Compared to chemically produced substances, biotech products are special in two aspects: the molecules are large, and the products are manufactured using living organisms. The main goal of biomanufacturing is to supply the products safely and cost efficiently.
Process analytics during fermentation serves to maintain consistent living conditions for the suspended cells or production microorganism. This includes monitoring and control of the physicochemical environment such as pH, dissolved oxygen, and dissolved carbon dioxide.
All in-line sensors in bioprocess engineering must be sterilizable. The sensors must be stable and reliable over days or even weeks, and they should have low-maintenance requirements in order to be attractive for industrial use.
pH is mostly identified as a key process parameter in fermentation processes because enzymatic activities and cellular metabolism are sensitive to pH changes. In-line pH measurement has been quite common for many years but is not without its risks. Should the electrode fail during a batch, the pH must be measured with off-line samples, which carries a risk of contamination and is labor intensive.
Mettler Toledo has developed a pH measurement system that not only eliminates the risk of electrode failure during production but also meets all stringent pharmaceutical requirements. The newest generation of Mettler Toledo pH electrodes features Intelligent Sensor Management (ISM) technology. A microchip within the head of the electrode continuously monitors the status of the sensor.
Can a sensor be used for the next batch? This is a crucial question for ensuring safe operation. Mettler Toledo’s M700 transmitter displays a sensor network diagram that shows the “health” of the connected sensors. If all corner points on the diagram lie to the extremes of the network the electrode is qualified for next batch operation. Corner points lying closer to the center indicate that maintenance is required. The sensor network diagram allows the status of an electrode to be ascertained without toggling through menus.
iSense Asset Suite
The number of sterilization cycles and cleaning cycles that the electrode has been exposed to are automatically counted for easy traceability of all sensors.
The PC-based iSense Asset Suite displays how many times an electrode has been sterilized and cleaned (Figure). In this example the electrode was sterilized 75 times, leading to increased glass membrane impedance. Electrode reactivation or replacement is recommended for safe measurement in future batches.
The microchip includes an analog-to-digital converter, as low impedance, digital transmission of the sensor signal is reliable, even in humid environments. ISM electrodes store calibration data inside the microchip and can be precalibrated outside the clean zone.
iSense Asset Suite stores data on all electrodes and sensors (pH, dissolved oxygen, and conductivity) allowing full traceability. The new ISM pH electrodes can also measure the redox potential simultaneously.
The redox potential is commonly used to maintain anaerobic conditions in a fermentation broth. It can be used to measure trace amounts of dissolved oxygen. Glucose-containing feed medium can be treated as a reducing source in oxidation-reduction of the culture medium. When the oxidation capacity is increased, the redox potential level will elevate to a higher value. Likewise, its value will become lower when the culture broth has a higher reducing capacity.
Beyond temperature, agitation, and pH, dissolved oxygen is another key process parameter. Neglecting control of these parameters could potentially impact final product quality, so in-line measurement can be employed to maintain the culture in its optimal state.
Cell cultures require oxygen for the production of energy from organic carbon sources—e.g., glucose. Given oxygen’s poor solubility in water, the control of oxygen flow is carefully regulated to ensure it does not become a rate-limiting factor in the process.
In contrast, a hyperoxygenated bioreactor air supply can irreversibly impact culture performance. The pO2 value is usually monitored with an amperometric, membrane-covered electrode (Clark type). Given the nature of amperometric DO probes, it is necessary for them to be polarized prior to their use.