Measuring the k_La (Oxygen Volumetric Mass Transfer Coefficient) of Cell Culture Bioreactors

What is k_La

The volumetric mass transfer coefficient (k_La) is a parameter that describes the efficiency of the gas-liquid mass transfer in a bioreactor. The ‘k_L’ is expressed as the rate of molecular diffusion through the gas-liquid interface area ‘a’ per liquid volume. The k_La is defined by the geometrical properties of the reactor and the operational process aspects. It is a powerful parameter to provide information about gas supply limitations in a bioreactor, in particular for oxygen in aerobic processes. The k_La values can be used for comparison of the performance of bioreactors with similar designs or scale-up strategies [1]. The parameter is important for DO control in bioreactors .

How to determine the bioreactor k_La

Several methods using reagents or organisms were developed to measure k_La values [2]. This protocol describes the gassing-out method recommended by the German Society for Chemical Engineering and Biotechnology (DECHEMA) which is an easy and relatively quick method without the need for hazardous chemicals or organisms [3].

As organisms are not needed in this cost-effective method, the experiments can be carried out under non-sterile conditions and without medium exchange for several experiments. It is suggested to use 1 × phosphate-buffered saline (PBS) buffer at 37 °C as a closer representation of cell culture medium than water. The method is based on dynamic measurements of oxygen concentration. The first step includes the establishment of anaerobic conditions by gassing the PBS buffer with nitrogen. Next, nitrogen gassing is stopped, the reactor headspace is replaced with air (21 % oxygen), and the PBS buffer is sparged with air at certain flow rates of interest. By monitoring the dissolved oxygen concentration in the PBS buffer as it rises from the minimum to its equilibrium value, the k_La value can be calculated. Removing excess nitrogen from the headspace before the actual k_La measurement ensures defined atmospheric conditions in the headspace and reliable preconditions for k_La measurements [3].

This short protocol is designed mainly to measure k_La values of cell culture bioreactors with typical small k_La values as the measurement is done under slow agitation and low gassing rates to mimic real cell culture conditions with minimized shear stress. To provide readers with realistic examples, the short protocol also includes the determination and comparison of k_La values of BioBLU 1c Single-Use Bioreactors with one impeller (Figure 1).