In scaling up batch culture systems, certain fundamental laws of microbial cell systems can be applied to mammalian cells where the suspension cultures contain the anchorage-dependent cells. This is not the case with animal cells which are sensitive to the effects of heavy metal ion concentration, shear force of impeller agitation or air sparging, and are dependent on serum or growth factors. For these reasons, the materials for construction of fermenters are 316 low carbon stainless steel, silicone and teflon. Different agitation systems such as marine-blade impeller types, vibromixer and air-lift are recommended to mitigate the shear stress. The maximum cell growth for large scale cell suspension using ajar fermenter is governed by several critical parameters listed in Table 9.
Table 9. Critical Parameters of General Cell Culture
1) Chemical parameters:
Decrease of general critical nutrients: glutamine and glucose
Increase of inhibitory metabolites:
ammonium ions and lactic acid (pH control)
gas sparging, chemically by adding cysteine, ascorbic acid and sodium thioglycollate, etc.
2) Physical parameters:
Decrease of dissolved oxygen:
aeration volume, agitation speed and oxygen contents of gas phase
Temperature and pressure: optimum condition control.
control of additional ion concentration etc.
3) Physiological parameters:
contamination of cytotixic compounds
increase of inhibitory metabolites and chalone like substance, ratio of fresh medium and cell adhesive surface
cell density and induction conditions, etc.
For each parameter, the pH, DO (dissolved oxygen), ORP (oxidation-reduction potential), temperature, agitation speed, culture volume and pressure can be measured with sensors located in the fermenter. The output of the individual sensors is accepted by the computer for the on-line, continuous and real-time data analysis. Information stored in the computer control system then regulates the gas flow valves and the motors to the feed pumps. A model of a computer control system is shown in Fig. 11. The computer control systems, like the batch systems for mammalian cell culture, seem to level out at a maximum cell density of 106 cells/ml. It may be impossible for the batch culture method to solve the several limiting factors (Table 10) that set into high density culture where the levels are less than 107 cells/ml.'351
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