SCALING UP ADHERENT STEM-CELL PRODUCTION
There are currently three main options available for scaling up adherent cell production. These include keeping a 2D surface
with some form of plate-based system, using microcarriers in stirred tank bioreactors, where the cells grow on the surface
of the microbeads, or using some form of three-dimensional (3D) technology where a sophisticated scaffold is used to hold
the cells, such as a microfiber or a hollow fiber.
There are two main criteria that must be considered when selecting a system for stem-cell bioprocessing and recovery. The
growth, integrity, and quality of the cells must be preserved and controlled, and it must be possible to harvest and recover
the cells smoothly.
For scale-up, it is not as simple as merely providing a larger surface area. Fragile, adherent stem cells are sensitive to
the niche microenvironment, which can affect the way they differentiate. For cell-therapy applications, it is vital to prevent
the cells from making unwanted differentiations.
Changing the niche environment in which they grow by altering the surface can have a large effect on the cell behavior. Stem-cell
culture and differentiation are sensitive to many different factors. These factors include the physicochemical environment,
pH, dissolved oxygen, shear stress, and the concentration of metabolites within the culture. The surface composition and geometry
affect the cells' connections and the extracellular matrix. The cell density can also have an effect.
Another point that must be considered is the ability to recover the stem cells smoothly. Most of the systems used to grow
cell lines were designed for the production of vaccines or proteins, with no need to harvest the cells at the end of the process.
Overall, scale-up from the R&D laboratories to an industrial process requires the ability to control the physicochemical parameters,
to minimize the change in the cell surface, and to monitor cell density. The following sections describe three common scale-up
The main feature of a plate-based system, such as the ATMI LifeSciences Integrity Xpansion multiplate bioreactor, is that
the surface remains flat and is made from hydrophilized polystyrene. The plates are similar to those on which stem cells are
grown on multiplate stacks in the laboratory. This bioreactor offers a compact version of a laboratory multiplate stack and
enables one to replace multiple devices by a single container. One Xpansion 200 plate offers a surface of 120,000 cm2 (equivalent to 20 standard stacks of 10 plates) in a 60x35 cm area. In this model, the bioreactor enables stem-cell expansion
in a closed system with fewer operations needed. The gas exchange is not between the plates, but rather in a central column
with channels along the plates through which the medium circulates, consequently reducing the footprint.
Temperature, pH, and dissolved oxygen levels are all carefully monitored and controlled. Light microscopy is then used to
study the morphology of the cells and enable cell density to be calculated.
Harvesting the cells is also straightforward. To remove them from the plates, a solution of an enzyme, such as trypsin, is
added to separate the cells from the surface and prepare them for concentration and washing.
Multiplate-based technologies provide one solution for both large autologous batches and small, allogenic batches containing
a few billions of cells per batch.