Polymer cushion

Adhesion of different immune cells to one another or to epithelial cells has also been studied using planar bilayer models. For example, lymphocyte function-associated protein-1 (LFA-1) promotes cell adhesion in inflammation [i.e., a reaction that can be mimicked by binding to purified ICAM-1 in supported membranes (70)]. Similarly, purified LFA-3 reconstituted into supported bilayers mediates efficient CD2-dependent adhesion and differentiation of lymphoblasts (71). This work was followed by a study in which transmembrane domain-anchored and GPl-anchored isoforms of LFA-3 were compared (72). Because this research occurred before the introduction of polymer cushions and because the bilayers were formed by the simple vesicle fusion technique, the transmembrane domain isoform was immobile, whereas the GPl isoform was partially mobile. By comparing results with these two isoforms at different protein densities in the supported bilayer, the authors showed that diffusible proteins at a sufficient minimal density in the supported membrane were required to form strong cell adhesion contacts in this system. 

With only a few exceptions, metal-supported biomimetic membranes consist of a more or less complex architecture that includes a lipid bilayer. In order of increasing complexity, they can be classified into solid-supported bilayer lipid membranes (sBLMs), tethered bilayer lipid membranes (tBLMs), polymer-cushioned bilayer lipid membranes (pBLMs), S-layer stabilized bilayer lipid membranes (ssBLMs), and protein-tethered bilayer hpid membranes (ptBLMs). 

Next, the use of polyelectrolyte multilayers, prepared by the layer-by-layer deposition protocol, as well as the use of polymer cushions prepared by plasma-polymerization is introduced. Evidence for the proper structural and functional characteristics of the corresponding tethered bilayers is derived from neutron reflectivity and from IR data, and by the observation of the functional incorporation of proteins. 

Keywords Tethered lipid bilayer membrane Polymer cushion Lipopolymer... 

Fig. 2 The construction of a polymer-cushioned lipid bilayer membrane. (A) Architecture constructed in a sequential way first, onto the functionalized substrate a polymer layer (cushion) is deposited by adsorption from solution and covalent binding, followed by the (partial) covalent attachment of a lipid monolayer containing some anchor lipids as reactive elements (B) able to couple the whole monolayer to the polymer cushion. (C) Alternatively, a lipopolymer monolayer, organized, e.g., at the water-air interface can be co-spread with regular low-mass amphiphiles and then transferred as a mixed monolayer onto a solid support, prefunctionalized with reactive groups, able to bind covalently to the polymer chains of the lipopolymer molecules, (B). (D) By a fusion step (or a Langmuir Schaefer transfer) the distal lipid monolayer completes the polymer-tethered membrane architecture...

Fig. 4 (a) The reactive-ester analogue of a carboxy-terminated monochloro-silane derivative self-assembles onto a glass substrate to result in a reactive monolayer, (b) Onto this, an ethyleneimine-containing polymer coil, obtained by the partial conversion of a polyoxazoline precursor polymer binds covalently after adsorption from solution to give a stable polymer cushion for the binding of a monolayer of a reactive amphiphile, a reactive ester derivative of a fatty acid in the example given in (c)... 

Fig. 5 Surface-plasmon resonance curves, i.e., reflectivity-vs-incident angle scans of the bare substrate, a Ag coated glass slide with a thin SiC>2 layer evaporated on top (A), after the self-assembly of a reactive monochlorosilane derivative (cf. Fig. 4a) (B), after the adsorption (from solution), covalent binding, and soxhlet extraction of the polymer cushion (C), and after the deposition of a model lipid monolayer (a layer of reactive ester derivatives of a fatty acid) (D)...

The degree of swelling of the tethering system, i.e., the amount of water that can be accommodated by the polymer cushion, can be followed by surface-plasmon optics. Figure 9 gives an example of the swelling of a polymer-tethered lipid monolayer in... 

Fig. 9 The polymer-tethered lipid monolayer (a) can swell by the up-take of significant amounts of water (b) surface-plasmon optical evidence of the swelling behavior of a polymer-cushioned monolayer by the exposure to air of different humidity (% relative humidity as indicated). Upon exposure to dry air the tethered monolayer collapses to its water-free thickness (however, can swell again if exposed to humid air (not shown))...

It has been shown that the chemical nature of the polymer cushion can be very flexible with examples ranging from polyelectrolytes to carbohydrate-containing macromolecules and cross-linked hydrogels. So far, the cushion has played only a rather passive role in that it was almost exclusively used as a structural element in the build-up of the tethered membrane architectures. The real potential, however, lies in the fact that these polymer systems could play a crucial functional role for these... 

Another most interesting aspect concerns the mechanical coupling of the polymer cushion with the membranes and their incorporated proteins. This could lead to interfacial architectures that show interesting features of structure formation by the coupling of the specific entropy driven properties of polymers in general with the self-organization capability of lipid bilayer structures. Experiments along these lines are under way. 

Sackmann, E., Tanaka, M. Supported membranes on soft polymer cushions fabrication, characterization and applications. Trends Biotechnol. 18(2), 58-64 (2000)... 

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