Enter Membrane Chromatography
Chromatography is generally understood to be a unit operation performed on a porous, beaded resin derivatized with appropriate
binding ligands which confer the properties of ion exchange, HIC, affinity etc. Most separations are performed on beaded agarose
and agarose–polymer matrices because the agarose does not generally interfere with the separation. The vast majority of potential
binding sites are found in the pores, and one of the disadvantages of traditional packed-bed chromatography is that the separative
process relies on pore diffusion to bring solute molecules into contact with their binding sites (Figure 4).
Figure 4. Comparison of diffusion in conventional bead resins and membrane adsorbers. In conventional beads, most of the
binding sites are within the pores and pore diffusion is the major process by which target molecules bind to their ligands.
In membrane adsorbers and perfusion chromatography beads, the pores are large enough for target molecules to reach their ligands
by way of convection currents, with very little pre-diffusion necessary.51
Such diffusion-dependence results in long process times at higher resin volumes.56 To a certain extent, the diffusion problem can be addressed by using smaller beads, which have a lower surface area to volume
ratio. However, this introduces the further issues of pressure drop and bead compaction in the column, particularly at high
flow rates. The pressure drop can be severe in long columns significantly reducing process efficiency.
In the 1970s, efforts were made to overcome the flow limitations posed by non-rigid resins by using the short, wide columns
discussed by Janson.24 Twenty years later, the concept of "perfusion" chromatography was introduced with the benefits of highly porous particles
with 8,000–10,000 Å "through-pores" giving linear flow rates of up to 1000 cm/h and more. Monolithic structures have also
been developed in the 1990s in pursuit of resolution of the same problem.
The idea of membrane chromatography was born from the idea of combining the convective flow—low pressure advantage of membranes
with their mass transfer capacity and led initially to the stacked membrane solutions. Membrane adsorbers are thin, synthetic
microporous or macroporous membranes, which are chemically activated to fulfill the same function as chromatography resins.
The development of the first membrane adsorbers by Brandt and colleagues in 198857 and other pioneers58–62 can be viewed as the equivalent of shortening the column to near zero length, allowing large-scale processes to run with
only a small drop in pressure even at high flow rates, and therefore resulting in higher productivity. Additionally, because
the transport of solutes to their binding sites occurs mainly by convection (while pore diffusion is minimal), the mass transfer
resistance is reduced, so that capture is rapid and largely independent of flow rate (Figure 4). This allows very high flow
rates to be used, reducing the overall process time by up to 100-fold. The most significant improvements have been seen with
large molecules, which are often unable to migrate into the pores of traditional media and tend to bind only to ligands displayed
on the bead surface.
The difference between bead and membrane-based chromatographic processing solutions is shown in Figure 5. It shows a scanning
electron micrograph showing typical ion exchange chromatography resin on the surface of a derivatized membrane.
Figure 5. Scanning electron micrograph showing ion exchange chromatography beads on the surface of a Sartobind Q membrane.