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Selecting Filters To Address Your Bioburden Challenges

By Stephanie Ferrante and David Beattie, MilliporeSigma

Selecting Filters To Address Your Bioburden Challenges

Biologics development and manufacturing is inherently complex and challenging. Compounding this is the added pressure to be first to market in a highly competitive healthcare landscape, where delays can put the success of a product and company at risk. The implication for process development engineers is that they are tasked with rapidly designing a purification process for a drug product that meets safety and quality standards for human use. 

In the growing market of biologics, the sterile filtration of liquids is a key component of many operations. This step is vital to minimizing microbial contamination and ensuring product safety and integrity.  Preventing microbial contamination in upstream processes reduces the risk of bioreactor contamination and subsequent business disruption. Selecting the right filter for downstream processes can significantly impact operational efficiency and cost. Understanding the different criteria for optimum filter selection helps narrow the options and streamlines product selection.

Technological innovations have opened the doors to many new approaches to drug development. Progress has also been made in the development of improved technologies for producing and manufacturing biologics. Advances in membrane technology and device design have resulted in new options for sterile filtration. Rather than a traditional one-size-fits-all sterilizing filter, more specialized filters have been developed for optimum performance in specific unit operations or with particular fluid streams.

Opportunities to consider and evaluate new filters most often occur in either the early stages of process development with a new molecule, or after approval when there is a change to production processes. By choosing a filter that best fits the goals of the process, it is likely to be reliable, sustainable, and scalable for the life of your drug product.

Considerations for Filter Selection

There are many filtration options available for today’s biologics manufacturers, and the following considerations can help narrow down choices: filter compatibility, retention requirements, fluid stream characteristics, filter format and scale-up needs (Figure 1).

Figure 1: Criteria for Filter Selection


The first and, perhaps, most important parameter to consider when selecting a filter is its chemical compatibility with the fluid stream. This is very important for fluid streams that have a very high or very low pH or contain solvents and/or surfactants. Performing a chemical compatibility assessment ensures the membrane selected will not be chemically altered and/or potentially shed particles when exposed to a specific fluid stream. Another compatibility consideration is the potential for binding of the product or a component of the product to the membrane. For example, if the fluid stream is a monoclonal antibody, a low-protein binding membrane should be selected to minimize product loss. Similarly, if the product to be processed contains a preservative, it is important to minimize binding of the preservative or other active ingredient in the fluid stream to the membrane.

Microbial Retention

When selecting filters, it is important to understand the goal of the filtration at each process step. There are filters that are designed to remove particulates and reduce bioburden (bioburden reduction), filters to completely remove bacteria (sterilizing filters), and filters designed to remove mycoplasma and reduce the levels of adventitious virus. Figure 2 illustrates the various operations in a monoclonal antibody (mAb) production process including where sterilizing filters might be implemented.

Figure 2: mAb Production Process Operations Including Sterilizing Filtration Steps.

Bioburden reduction

As purification processes can span several days, there is always a risk of microbial ingress to the system, which can challenge the integrity of any manufacturing processes. Filters that offer bioburden reduction can be implemented at multiple points throughout manufacturing processes to reduce this risk. Many downstream purification operations are not sterile operations, and bioburden reduction filters may be sufficient to minimize the risks of microbial contamination. Examples of operations include buffer filtration or filtration of process streams before chromatography operations. Bioburden reduction filters do not provide the same sterility assurance levels as sterilizing filters, but they are generally less expensive and can be an effective option for many operations.


Complete sterilization is required at a few critical steps in biomanufacturing processes, such as the filtration of cell culture media before the bioreactor and the final sterile filtration step prior to filling.Regulatory expectations for sterilizing-grade filters are outlined in the FDA Aseptic Processing Guidelines1 as well as ASTM® standards2 that define a sterilizing-grade filter as one that, when challenged with the bacterium Brevundimonas diminuta at a minimum concentration of 107 colony forming units (CFU) per cm2 of filter surface area, will produce a sterile effluent. Currently, sterilizing-grade filters usually have a rated pore size of 0.2 μm or smaller, but it is important to note that not every 0.2 μm filter is a true sterilizing-grade filter. Clarification filters, prefilters, as well as bioburden reduction filters, can all have 0.2 μm ratings, yet they are not always sterilizing grade. All sterilizing-grade filters should be supplied with a certificate of quality that shows the membrane meets bacterial-retention testing requirements outlined in ASTM® standards.2

Mycoplasma and virus removal

Sterile filtration of cell culture media and supplements presents different challenges than the sterile filtration of process intermediates, buffers, or final drug product. Raw materials in some cell culture media and supplements can contain mycoplasma contaminants in addition to bacteria. Mycoplasmas, which lack a cell wall, are the simplest and smallest self-replicating prokaryotes which gives them the ability to pass through a 0.2 μm sterilizing-grade filter. For this reason, sterilizing-grade filters with a 0.1 μm pore size rating, which, in some cases, have been validated to retain mycoplasma, are often selected for cell culture media applications.

