Shear sensitivity

Collisions/impacts The relatively very small size and mass of free cells is unlikely to result in cell colUsions of high impact strength that can cause damage to the cell wall. 

The shear generated by the impeller itself is not responsible for cell death. However, the smaller bubbles produced by the shearing action of the impeller can contribute significantly to ceU death. Experiments in unsparged baffled reactors in which oxygen was suppUed from headspace did not show any ceU damage. 

STIRRED TANK REACTORS FOR CELL CULTURE TECHNOLOGY 

The conclusions in (2) and (3) were further confirmed for TB/C3 hybridoma by Oh et al. (1989, 1992) who used a high shear Rushton turbine with and without gas sparging. Viability studies in the absence of air sparging showed no noticeable cell damage with a maximum power input of 0.25 W/kg. In contrast, relatively low level of aeration resulted in rapid cell death even at much lower impeller power. These results again prove that impeller-induced shear alone does not result in cell death, whereas the presence of gas bubbles results in significant cell death. 

In an extension of the earlier work, Kioukia et al. (1996) cultivated Sf9 insect cells in agitated laboratory bioreactors with and without Pluronic F68 and with and without 

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Acrylamide copolymers designed to reduce undesired amide group hydrolysis, increase thermal stability, and improve solubility in saline media have been studied for EOR appHcations (121—128). These polymers stiH tend to be shear sensitive. Most copolymers evaluated for EOR have been random copolymers. However, block copolymers of acrylamide and AMPS also have utiHty (129). 

Disk Pumps. When pumping shear-sensitive or highly viscous fluids, it is desirable to reduce internal turbulence caused by the vanes. The disk pump design rehes on the centrifugal frictional effect of a vaneless disk. Whereas the efficiency of this pump is lower than that of similar centrifugal pumps having vanes, it is often the only solution to certain pumping appHcations. 

Flexible Tube. The simplicity of design and the absence of seals and valves make the flexible tube or peristaltic pump a good choice for low capacity and low pressure appHcations in the pharmaceutical industry or wherever shear-sensitive or moderately abrasive fluids are pumped. Because of the continuous flexing of the tube, the tube material of constmction presents a challenge regarding life cycle. For the same reason, pressures are kept relatively low. 

Metering Pumps. For small flow rates, such as dosing chemical additives where precise control is requited, progressive cavity self-contained pumping units are used. These can often handle shear-sensitive fluids or Hquids containing abrasive particles. These pumps are not as widely pubHci2ed or generally as well known in the Hterature as other pump types. 

Recombinant DNA technology has already provided several products of therapeutic interest from mammalian cells. Table 2 gives examples of products from mammalian cells, the use, and the technology used for production. Technology development for these products has centered around the differences in characteristics of mammalian versus microbial cells, notably, the shear sensitivity and susceptibiUty to contamination of the mammalian lines. 

Is the product shear sensitive, i.e., will it harden due to turbulence ... 

Many reactions involve shear-sensitive materials, which severely limit the maximum mixing rate and make impeller and reactor design important. Mixing becomes the limiting factor. 

PVC on its own is extremely heat- and shear-sensitive and cannot be processed into finished goods, as it starts degrading at temperatures considerably lower than those required to process it. The processing of PVC requires a number of additives. These include heat stabilizers, impact modifiers, processing aids, and lubricants. 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

The constant shear concept has been applied for bioreactor scale-up that utilises mycelia, where the fermentation process is shear sensitive and the broth is affected by shear rate of impeller tip velocity. For instance, in the production of novobicin, the yield of antibiotic production is dependent on impeller size and impeller tip velocity. 

Traditional Apparatus. The above reference need only be expanded to Include the common use of cylindrical culture vessels (1), in place of the spherical reaction flasks, for shear sensitive materials and simple boiling water type baths (2) as their heating medium. 

Tramper J, Joustra D, Vlak, JM (1987) Bioreactor design for growth of shear-sensitive insect ceU. In Webb C, Mavitima R (eds) Plant animal ceU ciUtiu es process possibUities. Ellis Horwood, England... 

Shear Sensitivity Studies Under Defined Flow Conditions.151... 

Shear Sensitivity Studies in Stirred Tank Reactor Configurations. . 163... 

The purpose of this chapter is to review relevant work in the area, including an evaluation of methods employed for shear sensitivity studies, comparison of bases for data analysis and an outline of current knowledge of the interface between the hydrodynamic environment and the plant cells themselves. Coverage is limited to cell suspension cultures and does not extend to other tissue or organ systems. 

The perceived sensitivity of plant cells to the hydrodynamic stress associated with aeration and agitation conditions is typically attributed to the physical characteristics of the suspended cells, namely their size, the presence of a cell wall, the existence of a large vacuole, and their tendency to aggregate. Table 1 illustrates some of the differences between plant cells and other biological systems. Chalmers [19] attributed shear sensitivity in mammalian cultures at least in part to the fact that these cells occur naturally as part of a tissue, surrounded by other cells. The same is true for plant cells. The more robust microbial systems, on the other hand, exist in nature as single organisms or mycelial structures, very close to the forms they assume in submerged culture. 

From a shear sensitivity perspective, it is particularly interesting to note that the OB has also been observed to occur in response to other, non-pathogenic stimuli, including wounding [176], agitation [177-180], pressure [181,182] and osmotic stress [180,183]. Relevant studies are summarized in Table 4. 

BIOREACTOR DESIGN FOR SHEAR SENSITIVE CELL CULTURES... 

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