S-Layer Ultrafiltration Membranes Exhibit Precise Extrusion Values
Pores passing through crystalline bacterial surface layers, or S-layers, are identical in size and morphology. This property, in addition to the alignment of functional groups in well-defined lattice positions, makes S-layers a superior alternative to amorphous polymers in ultrafiltration and cross-flow applications, according to Margit Sara, assistant
professor at the Center of Ultrastructural Research in Vienna. She participated with Uwe Sleytr in research on S-layer technology, which has been licensed to Nanosearch Membrane GmbH (Vienna). That company will begin marketing an S-layer ultrafiltration membrane (SUM) early this year.
"Crystalline bacterial cell surface layers are the outermost cell envelope component in many eubacteria and arachaebacteria," explains Sara. "When we started to investigate them about 15 years ago, we saw that the S-layers were highly porous and that the pores show an identical size and morphology, indicating that S-layer technology held great promise in the biotechnology field. On the one hand, you have amorphous polymers, a rather low-defined material with pore-size distribution, average values, and random distribution of functional groups; on the other, you have these S-layer lattices with very precise exclusion values," says Sara. "But it took us about 10 years to find an appropriate production process."
That process is now operational. "We are able to produce S-layer ultrafiltration membranes in dimensions of 30 * 60 cm," explains Sara. "From this, we can punch out smaller samples and insert them into ultrafiltration cells, cross-flow apparatus, and so forth."
The process involves depositing the S-layers onto a porous supporting layer, typically a membrane with a spongy structure used in sterile filtration applications. Following deposition, the S-layer crystals are cross-linked with glutaraldehyde for stabilization purposes. "This is necessary," says Sara, "because the S-layer protein molecules are only linked by noncovalent bonds in the crystals. The procedure results in a very stable mechanical and chemical matrix; in a subsequent step, the Schiff bases are reduced with sodium borohydride," she adds.
Thus far, the technology has generated a great deal of interest for cross-flow applications, primarily because of its sharp molecular weight cutoff in the 30,000 to 40,000 range, says Sara. In addition, the particles that collect on the surface are swept away by the cross-flow stream. This represents a distinct advantage over "dead-end" filtration processes, she adds, which result in a cake forming on the membrane surface, contributing to a loss of flux.
Other applications in development include the use of S-layers as an immobilization matrix in dipsticks. According to Sara, the advantages of SUM-based t-PA and birch-pollen-specific IgE dipsticks over conventional dipsticks is the presence of strong signals in immunoassays, virtually complete elimination of background, and an absence of diffusion.
Current research at the university revolves around the use of S-layer proteins for the coating of liposomes. Stabilized with S-layers, the vesicles can escape detection by the body's immune system and deliver encapsulated drugs to specific organs.
While Sara feels that SUMs have far-reaching potential for medical devices, she notes some reluctance from industry to consider these new options. "Interestingly, if you speak about bacterial cell surface membranes, engineers tend to be put off," she says. "For the time being, they seem to prefer conventional polymers with exact chemical structures." The door, however, is by no means shut. "Whenever I give presentations at biomedical engineering conferences, there is always interest in S-layer technology. It is a cautious interest, but it's definitely there."
