Membrane Material

Due to the specific properties of biomolecules, the membrane materials to be used for their separation should ideally possess the following characteristics:

• Macroporosity: This will allow biomolecules to cross the membrane and to access the affinity sites.

• Hydrophilicity: Using hydrophilic supports, nonspecific adsorption by hydrophobic interactions and denaturation of biomolecules can be avoided.

• Presence of functional groups: These are required for the coupling of an affinity ligand.

• Chemical and physical stability: The material has to withstand the sometimes harsh conditions during derivatization, operation and regeneration.

• Biocompatibility: This is particularly important if the membranes are used in extracorporeal devices, for example for blood treatment.

• Large surface area relative to membrane volume: This will allow for the construction of small, integrated devices with high operational capacities.

Cellulose and cellulose acetate were among the first materials that have been used for affinity membrane preparation. They are hydrophilic and biocompatible, and due to the presence of hydroxyl groups, ligand coupling can be easily achieved using for example CNBr or carbonyldiimidazole activation. In order to improve the mechanical and chemical stability of cellulose membranes, chemical cross-linking with epichlorohydrin is sometimes carried out. Cellulose membranes normally have a rather small pore size, resulting in a high pressure drop. Attempts to produce membranes with larger pores using coarse cellulose fibres have resulted in a less uniform membrane structure.

Polysulfone is another suitable membrane material which has good film-forming properties. It is of sufficient physical, chemical and biological stability, and ligands can be coupled after chloromethylation-amination or acrylation-amination.

Microporous polyamide (nylon) membranes have also been used for the preparation of affinity membranes. This material is mechanically stable and has a rather narrow pore size distribution. It contains only a small number of terminal amino groups for ligand coupling, which can, however, be increased by partial hydrolysis of the amide functions.

A suitable membrane material is polyvinyl alcohol, in particular because of its hydrophilicity and bio-compatibility. Poly(ethylene-co-vinyl alcohol), which has a somewhat higher chemical stability, has also been used. Both materials contain hydroxyl groups and can be activated by the CNBr method, allowing immobilization of affinity ligands having an amino function. Ligands can also be coupled using epichlorohydrine or butanediol diglycidyl ether-activation.

Other materials that have been used for affinity membranes are poly(methyl methacrylate), poly(hy-droxyethyl dimethacrylate), polycaprolactam, poly (vinylidene difluoride), poly(ether-urethane-urea) and silica glass. Table 1 shows a list of membrane materials and the appropriate ligand-coupling chemistries.

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Solar Panel Basics

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