Cells and bubbles in sparged bioreactors

Physiological Microenvironmental Conditions in Different Scalable Culture Systems for Pluripotent Stem Cell Expansion and Differentiation

Human Pluripotent Stem Cells (PSCs) are a valuable cell type that has a wide range of biomedical applications because they can differentiate into many types of adult somatic cell. Numerous studies have examined the clinical applications of PSCs. However, several factors such as bioreactor design, mechanical stress, and the physiological environment have not been optimized. These factors can significantly alter the pluripotency and proliferation properties of the cells, which are important for the mass production of PSCs. Nutritional mass transfer and oxygen transfer must be effectively maintained to obtain a high yield. Various culture systems are currently available for optimum cell propagation by maintaining the physiological conditions necessary for cell cultivation. Each type of culture system using a different configuration with various advantages and disadvantages affecting the mechanical conditions in the bioreactor, such as shear stress. These factors make it difficult to preserve the cellular viability and pluripotency of PSCs. Additional limitations of the culture system for PSCs must also be identified and overcome to maintain the culture conditions and enable large-scale expansion and differentiation of PSCs. This review describes the different physiological conditions in the various culture systems and recent developments in culture technology for PSC expansion and differentiation.

Modeling shear-sensitive dinoflagellate microalgae growth in bubble column photobioreactors

The shear-sensitive dinoflagellate microalga Karlodinium veneficum was grown in a sparged bubble column photobioreactor. The influence of mass transfer and shear stress on cell growth and physiology (concentration of reactive oxygen species, membrane fluidity and photosynthetic efficiency) was studied, and a model describing cell growth in term of mass transfer and culture parameters (nozzle sparger diameter, air flow rate, and culture height) was developed. The results show that mass transfer limits cell growth at low air-flow rates, whereas the shear stress produced by the presence of bubbles is critically detrimental for air flow rates above 0.1vvm. The model developed in this paper adequately represents the growth of K. veneficum. Moreover, the parameters of the model indicate that bubble rupture is much more harmful for cells than bubble formation.

High Efficiency Hydrodynamic DNA Fragmentation in a Bubbling System

DNA fragmentation down to a precise fragment size is important for biomedical applications, disease determination, gene therapy and shotgun sequencing. In this work, a cheap, easy to operate and high efficiency DNA fragmentation method is demonstrated based on hydrodynamic shearing in a bubbling system. We expect that hydrodynamic forces generated during the bubbling process shear the DNA molecules, extending and breaking them at the points where shearing forces are larger than the strength of the phosphate backbone. Factors of applied pressure, bubbling time and temperature have been investigated. Genomic DNA could be fragmented down to controllable 1-10 Kbp fragment lengths with a yield of 75.30-91.60%. We demonstrate that the ends of the genomic DNAs generated from hydrodynamic shearing can be ligated by T4 ligase and the fragmented DNAs can be used as templates for polymerase chain reaction. Therefore, in the bubbling system, DNAs could be hydrodynamically sheared to achieve smaller pieces in dsDNAs available for further processes. It could potentially serve as a DNA sample pretreatment technique in the future.

Equipment characterization to mitigate risks during transfers of cell culture manufacturing processes

The production of monoclonal antibodies by mammalian cell culture in bioreactors up to 25,000 L is state of the art technology in the biotech industry. During the lifecycle of a product, several scale up activities and technology transfers are typically executed to enable the supply chain strategy of a global pharmaceutical company. Given the sensitivity of mammalian cells to physicochemical culture conditions, process and equipment knowledge are critical to avoid impacts on timelines, product quantity and quality. Especially, the fluid dynamics of large scale bioreactors versus small scale models need to be described, and similarity demonstrated, in light of the Quality by Design approach promoted by the FDA. This approach comprises an associated design space which is established during process characterization and validation in bench scale bioreactors. Therefore the establishment of predictive models and simulation tools for major operating conditions of stirred vessels (mixing, mass transfer, and shear force.), based on fundamental engineering principles, have experienced a renaissance in the recent years. This work illustrates the systematic characterization of a large variety of bioreactor designs deployed in a global manufacturing network ranging from small bench scale equipment to large scale production equipment (25,000 L). Several traditional methods to determine power input, mixing, mass transfer and shear force have been used to create a data base and identify differences for various impeller types and configurations in operating ranges typically applied in cell culture processes at manufacturing scale. In addition, extrapolation of different empirical models, e.g. Cooke et al. (Paper presented at the proceedings of the 2nd international conference of bioreactor fluid dynamics, Cranfield, UK, 1988), have been assessed for their validity in these operational ranges. Results for selected designs are shown and serve as examples of structured characterization to enable fast and agile process transfers, scale up and troubleshooting.

