Biomedical Engineering Reference
In-Depth Information
cells which grow in suspension. However, the exact yields vary depending on the
type of cell used. It is not practicable to produce very large quantities of cells us-
ing T-flasks, due to a limited surface area and problems associated with gaseous
exchange (particularly oxygen depletion and low solubility of oxygen in the water).
As more cellular products are commercialized, increasing demand for the con-
sistent supply of clinical-grade material of tens of kilograms is necessary. Further-
more, tissue engineering strategies need improved mass transport throughout 3D
porous matrices. Many parts of the body are exposed to stresses either due to the
weight they carry (such as bone), the function they perform (such as bladder and
cartilage), or the flow of fluid (lung and blood vessels). For example, cells coloniz-
ing the bladder are constantly under a mechanical strain as the bladder cyclically
expands and deflates. Thus, it is important to grow the cells outside the body by
exposing them to the same conditions that they are exposed to within the body. For
this purpose, large-scale production is performed in devices referred to as bioreac-
tors, which provide tightly controlled environmental (e.g., pH, temperature, nutri-
ent supply, and waste removal) for cell growth. However, a number of parameters
need to be considered while scaling up a cell culture process. These include prob-
lems associated with nutrient depletion, gaseous exchange (particularly oxygen),
and the buildup of toxic byproducts such as lactic acid. Optimal fluid movement
and chemical conditions within the bioreactor are essential for the proper growth
and development of the cells and tissues. Nonuniformity in the bioreactor microen-
vironment can lead to undesirable differences in the cell growth rate. To maximize
cell growth, one must adjust the temperature and provide sufficient aeration with
gases such as oxygen and nitrogen.
7.4.1 Different Shapes of Bioreactors
Different types of bioreactors have been designed to regenerate tissues with the
intention of improving the nutrient distribution while applying mechanical stimuli.
Some of the configurations are described next.
7.4.1.1 Extend T-Flask Technology
Few designs emulate the routinely used T-flask technology in scaling up operations.
There is a 500-cm
2
triple flask, a three-tiered chamber inoculated [Figure 7.9(b)]
and fed like a traditional T-flask, with the same footprint as a T175 flask. There are
a series of 632-cm
2
stackable chambers welded together as a single unit, with com-
mon fill and vent ports connected by channels to allow liquid and air flow between
them. The mass transport of gases such as oxygen and ammonia is the greatest
impediment to static cultures in large flasks. Cells grow best when the culture me-
dium is constantly moving, allowing improved aeration at the surface. Cell growth
is increased in response to mechanical stresses such as fluid shear (see Chapter 4),
as compared to those grown under static culture conditions. Parallel plate reactors
[Figure 7.9(c)] that apply controlled mechanical forces are used as model systems
of tissue development under physiological loading conditions. The medium is per-
fused directly on the cells using a pump. The ease of monitoring flow distribution,
the ease of varying the shear stress, and the ease of nondestructively observing cells
