Introduction to Tangential Flow Filtration
Tangential flow filtration (TFF) is a rapid and efficient method for separation and purification of biomolecules. It can be applied to a wide range of biological fields such as immunology, protein chemistry, molecular biology, biochemistry, and microbiology. TFF can be used to concentrate and desalt sample solutions ranging in volume from 10 mL to thousands of liters. It can be used to fractionate large from small biomolecules, harvest cell suspensions, and clarify fermentation broths and cell lysates. This report describes the basic principles that govern TFF and the use of TFF capsules and cassettes in laboratory and process development applications.
Tangential Flow Filtration OverviewMembrane filtration is a separation technique widely used in the life science laboratory. Depending on membrane porosity, it can be classified as a microfiltration or ultrafiltration process. Microfiltration membranes, with pore sizes typically between 0.1 µm and 10 µm, are generally used for clarification, sterilization, and removal of microparticulates or for cell harvesting. Ultrafiltration membranes,
with much smaller pore sizes between 0.001 and 0.1 µm, are used for concentrating and desalting dissolved molecules (proteins, peptides, nucleic acids, carbohydrates, and other biomolecules), exchanging buffers, and gross fractionation. Ultrafiltration membranes are typically classified by molecular weight cutoff (MWCO) rather than pore size.
There are two main membrane filtration modes which can use either microfiltration or ultrafiltration membranes: 1) Direct Flow Filtration (DFF), also known as ”dead-end” filtration, applies the feed stream perpendicular to the membrane face and attempts to pass 100% of the fluid through the membrane, and 2) Tangential Flow Filtration (TFF), also known as crossflow filtration, where the feed stream passes parallel to the membrane face as one portion passes through the membrane (permeate) while the remainder (retentate) is recirculated back to the feed reservoir.
An analogy for understanding the theory behind TFF can be seen in trying to separate sand from pebbles using a sifting screen. The holes in the screen represent the pores in the membrane while the sand and pebbles represent the molecules to be separated. In DFF, the sand and pebble mixture is forced toward the holes in the screen. The smaller sand grains fall through the pores in the screen, but the larger pebbles form a layer on the surface of the screen. This prevents sand grains at the top of the mixture from moving to and through the holes (Figure 1A). With DFF, increasing the pressure simply compresses the mixture without increasing the separation. In contrast, operating in a TFF mode prevents the formation of a restrictive layer by re-circulating the mixture. The process acts like a shaking sifter to remove the pebbles that block the holes in the screen, allowing the sand grains at the top of the mixture to fall toward and through the holes in the screen (Figure 1B).
Figure 1Separation of Sand and Pebbles Using a Sifting Screen
(A) Applying direct pressure to the mixture allows the sand grains at the bottom to fall through. A layer of pebbles builds up at the screen surface preventing sand grains at the top from moving to and through the screen.
(B) Shaking the screen breaks up the aggregated pebble layer at the bottom of the mixture and allows for complete fractionation. The crossflow dynamic of the feed stream in tangential flow filtration serves the same purpose as shaking in this example.
In solution, the same effect is encountered for DFF (Figure 2) and for TFF (Figure 3). The flow of sample solution across the membrane surface sweeps away aggregating molecules that form a membrane-clogging gel (gel polarization), allowing molecules smaller than the membrane pores to move toward and through the membrane. Thus, TFF can be faster and more efficient than DFF for size separation.
Figure 2Direct Flow Filtration Process
(A) The feed is directed into the membrane. Molecules larger than the pores accumulate at the membrane surface to form a gel, which fouls the surface, blocking the flow of liquid through the membrane.
(B) As the volume filtered increases, fouling increases and the flux rate decreases rapidly.
Figure 3Tangential Flow Filtration Process
(A) Sample solution flows through the feed channel and along (tangent to) the surface of the membrane as well as through the membrane. The crossflow prevents build up of molecules at the surface that can cause fouling.
(B) The TFF process prevents the rapid decline in flux rate seen in direct flow filtration allowing a greater volume to be processed per unit area of membrane surface.