Type of supports Used

There are many different ion exchanger separation supports for biopolymers on teh market. The most common are of beaded type. The particles can be packed in a column with one inlet and one outlet. By using particles with a large portes, it is possible to achieve very fast separations without diffusion limitations. Pourous bodies are another fast separation system. A porous continous rod or a sponge is packed into the column and is a suitable tool to make separations without diffusion problems. A third gorup is membranes such as stacked flat sheet membranes. Muller (J. Molecules Recognition, 11, 273-278 (1998)

Specific Types of Materials Used

Ion exchange resins were originally limited to use of natural product such as cellulose, clay and other minerals containing mobile ions that would exchange with ionic materials in the surrounding solute phase. Because of the low exchange capacity of these natural products, however, synthetic organic polymers capable of exchanging ions were developed. Among the first generation of synthetic ion exchange materials were the ion exchange resins which are an elastic 3 D hydrocarbon network comprising ionizable groups, either cationic or anionic, chemically bonded to the backbone of a hydrocarbon framework. Typical examples of commercially available ion exchange resins are the polystyrenes cross-linked with DVB (divinylbenzene), and the methacrylatescopolymerized with DVB. (Hou, US 4,663163)

The resistance to flow exhibited by synthetic ion exchange resins is controlled by the degree of crosslinking. With a low degree of crosslinking, the hydrocarbon network is more easily stretched, the swelling is large and the resin exchanges small ions rapidly and even permits relatively large ions to undergo reaction. Conversely, as the crosslinking is increased, the hydrocarbon matrix is less resilient, the pores are the resin network are narrowed, the exchange process is slower, and the exhanger increases its tendency to exclude large ions from entering the structure. The ion exchange resins made from polymeric resins have been successfully applied to the removal of both organic and inorganic ions from aqueous media but are normally suitable for the separation of bioppolymers such as proteins.

The third generation of ion exchange materials were the ion exchange gels. These gels comprise large pore gel structures and include the commerically known material Sephadex, which is a modified dextran. The dextran chains are crosslinked to give a 3D polymeric network.

Ion exchange gels made from synthetic polymers have also been used and include crosslinked polyacrylamdie (Bio-Gel P), microreticular forms of polystyrne (Styragel), poly(vinyl acetate) (Merk-o-Gel OR), crosslinked poly (2-hydroxy ethylmethorrlate) (Spheron) and polyacryloylmorpholine (Enzacryl).

The failure of single components to have both capacity and dimenstional stability led to yet another generation of ion exchange materials comprising composite structures, e.g., hydrid gels. Hybrid gels are made by combining a semi rigid component, for the purpose of convering mechanical stability, with a second component, a softer network, which is responsible for carrying functional groups. Agarose gel, which would otherwise be very soft and compressible, can be made stronger through hydridizing with cross linked polyacrylamide. The crosslinked polyacrylamide component is mechanically stronger than the agarose, improves the gel flow properties and reduces the gel swelling, but it sacrifices molecular fractionation range. Examples of hybrid gels other than polyacrylamide/agarose (Ultrogels) are polyacryloylmorpholine and agarose (Enzacryl) and composite polystyrenes with large pore polystyrenes as a framework filled with a second type of lightly crosslinked polymer.

Yet another composite gel structure is achieved by combining inorgnanic materials coated with organics and are the types known as Spherosil. Porous silica beads are impregnated with DEAE dextran so that the product will have the mechanical properties of silica, with the ion exchange properties of DEAE dextrans. These composites, however, have severe channeling defects arising out of particle packing, and they have capacity limitations on the coated surfaces.

Totally rigid inorganic supports such as porous silica or porous glass which are not susceptible to degradation have also been sued to provide high porosity and high flow rate systems. The major problem, however, is nonspecific adsorption of proteins due to the silanol groups on the silica surface. Since the hydroysis of silica is directly related to the pH conditions, the nonspecific adsorption by silica is minimal at neutral pH, but increases as the pH changes.

Commercially available 

Several commonly employed ion exchange resins are commercially available. The resin matrix backbone includes agarose and dextran (GE Healthcare), glycidyl methacrylate (Macroprep, Bio-Rad), polysyrenedivinylbenzene (Poros, Applied Biosystems) and polymethacrylate (Fractogel, EMD Chemical and Totopearl, Tosoh). (Liu, “Recovery and purification process development for monoclonal antibody production” mAbs,  2:5: 480-499 (2010). O’Donnel “A high capacity strong cation exchange resin for the chromatographic purificaiton of monoclonal antibodies and other proteins” PREP 2007) teaches “Toyopearl GigaCap S-650M” which is syntehsized on a 1000 Angstrom polymethacryate base with nominal particle size of 75 um. The resin has high capacities for both large (igG) and small (lysozyme) proteins.

Strong CEX:  Mustang S and Sartobind S are strong CEX membranes. The are modified with a form of sulfonic acid. The Mustang Q is made of polyethersulfone (PES) with u pores. The Sartobind S is made of regenerated cellulose with 3-5 um pores. Brown (WO2010/019148)

Fratogel SE Hicap and SP Sepharose Fast Flow resins are strong CEX. Fractogel SE Hicap resin is made of cross linked polymethacrylate particles of 40-90 um diameter with pore size of about 800 A. The functional ligand is covalently attached to the particle with a long, linear polymer chain. The SP Sepharose Fast Flow resin is made of highly cross linked agarose partciles of 45-165 um diameter. Sepharose Fast Flow is a cross linked derivative of Sepharose with a sulfopropyl ligand as the functional group. (Bill, 14,365449). 

Strong AEX: Mustang Q is a strong AEX membrane. It is modified with with a form of quaternary amine. It is made of polyethersulfone (PES) with 0.8 um pores. Brown (WO2010/019148)

Pore Size

In general, IEX membranes have pore sizes of 0.1-100 um. As a reference, Sartobind Q (Sartorius AG) is a strong AEX haing a nominal pore size of 3-5 um and is commercially available in a single or multiple layer format. Mustang Q is a strong AEX having a nominal pore size of 0.8 um. Sartobind S is a strong CEX haivng a nominal pore size of 3-5 um and Mustang S is a strong CEX having a nominal pore size of 0.8 um. Brown (WO2010/019148)

How the Polymer is attached to the support

Polymer coatings can be produced by several routes: (a) by reaction of the surface with tailor-made polymers and crossllinking; (b) by physical adsorption and crosslinking: (c) by graft or block polymerization. Muller (J. Molecules Recognition, 11, 273-278 (1998) discloses that polyamide hollow fibre membranes can be modifed with ion exchanger groups by a block polymerization procedure.