At Least 11 Things You Didn’t Know about Filtration (but you should, because you probably do it all the time)

Even people in early civilizations filtered liquids, usually by using woven materials like cloth to strain water. Modern filtration using synthetic membranes dates to the early 1950s, when the U.S. Army Chemical Corps commissioned the Lovell Chemical Company (later the Millipore Corporation) to design and manufacture nylon membrane-based devices for rapid analysis of microorganism contaminants in drinking water. Today’s filtration for biomedical and life scientists has evolved to include membrane materials and pore architecture tailored for optimal clarification and sterilization for a wide variety of solvents, and what is suspended in them. As cell and tissue culture have become fundamental techniques for modeling biological systems in the life sciences, toxicology, and pharma testing, the need for efficient and dependable devices for sterile filtration in the lab is more important than ever.

What is that membrane filter actually doing? Materials, pore size, rating, and more

Whether you just never gave it much thought and used whatever was available in the lab, or realize that you might want to make an informed selection from the wide array of membranes and devices currently available, understanding why there are so many filtration choices today could lead to finding the right filter. If you find after reading this that you’re not using the right membrane, making an informed choice can even lead to resolving contamination or other problems you might be experiencing in your application now.

Scanning electron micrographs of membranes commonly used for tissue/cell culture and sterile filtration applications.

From left to right: nylon net for cell collection and tissue prep; mixed cellulose ester (MCE) for some sterile filtration tasks; fast flow polyethersulfone (PES); very-low-protein-binding polyvinylidene fluoride (PVDF). Far right, asymmetric top-to-bottom (or feed, left to filtrate, right) pore architecture—here, in a PES membrane— is one way to get faster flow and still filter out small particles or microbes.

Why so many different kinds of membranes? A quick guide

Filters made of nylon are of course still available, as nylon is still an excellent choice for tasks like straining and dissociating tissue in tube-top strainers, as well as countless applications for nonsterile syringe filtration.

For applications like sterile filtration of aqueous solutions, newer chemistries are used to make membranes because they have properties optimized for frequent sterile filtration of aqueous solutions:

• Low protein binding to ensure that essential culture media factors are not unintentionally depleted by filtration
• Faster flow to minimize exposure of sterile media during filtration, and speed up everyday filtration tasks
• Low extractables to guarantee that unwanted chemicals leached from the membrane or device itself don’t end up in media for cells.

Polyethersulfone (PES) is ideal when fast flux is needed to filter volumes quickly, and is also low-protein-binding. When you have particularly stringent needs that dictate that the concentration of essential proteins in media or buffers is unaffected by filtration, the highly nonreactive polymer polyvinylidene fluoride (PVDF) is the best choice, as it demonstrates extremely low protein binding.

Why does filter membrane pore size matter?

You may have filter devices in your lab marked with specifications like 0.1, 0.22, or 0.45 µm. These pore size designations contribute to the filter’s rating, or what particle size may pass through the filter under laboratory conditions. The scale below helps to put macromolecules, common particles, and microbes into size perspective.

Filters with different pore sizes can be used to clarify solutions by excluding particulate matter, or to sterilize by preventing bacteria, fungi, and other contaminants—and when needed, two different filters can even be layered for simultaneous clarification and decontamination (check manufacturer’s instructions).

Some general guidelines for common lab tasks:

• 0.45 µm membrane pores work well for clarification
• 0.22 µm membrane pores will exclude most microbial contaminants (but not mycoplasma)
• 0.1 µm membrane pores are typically rated for mycoplasma and other small microbe—but flow more slowly, so are not the most efficient or necessary for routine applications.

Does it leach, tip, slip, seal? How filter design can save money, time, and ensure sterility

Let’s be frank—basal medium, serum, supplements, and other components necessary for successful cell culture are not inexpensive. Most labs can’t afford to discard a bottle of freshly prepared media because the lid didn’t seat and seal properly, and spilled media mishaps in the hood can mean hours of cleanup to restore sterile conditions. Scientists who spend hours working in biosafety cabinets with liquids have helped engineers at MilliporeSigma to optimize sterile filtration device features that increase stability and minimize awkward handling that can lead to spills.

Features to look for in sterile filtration funnel/bottle devices include:

• Base stability and lower profile to minimize spills and awkward reaching in the culture hood
• Clear membrane spec labeling on the product itself enables accurate selection for the application
• Optimal writing surfaces for in-lab labeling of filtered contents minimizes accidental wrong-media applications
• Precise machining and closure indicator for bottle cap protects from compromise of sterile contents
• Ergonomic design of funnel-bottle threading and vacuum ports to minimize handling and spills
• Manufacturer-documented testing of sealants and materials to ensure no or low extractables

We hope these tips about membrane characteristics and filter devices will help you navigate sterile filter choices depending on your lab’s needs—but there’s still much more to learn about membrane filtration. If you’re hungry for more, visit sigmaaldrich.com/stericupquickrelease to request a complimentary copy of our comprehensive Clarifying Guide to Sterile Filtration.