Non‐Newtonian rheology in suspension cell cultures significantly impacts bioreactor shear stress quantification

• Published in: Biotechnology and bioengineering 2018-08, Vol.115 (8), p.2101-2113
• Main Authors: Wyma, Alex, Martin‐Alarcon, Leonardo, Walsh, Tylor, Schmidt, Tannin A., Gates, Ian D., Kallos, Michael S.
• Format: Article
• Language: English
• Subjects: Bioreactors; Cell culture; Cell suspensions; Conditioning; Growth kinetics; Kinetics; Proteins; Regenerative medicine; Rheological properties; Rheology; Shear stress; Shear thinning (liquids); stem cell; Stem cells; stirred suspension bioreactor; Tissue engineering; Viscosity
• ISSN: 0006-3592; 1097-0290
• DOI: 10.1002/bit.26723

The fields of regenerative medicine and tissue engineering require large‐scale manufacturing of stem cells for both therapy and recombinant protein production, which is often achieved by culturing cells in stirred suspension bioreactors. The rheology of cell suspensions cultured in stirred suspension bioreactors is critical to cell growth and protein production, as elevated exposure to shear stress has been linked to changes in growth kinetics and genetic expression for many common cell types. Currently, little is understood on the rheology of cell suspensions cultured in stirred suspension bioreactors. In this study, we present the impact of three common cell culture parameters, serum content, cell presence, and culture age, on the rheology of a model cell line cultured in stirred suspension bioreactors. The results reveal that cultures containing cells, serum, or combinations thereof are highly shear thinning, whereas conditioned and unconditioned culture medium without serum are both Newtonian. Non‐Newtonian viscosity was modeled using a Sisko model, which provided insight on structural mechanisms driving the rheological behavior of these cell suspensions. A comparison of shear stress estimated by using Newtonian and Sisko relationships demonstrated that assuming Newtonian viscosity underpredicts both mean and maximum shear stress in stirred suspension bioreactors. Non‐Newtonian viscosity models reported maximum shear stresses exceeding those required to induce changes in genetic expression in common cell types, whereas Newtonian models did not. These findings indicate that traditional shear stress quantification of cell or serum suspensions is inadequate and that shear stress quantification methods based on non‐Newtonian viscosity must be developed to accurately quantify shear stress. The level of shear stress experienced by stem cells in bioprocessing applications can have a profound impact on bioprocess quality. In this study we demonstrated that suspensions of fetal bovine serum, single cells, or combinations of the two have non‐Newtonian viscosity. Shear stress predictions determined using non‐Newtonian viscosity were profoundly different compared to shear stress calculated with Newtonian viscosity. It was shown that non‐Newtonian shear stress predictions present an improved metric to ensure bioprocess quality.