Permeability, polymerized bilayers

Polymeric phospholipids based on dioctadecyldimethylammonium methacrylate were formed by photopolymerization to give polymer-encased vesicles which retained phase behavior. The polymerized vesicles were more stable than non-polymerized vesicles, and permeability experiments showed that vesicles polymerized above the phase transition temperature have lower permeability than the nonpolymerized ones [447-449]. Kono et al. [450,451] employed a polypeptide based on lysine, 2 aminoisobutyric acid and leucine as the sensitive polymer. In the latter reference the polypeptide adhered to the vesicular lipid bilayer membrane at high pH by assuming an amphiphilic helical conformation, while at low pH the structure was disturbed resulting in release of the encapsulated substances. 

The permeability of solutes across lipid bilayers is a product of the partition coefficient and the transverse diffusion coefficient [30]. Bilayer polymerization can alter solute diffusion by modifying either or both of these processes. In order to examine the effect of polymerization on bilayer permeability a nonionic solute of moderate permeability, [3H-glucose], was encapsulated in the vesicles prior to polymerization, removed from the exterior after polymerization, and its permeation across the bilayer was measured periodically [31]. Quantitative measurements of the 3H-glucose leakage revealed that the formation of linear polymer chains from methacryloyl lipids reduced the permeability coefficient to 0.3 to 0.5 of that of the unpolymerized lipid vesicles. A larger reduction (two orders of magnitude) was only found when crosslinked polymer networks were formed [31]. 

The need for increased stabilities and for controllable permeabilities and morphologies led to the development of polymerized surfactant vesicles [55, 158-161]. Vesicle-forming surfactants haw been functionalized by vinyl, methacrylate, diacetylene, isocyano, and styrene groups in their hydrocarbon chains or headgroups. Accordingly, SUVs could be polymerized in their bilayers or across their headgroups. In the latter case, either the outer or both the outer and inner surfaces could be polymerized separately (Fig. 38). Photopolymerization links both surfaces selective polymerization of the external SUV surface is accomplished by the addition of a water-soluble initiator (potassium persulfate, for example) to the vesicle solution. 

A related system is that of the lipid-bilayer corked capsule membranes which are formed from ultrathin (about 1 pm thick), spongy, 2.0- to 2.5-mm-diameter, more-or-less spherical nylon bags in which multiple bilayers are immobilized (Fig. 43) [343-345]. They were considered to combine the advantages of mechanical and chemical stabilities of polymeric membranes with the controllable permeabilities of surfactant vesicles. Polymerization of the bilayers, in situ,... 

Fig. 2. Comparison of the selectivities of neutral-carrier-modified solvent polymeric- [43] and bilayer membranes. The permeability ratios PJP (at equilibrium" (Ref. 18) as far as available) fulfilled for the glyceryl dioleate BLM s are taken from Figs. 10 and 11 in Ref. 18. Values on the SPM s were obtained using 0.1 M solutions of the aqueous chlorides and membranes of the composition 33.1 wt.% polyvinyl chloride, 66.2 wt.% dioctyl adipate, 0.7 wt.% carrier. For the macrotetrolides I2 NH( for valinomycin IZ K. ...

Polymer films have been obtained by plasma polymerization of hexafluorobenzene, N-vinylpyrrolidine, and chloracrylonitrile (Munro). Higuchi et al. have shown that irradiation of an azobenzene-modified poly(Y-methyl-L-glutamate-CO-L-glutamic acid) in bilayer membrane vesicles of distearyldimethylammonium chloride leads to trans-cis isomerization of the polymer this leads to transfer of the polypeptide from the hydrophobic bilayer membrane interior to the hydrophilic surface. As a result, there was a decrease in the ion permeability through the bilayer membrane and the formation of intervesicular adhesion. Eisner and Ritter have prepared photosensitive membranes from an aromatic polyamide and a cinnamate that incorporates a liquid crystalline component. 

BMA and EGDMA monomers and UV-initiated polymerization to generate a cross-linked poly(methacrylate) network in the POPC bilayer. The substrate, ampicillin, diffused into liposomes through the OmpF channels and was converted to ampi-cillinoic acid. Thus a polymer-stabilized, vesicle-sized bioreactor with selective permeability was created, allowing for retention of the enzyme and ingress/egress of substrate and product. 

After 5 min, during which the alamethicin equilibrated with the lipid bilayer and its CD spectrum did not change, different amounts of 0.1 N (normality equals molarity in monomeric units) of degree of polymerization 100,000, sodium polyacrylate (PA-) solution were added. Free alamethicin in the presence of salt tends to interact with PA- to form gels. Lack of gelation with added PA- indicates that the alamethicin is incorporated into the membranes. The final polyacrylate concentrations were between 0 and 0.1 N. The concentrations of Na+ and glucose added up to 0.1 M to maintain isotonicity. Due to the permeability of alamethicin channels to small ions, different polyacrylate concentrations resulted in different Donnan potentials across the membrane. 

Formation of microcapsules by in situ interfacial polymerization (where the monomers are entirely in the oil phase of the capsule core) yields microcapsules with a high core-to-wall ratio and a bilayer wall with an outer layer (about 0.05 urn) and an inner reinforcing spongy layer (0.5 fim). This method has been used to encapsulate a range of insecticides, pheromones, and herbicides, many of which have been available commercially (37). The capsule size may be varied from submicrometer to 100 /um diameter and the permeability selected for rapid or slow release of... 

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