Bioreactor engineering fundamentals for stem cell manufacturing

Cell-based therapies have the potential to address unmet healthcare needs and improve quality of patient care; effective manufacture is therefore essential. There are, however, many challenges to overcome before this can become a reality and a better understanding of the manufacturing requirements for cell-based products is required. A range of stem cell types are considered as useful cell therapy candidates for various reasons including human embryonic stem cells, induced pluripotent stem cells, and human mesenchymal stem cells (hMSCs). While all have potential as cell-replacement therapies, hMSCs are of particular interest because of their additional immunomodulatory and immune-evasive properties. Until recently, these cells were cultured as a monolayer in tissue culture-treated flasks. However, to produce the number of cells required to meet economically potential demand, it is important to provide greater surface area per unit volume of medium on which the cells can grow. This need has led to the use of microcarriers, particles of some 200. ce:hsp sp=0.25/μm in diameter on which the cells can grow in suspension-based bioreactor systems. After growth, with medium exchange and passaging, the cells have to be harvested. Given the flexibility of stirred tank reactors and the extensive knowledge available from studies of particle suspension within them and their use in free suspension culture, they have become the configuration of choice for culturing adherent cells at increasing scale. This chapter discusses, in detail, the underlying basic principles of stirred tank bioreactors including particle suspension, mass transfer, and the fluid dynamic stresses, which potentially might damage cells. These basic concepts are then related to the cultivation of hMSCs on microcarriers, which has been demonstrated up to the 1000. L scale. It is also shown how concepts from crystallization in stirred tank reactors involving microcarrier-microcarrier and microcarrier-impeller impacts have enabled a successful, insitu harvesting strategy to be developed. The overall concept of growing hMSCs at the minimum agitator speed required for suspension and harvesting them by a short burst of intense agitation in the same bioreactor is introduced. Finally, data are presented showing the success of that approach in 22 combinations involving bioreactors from 15. mL to 5. L with different cell donors, microcarriers (with and without coatings), and detachment enzymes.