The need for better characterization of filtration membranes
Historically, the filtration industry has relied on various testing methods — bubble-point measurements, microscopy, and membrane challenge tests — to characterize and qualify products. These techniques give valuable information about membrane properties and performance. But each is limited in its ability to provide quantitative information on membrane characteristics. Table 1 summarizes their uses and limitations.
Table 1. Uses and limitations of historical methods for testing filters
|What it does||How it’s limited|
|Bubble-point testing||Characterizes the largest pore in an integral membrane||
|Microscopy||Provides qualitative information about the specific pore structure||
|Challenge testing (e.g., particle or solute rejection)||Provides quantitative information on the performance characteristics of the filtration media||
Capillary flow porometry (CFP) offers many benefits, such as speed, ease of use, and reliability. The method provides quantitative information about essential membrane characteristics that are not accessible by other methods. These include the largest, smallest, and average pore sizes as well as detailed information on pore size distribution. Cytiva has developed a capillary flow porometry test to confidently and efficiently characterize the pore structure of hollow fiber filtration membranes before manufacturing the cartridges. Capillary flow porometry is particularly effective in measuring the pore size distribution for the full range of Cytiva microfiltration products down to and including 0.1 µm. It is minimally effective at characterizing Cytiva ultrafiltration products with a lower limit of 500 kDa nominal molecular weight cut-off, which is the lowest molecular weight (in Daltons) at which greater than 90% of a solute with a known molecular weight is retained by the membrane.
Examples of Cytiva’s capillary flow porometry information
Capillary flow porometry applies a relationship between the membrane pore diameter and the pressure required to expel a specified wetting fluid from the membrane pores. This is derived from the Young-Laplace relationship, and the diameter and distribution of the membrane pores may be determined. It is important to note that the choice of wetting fluid is essential because the pore size calculation depends on the surface tension and contact angle of the test fluid. Results from different wetting fluids will change the outcome of the membrane analysis.
Figure 1 shows an illustration of the capillary flow porometry technique. Figure 1B demonstrates the process of deducing pore-size distribution by analyzing pressure flow characteristics from capillary flow porometer data, as follows:
- At minimally applied pressures, the membrane is fully impregnated with fluid and no flow characteristics can be measured.
- Higher applied pressures gradually begin to force fluid out of the pores of the membrane, starting with the largest pore in the membrane.
- As the applied pressure grows, increasingly smaller pores are evacuated of fluid.
- The smallest pores are the last to be emptied by the applied pressure.
- The final portion of the curve represents a membrane that had been completely evacuated of fluid.
The pore size distribution is then calculated by comparing measured flow rates of the wetted membrane to the completely evacuated membrane, as shown in Figure 1C. The figure shows the smallest pore size (SP), mean-flow - or average - pore size (MFP), and largest pore size (LP) of the hollow fiber membrane. The relationship between pore size, wetting-material contact angle, and the applied pressure is demonstrated by the expression shown.
Fig 1. Illustration of the mechanism of porometry. (A) illustrates the chosen hollow fiber sample preparation morphology and the mechanism of gas flow through the membrane pores as applied by the CFP instrumentation. (B) demonstrates the process of deducing pore-size distribution by analyzing pressure flow characteristics from capillary flow porometer data. Numbers 1–5 are described in the text. (C) displays the pore size distribution as acquired by CFP.
The following pore size characteristics of the membrane can be determined from this raw data, using the Young-Laplace relationship:
- Bubble point pressure (bar) – pressure at which the largest pore diameter is evacuated of wetting fluid.
- Bubble point pore size (µm) – maximum pore size – the experimentally derived largest pore size of a membrane determined by CFP. This is the point at which the lowest applied pressure is able to evacuate a fluid from the membrane pores.
- Mean flow pore size (µm) – experimentally derived average pore size determined by CFP. This is indicated by the crossover of the experimental pore evacuation curve with the extrapolated ”half-dry curve”.
- Wet curve – experimentally derived pressure-flow curve demonstrating the response of the wetting fluid being evacuated from the pores of the membrane.
- Dry curve – linear response curve obtained when increasing pressures are applied to a dry fiber. This represents the response of when all the wetting fluid has been evacuated from the fiber pores.
- Half-dry curve – curve obtained by dividing the experimentally derived gas flow values from the dry curve by 2 and plotting this resulting curve over the experimentally derived data. The point at which the half-dry curve crosses the experimental wet curve allows the calculation of the mean flow pore size.
- Smallest pore pressure (bar) – pressure at which the smallest pore diameter is evacuated of wetting fluid.
- Smallest pore size (µm) – experimentally derived smallest pore size determined by CFP. This is the point at which the applied pressure is able to evacuate any remaining fluid from the smallest pores of the membrane.
- Pore size distribution – range of pore sizes present in the membrane as determined by CFP.
Capillary flow porometry provides pore size distribution of the membrane material. Unlike other techniques, it provides a more complete understanding of the membrane characteristics that can directly influence the quality and performance of filtration products.
When compared to other methods of membrane characterization, CFP provides many benefits in terms of practicality and the amount of valuable information regarding membrane structure. Using this technique as an in-process testing method can improve the confidence in membrane performance before final unit fabrication.
Modern bioprocess applications require strict standards for quality and reliability in all aspects of laboratory and manufacturing processes. To meet specific needs, Cytiva offers a wide variety of tangential flow filtration (TFF) products, including polysulfone hollow fiber filter cartridges.
- ASTM International. ASTM F316-94. Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test https://www.astm.org/Standards/F316.htm. Updated December 17, 2019. Accessed May 3, 2021.
- Technical note: How to determine the first bubble point. Porometer. 2020.
- Application note: Characterisation of pore size distribution of track etched membranes. Porometer. 2020.
- Training course: Gas-liquid scan system. Porometer. 2020.
- Application note: Calculations used in porometry, Porometer. 2020.
- Technical note: Effects of the wetting liquid on capillary flow porometry results. Porometer. 2020