The innovative design of single-use stirred-tank bioreactors has helped overcome some of the oxygen transfer limitations seen with rocking 2-D bioreactors. Impeller-based mixing, aeration via sparger, and overlay in novel single-use designs are among the advances. These designs are available in various configurations, including top-driven motors that require insertion of a stainless steel rod into an off-center single impeller shaft to prevent vortices.
Other designs are single bottom-driven impellers without a stirrer shaft, located off center to prevent vortices. Additional innovations include box-shaped vessels with paddle impellers. These systems typically utilize reusable sensor technology or, in some cases, may rely on single-use sensors that still have to be aseptically inserted into the bioreactor chamber. The insertion of sensors into a disposable system always introduces a sterility risk to the process.
The Biostat CultiBag STR is a single-use bioreactor with a classical stirred-tank design. It features a central stirrer shaft, sparger, and dual impeller design. The design of the impellers, and location of the shaft and the impellers on the stirrer shaft, mimic a traditional bioreactor. Impeller designs include marine-type impellers and rushton-type impellers as well as typical ring spargers.
The Biostat CultiBag STR has a turn-down ratio of 25%, allowing smaller minimum volumes in fed-batch operation. Since the system is intended for single-use applications, where convenient operation is key, additional user-friendly handling features are part of the design. This includes a bag holder with clam shell opening for easy installation of the bag and a motor lift and separate platforms for the bag and controllers, which facilitates faster turnaround by allowing additional bag holders to be processed downstream while a new bag holder starts upstream.
Like the CultiBag RM, this system also utilizes single-use sensors for pH and DO control. The design was modeled on traditional bioreactors incorporating the same aspect ratios—impeller diameters and location of the shaft and impellers on the shaft—so that the universally accepted tip ratios and power input per volume criteria and KLa’s can be applied for easier transition from stainless steel to single use (Figure).
Through the introduction of traditional bioreactor-like control platforms to the CultiBag RM and STR, real process development capabilities are now possible. These control capabilities can not only supply the user with additional information from continuous data-logging capabilities but they may also help to extend batch age and increase cell growth, product titers, and even specific productivities.