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Conventional synthetic membranes, fashioned for the most part from rather unremarkable polymeric materials, are essentially passive structures that achieve various industrial and biomedical separations through simple and selective membrane permeation processes. Indeed, simplicity of membrane material, structure, and function has long been perceived as a virtue of membranes relative to other separation processes with which they compete. The passive membrane separation processes -- exemplified by micro- and ultrafiltration, dialysis, reverse osmosis, and gas permeation -- differ from one another primarily in terms of membrane morphology or structure (e.g., porous, gel-type, and nonporous) and the permeant transport mechanism and driving force (e.g., diffusion, convection, and 'solution/diffusion'). The passive membrane separation processes have in common the fact that interaction between permeant and membrane material is typically weak and physicochemical in nature; indeed, it is frequently an objective of membrane materials design to minimize interaction between permeant and membrane polymer, since such strategies can minimize membrane fouling. As a consequence, conventional membrane processes often provide only modest separation factors or permselectivities; that is, they are more useful in performing 'group separations' (i.e., the separation of different classes of material) than they are in fractionating species within a given class. It has long been recognized within the community of membrane technologists that biological membrane structures and their components are extraordinarily sophisticated and powerful as compared to their synthetic counterparts. Moreover, biomembranes and related biological systems have been 'designed' according to a very different paradigm -- one that frequently maximizes and capitalizes on extraordinarily strong and biochemically specific interactions between components of the membrane and species interacting with them. Thus, in recent years synthetic membrane scientists have become intrigued with the notion of mimicking biological membrane structure and function where feasible -- and with 'activating' membranes by incorporating various biocatalytic or adsorptive entities within them. Generally, the objective has been to render synthetic membranes capable of performing separations more selectively or of increasing the range of function (e.g., chemical conversion) that they are capable of carrying out. At this point, the design of a number of novel synthetic membrane structures and processes has been guided by precedents afforded by biological systems. Several examples of this strategy as applied to synthetic membrane and membrane process development are enumerated.
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