A novel in situ probe for oxygen uptake rate measurement in mammalian cell cultures

Online monitoring of the cell-specific oxygen uptake rate with an in situ combi-sensor

In a biotechnological process, standard monitored process variables are pH, partial oxygen pressure (pO₂), and temperature. These process variables are important, but they do not give any information about the metabolic activity of the cell. The ISICOM is an in situ combi-sensor that is measuring the cell-specific oxygen uptake rate (qOUR) online. This variable allows a qualitative judgement of metabolic cell activity. The measuring principle of the ISICOM is based on a volume element enclosed into a small measuring chamber. Inside the measuring chamber, the pO₂ and the scattered light is measured. Within a defined measuring interval, the chamber closes, and the oxygen supply for the cells is interrupted. The decreasing oxygen concentration is recorded by the pO₂ optode. This measuring principle, known as the dynamic method, determines the oxygen uptake rate (OUR). Together with the scattered light signal, the cell concentration is estimated and the qOUR is available online. The design of the ISICOM is focused on functionality, sterility, long-term stability, and response time behavior so the sensor can be used in bioprocesses. With the ISICOM, measurement of online and in situ measurement of the OUR is possible. The OUR and qOUR online measurement of an animal cell batch cultivation is demonstrated, with maximum values of OUR = 2.5 mmol L^-1 h^-1 and a qOUR = 9.5 pmol cell^-1 day^-1. Information about limitation of the primary and secondary substrate is derived by the monitoring of the metabolic cell activity of bacteria and yeast cultivation processes. This sensor contributes to a higher process understanding by offering an online view on to the cell behavior. In the sense of process analytical technology (PAT), this important information is needed for bioprocesses to realize a knowledge base process control.

New developments in online OUR monitoring and its application to animal cell cultures

The increasing demand for biopharmaceuticals produced in mammalian cells has driven the industry to enhance the productivity of bioprocesses through intensification of culture process. Fed-batch and perfusion culturing strategies are considered the most attractive choices, but the application of these processes requires the availability of reliable online measuring systems for the estimation of cell density and metabolic activity. This manuscript reviews the methods (and the devices used) for monitoring of the oxygen consumption, also known as oxygen uptake rate (OUR), since it is a straightforward parameter to estimate viable cell density and the physiological state of cells. Furthermore, as oxygen plays an important role in the cell metabolism, OUR has also been very useful to estimate nutrient consumption, especially the carbon (glucose and glutamine) and nitrogen (glutamine) sources. Three different methods for the measurement of OUR have been developed up to date, being the dynamic method the golden standard, even though DO and pH perturbations generated in the culture during each measurement. For this, many efforts have been focused in developing non-invasive methods, such as global mass balance or stationary liquid mass balance. The low oxygen consumption rates by the cells and the high accuracy required for oxygen concentration measurement in the gas streams (inlet and outlet) have limited the applicability of the global mass balance methodology in mammalian cell cultures. In contrast, stationary liquid mass balance has successfully been implemented showing very similar OUR profiles compared with those obtained with the dynamic method. The huge amount of studies published in the last years evidence that OUR have become a reliable alternative for the monitoring and control of high cell density culturing strategies with very high productivities.

Characterization and Application of a Disposable Rotating Bed Bioreactor for Mesenchymal Stem Cell Expansion

Recruitment of mesenchymal stromal cells (MSC) into the field of tissue engineering is a promising development since these cells can be expanded vivo to clinically relevant numbers and, after expansion, retain their ability to differentiate into various cell lineages. Safety requirements and the necessity to obtain high cell numbers without frequent subcultivation of cells raised the question of the possibility of expanding MSC in one-way (single-use) disposable bioreactors. In this study, umbilical cord-derived MSC (UC-MSC) were expanded in a disposable Z 2000 H bioreactor under dynamic conditions. Z was characterized regarding residence time and mixing in order to evaluate the optimal bioreactor settings, enabling optimal mass transfer in the absence of shear stress, allowing an reproducible expansion of MSC, while maintaining their stemness properties. Culture of the UC-MSC in disposable Z 2000 H bioreactor resulted in a reproducible 8-fold increase of cell numbers after 5 days. Cells were shown to maintain specific MSC surface marker expression as well as trilineage differentiation potential and lack stress-induced premature senescence.