Introduction to Tangential Flow Filtration
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.
在使用细分筛尝试将沙子从鹅卵石中分离的过程中，即可看到一个用以理解 TFF 背后理论的类比。筛子上的洞代表膜中的孔，而沙子和卵石则代表要分离的分子。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).利用 DFF，增加压力就可以在不增加分离的情况下压缩混合物。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).
(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.
图 2Direct Flow Filtration Process
(A) 进料被引入滤膜。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.
图 3Tangential Flow Filtration Process
(A) 样品溶液流过进料通道后既沿着（成切线）滤膜表面流动也流过滤膜。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.