Cell Separation Devices Based on Filtration

Cell separation by filtration has been used since decades. 'Classical' filtration where the cell broth flow is orthogonal to the membrane or dead end filtration is not suitable for perfusion since filter clogging happens very quickly. Several alternative filtration technologies have been developed to overcome this by putting in motion the filter or the cell broth: spin-filter, tangential flow or cross-flow filter, floating filter, vortex-flow filter. The shear created by this movement helps to free the filter pores. Filter fouling is an inherent drawback of any filtration technology. Fouling results in degraded filtration performances in terms of filtered fluid flow rate but can also generate a selective retention of the POI.

Spin-Filter

The spin-filter was introduced by Himmelfarb et al. (1969). It consists of a cylindrical screen, which is continuously rotating together with the bioreactor impeller or independently, see Fig. 13.2. The cell-free supernatant enters in the spin-filter while the cells are retained outside. The permeate is pumped from the spin-filter cage into the harvest line. It was first introduced mounted inside the bioreactor. The alternative of external placing is preferred today in particular for large-scale bioreactors. Outside the bioreactor, it can be replaced or cleaned and re-sterilized in case of fouling. Fouling of spin-filter has been often described. It has been showed that it is caused by a deposition of nucleic acid and dead cells (Esclade et al. 1991; Mercille et al. 1994).

The operation parameters are critical for the fouling: material such as hydrophobic plastic should be preferred to stainless steel for the screen (Avgerinos et al. 1990; Esclade et al. 1991). The addition of silicone polymer antifoam has been reported to rapidly cause filter fouling (Emery et al. 1995). Higher rotation speed of the spin-filter helps to reduce the fouling but can be associated to increased cell damage (Deo et al. 1996; Vallez-Chetreanu et al. 2007). Higher perfusion rates result in increased fouling (Tolbert et al. 1981). This factor is unfavorable for high cell densities since these require higher perfusion rates. As Yabannavar (Yabannavar et al. 1992) predicted experimentally, Figueredo-Cardero et al. (2009) showed by CFD that the liquid flow direction through the spin-filter screen is not only inwards the spin-filter cage but also outwards; the perfusion rate being the difference of both flows. The study showed as well that higher rotation speeds increased the outwards flow while increasing the perfusion rate had only a limited effect of reducing this flow. Vallez-Chetreanu et al. (2007) showed that larger filter pore size can improved the filter longevity and that applying ultrasoundvibration to the spin-filter had a positive effect to reduce the fouling, resulting in a doubled culture period.

Fig. 13.2 Representation of bioreactor equipped with an internal spin-filter. The spin-filter consists of a cylindrical screen, which is continuously rotating. The cell-free supernatant enters in the spin-filter from where it is pumped into the harvest line