Even at 500-fold magnification, pores in the beads cannot be seen, whereas the membrane pores are easily visible. More than
95% of binding sites on the beads are found inside pores, making them inaccessible to large molecules. In contrast, binding
sites on membrane adsorbers are found on a homogeneous film approximately 0.5–1 μm in thickness on the inner walls of a reinforced
and crosslinked cellulose network. The diffusion time in such adsorbers is negligible because of the large pores and the immediate
binding of target proteins to the ligands.63
Despite all their attractive properties, there is a downside to the use of membrane adsorbers—the reduced dynamic capacity
for those molecules that are not excluded from the pores of a typical resin. Membrane chromatography is therefore a niche
application that is ideally suited to capture large molecules from diluted feed streams.64 In flow-through polishing steps, these targets are critical impurities such as nucleic acid variants, viruses, endotoxins,
and many host cell proteins. The overall capacity requirements for these highly diluted contaminants is usually very low and
a disposable membrane device is typically 5% of the volume of a conventional column that needs to be oversized to accommodate
the volumetric flow rate. For virus clearance validation, with its increasing regulatory scrutiny, the higher capacity and
thus more efficient removal is actually good news and a main driver towards the use of disposable membrane adsorbers in the
polishing step.
The first membrane chromatography devices were single flat sheets that were placed perpendicular to the feed mixture in a
step-up analogous to that of a dead-end filter. Since these pioneering devices were first used, a number of different formats
have been developed with stacked sheets, hollow fibers, and radial flow devices being the most popular. Other than in development
stages for new membranes, single flat sheets tend not to be used because of the limited adsorbent volume that can be tolerated.
Where flat membranes are used, they are generally stacked in specially designed lenticular modules, which allow the use of
much higher volumes. In contrast, hollow fibers are tubes of up to 2.5-mm diameter that present the binding ligands on the
inner surface. A hollow fiber adsorber may consist of hundreds of fibers bundled together in a cartridge or module, and the
feed mixture flows through the lumen of each tube, parallel to rather than perpendicular to the membrane surface, analogous
to cross-flow filtration. Hydrostatic pressure forces the liquid against the membrane and facilitates adsorption. Such devices
provide a much greater surface area to volume ratio than even the largest lenticular modules, and prevent the build up of
particulate matter on the membrane surface, which can lead to fouling when using single membranes. Despite their advantages,
there are as yet no commercially available hollow-fiber chromatography modules. Radial flow adsorbers combine the features
of lenticular membranes and hollow fibers: they are spiral-wound modules in which a single flat sheet is wound around a central
porous cylinder to provide many layers. In operation, the hydrostatic pressure forces the feed through the membrane stack
where ligand binding occurs. Flow distribution can be quite challenging and the design of such devices incorporates features
to address this.
Although they have many technical advantages for flow-through applications, membrane adsorbers have been slowly accepted by
the industry, reflecting a mixture of general inertia and uncertainty about comparative costs, especially in terms of consumables.
However, with the current focus on disposable processing solutions, membrane chromatography is establishing a position in
the bioseparations arsenal, particularly in monoclonal antibody purification.65
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