Biomedical Engineering Reference
In-Depth Information
Direct Instrument Interface with Sample Stream.
The most common exam-
ple of an instrument that connects directly to a bioreactor or fermenter is a gas
chromatographic mass spectrometer. Progress has also been made in directly coupling
bioanalyzers or flow injection analysis units to bioreactors. Each of these systems is
capable of measuring off-gas analysis, such as CO
2
(waste aerobic cell cultures) and O
2
that is nutrient in aerobic cell cultures and used for calculations in oxygen uptake rate
(OUR). There are generally two methods of determining OUR: the first is calculated
(K
L
a and total air sparge) and the second is empirically determined (mass spectro-
scopy-based gas analysis). For the calculated, two assumptions are made that contribute
to high process variability; the first is that K
L
a for cleanwater and the culture broth are the
same. The second is the assumption of a pseudosteady-state where OUR is equal to OTR
(oxygen transfer rate). Oxygen transfer contributes to process variability by affecting the
overall cells' metabolic ability to convert carbon sources to energy in the formof ATP via
the Krebs cycle. Since these gases are used to monitor the overall health of a bioreactor
during the course of a multiple day run, it is worth exploring the option of empirically
determining these values to more precisely control the process.
In addition, bioanalyzers are capable of generating other valuable information with
respect to metabolites and waste products that may be used to control bioreactors. The
instruments use two types of electrochemical analyzers: potentiometric and ampero-
metric. Potentiometric sensors provide basic parametric measurements, such as pH, and
measure various other metabolic waste products such as ammonium, sodium, potassium,
and acetate. Amperometric sensors are the real heart of bioanalyzers and are configured to
measure metabolites and nutrients such as glutamine, glutamate, glucose, and lactate,
amongothers. Insome instruments, aphotometric sensor isused todetect compounds such
as phosphatesorglycerol.Keeping the correct balanceof essential nutrients andmetabolic
waste products can be optimized with such systems that allow control feedback loops to
adjust timing and amounts of nutrients delivered during the bioreactor runs. Without this
continuous stream of feedback, the result is the fed-batch paradigm that usually leads to
higher process variability and often higher product quality attribute variability.
12.3 PURIFICATION PAT TOOLS
12.3.1 Adaptive Control of Buffer Concentrate Dilution
and Chromatography Skids
PLC-controlled blending and dilution skids are commercially available that perform
precision in-line dilution of buffer concentrates often using the plantWFI system. Sensors
are used tomeasure the dilution or blending and adaptively control the blending streams to
arrive at the desired concentration or blend ratio. This level of precision allows tighter
control of conductivity and pH during the course of separation thus allowing less noise in
the UV trace. These technologies also offer the ability to minimize manufacturing plant
footprint by reducing the need for large storage tanks.
Similar technology is used to control process chromatography skids for higher
precisionmixing during isocratic, step, and gradient elutionwith the additional benefit of
