4.4.2 Adherent growth (w/ microcarriers) in a stirred tank
bioreactor
Use of microcarriers increase the efficacy of stirred tank bioreactors
for a number of reasons, including reducing media requirements
[31,32]. Yet, microcarrier utilization does have the drawback that
attached cells become particularly susceptible to damage from agitation
[32]. To model the utility of microcarriers of varying geometries
will require representing individual cells as agents adhering to a
microcarrier also represented as a distinct agent. In the literature,
work by Croughan et al. [32] identified and mechanistically modeled
some of the hydrodynamic effects in microcarrier cultures and sought to
quantitatively correlate these with fluid dynamic properties. The
rationale underpinning this work was that successful scale up of a
microcarrier-based stirred bioreactor system would require an in depth
knowledge of the mass transport and hydrodynamic phenomena. The critical
parameters to assess hydrodynamic effects on animal cells grown on
microcarriers in suspension cultures are the Reynolds number, Kolmogorov
eddy length, maximum mean aggregate size and maximum shear stress
[33].
In this work by Croughan et al. [32], the researchers found
individual cells on microcarriers are most likely damaged by small
intense eddies that can affect individual cells, but are too small to
move individual microcarriers. Overall, these researchers made scale up
recommendations/predictions for microcarriers in suspension system based
on their thorough analyses - i) if scale up at constant power input per
unit volume is employed with vessels that are not geometrically similar,
one should consider the role of vessel and impeller geometry in terms of
the power requirements for suspension and regional distribution of power
generation and ii) scale up at constant time-averaged outer radial shear
rate should not lead to detrimental effects from hydrodynamic forces.
Additional relevant research detailed a high aspect ratio vessel (HARV)
that was developed to study tissue and cellular engineering in a
low-shear, non-turbulent, simulated microgravity environment [34].
While not a microcarrier-based stirred tank system per se, microcarrier
beads are co-injected with cells into the HARV system and rotation is
initiated at a desired angular velocity; therefore, the computational
modeling of microcarriers in this study is of considerable value. Here,
computational modeling of microcarriers is used to recapitulate observed
behaviors where microcarriers with density greater than the surrounding
fluid medium follow a circular motion relative to the culture medium
combined with a persistent migration and eventual collision with the
outer wall of the reactor; whereas microcarriers with a density less
than the fluid medium follow a circular motion migrating toward the
central region of the reactor. When multiple microcarrier beads that are
lighter than water are inserted into the reactor, the centrally directed
migration results in the formation of clusters that are stabilized by
tissue bridges formed by osteoblasts seeded onto the microcarriers. We
propose to follow a similar modeling paradigm to understand the effects
of high-density on both flow and bridging behaviors.