Crystalline Protein Layers as Isoporous Molecular Sieves and Immobilisation and Affinity Matrices

During the last 30 years membrane technology has developed into a topic of major practical and theoretical importance. Research in membrane technology involves the disciplines of physics and chemistry on the one hand and biochemical and process engineering on the other hand. A major breakthrough in the history of membrane technology was the development of asymmetric membranes where a thin selective layer of the order of 0.1 µm to 1 µm is supported by a highly porous sublayer of the same material (for reviews see Lonsdale 1981; Strathmann 1982). The symmetric membranes which consisted of an approximately 100 µm thick selective layer produced before were never really applied in industrial and laboratory processes. Another breakthrough was the development of composite membranes with an asymmetric structure where a top layer is supported by a porous sublayer. In this case the two layers consist of different polymeric materials. The advantage of composite membranes is that each layer can be optimised independently to obtain optimal membrane performance with respect to selectivity, permeation rate and chemical stability. Nowadays membrane processes are classified into microfiltration, ultrafiltration and hyperfiltration. Microporous membranes with a pore size from > 50 nm to 2 µm are applied in microfiltration processes and mesoporous membranes with a pore size from 2 nm to 50 nm are used for ultrafiltration, whereas in hyperfiltration non porous solution diffusion membranes are used.