Rapid increases in upstream bioprocess cell densities and product titers have introduced unprecedented operational challenges for downstream purification trains. Conventional separation technologies are being pushed beyond their physical and operational constraints due to elevated particulate loads, heavier impurity burdens, and higher product concentrations.
To understand how bioprocess engineers can overcome these intermediate fouling challenges, maintain robust virus clearance, and implement scale-down models for continuous operations, we spoke with Nicholas Marchand, R&D Manager at Cytiva. In this interview, Marchand shares actionable insights on modern fluid dynamic mechanisms, single-use flow path evolution, and process analytical technology (PAT) requirements for next-generation facilities.
As upstream titers and cell densities continue to rise, downstream feeds become increasingly complex. What are the primary fluid dynamic mechanisms driving flux decay and membrane fouling during high-titer filtration steps?
The dependency may be easiest to understand for depth filtration, where increasing cell densities translate to increasing solids content that needs to be removed, thus decreasing filter capacity. Because existing facilities are quite limited on physical space, there has been a bigger emphasis recently on optimizing processes to maximize depth filter capacity, alongside filter media technology development to accomplish the same.
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These intensified upstream processes also create increased impurity loads that challenge the clearance capabilities of our traditional platforms. Higher impurity levels can create a larger precipitate challenge during virus-inactivated pool neutralization, reducing depth and/or membrane filter capacities at this intermediate downstream step as well.
To purify these intensified upstream processes within the same downstream suite footprint, drug manufacturers are increasing product concentration in intermediate streams. We are seeing greater adoption of single-pass tangential flow filtration (SPTFF) to concentrate these intermediate streams, as SPTFF requires relatively simple setups to operate.
However, higher product concentration in feedstreams for virus filters has increased fouling challenges. Higher feed concentrations inherently mean higher solution viscosity and often higher aggregate concentrations, affecting flux and fouling respectively. These issues are compounded by the industry’s shift toward more complex, often more hydrophobic modalities that are even more prone to self-association at elevated concentrations.
Self-association can shift viscosity curves such that initial flux is substantially reduced. It can also lead to increased levels of both irreversible and reversible aggregates. These two forms of aggregates combine to cause both irreversible membrane fouling via pore plugging and a reversible decline in membrane flux that appears similar to a viscosity or reversible pore-narrowing effect.
How can bioprocess engineers optimize primary clarification and secondary depth filtration to protect downstream viral removal filters without creating unnecessary footprint or product loss?
Over the last several decades, virus filtration research has generally indicated that fouling is primarily caused by product-related impurities rather than process-related impurities. Clarification processes are primarily tasked with clearing insoluble process-related impurities (for example, cells and debris), although given the increased impurity challenge, clarification is sometimes also tasked with the removal of soluble process-related impurities, such as host cell proteins (HCPs) or DNA. That is to say, in most processes, the clarification step has a relatively weak direct link to virus filter fouling.
However, it is possible for soluble process-related impurities to reach and foul virus filters, so some virus filtration processes may benefit from clarification schemes that decrease overall impurity load—either by limiting cell lysis or capturing soluble HCP. In rarer cases, optimizing a clarification process could also reduce the formation of product-related impurities, such as aggregates, by limiting shear exposure, thermal stress, and interfacial stresses throughout the process.
Virus removal filters are inherently sensitive to fouling. What process parameters are most critical for maintaining robust viral log reduction without bringing flow rates to a grinding halt?
A benchmarking survey conducted across 16 biologics manufacturers identified key process parameters critical to ensuring viral clearance. For virus filtration, the parameters identified were loading (either mass or volume per filter area), differential pressure, and flush volume. The article also discussed the importance of pressure interruptions in potential virus breakthroughs.
I would also add filter selection to that list. While filter selection is not strictly an operational process parameter, it has a significant impact on how sensitive virus clearance is to those parameters. It is well documented that many newer-generation virus filters are less susceptible to virus breakthrough at the extremes of these operating parameters.
How are pre-sterilized, single-use filtration flow paths evolving to handle the high-pressure dynamics and continuous throughput demands of modern intensified runs compared to traditional multi-use setups?
The individual system components have largely evolved alongside market needs, and full-solution suppliers have integrated them together. Flexible, single-use tubing now comes in a variety of high-pressure, braided options that can operate to at least 4 bar. Peristaltic pumps that cause tubing wear are being replaced with diaphragm or levitating centrifugal pumps featuring single-use heads.
A variety of sensors for parameters such as pressure, conductivity, and pH are now widely available in single-use formats. The robustness of pneumatic valves has improved, and single-use bags with integrated impellers for mixing are common. Sterile connector options continue to broaden for both smaller and larger fluid pathways. Crucially, all of these components have been developed with gamma stability in mind to support pre-sterilization.
What are the key best practices when using scale-down filtration models to accurately predict commercial-scale membrane performance and potential fouling behavior?
It is critical to understand the key scaling parameters for each technology. That often means normalizing process parameters to filter areas, such as loading volumes or flow rates. However, technology-specific parameters—such as pathlength for TFF—must also be kept constant across scales. In integrated or connected processing, this can require creative sizing strategies, as discrete equipment sizes rarely align perfectly across scales.
It is also important during process characterization to understand the performance variability originating from both the filter media (for example, lot-to-lot variability) and the feedstock across scales. Safety factors must account for those collective variabilities.
Finally, ensure the feedstock conditions are truly representative of the commercial scale. What temperature will the pool be kept at? Will it be mixed, and to what degree? Will it have gone through a freeze/thaw cycle prior to filtration?
As biomanufacturing moves toward intensified or continuous processing, what design adaptations are required for filtration assemblies to handle extended run times and variable feed streams safely?
Processes that have traditionally relied on peristaltic pumps will likely need to transition to alternative pump technologies where possible to handle extended run times without tubing failure.
Additionally, additive pressure must be carefully managed. In filtration processes that run at relatively high pressures, such as virus filtration or TFF, directly connecting filters to adjacent unit operations could push system pressures beyond established limits. In these setups, we will likely need to implement pressure breaks, such as additional booster pumps or intermediate hold tanks.
Finally, improved PAT is a critical requirement for our field to move forward with intensified and continuous processing. For filtration steps, this includes reliable inline turbidity sensors, UV sensors with wider dynamic ranges, and ultimately inline or at-line aggregate tracking.
Navigating the Future of High-Titer Downstream Filtration
As upstream intensification drives higher titers and cell densities, downstream bioprocessing relies heavily on technical innovation across media development, hardware design, and process characterization. By addressing fluid dynamic challenges such as aggregate self-association and viscosity early, leveraging robust next-generation virus filters, and adopting single-use flow paths engineered for high pressure and long run times, bioprocess engineers can effectively eliminate downstream bottlenecks. As the industry accelerates toward fully continuous operations, the integration of strategic pressure breaks and advanced PAT tools will be paramount in maintaining both operational safety and product purity at scale.
