Biomedical Engineering Reference
In-Depth Information
a build up of charge. The resulting capacitance can be measured: it is depen-
dent upon the cell type and is directly proportional to the membrane-bound
volume of these viable cells. The choice of
in situ steam-sterilizable probes
includes a single-use, sterilizable flow through the cell.
Electrochemical Sensors
Electrochemical sensors include potentiometric, conductometric, and volta-
metric sensors. Thick- and thin-film sensors, as well as chemically sensitive
field-effect transistors (ChemFETs), possess potential as potentiometric dis-
posable sensors in bioprocess control because they can be produced inex-
pensively and in large quantities.
Many pH-sensing systems rely on amperometric methods, but they require
constant calibration due to instability or drift. The setups of most ampero-
metic sensors are based on the pH-dependent selectivity of membranes or
films on the electrode surface.
While turbidity sensors detect the total amount of biomass concentration,
capacitance sensors provide information specifically about the viable cell
mass. The electrical properties of cells in an alternating electrical field are
generally characterized by an electrical capacitance and conductance. The
integrity of the cell membrane exerts a significant influence on the electri-
cal impedance, so that only viable cells can be estimated. The Biodis Series
for monitoring viable biomass in disposables applications is available from
Fogale (
www.fogalebiotech.com
)
and Aber (
www.aberinstruments.com
),
the latter now offers an integrated version with Dagsip Biotools Company
(DASGIP) (
www.dasgip.de
)
. The new Aber Futura Biomass Monitor has
been designed so that multiple units can easily be incorporated into bio-
reactor controllers and supervisory control and data acquisition (SCADA)
systems.
The sensor CITSens Bio (
http://www.c-cit.ch/
)
can monitor the consump-
tion of glucose and/or the production of l-lactate during cultivation. The
CITSens Bio utilizes an enzymatic oxidation process and electron transfer
from glucose or lactate to the electrode (anode) via a chemical wiring pro-
cess, which is catalyzed by an enzyme specific for ~-d-glucose or l-lactate
and a mediator. The sensor function is therefore not affected by oxygen con-
centration and produces an exceptionally low concentration of side prod-
ucts, such as peroxide. The working principle of this sensor is in contrast to
that of a number of well-known alternatives currently on the market, which
depend on a sufficient supply of oxygen for their operation as they mea-
sure the hydrogen peroxide produced during the bioprocess. The principal
feature of the CITSens Bio is a miniaturized, screen-printed electrode com-
prising a three-electrode system for amperometric detection of the current
