Nature provides an excellent palette of highly effective membranes capable of highly selective transport of a large number of molecular species. It is therefore striking that historically the membrane industry has developed synthetic separation membrane processes in a very different way.
Traditional separation membranes are mostly dense polymeric films where advanced chemistry is used to control the surface properties of the films produced. A wide range of polymers and production techniques are used resulting in a great diversity in structure and function of separation membranes tailored to a wide variety of applications. Separation is usually described in terms of pore/solute size, pore/solute charge and dielectric effects, coupled with diffusion or convective flow. Occasionally, more complex partitioning and transport mechanisms are used, however, most synthetic membranes may be broadly described as polymer sheets containing micron to nanometer sized holes.
This is in stark contrast to the bewildering complexity of biological membranes. 30 % of the human genome codes for membrane proteins, and a typical mammalian cell membrane hosts several hundred lipid types. Despite dramatic progress over the last decades in our understanding of the molecular basis for biological membrane transport, this complexity remains a major obstacle in our molecular understanding of how living cells maintain their integrity and perform their function.