Tangential Flow Filtration Explores New Niches
TFF has been around for a while, but its place in biopharma manufacturing is hardly static.
By Paul Thomas, Senior Editor
Tangential flow filtration is old school. It’s been used in a variety of industries for decades—take blood and food processing, for example—and has become standard in biopharmaceuticals as an efficient and cost-effective means of separation. But you can’t really say that tangential flow filtration (TFF) has found its niche in biopharma, because as technologies change, so do applications.
The niche shifts, expands and contracts. Biopharmaceutical developers and manufacturers are using TFF in place of, or in tandem with, ultrafiltration, microfiltration, and diafiltration. It has found favor as a means of concentrating sample solutions (large and small), fractionating molecules, harvesting cells, and clarifying fermentation broths or cell lysates. In some biomanufacturing plants, tangential flow is replacing the need for material centrifugation.
North Carolina-based NCSRT, Inc. develops and optimizes filtration systems for a number of smaller and larger manufacturers, and lately the majority of its business is in patented two-step TFF systems, which offer the promise of higher yields, fewer purification steps and shorter operating time. “I used to do 40-liter batches of Epstein-Barr virus in a centrifuge for one month, and get a 10% yield,” founder and chief technology officer Hank Kopf says. “Now [through tangential flow filtration], I’m getting 90% of the virus and eliminating contaminants.”
NCSRT has teamed with Winnipeg’s Viventia Biotech to document the advantages of its SmartFlow TFF over centrifugation for clarifying and recovering Viventia’s Phase I/II VB6-845 antigen-binding fragment (Fab) (See Figure 1 below). It’s one example of how TFF is making inroads into new areas of bioprocessing. The adoption of TFF is not wholesale, however, notes Kent Iverson, an independent biologics CMC consultant. Take clarification of cell culture broth.
TFF is actually being replaced by centrifugation in many plants, and by direct flow “dead-end” filters in others—they’re often simpler and cheaper, Iverson says. That’s not to say that there isn’t a trend developing here. “The tangential flow filtration market will continue to grow,” Iverson says, especially as a means of ultrafiltration. In addition, intriguing TFF-based processes will continue to crop up in as-of-yet untested areas (box at right).
Killer Apps for TFF
Kent Iverson, an independent biologics CMC consultant, discusses three unique and rediscovered applications of tangential flow filtration:
Two-phase extraction: Protein of interest can be extracted from a process fl uid by adding an immiscible solvent (which can be aqueous), as long as the product has higher solubility. TFF using hydrophobic membranes can be used to separate the two liquid phases from one another. A company developing two-phase
extraction technologies for proteins is Analiza, Inc. (Cleveland, OH).
Suspension perfusion cell culture: Mammaliancell cultures can be grown continuously at very high cell densities, by aseptic dialysis with fresh medium. Cell culture fluid is recirculated from a bioreactor through a sterilized TFF microfiltration module; “spent” media is removed as filtrate from the module, while fresh medium is added to the bioreactor to maintain a constant culture volume. Some manufacturers, such as contract manufacturer Pacifi cGMP (San Diego), are conducting this method with a completely disposable bioreactor and TFF components.
Inclusion-body recovery: The insoluble protein product expressed in some intracellular bacterial expression systems can be diffi cult to separate from host cell contaminants such as endotoxin. After recovery and lysis of the bacterial biomass, the inclusion body fraction can be “purified” by circulating the lysate through a high molecular weight cut-off TFF ultrafilter, or in some cases a microfilter, and dialyzing the retentate with a dilute chaotropic agent or solvent until a suitable level of purity of the target protein is achieved.
A Tested Technology
As its name implies, tangential flow filtration involves a feedstream which flows parallel to the filtration membrane surface. Permeate passes through the membrane, while retentate returns to the original feed tank (Figure 1). The cross flow (TFF is also known as cross flow filtration) means less molecule buildup and fouling of the membrane surface. Applications depend upon multiple factors: among them, the nature of the feed material (e.g., the viscosity or density of cells), the geometry of the feed channel flow path, feed velocity, and filter material, size and configuration.
Contract R&D and manufacturing outfits, which tend to have greater product turnover and a willingness to showcase novel technologies, deserve credit for testing and popularizing TFF of late, NCSRT’s Kopf says. While single-use or disposable TFF systems are gaining traction in labs and pilot plants—with membranes, housing, valves and tubing all capable of being disposed of rather than cleaned and reused (more on this later)—most manufacturers are highly invested in their tried-and-trusted production platforms. And they’re hesitant to devote the time and effort required for the thorough extractable and leachable studies needed to qualify disposable materials.
What’s more, technology for reusable TFF continues to advance, whether the system is based upon flat-sheet cassettes or cylindrical hollow-fiber modules. Improved cleanability and flexibility are two key developments. Sartorius Stedim’s Sartocon series of cassettes, for example, now have steam-in-place capability for aseptic processing, says Christian Manzke, the company’s director of purification technologies. For non-sterile applications, the company’s Hydrosart regenerated cellulose filters can withstand caustics ranging in pH from 1 to 14.