Application Note - Real-Time DO Control Thanks to Optical Sensors

In-line Measurement of Critical Process Parameters

Manufacturing of biobased products such as biochemicals and biopharmaceuticals is a complex process. Real-time control of fermentation’s critical parameters is paramount to make them more reproducible and ultimately optimize product yield, whether the production is for agricultural, chemical, or pharmaceutical applications. In order to identify such optimal conditions, it is a common strategy to perform scale-up and scale-down studies to simulate on a smaller scale what can then be implemented at a larger scale. Focusing just on the biopharmaceutical industry, multiple parameters are monitored in order to conduct statistically-driven bioprocess optimization, to comply with current regulatory bodies' initiatives aimed at pushing innovation, such as PAT (Process Analytical Technologies). Among those are the CPPs (Critical Control Parameters), most important because their variability has a direct impact on the product’s therapeutic properties. Therefore, CPPs should be controlled in real-time to ensure the process delivers the desired product quality.

Microbial fermentations, for example, go through lag and log phase of growth within hours, which is why CPPs such as pH, temperature, feed rates and most important dissolved oxygen (DO) need to be continuously measured and controlled in order to push the process to maximum yield performances. Regarding DO, for the growth of microbials such as the yeast Saccharomyces cerevisiae, the oxygen introduced by air sparging in the bioreactor, is used, among other functions, to synthesize, for example, sterols and unsaturated fatty acids (UFAs) in lag phase.

One possibility, for automated air (therefore oxygen) addition is to perform cascade control around a set-point using the signal produced by in-line DO sensors. Of course, this is only possible under the assumption that the CPP measurements remain accurate and precise during the entire process: in other words, signal variations should come only from fermentation changes and not from the sensor drifting or not working properly.

In this application note, it will be clarified why the Scottish biotechnology company Ingenza, which operates GMP compliant laboratories for the construction, optimization and application of engineered microbial strains, has decided to move from electrochemical to optical sensors, in order to perform reliable in-situ automated control of DO for microbial cultures such as E. coli, P. putida, Bacillus sp, P. pastoris and S. cerevisiae, in laboratory scale bioreactors.

Figure 1: Detail of Hamilton VisiFerm ECS sensor into the Ingenza’s Electrolab Biotech Bioreactor FerMac 320.

Dissolved Oxygen Control of Bioprocesses with Optical Sensors

The traditional way to measure DO in a process was through polarographic sensors. They operate on the principle of reduction of oxygen diffusing through a membrane at the surface of a noble-metal electrode (cathode). This is the membrane covered electrode introduced by Clark. These sensors have some disadvantages such as:

• Long response time and need to wait hours for sensor polarization before the measurement can start
• Electrostatic and mechanical interferences to the signal, since they are normally analog-sensors
• The tip of the sensor’s membrane can get easily damaged 

Hamilton’s VisiFerm DO ECS optical dissolved oxygen sensors offer users like Ingenza a simple way to upgrade from traditional polarographic sensors and avoid the performance issues listed above. 

The sensor measurement principle offers increased accuracy and reduced maintenance while the ECS (Electrochemical Signal) output can be directly input into controllers or transmitters that accept the traditional nano-amp signal of a polarographic oxygen sensor. ECS signal is a smart solution to equip older bioreactors that require a nanoAmp (nA) sensor output with a digital simulation of such signal allowing for direct replacement of old-technology sensors with all the benefits of newer optical sensors.

In-line Optical DO Sensors into Ingenza R&D Laboratory

Ingenza cooperated with Hamilton in order to plug the signal from VisiFerm sensors directly into their control towers, so that they are linked to the bioreactor RPM and aeration on a cascade set-point. This enabled Ingenza to bring the benefit of optical DO sensors into their existing bioreactor (see Figure 1).
According to Alison Arnold, Ingenza’s Head of Fermentation and Microbiology, besides the robustness of the digital signal, the main benefits of such a solution, compared to an analog one, are the following:

• Time-saving: no need to wait for polarization time after autoclaving the sensor
• No downtime risk: the optical sensor membrane can survive all fermentation processes
• Automated efficiency: precise set-point control of dissolved oxygen due to an accurate digital signal sent from the sensor directly to the control tower
   

These benefits enable Ingenza to perform their studies with total safety regarding in-line DO measurement reliability and therefore enable real-time control strategies of CPPs. This makes the company’s fermentation processes more efficient: shorter processes generate a quicker turnaround in time and therefore provide financial improvements.

Author
Alison Arnold, Head of Fermentation and Microbiology

Company
Ingenza Ltd

Roslin Innovation Centre
Charnock Bradley Building
Easter Bush Campus,
Bush Farm Road
Roslin
EH25 9RG.

Phone: +44(0)131 651 9681
[email protected]
www.ingenza.com