Transport Phenomena in Natural and Synthetic Membranes

Abstract

Although the differences between natural and synthetic membranes are as yet far more numerous than their similarities, the relationship of structure to function in certain instances is strikingly parallel. In principle there seems no reason why some of the specific functions of natural membranes should not eventually be emulated to great advantage on an engineering scale, and indeed there are encouraging indications that the initial steps have been taken in this direction. The most important general property of natural membranes is that they function as “active” elements (to use electrical network terminology). The transport of a given species is frequently driven by an input of metabolic energy, and in many cases the flow may be non-conservative—i.e., reaction and diffusion occur simultaneously within the membrane. In contrast, most synthetic membranes function as “passive” elements (this is true of the numerous types which have been used in purely physico-chemical studies, as well as those developed for specific purposes such as desalination, separations technology, or biomedical engineering). The permeability characteristics of these membranes are the only parameters of significance; no coupling to chemical reaction occurs and transport across them is always conservative. With one or two notable exceptions, moreover, they do not approach the extraordinary selectivity of natural membranes.