A Bioreactor System Based on a Novel Oxygen Transfer Method

JUNE 2008 B ioreactor technologies for mammalian cell culture may seem quite sophisticated (1). In fact, efficient mammalian cell culture is simple and requires just two major elements: mixing and oxygen (O2) transfer. Traditional methods for mixing use an impellor, and the classical O2 transfer method features a sparging stone that expels air bubbles to increase the O2 transfer surface area in contact with a culture medium. Mammalian cells are sensitive to shear forces and also to the toxicity of pure or enriched O2 atmospheres. A classical steel deep-tank bioreactor usually has a sophisticated control tower to control O2 and impellor mixing speed (2). Reasonably, the use of ambient air is ideal for bioreactors because it is nontoxic to mammalian cells. But because of the inefficiency of sparging-stone O2 transfer, an enriched or pure O2 supply is usually necessary. We might try to increase the flow rate of O2 (thus increasing O2 transfer), but that causes cell death when bubbles burst at the surface. So an improved O2 transfer method is desirable for mammalian cell culture bioreactors. The shear force sensitivity of mammalian cells to impellor tip speed becomes a technical issue during scaleup. In addition, deep-tank bioreactors usually require production downtime for cleaning, sterilization, and validation before each fed-batch production. To address the above technical problems, the single-use plastic bag “Wave” bioreactor (www.wavebiotech. com) was developed by Vijay Singh (3, 4). Its single-use plastic bag eliminates production downtimes. Singh’s design has no impellor, so it creates no shear force. It uses no sparging and thus operates without bubble-burst cell damage, so it performs well for mammalian cell culture. Wave bioreactors are widely used in production of preclinical material, seed trains for larger production-scale cultures, and closed-system vaccine production. Pioneering work by Dr. Singh opened new doorways for mammalian cell culture with disposable plastic bags. Currently many forms of singleuse bioreactors have been created, most with either a plastic bag inner lining to transform classical deep-tank designs into single-use bioreactors or to create wave-style imitations that may violate Wave patents without significant improvement. We have analyzed the advantages and drawbacks of the Wave bioreactor and found that the company did not precisely define and optimize its O2 transfer method. The wave motion was hypothesized to increased O2 transfer through an increased medium surface area. Here, we disclose a novel O2 transfer method based on maximizing the interface surface area between a bioreactor vessel’s surface and the current of culture medium through it. Based on this novel method, we have manufactured and tested different disposable plastic-bag bioreactors. We designed them to offer more efficient O2 transfer and a better mixing method with a mechanical advantage over the Wave bioreactor. These novel bioreactors represent new intellectual property based on our novel O2 transfer method. They hold great commercial promise for cost-effective mammalian culture and protein expression. V E N D O RVoice