Biomedical Engineering Reference
In-Depth Information
transmitted to the anode (working, counter, and reference electrode). This
three-electrode system ensures a reliable electrical signal with long-term sta-
bility. The chemical components, including the enzyme, are deposited onto
the active field of the working electrode, and the enzyme is cross-linked to
form protein and hence is immobilized in this network. The immobilization
process itself has an antimicrobial effect. A dialysis membrane is cast over
the sensing head to create a barrier between the sensor and the cultivation
medium.
Pressure Sensors
Another important process parameter that is frequently monitored during
bioprocess unit operations such as filtration, chromatography, and many
others is pressure. Using a traditional stainless steel pressure gauge in con-
junction with a disposable experimental setup is possible, but has the draw-
back that the pressure gauge has to be sterilized separately. Furthermore,
the connection of the sensor to the previously gamma-radiated disposable
assembly can be problematic.
Many bioprocess unit operations are either controlled based on pressure
or have significant pressure-related safety issues. Traditional stainless steel
reactors are monitored and controlled for pressure, as pressure is used as a
means of influencing mass transfer and preventing contamination. In addi-
tion, a high-pressure event is a potentially hazardous situation. Single-use
bioreactor systems, on the other hand, are frequently not monitored or con-
trolled for pressure because stainless steel pressure transducers are not com-
patible or cost effective when applied to disposable bioreactors. As a result,
a clogged vent filter on a bioreactor can easily rupture bags, spilling the con-
tents of the reactor and exposing the operators to unprocessed bulk.
Another application where pressure monitoring is central to process per-
formance is depth and sterile filtration. A filter's capacity is primarily mea-
sured by either flow decay or pressure increases, although adding reusable
traditional pressure transducers to a process train defeats the purpose of a
single-use process setup. Depending on the process application, the product
contact surface of a traditional device requires either sanitization or moist
heat sterilization.
There are traditional devices that are compatible with steam in place (SIP),
where only the product contact surface is exposed to steam, and even devices
that can be placed in an autoclave where the entire device is exposed to
steam. Many single-use process components, however, are not compatible
with moist heat sterilization temperatures, so there may be a requirement
for separate sterilization of the stainless steel device and possibly less than
optimal connection to a presterilized disposable assembly.
