Biomedical Engineering Reference
In-Depth Information
dynamic reaction period, cells, substrates (including the nutrient salts and vita-
mins), and concentrations of the products vary with time. Proper mixing keeps the
differences in composition and temperature at acceptable levels. Cell growth ceases
due to an accumulation of toxic metabolites and a depletion of nutrients. The dis-
advantages include: (1) a decreased productivity due to time for filling, heating,
sterilizing, cooling, emptying, and cleaning the reactor, (2) a large expense incurred
in preparing several subcultures for inoculation, (3) an increased labor cost, and (4)
an increased focus on instrumentation due to frequent sterilization.
7.4.2.2 Fed-Batch Bioreactor
Eliminating the cell-growth restriction by a nutrient supply is replenishing depleted
nutrients to enable the production of cellular products for a prolonged period of
time. This is the principle of fed-batch operation [Figure 7.11(b)], which, in gen-
eral, produces antibodies at a higher concentration due to the accumulation of
the product. A more frequently used method is initiating the reactor in the batch
mode until the growth-limiting substrate has been consumed. Then the substrate
is fed to the reactor as specified (batch) or is maintained by an extended culture
period (continuous). For a secondary metabolite production, in which cell growth
and product formation often occur in separate phases, the substrate is typically
added at a specified rate. This mode of operation provides an increased opportunity
for optimizing environmental conditions in regard to the growth phase or the
production and the age of the culture. The volume throughput per run is restricted
by the size of a bioreactor and is rate-limited by its prolonged operation time. To
compensate for the low-volume throughput, a much larger volume bioreactor is
often required. The controlled feeding of key nutrients is often performed and the
nutrient concentration in cultures is maintained at low levels.
7.4.2.3 Perfusion Bioreactors
In the perfusion mode, cells are retained by some type of retention device, whereas
the cell-free medium containing the product is removed continuously from the bio-
reactor. A fresh medium is added to maintain a constant volume [Figure 7.11(c)]. As
a result, nutrients are constantly replenished, and toxic metabolites are removed. By
comparison, a perfusion operation can produce a large volume of product from a
size-limited bioreactor on a continuous basis. However, the product concentration
is lower than that in a fed-batch operation due to the dilution effects resulting
from a constant medium replenishment. The specific metabolite production rate is
estimated from the expressions:
dC
(7.29)
dt
=−
qX
pC
where
C
is the metabolite concentration,
p
is the normalized perfusion rate (1/day),
and
q
is the specific metabolite production rate. The perfusion rate (also referred
to as the dilution rate) is calculated as the ratio of volumetric flow of the media to
