Controls - Disposable Bioprocessing Systems - page 144

The YSI 8500 CO 2 monitor measures dissolved carbon dioxide in biopro-
cess development applications. Engineered to fit within a variety of bioreac-
tors, the unit delivers precise, real-time data that increase an understanding
of critical fermentation and cell culture processes. This data can help in gain-
ing insight into cell metabolism, cell culture productivity, and other changes
within bioreactors.
An in situ monitor based on the reliable optochemical technology was
developed by Tufts University and YSI Incorporated ( www.ysilifesciences.
com ) . The technology involves the use of a CO 2 sensor capsule consisting of
a small reservoir of bicarbonate buffer covered by a gas-permeable silicone
membrane. The buffer contains hydroxypyrene trisulfonic acid (HPTS), a
pH-sensitive fluorescent dye. CO 2 diffuses through the membrane into the
buffer, changing its pH. As the pH changes, the fluorescence of the dye
changes. The model 8500 monitor compares the fluorescence of the dye at
two different wavelengths to determine the CO 2 concentration of the sam-
ple medium. The sensor can be autoclaved multiple times. It will measure
dissolved CO 2 over the range of 1% to 25%, with an accuracy of 5 % of
the reading, or 0.2% absolute. Previously, CO 2 was measured either in the
exit gases from the fermentation process or by taking a manual sample.
The new optical-chemical technology uses a fiber-optic cable transfer light
through a stainless steel probe into a disposable sensor capsule, which con-
tains a pH sensitive dye. The dissolved CO 2 diffuses through a polymer
membrane to change the color of the dye, which is then relayed by fiber-
optic cable back to a rack-mounted monitor that determines and displays
the dissolved CO 2 level.
Biomass Sensors
Information about the biomass concentration can also be obtained via turbid-
ity sensors. Generally, these sensors are based on the principle of scattered
light. Most turbidity sensors have the disadvantage that there is only a linear
correlation for low particle concentrations. But sensors that use backscatter-
ing light (180°) also have linear properties for high particle concentrations.
A window that is translucent for the desired wavelength in the IR region is
necessary for the use in disposable reactors. The S3 Mini-Remote Futura line
of biomass detectors ( www.applikonbio.com ) makes it possible to incorpo-
rate sensors inside disposable bioreactors. This system incorporates an ultra
lightweight preamplifier for connecting to the Aber Instrument Company
(ABER) disposable probe. The main Futura housing can be mounted away
from the single-use bioreactor vessel. Cells with intact plasma membranes in
a fermenter can be considered to act as tiny capacitors under the influence of
an electric field. The nonconducting nature of the plasma membrane allows