Bubble stimulation efficiency of dinoflagellate bioluminescence

Dinoflagellate bioluminescence, a common source of bioluminescence in coastal waters, is stimulated by flow agitation. Although bubbles are anecdotally known to be stimulatory, the process has never been experimentally investigated. This study quantified the flash response of the bioluminescent dinoflagellate Lingulodinium polyedrum to stimulation by bubbles rising through still seawater. Cells were stimulated by isolated bubbles of 0.3-3 mm radii rising at their terminal velocity, and also by bubble clouds containing bubbles of 0.06-10 mm radii for different air flow rates. Stimulation efficiency, the proportion of cells producing a flash within the volume of water swept out by a rising bubble, decreased with decreasing bubble radius for radii less than approximately 1 mm. Bubbles smaller than a critical radius in the range 0.275-0.325 mm did not stimulate a flash response. The fraction of cells stimulated by bubble clouds was proportional to the volume of air in the bubble cloud, with lower stimulation levels observed for clouds with smaller bubbles. An empirical model for bubble cloud stimulation based on the isolated bubble observations successfully reproduced the observed stimulation by bubble clouds for low air flow rates. High air flow rates stimulated more light emission than expected, presumably because of additional fluid shear stress associated with collective buoyancy effects generated by the high air fraction bubble cloud. These results are relevant to bioluminescence stimulation by bubbles in two-phase flows, such as in ship wakes, breaking waves, and sparged bioreactors.

Influence of pluronic F68 on oxygen mass transfer

Pluronic F68 is one of the most used shear protecting additives in cell culture cultivations. It is well known from literature that such surface-active surfactants lower the surface tension at the gas-liquid interface, which influences the mass transfer. In this study, the effect of Pluronic F68 on oxygen mass transfer in aqueous solutions was examined. Therefore, the gassing in/gassing out method and bubble size measurements were used. At low concentrations of 0.02 g/L, a 50% reduction on mass transfer was observed for all tested spargers and working conditions. An explanation of the observed effects by means of Higbie's penetration or Dankwerts surface renewal theory was applied. It could be demonstrated that the suppressed movement of the bubble surface layer is the main cause for the significant drop down of the kL a-values. For Pluronic F68 concentrations above 0.1 g/L, it was observed that it comes to changes in bubble appearance and bubble size strongly dependent on the sparger type. By using the bubble size measurement data, it could be shown that only small changes in mass transfer coefficient (kL ) take place above the critical micelle concentration. Further changes on overall mass transfer at higher Pluronic F68 concentrations are mainly based on increasing of gas holdup and, more importantly, by increasing of the surface area available for mass transfer.