Similarly, raw materials in cell culture media and feeds are susceptible to adventitious virus contamination, and virus removal filters have been developed to efficiently process these materials. These upstream virus filters reduce the risk of a potential bioreactor contamination, which could have significant business impact in terms of production interruption and reduced drug availability. Both the 0.1 μm pore size sterilizing filter, and the virus filter for cell culture media and feeds, are specifically designed for ‘protecting the bioreactor’ and would not be efficient options for downstream filtration.

Understanding the contamination risks and the processing and application needs of different operations helps define the appropriate filter requirements.

Fluid Stream: Does it Impact Selection?

Aside from the level of microorganism retention required, the complexity of the fluid stream influences filter selection. Fluid streams can be generally classified as non-plugging or plugging.

If processing a non-plugging stream, such as a buffer or water, a filter designed for high-flux processing is generally recommended. These filters can efficiently process a large volume of non-plugging liquid through a small area of membrane, resulting in a small filter footprint. To predict the process-scale filter size, most filter manufacturers provide water flow curves along with other buffer sizing tools.

For processing plugging or viscous streams, like cell culture media, filters designed to maintain flux while retaining particulates are recommended and process-scale performance is best predicted by running a trial using a small-scale filtration tool. For these challenging streams, high-capacity filters that contain integrated prefilters can be used to remove particulates and protect the downstream sterilizing filter from plugging. An alternative might be a high-area filter where membrane is configured in an “M pleat pattern” resulting in more membrane area in the same size filter, reducing filter footprint as compared to standard area filters (Picture 1).

Picture 1: Pleat Patterns of Standard and High Area Filters (M-Pleat Pattern)

Not considering the type of fluid stream being filtered could have consequences. Incorrect sizing can result in oversizing, which results in paying for more filter area than necessary, as well as increasing the amount of unrecovered product trapped in the membrane (hold up). Conversely, under-sizing filters can result in the filter plugging before the filtration is finished.  Ideally, manufacturers should use the lowest filtration area that meets their process needs. Not only will this reduce costs, but it will also result in less hold-up volume, ultimately resulting in higher product yield.

Filter Formats and Device Scale

Independent of the considerations outlined above, but no less important, is the format of the filter. There are two main formats available: cartridge filters, used in stainless-steel housings, and stand-alone single-use capsule filters. The choice of cartridge or capsule is largely driven by the manufacturing plant and setup. Cartridge filters have traditionally been used in biologics manufacturing; however, more manufacturers have moved away from stainless steel and are implementing single-use technologies to increase manufacturing flexibility and efficiency.  

In addition to the choice of cartridges or capsules, it is important to consider future needs. The filter selected should be available in the sizes needed for current processes and also in sizes that enable scaled-up production in the future.  In general, considering the long-term goals of a project early may prevent having to redevelop the filtration process later, potentially saving valuable resources and mitigating the risks associated with design changes.

Today, filters are selected to meet the needs of different biologics manufacturers, applications, and process steps. An experienced filter supplier can be a partner that helps identify the most suitable sterile filtration products to maximize the efficiency of your operation, assure successful validation of performance, and provide supporting product documentation to streamline regulatory filing.

Stephanie Ferrante
Stephanie Ferrante is the Associate Director of Biosafety Technology Management. She has over 15 years of experience supporting biopharmaceutical filtration processes, with a focus on sterilizing filtration and aseptic processing. Stephanie contributes to the Aseptic Processing training courses at PDA and has a B.S. in Microbiology and Public Health from The University of Massachusetts, Amherst and a M.S. in Environmental Science from Oklahoma State University in Stillwater, OK.

David Beattie
David Beattie is the Vice President of R&D for BioProcessing at MilliporeSigma, the laboratory and process solutions division of Merck KGaA, Darmstadt, Germany, where he is responsible for technology and product development related to cell culture, chromatography, purification, single use processing, system hardware, filtration and virus safety.  Previously, he was Global Director of Biotechnology Process Sciences at EMD Serono, where he managed all biologics drug substance process development, generic platform process innovation and quality by design activities at EMD Serono’s four biotechnology development sites.  Prior to joining the MilliporeSigma, he was Assistant Director of Process Development at Ipsen from 2002-2005.  From 1998-2002, he was Director of Process Development at Avant Immunotherapeutics, and from 1992-1998, he was a Research Manager for VRI, which merged with T-Cell Sciences to create Avant.  David received a B.A. in Molecular Biology from Colgate University, a Ph.D. in Microbiology and Molecular Genetics from Harvard Medical School and an M.B.A. from Boston University.

  1. FDA, Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing — Current Good Manufacturing Practicehttps://www.fda.gov/downloads/Drugs/Guidances/ucm070342.pdf
  2. American Society for Testing and Materials, ASTM F838: Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized for Liquid Filtration https://www.astm.org/Standards/F838.htm

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