Charged Membranes: Adding Another Dimension
Chemical modification of polymer filter materials can provide a measure of affinity or repulsion between the membrane and biomolecules, potentially enhancing performance.
Charged ultrafiltration and charged membrane chromatography, both specialties of Mark Etzel, Ph.D., associate professor of www.wisc.edu.com), may one day lead to improved impurity clearance and compact even capture media. These membranes separate species on charge as well as size thereby improving selectivity and flux. They reject, that is retain, molecules with the same polarity through electrostatic repulsion.
In theory, processors can use larger-pore membranes, which should better retain the target molecule while permitting more noncharged and oppositely charged species to flow through. “In other words, you can use less membrane area to do the same job,” Dr. Etzel says.
Another advantage of charged ultrafiltration membranes is the ability to separate similarly sized proteins based on different charges, which is not possible using uncharged membranes. For example, a positively charged membrane will retain a basic protein and allow an acidic protein to pass through. Similarly, positively charged mAbs will be retained and concentrated by a positively charged membrane. “This greatly increases a membrane’s separation power, which is a big step forward in ultrafiltration,” Dr. Etzel adds.
Much of the recent ground-breaking work on charged membranes was done by Robert van Reis at Genentech (www.genentech.com), who holds chemistry patents for attaching charges to cellulose membranes. The idea of charged ultrafiltration membranes is not new; van Reis took an under-exploited idea and made it into something that turned out quite useful for bioprocessors.
Charged membrane chromatography, a related idea, has come into and gone out of fashion, according to Dr. Etzel, who has worked in this field for the last 15 years. “It was hot during the early 1990s, then faded away, then came back during the last two or three years.”
Charged membranes’ waxing and waning fortunes are, in part, due to an unreasonable expectation for their use in protein capture to support or replace column chromatography. “That was clearly the wrong job for them,” states Dr. Etzel.
Membranes have to compete favorably with packed-bed chromatography, which they do for clearance of viruses, DNA, endotoxins, and other contaminants that are too large to diffuse into beads in a packed bed.
Dr. Etzek admits that his position is somewhat controversial. “Many people are trying to use membrane chromatography for protein purification vs. contaminant clearance, and that may happen someday for large-scale processes, because membrane capacities are increasing all the time.”
For now, though, Dr. Etzek argues, membrane adsorbers are best confined to small-scale capture and polishing. “Column chromatography still works pretty well and is entrenched at biomanufacturing facilities. It’s hard to unseat an established technology.”