Scale-up of mouse embryonic stem cell expansion in stirred bioreactors

The aim of this study was to develop a robust, quality controlled and reproducible large-scale culture system using serum-free (SF) medium to obtain vast numbers of embryonic stem (ES) cells as a starting source for potential applications in tissue regeneration, as well as for drug screening studies. Mouse ES (mES) cells were firstly cultured on microcarriers in spinner flasks to investigate the effect of different parameters such as the agitation rate and the feeding regimen. Cells were successfully expanded at agitation rates up to 60 rpm using the SF medium and no significant differences in terms of growth kinetics or metabolic profiles were found between the two feeding regimens evaluated: 50% medium renewal every 24 h or 25% every 12 h. Overall, cells reached maximum concentrations of (4.2 ± 0.4) and (5.6 ± 0.8) ×10(6) cells/mL at Day 8 for cells fed once or twice per day; which corresponds to an increase in total cell number of 85 ± 7 and 108 ± 16, respectively. To have a more precise control over culture conditions and to yield a higher number of cells, the scale-up of the spinner flask culture system was successfully accomplished by using a fully controlled stirred tank bioreactor. In this case, the concentration of mES cells cultured on microcarriers increased 85 ± 15-fold over 11 days. Importantly, mES cells expanded under stirred conditions, in both spinner flask and fully controlled stirred tank bioreactor, using SF medium, retained the expression of pluripotency markers such as Oct-4, Nanog, and SSEA-1 and their differentiation potential into cells of the three embryonic germ layers.

Equipment design considerations for large scale cell culture

Equipment design is frequently recognized as a key component in the success of GMP biologics manufacturing, but is not always implemented with full appreciation of the processing implications. In the case of mammalian cell culture, there are some recognized issues and risks that develop when transitioning to a large scale of operation. The developing demand for cell culture production capacity in the biopharmaceutical industry has led to a progressive increase in the scale of operation in the last decade. This review will provide a high level summary of the documented process difficulties unique to serum-free large scale (LS) cell culture, analyze the engineering constraints typical of these processes, and suggest some practical equipment design considerations to enhance the productivity, reliability and operability of such systems under GMP manufacturing conditions. A systems approach will be used to establish a good LS bioreactor design practice, providing a discussion on gas distribution, agitation, vessel design, SIP/CIP and control issues.

Scale-up analysis for a CHO cell culture process in large-scale bioreactors

Bioprocess scale-up is a fundamental component of process development in the biotechnology industry. When scaling up a mammalian cell culture process, it is important to consider factors such as mixing time, oxygen transfer, and carbon dioxide removal. In this study, cell-free mixing studies were performed in production scale 5,000-L bioreactors to evaluate scale-up issues. Using the current bioreactor configuration, the 5,000-L bioreactor had a lower oxygen transfer coefficient, longer mixing time, and lower carbon dioxide removal rate than that was observed in bench scale 5- and 20-L bioreactors. The oxygen transfer threshold analysis indicates that the current 5,000-L configuration can only support a maximum viable cell density of 7 x 10(6) cells mL(-1). Moreover, experiments using a dual probe technique demonstrated that pH and dissolved oxygen gradients may exist in 5,000-L bioreactors using the current configuration. Empirical equations were developed to predict mixing time, oxygen transfer coefficient, and carbon dioxide removal rate under different mixing-related engineering parameters in the 5,000-L bioreactors. These equations indicate that increasing bottom air sparging rate is more efficient than increasing power input in improving oxygen transfer and carbon dioxide removal. Furthermore, as the liquid volume increases in a production bioreactor operated in fed-batch mode, bulk mixing becomes a challenge. The mixing studies suggest that the engineering parameters related to bulk mixing and carbon dioxide removal in the 5,000-L bioreactors may need optimizing to mitigate the risk of different performance upon process scale-up.

Special Engineering Aspects

The specific characteristics of mammalian cells discussed in Chap. 2 require adapted solutions for bioreactor design and operation. Especially, cell damage by shear stress and aeration has to be considered. Therefore this chapter starts with a detailed discussion of shear stress effects on mammalian cells (anchorage-dependent and suspendable cells) in model systems and bioreactors, respectively, and consequences for reactor design. Appropriate oxygen supply is another critical issue, as adapted oxygen supply systems are required. Techniques for immobilization of cells, either grown on microcarriers in suspension culture or within macroporous carriers in fixed bed or fluidized bed reactors, are discussed as well. With respect to the operation of bioreactors, the characteristics of different culture modes (batch, fed-batch, chemostat, perfusion) are introduced and practical examples are given. Finally, concepts for monitoring of bioreactors, including offline and online methods as well as control loops (e.g. O₂, pH), are considered.

