ABSTRACT
After cell cultivation, a cell separation step is generally needed to process the culture medium to purify the product. This step is performed either to obtain a cell-free medium for an extracellular product or to obtain a cell concentrate if the product is intracellular. In animal cell cultures, the product is normally secreted. Thus, after cell separation, the product is in solution and can be processed by chromatographic techniques or other protein purification methods, as presented in Chapter 12. When the culture is operated in perfusion mode (Chapter 9), the
retention device may be placed inside or outside the bioreactor (Figure 11.1). When the cell culture is operated in batch or fed-batch mode, the cell separation is conducted after the end of the cultivation period (Figure 11.2). This is the first step of downstream processing. Animal cell separation is far from a trivial task for many reasons.
Animal cells have no cell wall. They are protected only by the cellular membrane (lipid bilayer). For this reason, they present high sensitivity to shear stress. When cultivated in suspension, the cells tend to attach to the device walls, which may generate clogging problems. Their density is not much higher than water (1.06-1.14 g cm-3) and cell size is in the range of 8-40 m, which results in low settling velocities (Medronho, 2003). For example, Luebberstedt (2000) worked with HeLa cells with a density of 1.06 g cm-3 and varying from 10 to 28 m in size. Schultz (1996) found BHK-21 cell sizes in the 11-25 m range.
