Alternating Tangential Flow Filtration

Hollow fiber and cassette membranes are the two most common membranes for flow filtration. Hollow fiber membranes are cylindrical, with the membrane embedded as fibers in the cylinder. The fluid passes through the cylinder, with ports to separate permeate and retentate at either end of the cylinder. Cassette membrane layers are stacked and separated by screens. The fluid is flowed through the layers and the permeate is removed out of the side, while the retentate continues to flow.

Membranes are classified by their average pore size. Those used in bioprocessing are generally for either microfiltration or ultrafiltration. Microfiltration membranes have pores with diameters greater than 0.1 µm, and are generally used to separate proteins and other small molecules from cells (the larger cells are discarded in the retentate). Ultrafiltration membranes have pores with diameters smaller than 0.1 µm, and are often used to concentrate proteins. The proteins are in the retentate in this case, while water and various small solutes are discarded in the permeate.

Dead‑ended filtration flows samples directly through a membrane, oftentimes resulting in a clogged filter. The tangential flow of TFF prevents clogging and is much faster and more efficient than traditional filtration methods.

In TFF, the flow stream with the desired product can be recycled and looped through the system as many times as necessary for complete separation of the permeate and retentate. This process can also be used to clean the filter, by flowing pure buffer through the system.

As with dead‑ended filtration, the size of the membrane pores determine which particles are able to pass through the membrane. Pore size can be optimized to isolate the desired particles either in the permeate or the retentate.

• When centrifugation isn’t available or practical, TFF is a convenient way to harvest cells. Microfiltration membranes will retain the whole cells while allowing for selective recovery of the supernatant.
• After cells are lysed, TFF is useful for lysate clarification to remove debris such as unlysed cells and membrane fragments. This is often done as an intermediate purification step before chromatography.
• Smaller and larger proteins may be separated using ultrafiltration membranes. The proteins must be of sufficiently different sizes for the membrane to adequately filter the components.
• While chromatography often requires diluting protein samples, final products may require higher concentration. In this case, ultrafiltration may remove permeate while retaining the proteins at a higher concentration.