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.