Selecting the right filters is crucial for efficient purification and safety of biopharmaceuticals, and filterability testing is an important part of the selection process. This article guides you through the essentials of choosing a filter and running filterability tests. Whether you’re purifying monoclonal antibodies (mAbs), viral vectors, or other therapeutic molecules, the information here will help you navigate the complexities of filter selection and optimization for a successful biomanufacturing process.
Choosing a filter
There are many filters to choose from, with lots of different filter membrane materials and specifications. Only a subset of these will be suited for a given process step or purpose, however. So, defining the purpose of the filter is the first step. From there, the process requirements and the feed material will help you narrow your options further.
Example filter selection path:
- What is the goal of the filtration: sterility, bioburden control, removal of certain impurities, concentration or buffer exchange?
- What is the target molecule, size, and charge?
- What are the process liquid excipients, pH, and viscosity?
- What are the requirements and constraints (e.g., batch volume, flow rates, process time, equipment compatibility)?
The answers to these questions will determine the type of filter and specifications (e.g., pore size, retention rating, membrane material) best suited for your purpose. To find filters based on target molecule and process step, take a look at this selection guide.
Filterability testing at a glance
A filterability test (or trial) is a small-scale filtration assessment on the bench, using as little product as possible. This test helps you choose the best-performing filter and establish operating parameters for your process. Below is an overview of the general procedures for filterability testing for direct flow filtration and tangential flow filtration (TFF).
Direct flow filtration
For direct flow filtration tests, it’s important to focus on key parameters such as flow rate, pressure, and filter capacity. This will allow you to determine the most suitable filter for your product during testing and will allow for filter sizing for your large-scale process.
There are two methods for how you can approach a direct flow filtration test (Fig 1), which will depend on your product type and scale-up requirements. It is important to assess here whether the filtration will be performed using a pump or a pressure source. Typically, you’ll use the same method at large scale as you use for small-scale filter tests.
Constant pressure filtration: Use a pressure source to maintain constant feed pressure throughout the study. When you see a significant drop (at least 75%) in flux (flow rate by unit of membrane area), the filter has reached its maximum capacity, and the test can be stopped.
Constant flow filtration: Use a pump to keep a steady feed flow rate. When back pressure rises significantly (typically 1.5-2.0 bar for filter tests), this signals the filter’s maximum capacity has been reached.
The capacities you determine during the study, along with product quality metrics such as turbidity, host cell protein concentration, endotoxin, or DNA levels, will enable you to accurately select and size your filter.
Fig 1. Comparison of constant pressure filtration vs constant flow filtration setup. For constant pressure filtration, throughput is assessed based on flux decline, whereas for constant flow filtration, throughput is assessed based on pressure increase.
Tangential flow filtration
Small-scale trials for TFF start with a flux excursion (flux vs time) to determine what level of fouling to expect with your product. This involves measuring the filtration flow over a period of time to see how it changes as foulants accumulate on the filter.
If the fouling rate is minimal, the process may be operated under transmembrane pressure (TMP) control. For products with higher fouling rates, or for filters with very high permeability, restricting the permeate flow (permeate flow control, PFC) will typically yield more stable operation for a longer time than can be achieved with unrestricted permeate flow.
Fig 2. Comparison of transmembrane pressure control vs permeate flow control setup. This is the same for hollow fiber or flat sheet cassettes.
For TMP control, optimal working TMP — i.e., flux vs TMP — and the sensitivity to cross-flow rate (volumetric flow rate vs differential pressure) are evaluated. This entails measuring the filtration flow at different TMP values (starting with a low TMP) until it plateaus (that is, until there is no longer an increase in filtration flow as the TMP is increased).
Fig 3. Example flux excursion using TMP control strategy
For permeate flow control (PFC), a pump is used to restrict and control the permeate flow rate. Restriction of the permeate flow increases pressure within the filter permeate space and lowers the TMP. A permeate flow rate excursion identifies the flux at which the TMP becomes unstable, known as the critical flux. Process operation is performed at a permeate flow rate lower than the critical flux. (Learn more about the methods and parameters described here in our TFF handbook.)
Fig 4. Example permeate flow rate excursion to identify the critical flux (yellow bar) and the optimal operating flux (green bar) which is far enough below critical flux to allow robust performance
After the trial, a cleaning protocol determines how to clean the cassette, and an integrity test before and after the filtration determines the filter’s suitability for reuse.
Filterability testing can be completed with standard manual lab equipment. Your setup should include pumps for flow of the product onto the filter, scales to determine the filtrate flow rate (or flux), and pressure sensors to monitor the pressure throughout the study. Automated filter testing is also an option with equipment such as ÄKTA™ chromatography systems (for direct flow filtration testing) or the ÄKTA™ flux system (for TFF testing).
Want to test a filter or get more guidance?
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FAQs
What is the difference between a filterability test and a filter integrity test?
Filter integrity tests are used to confirm filter performance in an established process, for example to confirm the performance of the sterile filter to produce a sterile final drug product. Filterability tests, in contrast, are part of process optimization and development and are related to finding the right filter for a given operation. Both types of testing are used widely in bioprocessing as well as in food and beverage applications.
What is filterability?
Filterability refers to the ability of a fluid to pass through a filter.
What is filtration in bioprocessing?
Filtration is used in bioprocessing for a multitude of processes as part of an overall bioburden control strategy and in downstream purification steps. This includes removal of contaminants such as cell debris, viruses, and bacteria; concentration and buffer exchange; and removal of airborne contaminants from labs and workspaces.