ABSTRACT
The use of membranes during the purification and separation of proteins is becoming increasingly important both in academic sciences and in industrial process engineering. Thus, membranes play an important role in different downstream processes, starting with cell separation from the bioprocess broth up to final polishing of the product. A pressure driving force and a semipermeable membrane are used to effect a separation of components in a solution or colloidal dispersion. The separation is based mainly on molecular size, but also to a lesser extent on shape and charge. As a pressure-driven process the membrane filtration allows processing of proteins at ambient temperatures and without exposing them to thermal stress or alterations of its chemical environment. Membrane processes are easily scaled up, and the opportunity of using the same materials and configurations in different sizes from laboratory to process scale reduces the validation effort enormously. However, filtration processes are limited in selectivity. Fractionation of proteins can only be achieved at large differences in molecular weight of the proteins, and it is important to keep in mind that a certain difference in molecular weight between two proteins does not mean the same degree of difference in molecular size. Proteins that differ in molecular weight by 10 times may differ in size by only 3 times when in the globular or folded form.
