range of biological processes including cell retention for perfusion cultures. In the

TFF system, cell broth is pumped through a dip tube in the lumen of a hollow fiber

module via a peristaltic pump. A simplified overview and description of the setup is

shown in Figure 6.5. This leads to a perpendicular flow of the cell broth to the

permeation direction [19,52]. The hollow fiber module consists of multiple fibers

that are mounted in parallel in the cartridge. Each cartridge has a specific cut-off

(typically 0.1–10 μm) retaining the cells within the hollow fibers, while spent

medium is withdrawn as permeate that is passing through the membrane. Due to the

relatively high shear stress at the filter surface caused by the tangential flow of the

cell broth, filter fouling is reduced, allowing continuous operation and high filtration

flux. Indeed, TFF systems have shown reduced fouling compared to spin filters.

However, shear rates and thus feed flow rates are limited by the shear stress sen-

sitivity of animal cells (Castilho 2002). Although various mammalian and insect

cell lines were shown to survive hydrodynamic stress up to γ = 3,000 s⁻¹ [68] or

γ = 4,000 s⁻¹ [69], values as low as γ = 620 s⁻¹ were shown to be harmful for

suspension HEK293 cells after adenovirus infection [60]. Moreover, since re-

circulation is based on peristaltic pumps, high local shear stress areas are generated

by the pumps further limiting the use for the cultivation of shear stress sensitive cell

lines. This issue can be overcome by implementation of low shear stress centrifugal

Feed pump

HF Membrane

Impeller

pump

Balance

Weight control

Permeate pump

Spent medium

Feed

medium

FIGURE 6.5 Schematic illustration of a setup of a TFF as CRD. Peristaltic pumps or

centrifugal pumps are typically operated in constant or pulsed unidirectional flow. Cell-free

supernatant is constantly removed in a direction orthogonal and radial to the hollow fiber

filters, allowing fresh medium to be added to the bioreactor. Compared to ATF setups (see

Figure 6.6) the working volume in the bioreactor remains constant. Figure adapted from [ 65].

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Bioprocessing of Viral Vaccines