Fluid mixing in shaken bioreactors: Implications for scale-up predictions from microlitre-scale microbial and mammalian cell cultures

Pressures on pharmaceutical companies to speed bioprocess development have led to significant interest in small scale, parallel experimentation. A particular focus is cell cultivation and the optimisation of protein synthesis because of the number of biological and engineering variables involved. In this work, we briefly review the current understanding of mixing and mass transfer phenomena in shaken bioreactors with a view to defining criteria for the scale-up of results obtained in shaken microwell systems to conventional laboratory scale.

Scale-up approaches are illustrated for two different cell cultures. The first involves an automated microscale process (1000μl) for the aerobic fermentation of E. coli JM107:pQR706 overexpressing transketolase (TK) which is subsequently used for asymmetric carbon–carbon bond formation. The kinetics of both the fermentation and bioconversion stages are first quantified as a function of fermentation medium composition (LB or LB-glycerol) and shaking frequency with oxygen transfer rates being identified as rate limiting in certain cases.

Successful scale-up of the microwell process (in terms of maximum cell growth rate, biomass yield and specific TK activity) to a 1.4l scale mechanically stirred bioreactor is then demonstrated based on experiments performed at constant kLa values. The second process investigated involved antibody production in suspension cultures of VPM8 hybridoma cells.

Initial results suggest that experiments performed at constant mean energy dissipation rates provide a satisfactory basis for scale translation from shaken microwells (800μl) to conical flasks (100ml) and are indicative of results obtained in a mechanically stirred bioreactor (3.5l). Overall this work provides an initial insight into the engineering characterisation of shaken bioreactors and how key parameters may be used to define suitable scale-up criteria for different cell cultures.