct flow filtration, expanded bed chromatography and tangential flow filtration (TFF). One of the most popular techniques is TFF. The two key applications are whole cell harvesting without cell breakage and lysate clarification where the effective transmission of product is paramount whilst retaining cell debris and precipitates. The major requirements of the membrane should be: low non-specific protein binding, resistance to fouling, compatibility with cleaning agents and a wide range of pore sizes or molecular weight cutoffs. TFF membranes should also be available in scalable formats, so that processes can be modelled at lab scale and scaled directly to process scale. A critical requirement for TFF systems in cell harvesting is control of transmembrane pressure (TMP) which should be maintained at a constant value across the entire membrane surface. Figure 1 shows a typical TFF module with ports for feed, retentate and filtrate. If the filtrate ports are fully open, a typical pressure profile shown in Figure 1 (a) is obtained. Because the filtrate pressure is zero, there is a non-uniform transmembrane pressure across the entire membrane surface as shown in Figure 1 (b) and this will result in decreasing flux as the gel layer forms. If the feed filtrate port is closed and the retentate filtrate port restricted, a pressure profile as shown in Figure 2(a) can be achieved. The resultant transmembrane pressure in Figure 2(b) is more uniform across the entire membrane and will ensure consistent membrane performance. Using this operating principal of low-pressure filtrate control, we can then apply TFF successfully to cell harvesting applications. However, optimisation of transmembrane flux is also essential as shown in the following example.