Figure 6. Figure 6 shows the difference between the flow pattern of packed bed chromatography and membrane chromatography,
which results in different frontal surface area to bed height ratios. The hydrodynamic properties presented by membrane adsorbers
overcomes mass transfer limitations at low pressure but the lower total surface area dictates their use mainly in the removal
of impurities or the capture of low concentration targets.
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The history of membrane chromatography shows the immense progress that has been made since 1988, producing the diversity of
membrane structures, surface chemistries, and configurations that are seen today. Commercial membranes have excellent manufacturing
tolerances, which make them quite uniform in thickness and pore-size distribution, with any remaining unevenness negated by
the use of stacks containing large numbers of sheets. For polishing applications, it has been shown that membrane chromatography
has several clear advantages over resins, but in the future it will be necessary to address certain limitations that frustrate
the wider adoption of this technology platform, particularly in large-scale industrial applications. These challenges include
relatively low binding capacity and issues surrounding integrity testing. Although column packing is no longer necessary and
diffusion-based integrity control is the state-of-the-art in membrane applications, additional functionality tests are mandatory
to demonstrate even flow-distribution and the validity of scale-down experiments.66 Flow-distribution can be a significant challenge in membrane chromatography because of the large frontal area compared with
the bed height (Figure 6). Fortunately, functionality tests are now becoming available and demonstrate that even at bed heights
of less than 1 cm, membrane stacks are extremely reproducible and robust.
Disposable membrane chromatography can only live up to its expectations if its potential is not exaggerated and its use restricted
to areas where membranes are clearly beneficial.
Beyond Chromatography?
It has been noted that "Process chromatography has the notoriety of being the single largest cost center in downstream processing"
and Przybycien et al. have asked, "Is there life beyond packed bed chromatography?"67 Of the current alternatives, few technologies are likely to have a major impact on downstream processes, and process chromatography
will remain the workhorse of the industry. However, protein precipitation with concomitant virus kill68 is a likely complement or alternative in protein purification, and plasmid DNA may be produced most efficiently by differential
precipitation.69 Perfusion chromatography70 perhaps never lived up to its expectations, but the struggle to overcome mass transfer limitations in chromatography is
currently being addressed by monoliths,71 which still suffer from low capacities for bulk protein purification. Displacement chromatography72 using low molecular weight displacers is of increasing interest now that commercial displacers are available. Because of
its selectivity, affinity chromatography still shows promise for the future, particularly when alkaline-resistant, synthetic
ligands are used instead of protein or peptide ligands.73
Various engineering solutions have been attempted over the years to address various challenges of unit operations. Examples
include continuous (annular) chromatography and fluidized, rather than expanded beds, and integrating process steps into continuous
processes. With improved engineering, radial flow columns provide an interesting opportunity to maximize adsorbent performance
and reduce dilution during elution.74
A vision for the future of process chromatography is expressed by Jan-Christer Janson:
"In a world where not only the pure technical problems are important to the biochemical engineer but where regulatory constraints
have become more and more an issue, a relevant vision for the future would be that systems—columns and media integrated—will
be available that allow continuous scaling up for production and scale down for various process validation reasons, such as
virus clearance studies and trouble shooting".75
Editor's note: This article is an expanded version of an article previously published in BioPharm International.
John Curling is the president of John Curling Consulting AB, Uppsala, Sweden, and is senior advisor to ProMetic BioTherapeutics, Inc.,
Gaithersburg, MD, +46 18 290620, john@consultcurling.se
. Uwe Gottschalk, PhD, is vice president of purification technologies at Sartorius Stedim Biotech GmbH, Göttingen, Germany. Both are also members
of BioPharm International's editorial advisory board.
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