Biomedical Engineering Reference
In-Depth Information
under a light microscope are attractive features of parallel flow reactors. They are
widely used to study cell migration, function, and proliferation in cardiovascular
applications. However, the major drawback is related to the scale-up issues, spe-
cifically the total volume of media required to perfuse all the cells. The volume of
the medium required is directly proportional to the total surface area on which the
required number of adherent cells is growing. Although these limitations have been
addressed by stacking the plates, it is not an attractive configuration if the focus is
secreted products.
7.4.1.2 Roller Bottles
A gentle stirring or rocking may be ideal for cells that grow in suspension in a small
scale, but many cells grow only on a substrate. Roller bottles [Figure 7.10(a)] are
developed converting T-flasks to bottles whose entire inner surface is available for
cell growth. A special apparatus slowly rotates (between 5 and 60 revolutions per
hour) the bottles, and the cells are alternately bathed in the medium and exposed to
the atmosphere, which in turn allows for a larger usable surface area and increased
oxygenation. The associated mixing also prevents formation of concentration gra-
dients of nutrients. Most roller bottles accommodate standard-sized (~ 1,050 cm
2
)
bottles. Expanded surface roller bottles that use different patterns to obtain a larger
surface (~4,200 cm
2
) are developed. However, the size of the roller bottles is a
problem since they are difficult to handle inside the biological safety cabinet. Also,
uniform cell seeding and the inability to routinely observe cell cultures under light
microscopy are additional problems.
7.4.1.3 Spinner Flasks
For cells that have been engineered to grow in suspension, the simplest device is a
spinner flask [Figure 7.10(b)]. It is a wide cylinder with ports for the addition and
removal of cells and media and gases with CO
2
enriched air. To achieve a sufficient
air exchange, tanks are constantly agitated by magnetic stir bars of different shapes
and are either kept small enough for the headspace to provide ample oxygen or are
equipped with an aeration device such as a sparger to provide a continuous flow of
oxygen. Spinner flasks have been used for culturing cells inside porous 3D matrices.
During seeding, cells are transported to and into the scaffold by convection. During
culture, stirring enhances an external mass transfer but also generates turbulent
eddies, which could be detrimental for the development of the tissue. Spinner flask
systems designed to handle culture volumes of 1 to 12 liters are available. For large-
scale operations, spinner flask technology is extended into large bioreactors where
cells are kept in suspension by either a propeller in the base of the chamber vessel
or by air bubbling through the culture vessel. However, excessive fluid-shear or
turbulence damages cells. It is important to determine and control the fluid stress
level inside the bioreactor.
For culturing adherent cells, microcarrier (30-200
m in diameter) beads (ei-
ther porous or smooth surface) have been developed. These offer a large surface
area per unit volume onto which cells can attach and grow. This technology has a
vastly expanded cell culture capacity. The surface properties of the beads should
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