Evaluation of Single-Use Fluidized Bed Centrifuge System for Mammalian Cell Harvesting - This article discusses the evaluation of a novel single-use fluidized bed centrifuge for harvesting of antibodi


Evaluation of Single-Use Fluidized Bed Centrifuge System for Mammalian Cell Harvesting
This article discusses the evaluation of a novel single-use fluidized bed centrifuge for harvesting of antibodies.

BioPharm International
Volume 25, Issue 11, pp. 34-40

Compared with cell harvesting alternatives that rely on the use of conventional centrifuges such as disc-stack or filtration devices to separate cells from the supernatant, the FBC has several advantages over both conventional methods. First, the FBC eliminates the need for cleaning and sterilizing cycle validations. Second, there is no risk for cross-contamination between batches because the product-contact components are single-use only. Third, there is lower shear stress induced by the FBC during operation compared with conventional centrifugation and filtration. Because of the establishment of the fluidized bed during operation, FBC's g-force does not result in cell lysis because cells are not packed against the centrifuge wall. Finally, the washing option available with the FBC provides maximal recovery of the product without diluting the centrate. In addition to these advantages, historical clarification efficiency data from disc-stacks have been shown to be comparable with the expected clarification efficiency performance of the FBC (7).

The FBC is based on a balancing act of two counteracting forces within the system, the centrifugal force (Fg) versus the fluid flow (VQ). The physics of the FBC follows Stokes' Law for spherical particles in a continuous fluid (Equation 1), or in the case of bioprocessing, a cell suspended in a medium:

where Vs is the settling velocity of the particle, ρp is the density of the particle, ρf is the density of the fluid, μ is the dynamic viscosity of the fluid, D is the diameter of the particle, and g is the gravitational acceleration of the particle. The centrifugal force (Fc) and fluid flow velocity (VQ) are defined in the following equations (Equations 2 and 3):

Figure 2: FBC schematic demonstrating principles of FBC’s fluidized bed. (FIGURES 2–6 ARE COURTESY OF THE AUTHORS)
where ω is the angular velocity, R is the radius, Q is the flow rate, and A is the cross-sectional area of the chamber. When the settling velocity (VS) due to the centrifugal force is balanced against the fluid flow velocity (VQ), a fluidized cell bed is created within the chambers while the supernatant is discharged from the chamber as the centrate. Figure 2 shows a schematic of the inner workings of the FBC.

The overall preparation time for the FBC is relatively short compared to the cycle times for cleaning and sterilizing stainless-steel equipment. A single-use set that contains four interconnected chambers and the valve set are first inserted into the FBC prior to an operation. After loading the single-use set into the FBC, individual tubing ends from the single-use set can be sterile-connected to respective vessels such as the bioreactor, buffer, centrate vessel for harvested product, and waste container. Pinch valves and bubble sensors are built-in as part of the FBC and used to automate the cell-harvesting process. The built-in human-machine interface (HMI) screen on the FBC allows run parameters such as centrifuge speed, flow rates, and harvest volumes to be set in individual recipes. Cell-harvesting runs can be nearly fully automatic once the recipes are configured, but manual controls are also available if required.

Figure 3: Process flowchart for harvest clarification using the FBC.
Figure 3 shows the process flow diagram of a typical harvest clarification recipe. Step one in the process is to prime the single-use set with a buffer. All tubing and chambers are filled with the buffer to displace air from the system. Step two in the recipe directs the system to displace the buffer initially in the single-use chamber(s) into the waste vessel. In doing so, the buffer is not introduced into the centrate vessel and will not dilute the product. Harvest clarification begins in step three, where cells and cell debris are retained in the chamber(s) and the clarified liquid is separated into the centrate vessel. When the chamber(s) are nearly filled with cells, in step four, the bioreactor feed temporarily pauses, and the buffer is used to flush the chamber(s) to recover the product that is still present in the cell bed. In step five, after the wash, pump directions are reversed to discard the cell bed into the waste vessel. In the case where the bioreactor volume is larger than what all four single-use chambers can handle in one batch, steps two through five can be repeated in multiple cycles to finish processing the full volume of the bioreactor. At the end of the process, the system purges all liquids from the single-use set and prompts the operator to seal all tubing prior to the disposal of the single-use set. The remainder of this article discusses the results obtained from the evaluation runs conducted using the FBC for cell harvesting and clarification.

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