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.
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