Filtration Nonwoven Media Versus Membranes in Medical Device Applications

Author: Jacob Andrews – Technical Director for Filtration Technologies

As described in our previous blog post “Important things to consider when choosing a filter for your medical device application,” there are many aspects to consider when choosing or designing a filter that is fit-for-purpose in your medical device application. In this post we begin to examine the different types of filtration materials commonly employed in medical filter devices, specifically focusing on the differences between two major classes of filtration materials – nonwoven media and cast membranes. 

Figure 1, shows example SEM images of a nonwoven media (Figure 1.a, nonwoven Polypropylene) and a membrane (Figure 1.b, Polyethersulfone). These images depict some key morphological differences between nonwovens and membranes. As a collection of fibers, nonwovens create a tortuous path of pores between the fibers through which fluid can flow and particles can be captured. The size and packing density of the fibers may be tuned to create the desired performance attributes of flow, retention, and throughput necessary for the application. Generally speaking, more densely packed fibers will increase the retention efficiency and decrease the flow rate and smaller sized fibers will increase the flow potential at given retention efficiency. 

Membranes on the other hand appear more as a solid film with a tortuous path of pores created in more discrete channels with less opportunity for cross-flow as compared to nonwovens. The various manufacturing methods employed to make membranes are used to tune the size and quantity of pores, membrane thickness, chemical properties and other attributes as required to achieve the desired flow and retention performance necessary in the application. 

Figure 1: 

1.a 

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1.b 

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SEM Images of nonwoven and membrane. 

Figure 1.a shows an SEM image of nonwoven PP media 

Figure 1.b shows an SEM image of a Polyethersulfone (PES) membrane. 

One of the main advantages of using nonwovens over membranes in filter applications is that nonwovens typically provide higher dirt holding capacity (DHC) compared to membranes. This is due to the greater thickness and cross-flow ability of nonwovens which, when properly selected, allows particles to be captured throughout the depth of the media rather than at its surface (Figure 2). 

Whether particles are captured on the surface of a filter material or within its pore structure is ultimately dependent on the size of the pores compared to the size of the particles being captured. In the case of symmetric filtration materials, if the particles are much larger than the average pore size of the filter material, the majority will be captured at the upstream surface. If the particles are much smaller than the pore size of the filter material, the majority will pass through to the downstream side. And finally, if the particles are similarly sized to the pore size of the filter material, they are likely to be captured throughout the thickness of the filtration material. (See Figure 3) 

Figure 2: 

2.a 

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2.b 

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Diagram of particles captured on a surface and through the depth of a filtration material. 

Figure 2.a shows a diagram of a membrane with particles captured on the surface. 

Figure 2.b shows a diagram of a nonwoven with particles captured throughout the depth of the media. 

Figure 3: 

3.a 

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3.b 

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3.c 

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Diagram of different sized particles captured by a nonwoven filtration material. 

Figure 3.a shows a diagram of a nonwoven with large particles captured on its surface. 

Figure 3.b shows a diagram of a nonwoven with medium particles captured throughout its depth. 

Figure 3.c shows a diagram of a nonwoven with small particles which are not captured. 

Due to the increased thickness, relatively wider pore size distribution, and morphology of the tortuous path within nonwovens compared to membranes, when particles are captured throughout the thickness of a nonwoven, a relatively large number of particles can be captured prior to the material becoming plugged . As shown in Figure 4, the thickness of the nonwoven provides more volume within which particles may be captured; its wider pore-size distribution allows more particles to flow past the surface without becoming captured; and the morphology of its tortuous path allows for significant cross-flow, which means the fluid can more easily navigate around portions of the media which have been blocked by particles. By contrast, membranes typically are thinner, have a narrower distribution of pore size, and are more limited in the amount of cross-flow allowed through the morphology of the tortuous path. 

In combination, these factors generally reduce the ability of the filtered fluid to navigate around captured particles and results in the relatively lower DHC of membranes. 

Figure 4: 

4.a 

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4.b 

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Diagram of particles captured by a membrane and a nonwoven filter material, and the relative impact on fluid flow through the material. 

Figure 4.a shows a diagram of a membrane with particles captured and blocking much of the fluid flow through the material. 

Figure 4.b shows a diagram of a nonwoven with a similar number of particles (in fact slightly more) captured but with more channels for fluid flow through the material remaining. 

Despite the higher DHC of nonwovens, membranes offer several important advantages over nonwovens which make their use advantageous or required in many applications. One advantage of membranes over nonwovens is that the pore size achievable with a membrane is typically much smaller (and with a narrower distribution) than can be achieved with a nonwoven. In the case of sterilizing liquid filtration – where a filter is validated to remove 100% of specified microbial contaminants under controlled conditions, such as those specified in ASTM F838-20 – a membrane is typically required. In this application, a very narrow and well controlled pore size distribution is needed because even one oversized pore can lead to microbial passage which would result in failure and potentially risk patient safety. A very small pore size (typically 0.2 µm or less) is required due to the small size of the microbes typically targeted for complete removal. 

Another area where membranes often offer an advantage is in liquid repellency. Hydrophobic filters are typically used to filter gases due to the fact that they prevent the passage of liquids that are often undesirable in the gas stream. (See our related posts on “Hydrophilic vs Hydrophobic: What's The Difference and How To Select” and “Selecting the Correct Filter Media: What Questions Should You Ask?”) A filter material’s resistance to the passage of liquids is determined by measuring its Water Intrusion Pressure (WIP) – or the pressure required to force liquid into the pores of the filter material. In some applications, a high WIP is required to prevent contamination (by liquid) of the medical device and/or patient. Membranes with WIP values exceeding 50 psi are readily available and therefore are often chosen over nonwovens which typically exhibit much lower WIP values. 

Ultimately, the choice to use a nonwoven or a membrane in your medical device applications comes down the specific requirements of the application. And in some applications, the greatest performance can be achieved by using nonwovens in combination with membranes. In these applications, nonwoven materials are typically used upstream to act as a prefilter and extend the life of the downstream membrane filter, which may be required to gain the necessary retention or liquid repellency performance. Often the optimal media and membrane combination can be customized into a single filtration device that meets the specific needs of the application. This optimal combination is typically determined through both the application of theory as well bench-scale performance testing. 

Saint-Gobain offers engineered filtration solutions that can be customized for your application and is available to support your next medical filtration project with Applications and Development Engineering expertise. For more information contact us at www.medical.saint-gobain.com/contact-us

  • For the purposes of this post, the terms nonwoven and media refer to filtration materials other than cast membranes. They can include many types of nonwoven media classes including melt blown, electro spun, and wet-laid media. The term membrane, by contrast, refers to a typically cast or expanded porous film. The term filtration material is used to generally describe all such materials incorporated as the retentive layer (or layers) within a filter element – including nonwovens, membranes, and other classifications not described in detail within this post. 

  • The pores of a filter media refer to the channels created within the void areas of the material. Although pores are sometimes idealized as a cylindrical hole, in reality a filter’s pores are a tortuous path between the upstream and downstream surfaces of the filtration material. 

  • Typical membrane fabrication methods include Solvent Induced Phase Separation, Temperature Induced Phase Separation, and sintering followed by physical expansion. 

  • The dirt holding capacity or DHC of a filter is the amount of material (typically particles) that the filter can capture without blocking (i.e. without reducing flow beyond the acceptable limit). 

  • The term plugged refers to the blockage of flow through the filter which is caused by the accumulation of contaminants within a filter material. A filter is considered plugged, or fully loaded/exhausted, at the point where the pressure drop / flow ratio has increased beyond a value acceptable in the intended application. 

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