When developing custom filtration systems for medical devices and applications, selecting the right filter material is crucial. One of the key factors to consider is whether a hydrophilic or hydrophobic filter material will provide the performance you need. But what do these terms mean and how do you choose the right one? Read on as we explore the differences between hydrophilic and hydrophobic filter materials and provide some tips for selecting the ideal one for your application.

What does hydrophilic mean?

The term hydrophilic has an interesting etymology. “Hydro-” refers to water and “-philos” means friendship or affinity. When combined, hydrophilic translates to water-loving or having an affinity for water. Hydrophilic describes surfaces and materials with low contact angle (< 90⁰) and high surface energy that are attracted to water.

What does hydrophobic mean?

Building upon the root words mentioned above, “-phobic” means dread or fear. When combined, hydrophobic translates to water-fearing or having an aversion to water. Hydrophobic describes surfaces and materials with high contact angle (> 90⁰) and low surface energy that repel water.

Selecting the Right Filter Material for Your Application

The choice between hydrophilic and hydrophobic materials depends on your application and what you need the filter to do. For most medical applications, the following simple rules can be applied:

• Liquid application: Hydrophilic filter material 
  • Knowing that hydrophilic materials are water-loving, it makes sense that hydrophilic materials are commonly used for liquid filtration applications. 
  • Most liquids readily flow through hydrophilic membranes. This means you can avoid the use of pre-wetting agents and eliminate one potential source of contamination in your workflow.  
• Gas application: Hydrophobic filter material 
  • Hydrophobic materials have a strong aversion to water, which means they allow gas molecules to flow freely while preventing liquid water from passing through. 
  • Moisture and liquid droplets will not be absorbed by a hydrophobic membrane, optimizing the flow of gas/air. 

One of the most common hydrophilic materials used in medical filtration applications is polyethersulfone (PES). PES is a go-to material for sheath fluid filters in diagnostic equipment, pre-filters and sterilizing filters in endoscope reprocessing devices, and point-of-use water filters for dentistry and hospital use. PES is often utilized in a dual-layer configuration in which a larger pore size PES membrane (e.g., 0.45 um or 0.65 um) is layered over a smaller pore size PES membrane (e.g., 0.2 um or 0.1 um). Incorporating a built-in PES pre-filter can help extend the lifetime of the filter. Other hydrophilic materials we often see used in medical applications include nylon membrane and natural glass fiber.

Polytetrafluoroethylene (PTFE) is one of the most common hydrophobic materials used in medical applications. PTFE is a workhorse for sterile venting. Typical applications include respiratory therapy, in vitro fertilization, diagnostic equipment, and endoscope reprocessing devices. PTFE can be used alone or with various materials such as glass fiber and activated carbon. Incorporating a non-woven coalescing/pre-filter, like glass fiber, into a PTFE vent can prolong the life of the hydrophobic membrane. It is important to note, however, that PTFE is not gamma stable; for applications requiring a gamma stable hydrophobic material, polyethylene is a suitable alternative. Overall, hydrophobic materials commonly used for medical applications include PTFE, polypropylene, polyethylene, and glass fiber.

If you are unsure about the hydrophilic or hydrophobic nature of a membrane material, you can conduct a simple wettability test. Place a droplet of water onto the surface of the material and assess its behavior. If the droplet tends to spread out (contact angle < 90⁰), the material is likely hydrophilic; if the droplet tends to retain a spherical shape (contact angle > 90⁰), the material is likely hydrophobic.

Exceptions to the rules

In our experience, the rules outlined above apply to about 90% of medical device applications. However, there are important exceptions to consider for both hydrophilic and hydrophobic materials. 

• A hydrophobic membrane is recommended for liquid filtration when:
  • The liquid being filtered is chemically incompatible with hydrophilic materials. For example, it is not recommended to use PES membranes with dimethyl sulfoxide (DMSO) or strong acids like nitric or sulfuric acid. 
  • The application has heightened purity requirements. In this situation, utilizing hydrophobic material may reduce extractables. 

When using a hydrophobic membrane for liquid filtration, you may need to incorporate a pre-wetting step into your workflow. Pre-wetting ensures that the hydrophobic membrane becomes temporarily hydrophilic, allowing it to efficiently filter aqueous solutions. If the liquid surface tension is low (e.g., IPA with a surface tension of approximately 22.7 mN/m) or if the pore size is large enough, you can likely avoid pre-wetting. 

• A hydrophilic membrane is recommended for gas/air filtration when:
  • The gas being filtered is chemically incompatible with hydrophobic materials. For example, exposure to certain acids and bases may cause PTFE to swell and shrink. 
  • The application conditions are incompatible with hydrophobic materials.

Choosing Your Hydrophilic/Hydrophobic Membrane 

Selecting the right materials for construction for any medical application is a crucial factor that can greatly impact the performance and success of a product. When developing custom filtration systems, it is important to think about the configuration (disc, cartridge, capsule), the effective filtration area (EFA), the inlet and outlet fittings, the pore size rating of the filter material and, as discussed here, the hydrophilic vs. hydrophobic nature of the filter material.  

While there are numerous facets to consider, such as biocompatibility, chemical compatibility, mechanical properties, and ease of processing, it is important to collaborate with an engineering team that possesses an in-depth understanding of both material properties and application-specific requirements. 

We understand that selecting the right filter material can be a complicated process, and it is not always easy to make the best decision without the right expertise. Consult a technical expert who can provide guidance and recommendations based on the specific needs of your filtration project.