BIOMIMETIC MEMBRANES - NEW SEPARATION TECHNOLOGY TOOLS WITH ANCIENT ROOTS

Nature provides an excellent palette of highly effective membranes capable of highly selective vectorial transport of a large number of molecular species. It is therefore striking that the membrane industry has developed synthetic separation membrane processes in a very different way [1]. 

Traditional separation membranes are mostly dense polymeric films where advanced chemistry is used to control the surface properties of the films produced [2]. A wide range of polymers and production techniques are been 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 nanometre sized holes. 

     This is in stark contrast to the bewildering complexity of biological membranes. 30 % of the human genome codes for membrane proteins [3], and a typical mammalian cell membrane hosts several hundred lipid types [4].

Despite dramatic progress over the last decades in our understanding of the molecular basis for biological membrane transport (e.g. [5-7]), this complexity remains a major obstacle in our molecular understanding of how living cells maintain their integrity and perform their function [8].

One way leading to a better understanding of membranes and membrane transport is to focus on a few of its components and features. This understanding is crucial if we want to exploit – or mimic – nature’s tremendous capability for selective membrane transport. The term Biomimetic Membranes denotes the common denominator for such endeavours [9]. Recent examples of membrane biomimetics include low noise recording devices for ion channel research [10], free-standing triblock copolymer membranes [11, 12], enzyme-immobilization [13], and gas-extraction membranes [14].

In the development of biomimetic membranes it is important to know the morphological descriptors such as the amount and intrinsic properties of amphiphiles (lipidic or block coplymeric types) forming the membrane, the equilibrium thickness, and the coverage. Also important are the properties of interaction: the stability against mechanical perturbations (e.g. viscoelastic responses to changes in hydrostatic or osmotic pressure differences) [15, 16], the rate of regeneration (self healing) [17], the ease with which functional peptides or proteins can be adsorbed/incorporated [18] and, once incorporated, how proteins interact with the amphiphilic matrix [19]; and surface (e.g. electrostatic) energetics [20].

Perhaps the most challenging part of biomimetic membrane development is to understand the interaction between the membrane and its support – in particular when this support also is porous and thus can support mass transport across the membrane [21]. In Aquaporin's case the biomimetic membrane with embedded aquaporins must support pressures up to 10 bar and allow a water flux > 100 l /m2 h. Therefore the development of the Aquaporin Inside™ membrane is closely linked to the simultaneous development of suitable porous support materials. 

 

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