In general, a conventional submerged type of bioreactor/oxygenator with an agitation system has been used in cell cultivations.
The same type of oxygenator as used for the homogeneous cultivation of microorganisms may be applied for plant cell cultivation and for micro-carrier-supported animal cell culture.
There are three major types of homogeneous-oxygenator systems used for propagating microbial, plant, and animal cells. These are stirred tank reactors (STRs), thin-film rotary oxygenators, and airlift reactors (ALRs). All of these oxygenators are able to exert shear effects on cell cultures.
In general, animal cells are much more sensitive to shear stress as compared with microbial and plant cell cultures. In STRs, however, there is conflicting data about this degree of shear sensitivity. Conventional turbine or vibrator-agitated bioreactor oxygenators have been commonly used to produce mammalian cells which can grow independently of solid support. It has been shown that the growth of specific hybridoma cells decreased at an agitation rate of 240 rpm. This has been stated to be an indication of the sensitivity of hybridoma cells to excessive mechanical agitation. It has been shown that critical shear stress causing progressive loss of viability of insect cells was 1-4 N/m2.
The problem is that it is not an easy task to grow large quantities of mammalian and plant cells in an artificial medium. The well-established technology of industrial microbiology is adapted to the requirements of bacteria, yeasts, and molds. Every single cell of these microorganisms being encased in a tough cell wall serves as an independent metabolic factory with fairly simple nutritional requirements.
Microorganisms grow well dispersed and floating free in a liquid cultivation medium in a conventional STR with a capacity as high as 80,000-100,000 liters. In this conventional oxygenator design, microorganisms depending on nature resist damage even when they have proliferated to form a thick suspension and even when the suspension is agitated vigorously with a mechanical agitator. Mammalian and plant cells have different properties. They are larger than those of most microorganisms, more fragile, sensitive to shear and other mechanical forces, and more complex.
As can be seen from literature, the delicate plasma membrane that encloses an animal cell is not encased in a tough cell wall. Having these structural characteristics most animal cells will not grow at all in suspensions. They grow only when they can attach themselves to a surface. This requirement led to the development of a novel micro-carrier-anchored cell bioreactor.
Over the years techniques have been developed to grow mammalian and plant cells on a small scale in the laboratory bioreactors. A mammalian cell culture begins with mammalian tissue. The tissue is dissociated either mechanically or enzymatically, or by a combination of the two methods. This yields mixture of single cells and small clumps of cells. The mixture is inoculated into an appropriate liquid growth medium containing salts, glucose, certain amino acids, and blood serum. Although most mammalian cells need to be anchored to a solid support, cells that originate in blood or lymphatic tissue, along with most tumor cells and other transformed cells, can be adapted to grow in suspension.
Such cells can be cultivated in spinner bottles. It has been reported that a Waldhof-type and bubble column oxygenation bioreactor with a draft tube in the middle of the vessel has also been used successfully. Bioprocess engineering that involves mammalian cell culture has come to prominence recently with a growing number of bio-products obtained from such bioprocesses.
In order to explain the mechanisms of cell loss in conventional agitated and sparged oxygenation bioreactors, many experimental studies have been carried out to investigate the influence of culture conditions on the susceptibility of cell damage. It has been proposed that bubbles of sparged air cause most of the losses as they disengage from the broth. Thus, conventional and novel oxygenation bioreactors for microbial, mammalian, and plant cell culture systems possess several common and uncommon features.
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