Microcarrier systems
The most commonly used large-scale technology involves microcarriers. Here, the surface on which the cells grow is in the
form of microbeads, which are suspended in the culture medium within a stirred tank bioreactor. However, the environment is
modified, because continuous stirring increases the shear stress on the cells, and this leaves no guarantee that the morphology
of the cells will remain the same as when the cells are grown on static surface, such as plates.
Not all stirred tanks are suitable for use with microcarriers because it is imperative that the culture must be agitated very
smoothly and gently to ensure that the microbeads do not break, and shear stress must be minimized. To address this, a few
stirred tank bioreactors have been designed with mixing technology that provides efficient mixing with minimal agitation and
lower shear stress.
One drawback of microbeads is the seeding step. While they have great potential at a large scale, to be able to run a 200-L
bioreactor, it is necessary to have gone through several scale-up steps, with runs at 20 L and 50 L. Multiple scale-up steps
add to the complexity of the seeding and passages steps. Another drawback of the stirred tank is a technological one: harvesting
the cells. A sophisticated and gentle purification step becomes essential to separate the cells from the beads without damaging
or changing them.
Microbeads represent a solution for large-scale production, but process development is highly complex and takes time, and
cell observation is complex. The microcarrier technology, on the other hand, optimizes capacity and is promising for large
allogenic batches that have more than hundreds of billion cells per batch.
Packed-bed bioreactors
Another alternative for adherent cell growth is a packed-bed bioreactor. This type of bioreactor has a 3D scaffold structure
based on a specific material such as nonwoven microfibers, a biopolymer, or a hydrogel, and the cells are entrapped in this
fixed bed. A drawback is the difficulty of observing the cells as they grow. This technology provides a good environment for
the cells to grow in, but because of its 3D nature, there is an even greater possibility that the morphology of the stem cells
will be different and as a result, a significant amount of validation is required to ensure the final production process is
both robust and well defined. A further drawback of this technology comes with the harvesting process. During harvesting,
the cells need to be removed from the scaffold.
It has been proven that the cells grow in hollow fiber-based systems because they grow within the fibers, limiting the shear
stress. It is notable that these fibers are limited in terms of scale, and the pH and dissolved oxygen within them may not
be controlled. However, if the cells are particularly sensitive to shear stress, packed-bed bioreactors offer advantages over
stirred tanks.
The common drawback across all of these systems is that the surface has changed from 2D to 3D, which may lead to different
behavior in the cells. However, biopolymer, hydrogel, and innovative scaffolds may a means of solving the drawback of allogeneic
therapy and large-scale stem cell expansion.
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