Multiphase Bioreactor Design

Redox potential is measured potentiometrically with electrodes made of noble metals
(Pt, Au). The mechanical construction is similar to those of pH electrodes. Accordingly,
the reference electrode must meet the same requirements.

Biomass

Since an on-line generated signal for biomass concentration is decisive for control
purposes a series of sensors and methods that can be automated have appeared in recent
decades. Many of them rely on optical measuring principles, others exploit filtration
characteristics, density changes of the suspension as a consequence of cells, or
(di)electrical properties of suspended cells.
The nowadays commercially available OD-sensors are based on the determination of
either transmission, reflection or scatter of light, or a combination thereof. The theoretical
backgrounds why these OD measurements reflect the biomass concentration are rather
manifold, complicated and would constrain the application tremendously if few
simplifications could be reasonably applied. A direct a priori calculation of dry weight
concentration from any OD measurement must not be expected as realistic, but the
systems can be calibrated from case to case.
The measurement principle of the biomonitor (formerly called ßugmeter) relies on the
fact that the capacitance of a suspension at low radio frequencies is correlated with the
concentration of the suspended phase of fluid elements that are enclosed by a polarisable
membrane, i.e. intact cells. The capacitance range covered is from 0.1 to 200 pF, the
radio frequency some 200 kHz to 10 MHz. A severe limit to this principle is the
maximally acceptable conductivity.
Bioreactions are exothermic. The net heat released during growth represents the sum
of the many enzymatic reactions involved. A completely non-invasive method is to
exploit the heat generated during growth and other metabolic activities of organisms
which is also proportional to the amount of active cells in a reaction system (Boe and
Lovrien, 1990). Under well defined conditions, calorimetry can be an excellent method
for the estimation of total (active) biomass, even for such slow growing organisms as
hybridoma cells or for anaerobic bacteria growing with an extremely low biomass yield.
The method is so inherently sensitive that cell cycle dependent events can be analysed as
well.

Interfacing to ex-situ measurements

Samples removed from the reactor in some way can be analysed with devices that are not
(yet) suitable or available to be mounted in situ, but this is a reasonable work-around.
Depending on the analyte of interest, i.e. whether it is soluble or (in) the dispersed phase,
one needs to sample either the entire culture liquid or just the supernatant. The latter can
be acquired using, for instance, a filter. In this case, the filter is usually also the sterile
barrier.
When the dispersed phase, usually but not necessarily the cells, is of interest, no
separation of phases must take place during sampling. The system must be opened in a
way that allows no infections to enter the reaction space neither during sampling nor
between the sampling events.

Multiphase bioreactor design 62