Sensors are an important part of bioreactor process control, controlling temperature, pH, dissolved oxygen and stirring speed. As bioreactor design and control have advanced, the requirements for sensing technology have also changed. For example, single-use bioreactors (SUB, Single-use bioreactor) are becoming increasingly popular, and disposable solutions for pH and DO have also been launched. However, most of the sensors currently used for SUBs are still used in traditional stainless steel bioreactors, which to some extent are not suitable for SUBs design.
pH sensor: Culture pH is a critical variable in bioreactor operation. pH sensing technology can be roughly divided into the following categories: porous glass electrolyte-filled sensors based on electrodes, ISFET pH (Ion-sensitive field effect transistor pH) sensors based on MOSFET (Metal oxide semiconductor field effect transistor), pH based on optical properties Sensors, potentiometric sensors, electrochemical sensing technology sensors.
Glass pH electrodes are still widely used in most pH sensors due to their superior repeatability, durability, and precise Nernstian response. Ion selective electrode (ISE, Ion selective electrode), such as electrochemical pH electrode, is a major sensor subcategory based on the principle of potentiometric method. Potentiometric method is a method of measuring electrical potential in which there is no current flowing between the electrodes. The indicator electrode compares the change in potential across the solid-state membrane between the analyte in the internal solution and the reference electrode. Current pH sensor designs typically incorporate a reference electrode within the probe, resulting in a bulky structure. The main challenge of glass electrode pH sensors is the fragility of the glass material and the problem of scaling when used in complex media.
Electrochemical sensors use electrodes to convert analytes into measurable substances. For example, a gas sensor measures the concentration of a gas by oxidizing or reducing the target gas on an electrode and measures the resulting electrical current that is converted. The electrochemical sensor consists of three electrodes: working electrode, reference electrode and counter electrode. The working electrode undergoes a redox reaction with ions. In addition to the electrode, there is a breathable membrane in the sensor to separate the water-based components from the gas, adjust the amount of gas reaching the working electrode, and prevent internal leakage of the sensor.
Electrochemical sensors use electrodes to convert analytes into measurable substances. For example, a gas sensor measures the concentration of a gas by oxidizing or reducing the target gas on an electrode and measures the resulting electrical current that is converted. The electrochemical sensor consists of three electrodes: working electrode, reference electrode and counter electrode. The working electrode undergoes a redox reaction with ions. In addition to the electrode, there is a breathable membrane in the sensor to separate the water-based components from the gas, adjust the amount of gas reaching the working electrode, and prevent internal leakage of the sensor.
ISFET sensing technology uses field-effect transistors, which can be used to measure ion concentrations in solutions because they are ion-sensitive. The source and ground electrodes are grounded to the substrate and connected to the circuit. Attachment of analytes/ions to the gate membrane causes a change in potential between the source and ground electrodes, which change is a measure of ion/analyte concentration. The ISFET is considered to be the first biosensor field effect transistor used in biological solutions, and is therefore also called a biofield effect transistor.
Compared to electrochemical sensors, optical sensors only measure the activity of H3O+ ions. Optical sensors have several advantages: small size, continuous measurement, no need for a separate reference electrode, etc. Photobleaching is the most important factor affecting the accuracy of this type of sensor. Breakage of covalent or non-covalent bonds resulting from nonspecific binding triggered by excitation light can lead to photobleaching of the indicator dye, rendering it unable to emit light. Causing sensor inaccuracy over time. The main trend in optical sensor development is miniaturization. This will reduce costs and increase mass producibility. The optical pH patch is such a small optical sensor. Optical pH patches combine a pH sensor onto an adhesive disc that adheres to the bioreactor surface. Another recent area of research is the development of disposable optical sensors for use with disposable bioreactors.
Anaerobic processes rely heavily on “manual experimental analysis” and “qualified experimental operators”. Control measurements for this process involve spectroscopy, titration, etc. Currently, the types of biological processes being studied require different spectroscopic techniques due to the magnitude of the energy changes involved. Biological process spectroscopy technology monitoring often produces a large number of spectra, and the information content of each spectrum is significantly lower than the number of data. Quick extraction of useful information from a large amount of data is the key. Fluorescence spectroscopy emerged in the early 21st century. Fluorescence measurement of the reduced form of [NAD(P)H] is the most popular fluorescence sensor.
Temperature control in bioreactors is a mature technology and accuracy of ±0.5°C or better can often be achieved. Typical temperature sensors used in industry include thermocouples, resistance temperature detectors (RTDs), and thermistors. The choice of a specific temperature measurement instrument depends on the sensor’s stability, sensitivity, accuracy, linearity, and sterilization. PT100 is commonly used for temperature sensing in bioreactors. Platinum RTDs are used because they provide a nearly linear response to temperature changes, are stable and accurate, provide repeatable responses, and they have a wide temperature range. RTDs are often used in precision applications because of their accuracy and repeatability. Temperature control in animal cell bioreactors is generally simpler than in microbial fermenters because the cell culture is less metabolically active and requires less heat to be removed from the reactor.
Standard single-paddle impeller or single-baffle mixing tanks often suffer from the disadvantages of uneven shear characteristics and energy dissipation. It has a greater impact on sensitive microorganisms. In a multi-paddle impeller system, reducing the impeller speed to achieve equivalent power dissipation will result in a reduction in the maximum shear value generated. It should be noted that whether it is a single-blade impeller or a multi-blade impeller, the shear force that causes rupture at the bubble interface is the same. The overall cell destruction rate due to fluid shear is therefore expected to be lower in a multi-paddle impeller system consuming the same total power. Therefore, when microorganisms are very sensitive to shear, multi-paddle impeller systems will be the first choice. During the installation of the paddle wheel, an instrument such as a tachometer is often used to detect whether the required rotational speed matches the reading in the tachometer to verify whether the rpm value is close to the optimal value. Use CFD (Computational fluid dynamics) modeling and various feature mixture predictions to achieve the desired paddle wheel speed and control.
During the fermentation culture process, in addition to temperature, pH value, DO and stirring speed, online exhaust gas analyzer is also an important sensing technology for process control. Tail gas analyzers are usually utilized in VOC component analysis or specific gas analysis. Optical array sensing has excellent performance in detecting and identifying various analytes (including hazardous compounds) and can be used for detection and identification in bioreactors. VOCs collected in headspace. An exhaust gas analyzer with a CO2 sensor and a DO sensor can be used to calculate OUR, which is also one of the key parameters for microbial growth. This analysis technology can be divided into non-invasive sensing technologies, which are used to detect CO2 and O2 in the gas at the end of the tank, analyze the respiratory quotient OUR of the metabolic process, and monitor changes in the fermentation process. Online monitoring can speed up the analysis and does not require sampling or pre-treatment. It is directly connected to the fermentation tank exhaust gas discharge pipe (traditional exhaust gas analyzers require dehumidification and other pre-processing of the fermentation exhaust gas). Multi-channel exhaust gas can be selected for parallel bioreactors. Analyzers, such as four channels, eight channels, and dozens of channels. It can realize online parallel monitoring of multiple tanks at the same time.