The Effect of Single-Use Retentate Recycle Bag Design On Diafiltration Efficiency

Performance of ultrafiltration/diafiltration (UF/DF) systems depends, among other factors, on highly effective mixing in the retentate recycle tank. Retentate mixing requirements are unique as compared to mixing requirements for other bioprocessing unit operations such as buffer preparation, because UF/DF processes must achieve a high level of tank homogeneity and buffer exchange (e.g. > 99.9%) under continuously changing level, protein concentration, and/or buffer concentration conditions. In ultrafiltration processes, poor retentate mixing may result in a higher protein concentration in the feed stream than intended, leading to reduced flux and issues with protein quality or solubility. In diafiltration processes, poor retentate mixing can lead to reduced exchange of buffer components and inability to meet a final excipient specification. Consideration of effective retentate mixing is of particular importance for processes employing single-use flow paths since component and system availability is more limited and less established for rigorous downstream separation capability than more mature stainless steel system designs.

The design of two different styles of commercially available, single-use retentate recycle bags was experimentally investigated, specifically to assess the effect on diafiltration performance. The use of active versus passive agitation, different inlet/outlet configurations, and a range of process parameters such as tank level, recirculation rate, and fluid viscosity were evaluated using a simulated diafiltration process to characterize the effect on solute clearance. A mechanistic model of solute clearance during diafiltration is introduced which explicitly accounts for unmixed regions or deadlegs. The model is used to explain the experimental results and illustrates shortcomings of common practices used to evaluate tank mixing in UF/DF processes.