The focus in single-use bioreactor development is effective oxygen transfer, and the cultivation of cell culture processes and microbial cultures. Cell densities and productivities similar to conventional stirred tanks can easily be achieved in single-use bioreactors with the introduction of real process control. The Biostat CultiBag RM can reach KLa values of 43.2 h-1 and 12.9 h-1, in 2-L and 20-L cultures, respectively, at small scale. This article will show the productivities of Chinese hamster ovary cells (CHO), E. coli, and C. diphtheriae in the Biostat CultiBag RM using disposable sensor technology.
Early disposable bioreactors designs did not have good control capability, and growth was generally equivalent to shake flasks. Simple cultivation of cells and product for small-scale operations were possible, but a reusable stirred-tank system was required for real process optimization studies, where data could be logged and process parameters could be controlled and modelled. This was mainly because of the lack of good pH and dissolved oxygen (DO) control or at least a level of control similar to traditional stirred systems. Another limitation of rocking disposable bioreactors have been the limited aeration and agitation rates that could be achieved in systems that mix the culture by rocking back and forth and provide gassing by surface aeration alone. Such systems were mainly suited to cell cultures that exhibit low biomass concentrations and oxygen uptake rates. Also, mammalian cells are more fragile, which prevents the use of vigorous aeration and agitation strategies, making a rocking platform an ideal cultivation vessel. However, many modern cell culture processes require more strenuous aeration and agitation, and cells are becoming more robust. Therefore, the ability to have a good gas mixing strategy with feedback control is important. Microbial processes have a high demand for oxygen and are processed to high biomass concentrations. Most disposable systems cannot offer high processing rates. However, disposable rocking devices can be just as effective at the seed stage when good gas mixing and controls are available.
Recently, disposable stirred-tank bioreactor designs have been introduced that mimic traditional stirred-tank bioreactors (STR) and therefore gain more market acceptance. Most of these designs use reusable sensor technology with standard feedback control loops. The insertion of reusable sensors into a disposable system involves time-consuming tasks such as cleaning, sterilizing, and calibrating the sensors before aseptic insertion into the bioreactor. Disposable sensors are relatively new to the market and the market acceptance of such systems is slow because it requires detailed evaluation and validation. The operation of disposable sensors is different because no cleaning, sterilizing, or calibration is required at start-up. The sensors consist of membrane patches with an immobilized fluorescent dye that is able to detect the respective analyte (H+ or O2). The sensor patches are already part of the bag assembly. They come gamma irradiated and there is no break in the bag seal. An optical fiber transmits light of a specific wavelength to the sensor and returns the luminescence response from the sensor back to the measuring amplifier. This fiber optic works through the bag film, thereby maintaining sterility at all times. Standard feedback control loops are possible and recalibration functions to correct drift over time.
Sartorius Stedim North America
The use of superior gas mixing systems with disposable sensors, and tighter process control, can be used to cultivate both modern cell culture and seed stage microbial cells successfully. Process monitoring and control of cell cultivation processes are required for culture reproducibility, modeling scale-up parameters, increasing achievable cell densities, extending batch age, and increasing productivity and yield. This article shows that using disposable sensor technology, comparable cell densities, viabilities, and titers were achieved in several cell lines, and in some cases, were even higher than in stirred tanks.