Other Membrane Structures

Track Etched One of the most successful attempts to manufacture the best membrane (a sieve) is the track-etched membraney. For the production of track-etched membranes, dense polymeric films (polycarbonate or polyester) are randomly exposed to a high-energy ion bombardment or particle radiation. This bombardment damages the polymeric chain in the dense film, leaving small "tracks." Subsequently the polymer can be etched specifically (in an acid or alkaline solution) at the ends of the damaged track. The pores that are being formed are cylindrical channels and very uniform in size. However, the membrane has a low porosity, due to the fact that the chance for an overlap between two pores increases with the porosity (the density of the radiation). The membranes are often used in laboratories for analysis. See Figure 14.

Anodized Alumina Membranes The formation of the pores of an anodized alumina membrane is a self-assembling process. By anodization of aluminum in an acidic solution, a highly ordered structure of pores in the Al2O3 matrix can be obtained. Due to lattice expansion by the oxidation of the aluminum, an anisotropic potential distribution, and heat development during anodization, the self-organization process forms the pores. The membranes are relatively thick, resulting in long pores with a pore size ranging from 20 to 200 nm. The pore size is very uniform (see also Fig. 15). However, the membranes are unsupported and need, depending on the application, a second support.

Microsieve Most of the previously described porous membranes have a spongelike structure, which makes it difficult to clean the inside of the membranes. Also the retention of particles, proteins, and other coagulates is not only determined by the porous top layer, but also by the sponge support structure. Moreover, in certain applications, like clarification of beverages, the cleaning agents must be removed completely after cleaning to prevent contamination of the beverage. However, it is nearly impossible to remove cleaning agents that are left behind in dead-end pores. Until recently the best sieve structures were the track-etched and

Figure 14. SEM micrograph of polymeric track etched membrane filter. Courtesy of Aquamarijn Research.

Figure 15. SEM photograph of an anopore membrane with a pore size of 50 nm. Left: top view. Right: side view. Note the vertical pore/channel structure. Reprinted with permission from [201], H. Asoh et al., J. Elec-trochem. Soc. 148, 152 (2001). © 2001, The Electrochemical Society.

Figure 14. SEM micrograph of polymeric track etched membrane filter. Courtesy of Aquamarijn Research.

Figure 15. SEM photograph of an anopore membrane with a pore size of 50 nm. Left: top view. Right: side view. Note the vertical pore/channel structure. Reprinted with permission from [201], H. Asoh et al., J. Elec-trochem. Soc. 148, 152 (2001). © 2001, The Electrochemical Society.

the anopore membrane. However, both membrane types lack the possibility of adjusting the pore size freely in conjunction with the highest achievable porosity.

Using techniques used in semiconductor technology it is possible to fabricate thin membranes with perfectly uniform pores [3]. The membrane, the microsieve, was made using flat substrates, thin-layer deposition techniques, photolithography, and high-resolution etching methods.

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