Influence of power supply in the feasibility ofPhaeodactylum tricornutumcultures

The influence of fluid-dynamic conditions on the yield of Phaeodactylum tricornutum microalgal cultures was analyzed in two stages: first, the influence of air flow rate; second, the influence of using fluid-moving pumps for recirculating the culture. With respect to the air flow rate, the yield of the cultures increased with the aeration rate up to values of 2.0 v/v/min, then stress was observed and the yield of the cultures decreased. With respect to the influence of mechanical power supply for liquid impulsion, three different types of pumps--centrifugal, pulse, and peristaltic--were essayed at different power supplies. The cultures were stressed for the three types of pumps essayed. For each pump, the higher the power supply the lower was the Fv/Fm value and the higher was the stress at which cells were exposed. The highest measured stress was when the culture was moved with the centrifugal pump. Despite measured stress, for all the experiments stable steady states were reached, thus indicating that cells reduced their yield but did not die, as was verified by cell viability measurements. It was observed that the increase of the power supply improved the frequency of light exposition thus enhancing the yield of the cultures. However, the higher the power supply, the lower the microeddy length scale; therefore, stress could appear. Data demonstrated that the microeddy length scale was always much higher than cell size and therefore the turbulence was not responsible for stress. Also, the mass transfer was discarded as responsible for yield reduction. It was concluded that the shear rate was the factor determining the existence of stress phenomena. The evaluation of these shear rates demonstrated that values above 30-80 s(-1) damaged the cells strongly. These data were verified in an outdoor pilot-scale tubular photobioreactor that was implemented with the same type of pumps, thus demonstrating the necessity to take into account this factor in the design and scale-up of microalgal photobioreactors.

Reactor design for large scale suspension animal cell culture

The scale of operation of freely suspended animal cell culture has been increasing and in order to meet the demand for recombinant therapeutic products, this increase is likely to continue. The most common reactor types used are stirred tanks. Air lift fermenters are also used, albeit less commonly. No specific guidelines have been published for large scale (>/=10 000 L) animal cell culture and reactor designs are often based on those used for microbial systems. However, due to the large difference in energy inputs used for microbial and animal cell systems such designs may be far from optimal. In this review the importance of achieving a balance between mixing, mass transfer and shear effects is emphasised. The implications that meeting this balance has on design of vessels and operation, particularly in terms of strategies to ensure adequate mixing to achieve homogeneity in pH and dissolved gas concentrations are discussed.

Interaction between CO2-mass transfer, light availability, and hydrodynamic stress in the growth of phaeodactylum tricornutum in a concentric tube airlift photobioreactor

The microalga Phaeodactylum tricornutum was grown in a concentric tube airlift photobioreactor. A maximum specific growth rate of 0. 023 h-1 was obtained using a superficial gas velocity around 0.055 m/s. Lower or higher gas flow rates limited the culture performance. To establish if the observed limitation was due to CO2 or to the photosynthetically active irradiance, characteristic times for mixing, mass transfer and CO2 consumption, and the photon flux absorbed by the culture were analyzed. The CO2-gradients in the culture were shown to be responsible for the limitation during the exponential growth phase, and both CO2 and light irradiance were limiting in the linear growth phase. The decrease in specific growth rate relative to the maximum was found to be related to the specific gas-liquid interfacial area, the length scale of the microeddies and the shear rate. Copyright 1998 John Wiley & Sons, Inc.

Mechanisms of animal cell damage associated with gas bubbles and cell protection by medium additives

Animal cell damage arising from gas sparging is considered to be a major barrier to large-scale production of recombinant biologicals in animal cell culture. Understanding sparging cell damage is therefore of significance to the application of animal cell culture. The paper reviews the hydrodynamics of bubble rupture, mechanisms of cell-bubble interaction, mathematical modelling and quantification of the sparging damage. Another interesting topic addressed in the paper is the protective effects of various medium additives against fluid mechanical cell damage, especially those surface-active polymers such as pluronic polyols, methylcellulose and polyethylene glycol. Experimental results obtained recently by the author and other researchers were examined to reveal the mechanisms of additive protection. The interactions of additives with air-liquid interfaces and the animal cells were analyzed with respect to their physical properties and chemical structure.