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Design and Performance of Single- Use, Stirred- Tank Bioreactors. Figure 1: Sterilizing filter systems. Single- use components and systems have been incorporated into many bioprocesses as an alternative to cleanable, reusable systems. A wide range of publications have detailed the reasons for this trend toward a single- use approach.
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Justification in many cases comes from process- specific benefits such as increased manufacturing flexibility — especially for contract manufacturing organizations (CMOs) — enhanced sterility assurance, elimination of cleaning, reduced capital investment, faster processing times with increased productivity, faster start- up, and other benefits (1). One critical factor in the application of single- use technology for biopharmaceutical manufacturing has been a need to design and develop components that fully satisfy the performance, functionality, and validation requirements for user process systems. Meeting this requirement is particularly difficult for complex equipment such as bioreactors (1). Here, we discuss how the challenge can be addressed from a systems engineering point of view. Pall’s research and development program included the following key elements during development of the Allegro STR 2.
STR 1. 00. 0, and STR 2. CFD) modeling studies showing good mixing performance with the cubical design — matching that for the typical cylindrical format of stainless steel bioreactorsease- of- use considerations. Design Considerations. When designing a single- use alternative to an existing cleanable component, one simplistic approach would be to develop a version with similar form, fit, and function that incorporates disposable materials. For some products, that may be possible. For instance, a sterilizing filter system containing a filter cartridge in a stainless steel housing can be replaced by a disposable single- use capsule filter containing a similar type of cartridge inside a plastic housing (Figure 1).
Even in that example, however, it may be necessary to modify the filter cartridge’s materials of construction to provide gamma- radiation resistance needed for presterilized single- use products. For more complex components such as bioreactors, fundamental changes in design and engineering may be necessary. That gives product designers and developers the opportunity to create innovative designs that could enhance performance over traditional cleanable systems while offering practical benefits such as faster system set- up and improved product handling. Figure 2. A: Pall Allegro STR 2. Design Approach. The first consideration in the design of this bioreactor was to apply systems- engineering principles to ensure robust and reliable performance.
Such an approach requires excellent bioprocess, mechanical, production, and control engineering along with ergonomic considerations to ensure easy assembly and efficient operation of the equipment. That can be followed best by considering the overall layout of a bioreactor and its ancillaries. By focusing on individual components critical to achieving a final design (Figure 2), different considerations can be brought out. Figure 2. B: Pall Allegro STR 1. Geometry of Biocontainer: Most glass and stainless steel bioreactor vessels are cylindrical, the latter designed with dished heads. Such geometry was required to accommodate high- pressure steam for sterilization, and those bioreactors were designed to pressure- vessel standards that require a rounded shape for enhanced mechanical stability (2). Thus, cylindrical vessels became the standard geometry for bioreactors.
However, increasing acceptance of single- use technology and flexible biocontainers, together with elimination of steam- in- place (SIP) and clean- in- place (CIP) procedures, offers product designers the opportunity to consider alternative geometries. It also allows them to incorporate features that could prove beneficial to performance and other functions.
ALERT: Welcome to the New FIRE web site. This will be the active FIRE web site and will eventually contain all material from 1999 to the present date. The pharmacology and physiology of amphetamine and its derivatives are presented. Design Approach The first consideration in the design of this bioreactor was to apply systems-engineering principles to ensure robust and reliable performance.
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Assessment of the most suitable geometry for a single- use stirred- tank bioreactor led Pall to choose a square cross- sectional format. We favor this geometry not only for its specific bioprocess engineering features (described below), but also for the following reasons: Cubical single- use biocontainers have been used for many years in unit operations such as storage, product transfer, and mixing in a broad range of validated pharmaceutical processes. Pall had already gained valuable experience in the manufacture, supply, and validation of biocontainers with cubical geometry. Figure 3: Allegro STR 2. System features. Ergonomics of Biocontainer Handling and Protection (Figure 3): Flexible polymeric biocontainer film for single- use used in constructing a stirred- tank bioreactor can be damaged during manufacturing through final installation and operation. Thus, Pall paid special attention to features such as biocontainer geometry, materials of construction, seam welding, and packaging as well as design of the system as an integrated unit.
