Abstract
Tissue cells are known to be sensitive to mechanical stresses imposed on them by agitation in bioreactors. The amount of agitation provided in a microcarrier or suspension bioreactor should be only enough to provide an effective homogeneity. Three distinct flow regions can be identified in the reactor: bulk turbulent flow, bulk laminar flow, and boundary-layer flows. Possible mechanisms of cell damage are examined by analyzing the motion of microcarriers or free cells relative to the surrounding fluid, to each other, and to moving or stationary solid surfaces. The primary mechanisms of cell damage appear to result from (a) direct interaction between microcarriers and turbulent eddies, (b) collisions between microcarriers in turbulent flow, and (c) collisions against the impeller or other stationary surfaces. If the smallest eddies of turbulent flow are of the same size as the microcarrier beads, they may cause high shear stresses on the cells. Eddies the size of the average interbead spacing may cause bead-bead collisions which damage cells. The severity of the collisions increases when the eddies are also of the same size as the beads. Bead size and the interbead distance are virtually equal in typical microcarrier suspensions. Impeller collisions occur when the beads cannot avoid the impeller leading edge as it advances through the liquid. The implications of the results of this analysis on the design and operation of tissue culture bioreactors are also discussed.
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Abbreviations
- d cm:
-
particle diameter
- d icm:
-
impeller diameter
- d scm:
-
average surface-to-surface spacing between particles
- E cg cm2 s−2 :
-
energy of particle-to-particle collisions
- E c,ig cm2 s−2 :
-
energy of particle to impeller collisions
- g 980 cm s−2 :
-
gravitational constant
- k cm−1 :
-
surface to volume ratio term, Eq. (10)
- l ecm:
-
eddy size
- m g:
-
particle mass
- n s−1 :
-
impeller rotational speed
- n bcm−3 :
-
number of particles per unit volume
- n B :
-
number of impeller blades
- N ccm−3 s−1 :
-
particle-particle collision frequency per unit volume
- N c,ii cm−3s−1 :
-
frequency of particle to impeller collisions per unit volume
- N p :
-
power number
- Re :
-
Reynolds number
- P g cm s −1 :
-
agitator power consumption
- r cm:
-
radial distance in spherical coordinate system
- R cm:
-
particle radius
- R icm:
-
impeller leading edge radius
- S ucm−1 :
-
surface area of beads per unit volume
- SC g cm−1 s−3 :
-
severity of particle-to-particle collisions per unit volume
- SC i g cm−1s−3 :
-
severity of particle to impeller collisions
- t s:
-
time between particle collisions
- νb,r cm s−1 :
-
root mean square relative velocity between neighboring particles
- νe cm s−1 :
-
velocity of the smallest eddies
- νcm s−1 :
-
fluid velocity
- νt cm s−1 :
-
bead terminal velocity
- νφ cm s−1 φ:
-
component of fluid velocity around a spherical particle
- νt8 cm s−1 :
-
fluid approach velocity
- V cm3 :
-
reactor liquid volume
- V b,tot cm3 :
-
total bead volume in reactor
- w cm:
-
impeller blade width
- x cm:
-
distance from impeller leading edge
- y cm:
-
distance from impeller surface
- α :
-
volume fraction occupied by particles
- β g cm−1 s−1 :
-
parameter in shear stress definition
- γ s−1 :
-
shear rate
- δ cm:
-
boundary layer thickness
- δ l,δt :
-
laminar and turbulent δ, respectively
- ɛ cm2 s−3 :
-
energy dissipation rate per unit mass
- η cm:
-
size of smallest eddies
- θ :
-
angle in spherical coordinate system
- μ g cm−1 s−1 :
-
viscosity
- ν cm2 s−1 :
-
kinematic viscosity
- ρ f g cm−3 :
-
fluid density
- ϱb g cm−3 :
-
particle density
- τ g cm−1 s−2 :
-
shear stress
- τ w,l :
-
wall τ in laminar boundary layer
- τ w,t :
-
wall τ in turbulent boundary layer
- φ :
-
angle in spherical coordinate system
- ψ :
-
stream function
- avg:
-
average
- b :
-
bead
- f :
-
fluid
- max:
-
maximum
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Cherry, R.S., Papoutsakis, E.T. Hydrodynamic effects on cells in agitated tissue culture reactors. Bioprocess Eng. 1, 29–41 (1986). https://doi.org/10.1007/BF00369462
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DOI: https://doi.org/10.1007/BF00369462