Perfusion culture

Fig. 2. Schematic of a process for microcarrier perfusion culture using a spin filter device.

Fig. 5. A suspension perfusion culture process having ceU recycle. System employed at Amgen using CHO cells for production of a recombinant protein.

Perfusion cultures of submerged free cells are essentially continuous cultures with a cell retention device so that no cells exit in the effluent stream. The governing ODEs are... 

The procedure for the estimation of qs and qp is identical to the one presented for fed-batch and continuous cultures. The only difference is in the estimation of the specific growth rate (p). Since perfusion cultures behave as batch cultures as far as the biomass is concerned, p can be obtained as described earlier for batch systems. Namely p is obtained as the slope in the plot of / Xv(t,) versus t,. 

We finish this section with a final note on the estimation of the specific death rate (kd). The procedures presented earlier (for batch, fed-batch, continuous and perfusion cultures) provide an estimate of the apparent growth rate (pa) rather than the true specific growth rate (p). The true growth rate can only be obtained as (Pa+kd) where kd is the specific death rate. 

The dynamic mass balance for nonviable cells in a batch or perfusion culture yields. 

Xu Y, Sato K, Mawatari K et al (2010) A microfluidic hydrogel capable of cell preservation without perfusion culture under cell-based assay conditions. Adv Mater 22 3017-3021... 

Third type of culture is stroma-non contact . In this system primitive progenitor cells are sustained when cells are co-cultured with irradiated allogeneic stroma but separated from it by the 0,4 micron membrane in transwell inserts (Costar, Cambridge, MA). These cultures are maintained by daily supplementation of stromal feeder conditioned media (Roller et al. 1998, Verfaillie, 2001) successfiilly expanded umbilical cord blood cells in a novel automated perfusion culture system. Development these approaches followed in the studies of investigators who incorporated the stromal components into the expansion culture. Recently published trials by McNiece et al. 2000 are more encouraging where cells were expanded in static culture for 10 days in Teflon bags (American Fluoroseal, USA). 

Roller M., Manchel L, Maher R., Goltry K., Armstrong R., Smith A. (1998) Clinical scale human umbilical cord blood cell expansion in a novel automated perfusion culture system,. Bone marrow transplant. 21(7), 653-665. 

To avoid inhibition of proUferation and differentiation the pH in hematopoietic cell cultures has to be controlled carefully. Especially in non-perfused culture the pH shifts during the cultivation due to secreted acidic metaboUtes like lactate. However is has been reported recently that pH control alone is not sufficient to eliminate inhibition of cell growth and metaboUsm as other inhibitory factors also accumulate in un-fed cultures [51]. 

Cell Retention Devices for Suspended-Cell Perfusion Cultures... 

Keywords. Perfusion cultures, mammalian cell retention devices, filtration, centrifugation, sedimentation... 

Suspension systems can be operated in different modes batch, fed-batch, chemostat, and perfusion (Fig. 1). These operation modes differ basically in the way nutrient supply and metabolite removal are accomplished, which in turn determines cell concentration, product titer and volumetric productivity that can be achieved [8]. The intrinsic limitation of batch processes, where cells are exposed to a constantly changing environment, limits full expression of growth and metabolic potentials. This aspect is partially overcome in fed-batch cultures, where a special feeding strategy prolonges the culture and allows an increase in cell concentration to be achieved. In perfusion and chemostat processes nutrients are continuously fed to the bioreactor, while the same amount of spent medium is withdrawn. However, in perfusion cultures the cells are retained within the bioreactor, as opposed to continuous-flow culture (chemostat), which washes cells out with the withdrawn medium [9]. 

From an economic point of view, perfusion cultures of animal cells should be operated at high perfusion rates [17]. However, the high cell concentrations achieved in such cases result in several technical constraints, such as oxygen transfer, CO2 removal, medium formulation, and, especially, cell retention efficiency. 

Although perfusion cultures using RDF devices have not yet been reported, the results obtained in harvesting [113] and simulated perfusion experiments [104] indicate that it could be a promising cell retention device for suspendedcell perfusion processes. 

---