In addition, the design team incorporated special features to facilitate ease of installation and operation and to eliminate risks of operator error (3). It was evident during this development program that a cubical biocontainer would be easier to fold, protect, and package than a cylindrical format would be. That helps ensure the highest possible integrity of biocontainers during manufacturing, shipping, and installation. The cubical shape also presents distinct faces, which helps guide correct biocontainer alignment and orientation during installation into support hardware. The support hardware and biocontainer were engineered to enable simple installation and biocontainer inflation with minimal need for operator adjustments. That eliminates the inherent risk of damage from manual handling and operator error. Another critical requirement was to prevent contact between the biocontainer film and other system components, making external rigid protective elements necessary.
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For example, the Allegro STR 2. On Allegro STR 1. STR 2. 00. 0 systems, a rigid base contains and protects the film during shipping and installation. Additionally, the bottom- entry impeller format helped engineers design compact and protective biocontainer packaging that keeps the impeller and components from damaging the biocontainer. These bioreactors are designed to enable complete installation and inflation by no more than two operators, with step- by- step instructions and color- coded numbered labels to guide them through the process.
Minimal operator intervention reduces the risk of accidental damage. The front doors open fully to allow easy access during installation.
The door lock controls are based on fluid levels to prevent accidental opening of bioreactor when it is full of liquid. Single- use sensors are incorporated where available to eliminate installation, cleaning, sterilization, and calibration checks. For reusable probes, bellows were designed for aseptic insertion. Functional Features.
Impeller System: To meet bioprocess demands, the choice of impeller is critical to bioreactor design. This component must be able to impart enough power into cell culture medium to enable sufficient mass transfer and satisfy cellular oxygen demands while maintaining good media mixing and homogeneity. In this case, the impeller system design and location had to enable both the ergonomics associated with using a cubical biocontainer and the ease of installation and packaging. Figure 4: Pitched- blade impeller. Impeller Type: The preferred impeller type identified from published literature (2, 4) was a pitched- blade design (Figure 4). This type is common in bioreactors used for animal- cell culture because it is considered less likely to cause shear damage with optimal blade diameters and agitation speeds.
A pitched- blade impeller with an appropriate blade configuration also balances axial and radial flow. Blade Geometry: The geometry of this pitched- blade impeller was designed specifically to optimize performance in a cubical bioreactor. The large area and 4. For 2. 00- L Allegro systems, the D/T ratio was set to 0. W/kg) ensures that mixing times are shorter than for smaller impellers. Homogeneity is improved during p.
H control, and sparged air is dispersed more efficiently (4, 5). Impeller Clearance: The ratio of impeller clearance (C) from the bottom of a bioreactor (Figure 4) to fluid height in the bioreactor (H) affects flow patterns and mixing. The C/H ratio in mixing systems typically ranges from 1/3 to 1/6. For Allegro STR 2. That takes into account the need to maintain good axial flow and adequate mixing at both minimum (6. L) and maximum (2. L) working volumes.
Pumping Direction: The ability to operate an impeller for up- flow or down- flow pumping is an important requirement and was engineered into this system by means of a bidirectional drive motor. For most animal cell cultures, Allegro STR bioreactors would be programmed for up- flow pumping. But its down- flow capability can benefit adherent- cell cultures on microcarriers, where downward flow enables suspension of the microcarriers at low speeds with low specific power input. Drive System: For this impeller’s drive system, the design team chose a bottom- entry drive shaft over a top- entry option.
The shorter shaft for bottom entry reduces bending moments under power and imposes less stress on drive shaft bearings and dynamic seals for a sterile barrier. As mentioned above, a shorter shaft also allows for more compact packaging and minimizes the risk of container damage. Speed and Specific Power: The Allegro STR 2.
In qualification studies for this bioreactor, the power value (P) was measured using a rotary torque transducer fitted to the bioreactor shaft between the impeller and motor (6). The team measured power dissipated by the impeller rotating in air, subtracting the value from that obtained with it rotating in 2. L of water at 3. 7 .