Multiphase Bioreactor Design

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In serum-free medium the separation factor was larger than unity indicating cell-
bubble attachment, which is in agreement with previous results. The viability was
reduced with about 56% including 12% lysis of cells. Remarkably, only a fraction of the
cells were killed, indicating that the bursting of a foam is less detrimental than that of a
bubble on a surface, in


Table 15.7 Foaming experimental results from


Michaels et al. (1995b) (see text for symbols).


(^) d32 mm asep FL Column NCV Foam NCV
serum free 1.40 1.68 11.7 1.68 11.7 89.3 73.8 0.20
0.1% PVP 0.96 3.85 21.5 2.65 14.7 99.4 94.2 0.57
0.1% PEG 0.65 2.38 10.6 1.11 4.9 95.6 94.9 0.12
0.1% PVA 0.82 0.78 14.1 0.46 8.3 99.1 97.3 0.06
0.1% Pluronic 0.68 0.74 16.5 0.36 8.0 101.2 95.0 0.06
0.1% Methocel 1.21 0.64 8. 4 0.55 7.3 99.0 98.1 0.05
0.5% FBS 1.43 1.91 11.9 1.95 12.2 88.9 78.4 0.23
5%FBS 0.98 0.50 26.5 0. 35 18.6 100.1 95.6 0.09
which case all cells are killed. Addition of 0.1% PVA, Pluronic F68 and Methocel
reduced the specific separation factor to 0.55 or less, indicating the absence of cell-
bubble attachment and rapid draining of cells from the films. The retained viability in the
foam after rupture for the remaining cells was 100%. Addition of serum up to a
concentration of 5% led to a decrease in the specific separation factor to 0.35. Also the
retained viability in the foam increased from 78 to almost 100% at 5% FBS. Finally,
increasing serum concentrations led to an increasing fraction of fluid transferred.
Addition of PVP and PEG resulted in specific separation factors still higher than unity.
However, in this case also the viability in the foam was equal to the viability in the bulk.
This again indicates that the protective mechanism of additives is primarily the reduction
of hydrodynamic forces and strengthening of the cell rather than reducing the number of
cells in the danger zone.
For a serum percentage of 3% Cherry and Hulle (1992) found a separation factor of
0.6, which is comparable to the value obtained by Michaels et al. (1995b). They proposed
the following equation to describe cell death:


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where ψ (–) is the fractional death in a bubble film, Cvf (cells.m−^3 ) is the viable-cell
concentration in the film, Cvb (cells.m−^3 ) is the viable-cell concentration in the bulk, h (m)
is the thickness of the film, F (m^3 .s−^1 ) is the gas-flow rate, Rb (m) is the bubble radius and
V (m^3 ) is the reactor volume. Using the cell separation value of 0.6 and the 20% cell
death upon film rupture, Cherry and Hulle (1992) were able to accurately predict the
first-order death-rate constant in a sparging experiment.


Lethal effects of bubbles in animal-cell culture 481
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