Stress hydrodynamic

The main part of the report describes the results of systematic investigations into the hydrodynamic stress on particles in stirred tanks, reactors with dominating boundary-layer flow, shake flasks, viscosimeters, bubble columns and gas-operated loop reactors. These results for model and biological particle systems permit fundamental conclusions on particle stress and the dimensions and selection of suitable bioreactors according to the criterion of particle stress. 

Hydrodynamic stress exists independently of this. In the case of laminar flow, the stress is given by Newton s law (1) ... 

The equilibrium particle diameter in the case of non agglomerate particle systems or the enzyme activity of immobilised enzymes after a certain exposure of time is entirely due to the reactor-specific comminution process, and conclusions can therefore be drawn regarding the maximum intensity of hydrodynamic stress. 

The presented results for systematic studies on hydrodynamic stress in shake flasks, baffled stirred tanks, reactors in which boundary layer flow predominates (e.g. stirred tank with a smooth disc or unbaffled stirred tank), viscosi-... 

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. 

For certain conditions, the hydrodynamic stresses generated by the very rapid fluctuations in turbulent flow, the so-called Reynolds stresses, can be estimated as [70] ... 

Cell response to hydrodynamic stress has frequently been evaluated in terms of the release of intracellular components into the suspending fluid. An increase in... 

The effect of a particular cultivation environment on a system can be evaluated in terms of biomass (fresh/dry weight, cell number), secondary metabolite production [51,75,89,102,103,106,107] or substrate consumption (e.g. carbon source [57] or oxygen [53,108]). Using the Evan s Blue method to identify non-viable cells. Ho et al. [108] used viable cell density measurements to determine variations in specific growth rate attributable to hydrodynamic stress. 

Wongsamuth and Doran [58] exposed batch-cultivated suspensions of A belladonna to energy dissipation levels in the range 10 -10 Jm. Here again, a hierarchy of responses to hydrodynamic stress was identified, with membrane... 

Work on the OB has clearly demonstrated that plant cell response to stress, including hydrodynamic stress, is a more complex process than was originally anticipated. Moreover, it highlights a number of cellular processes which may define immediate system response and longer term performance prospects of plant cell suspensions. 

Agglomerates in a sheared fluid rupture when the hydrodynamic stress exceeds a critical value in dimensionless form the criterion for rupture is Fa > Facrjt. Rupture occurs within a short time of application of the critical stress, and thus can be distinguished from erosion, which occurs over much longer time scales. 

This new particle diameter represents the separation at which the hydrodynamic stress balances with the thermal and thermodynamic stresses ... 

This serai-empirical approach may be compared with a calculation based on the hydrodynamic stress gradient at the equator of a steadily moving drop with a rigid surface, and for Re < 1. The latter condition is easily satisfied for small drops. The tangential stress gradient is given (70, 77) by ... 

The impact of hydrodynamic stress on animal cells has been reviewed extensively (29,43). Most of the work reported in the literature on cell damage in agitated bioreactors has been done at bench-scale. Kunas and Papoutsakis (44) reported that in 1-2 L bioreactors equipped with a 7 cm diameter pitched-blade impeller, cell damage was not observed until the impeller rate was raised to above 700 rpm (tip speed 513cm/s), as long as air entrapment did not occur. However, it is not clear how these bench-scale observations translate into damaging impeller rates at manufacturing scale. 

In dispersive mixing the clusters of particles held together by cohesive forces (agglomerates) are successively broken apart by hydrodynamic stresses imposed on the external surfaces of the deforming liquid matrix, which in turn generate internal stresses within the cluster (40). A detailed review of dispersive mixing was given by Manas-Zloczower (41), and in this section we will follow her discussion. 

Cell culture systems must provide the physiological conditions for cell survival and proliferation. In vitro, animal cell growth is dependent on several factors, such as pH, temperature, osmolality, gas concentration (oxygen and CO2), available surface substrata, and state of the cells at inoculation (Freshney, 2005). Other factors that impact the culture are medium composition, which can differ extensively between cell lines and is discussed in detail in Chapter 5, as well as susceptibility to hydrodynamic stress, as discussed in Chapter 7. 

Animal cells do not have a cell wall and are, thus, highly susceptible to the effects of shear stress. Cells respond to hydrodynamic stress within minutes, altering their metabolism and the gene expression pattern (Nol-lert et al., 1991). Under sub-lethal levels of shear stress, there is initially an increase in passive transmembrane transport, simultaneously with damage to surface receptors. The plasma membrane is generally the main site for shear damage, and it may lose its capacity to mediate the transport of ions and molecules, so that the cell loses its viability. It has been demonstrated... 

The shear-sensitivity of plant cells has been observed in a number of cases [17, 52-58]. For example, Takeda et al. [58] reported decreases in respiration rate, ATP content and membrane integrity of both eucalyptus and safflower cells under hydrodynamic stress in a stirred-tank reactor. 

Embryogenic rice calli tend to form larger clumps during cultivation. Therefore, immobilization of the calli has hardly been carried out until now. Porous supports such as polyurethane foam have often been used for the immobilization of mycelial cells [64, 65] and plant cells [66-68]. In almost all cases, effective production of biological materials by the immobilized cells has been reported. To avoid the damage due to the hydrodynamic stress, we proposed the immobilization culture of rice callus using a macroporous urethane foam support. A turbine-blade reactor (TBR), which has been developed for hairy root culture, was also used in the culture. In the culture space, polyurethane foam was added as an immobilization support. 

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