Bioreactors cell maintenance

The subscripts on the usual process parameters indicate the positions in the process to which these parameters refer. The kinetics of yeast growth can be described by a Monod rate expression with = 0.625 h and Kg = 2 g/L. Cell death and cell maintenance effects are negligible, as is formation of products other than yeast cells. The yield coefficient x/s is 0.44. The effluent from the bioreactor flows directly to a membrane filtration apparams. The membrane is completely permeable to the substrate, so the concentrations of the substrate in the CSTBR, the effluent from the bioreactor, and the permeate from the membrane and in the recycle stream are all identical. The membrane rejects a substantial proportion of the yeast cells so that the ratio of the concentration of yeast in the recycle stream is a factor of 4 larger than that in the effluent from the CSTBR. Volumetric expansion and contraction effects may be considered negligible. 

The culture can be used directly for the conversion of phenylpyruvic add to resting cells L-phenylalanine. Therefore, a batch process with resting cells can be carried out, with some glucose added for maintenance (fed-batch fermentation). Another approach is to harvest the cells from the fermentation broth and to use them in a separate bioreactor in higher concentrations than the ones obtained in the cell cultivation. An advantage of the last method can be that the concentration of compounds other than L-phenylalanine is lower, so that downstream processing may be cheaper. 

As seen above, the artificial systems are only able to supply detoxication functions of the liver. In some cases, this might not be enough to save patients. An alternative is the design of bioartificial liver. A simplistic approach consists in considering such a device as a bioreactor based on synthetic elements able to offer an adequate environment to the liver cells. This environment would in turn lead to the maintenance of efficient functions of the cells aiming at liver supply, when placed in a bioreactor located in an extracorporeal circuit. The mandatory requirements for acceptable cell viability and functions in a bioartificial liver (BAL) are tentatively listed below, according to a biotechnological point of view ... 

All biochemical reactions are enzyme-mediated. The rate of an enzyme reaction depends on the substrate concentration at the location of the enzyme and thereby on the diffusion rate of a substrate to the enzyme. It is therefore important to permanently obtain an intimate contact between a cell or enzyme and substrate molecules. Additionally, the product generated in the bioreactor has to be extracted because it may under certain conditions inhibit its own production. In some processes there may also be even a prepurification in the bioreactor itself. If living micro-organisms have to be applied, it is necessary to provide sufficient nutrition and respiration gases in case of aerobic fermentation. All other reaction parameters such as temperature, pH-value and reaction time have to be controlled precisely. In many cases (generally with modem processes) the maintenance of microbiological integrity (sterile process) is absolutely mandatory for a successful fermentation. 

Three-phase fluidized beds can be used as bioreactors for aerobic biochemical processes, including both fermentation processes and wastewater treatment. The gas phase is air, required for biological growth, while the solid particles provide immobilized surfaces on or in which cell growth can occur. The aqueous liquid phase provides the culture medium needed for the growth and maintenance of the cells. Air may be introduced separately from the liquid, or be premixed with the aqueous medium. The liquid medium may exhibit non-Newtonian rheology. A disadvantage of three-phase... 

An important tenet for achieving highly productive processes is the achievement within the bioreactor of a high viable cell concentration and its subsequent maintenance for an extended period. The latter requires the death rate to be minimized. This section describes cell engineering approaches evaluated with GS cell lines to minimize the death rate. 

During the stationary phase the equation that describes how the mass of the microorganism present in the bioreactor declines as a consequence of the cell s requirements for maintenance energy and/or losses associated with lysis of cells can be written in differential form as... 

An alternative approach to consideration of the multiple routes by which the limiting substrate is consumed in a bioreactor is to reexamine the pathways shown in Figure 13.3 and to write an expression for the rate of consumption of the limiting substrate when it proceeds via a particular pathway. The total rate at which substrate is consumed can then be written as the sum of the rates of consumption via the individual pathways. Thus, in terms of our formulation of the convention for yield coefficients for metabolism of cells that takes place in a constant-volume closed system, the rate of consumption of substrate can be expressed as a sum of terms associated with the rates at which biomass and other products of metabolic processes are formed plus a maintenance coefficient, m, that characterizes the rate at which a cell in a resting state must consume substrate for maintenance activities if it is to remain aUve ... 

This relation indicates that the rate at which cells are produced during growth is balanced by the sum of the rates at which cells die (or must address maintenance metabolism effects) and the rates at which cells exit the bioreactor-settler combination in the clarified effluent and the excess sludge. 

Disadvantages of bag bioreactors include potential problems with respect to obtaining adequate mass transfer of oxygen as a substrate for cell growth and maintenance of sterile operating conditions by use of aseptic connections where analytical probes, ports for entry and exit of process streams, including the possibility of use of some form of disposable impeller and/or sparger. Some conunercially available bag bioreactors contain stirrers and sensors built into the bag to facilitate (pre)sterilization of these components. 

When bioreactor operation relies on the use of living cells for the production of given goods or biomass, cell growth and maintenance requirements have to be considered. These processes are more complex, since they involve simultaneous substrate consumption, product formation, and cell growth, which can be schematically represented as... 

GMP cell-culture bioreactors are often placed in a class D or even class C cleanroom, even though the process is closed and there is no regulatory requirement for it. However, cell cultures often run for many weeks as perfusion culture, and thus mitigating the risk of contamination is considered a justification for the selected cleanroom environment. For maintenance and cleaning purpose, it is desired to move the majority of technical installations into a nonclassified technical space with only access to vessel ports and probes from within the clean... 

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