rupture process, while apparently the hydrodynamic forces are still high enough to kill
the cells in these larger volumes. Therefore, it would be interesting to study whether this
still holds for bubbles larger than 6 mm, where the severity of the burst is even further
reduced and no jet formation is observed anymore.
Certain surfactants like Pluronic F68 and Methocel can completely protect the cells
against shear from bursting bubbles. These compounds adsorb on the cell membrane and
the bubble surface and protect the cells by a combination of mechanisms primarily
reduction of cell fragility and a decrease in hydrodynamic forces accompanying bubble
rupture. In addition, some additives prevent attachment of cells to bubbles and allow cells
to drain rapidly from bubble films. Thus, they cause a decrease in the cell concentration
in the killing volume. Although this does not seem required for protection it prevents the
physical loss of cells in case foaming occurs. With the application of additives like
Pluronic and Methocel the problem of cell death due to sparging is largely solved.
However, these compounds may have a negative effect on cell physiology and oxygen
transfer.
For reactor design purposes the height of the reactor is the main design parameter,
with higher reactors being more favourable for the growth of animal cells. With respect
to bubble diameter the influence on reactor performance depends on a number of yet
unknown effects and parameters and is therefore not clear. Finally, one of the major
remaining problems associated with sparging will likely be foaming due to high cell and
protein concentrations in the medium.
REFERENCES
Absolom, D.R., Lamberti, F.V., Policova, Z., Zing, W., van Oss, C.J., Neuman, A.W. (1983)
Surface thermodynamics of bacterial adhesion. Appl. Environ. Microb. 46, 90–97.
Abu-Reesh, I., Kargi, F. (1989) Biological responses of hybridoma cells to defined hydrodynamic
stress. J.Biotechnol. 9, 167–178.
Al-Rubeai, M., Oh, S.K.W., Musaheb, R. and Emery, A.N. (1990) Modified cellular metabolism in
hybridomas subjected to hydrodynamic and other stresses. Biotechnol. Lett. 12, 323–328.
Al-Rubeai, M., Emery, A.N. and Chalder, S. (1992) The effect of Pluronic F-68 on hybridoma cells
in continuous culture. Appl. Microbiol. Biotechnol. 37, 44–45.
Al-Rubeai, M., Emery, A.N., Chalder, S. and Goldman, M.H. (1993) A flow cytometric study of
hydrodynamic damage to mammalian cells. J.Biotechnol. 31, 161–177.
Al-Rubeai, M., Singh, R.P., Goldman, H. and Emery, A.N. (1995a) Death mechanisms of animal
cells in conditions of intensive agitation. Biotechnol. Bioeng. 45, 463–472.
Al-Rubeai, M., Singh, R.P., Emery, A.N. and Zhang, Z. (1995b) Cell cycle and cell size
dependence of susceptibility to hydrodynamic forces. Biotechnol. Bioeng. 46, 88–92.
Andrews, G.F., Fike, R. and Wong, S. (1988). Bubble hydrodynamics and mass transfer at high
Reynolds number and surfactant concentration. Chem. Eng. Sci. 43, 1467–1477.
Augenstein, D.C., Sinskey, A.J. and Wang, D.I.C. (1971) Effect of shear on the death of two strains
of mammalian tissue cells. Biotechnol. Bioeng. 13, 409–418.
Bavarian, F., Fan, L.S. and Chalmers, J.J. (1991) Microscopic visualization of insect cell-bubble
interactions: I: rising bubbles, air-medium interface, and the foam layer. Biotechnol. Prog. 7,
140–150.
Born, C., Zhang, Z., Al-Rubeai, M. and Thomas, C.R. (1992) Estimation of disruption of animal
cells by laminar shear stress. Biotechnol. Bioeng. 40, 1004–1010.
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