Vesicles polydisperse

Block copolymer vesicles, or polymersomes, are of continued interest for their ability to encapsulate aqueous compartments within relatively robust polymer bilayer shells (Fig. 7) [66, 67]. Eisenberg and coworkers were the first to report the formation of block copolymer vesicles from the self-assembly of polystyrene-h-poly(acrylic acid) (PS-h-PAA) block copolymers. They also have described the formation of a wide range of vesicle architectures in solution from the self-assembly of five different block copolymers PS-h-PAA. PS-h-PMMA, PB-h-PAA, polystyrene-h-poly(4-vinyIpyridinium methyl iodide), and polystyrene-h-(4-vinylpyridinium decyl iodide) [68]. Small uniform vesicles, large polydisperse vesicles, entrapped vesicles, hollow concentric vesicles, onions, and vesicles with hollow tubes in the walls have been observed and the formation mechanism discussed. Since vesicles could be prepared with low glass transition polymers such as PB [69, 70] and PPO [71], it has been established than these structures are thermodynamically stable and not trapped by the glassy nature of the hydrophobic part. 

Poly(L-arginine)6o-Wocfc-poly(L-leucine)2o (PLArg-/ -PLLeu, Fig. lOg) showed similar physical properties as PLLys60-fe-PLLeu2o and also formed micrometersized vesicles in aqueous solution. These vesicles were able to entrap water-soluble species, such as dextran, and could be extruded through polycarbonate filters to yield stable, low-polydispersity vesicles of controllable diameter down to 50 nm [53]. 

Polymersomes are polymer-based bilayer vesicles, also termed as nanometer-sized bags by scientists. The bilayer structure displayed is similar to liposomes and niosomes as shown in Figure 48.7. They can be considered as liposomes but of nonbiological origin. Amphiphilic block copolymers can form various vesicular architectures in solution. They can have different morphologies such as uniform common vesicles, large polydisperse vesicles, entrapped vesicles, or hollow concentric vesicles. ... 

Since the bending energy Fbend = 8tik is independent ofi , there is no distinguished equilibrium radius for i ves > Rmin- Afl er preparation one mostly obtains polydisperse vesicle samples. The increase in translational entropy will shghtly favor the formation of small vesicles. As will be outlined below, a control of the vesicle radius is possible by inducing an asymmetry between the outer and the inner monolayer. 

Figure 10 Representative TEM images of various vesicle types (a) uniform vesicles, (b) polydisperse vesicles, (c) entrapped vesicles, (d) hollow concentric vesicles, (e) onion-like vesicles, (f) vesicles with tubes in the walls. (Reproduced from Ref. 31. Wiley Periodicals, Inc, 2004.)...

Templates made of surfactants are very effective in order to control the size, shape, and polydispersity of nanosized metal particles. Surfactant micelles may enclose metal ions to form amphiphilic microreactors (Figure 11a). Water-in-oil reverse micelles (Figure 11b) or larger vesicles may function in similar ways. On the addition of reducing agents such as hydrazine nanosized metal particles are formed. The size and the shape of the products are pre-imprinted by the constrained environment in which they are grown. 

Ehwald, R., Heese, P., and Klein, U., Determination of size limits of membrane separation in vesicle chromatography by fractionation of a polydisperse dex-tran, /. Chromatogr., 542, 239, 1991. 

Phospholipid vesicles form spontaneously when distilled water is swirled with dried phospholipids. This method of preparation results in a highly polydisperse array of multicompartment vesicles of various shapes. Extrusion through polymeric membranes decreases both the size and polydispersity of the vesicles. Ultrasonic agitation is the most widely used method for converting the lipid dispersion into single-compartment vesicles of small size. 

Jiang Y, Chen T et al (2005) Effect of polydispersity on the formation of vesicles from amphiphilic diblock copolymers. Macromolecules 38 6710-6717... 

Bending moduli can in principle be obtained for two types of systems (i) extended, flat surfaces or interfaces, the subject matter of this section, and (ii) surfaces that are already strongly curved, and for which y is zero or extremely low, such as in vesicles or micro-emulsions. For instance such moduli can be inferred from shape fluctuations, from the Kerr effect (sec. 1.7.14] or from polydispersity using some scattering technique. We repeat that this type of measurement is often ambiguous because the bending contributions to the Helmholtz energy can only be estimated when all other contributions are accurately known. 

Store the final product at temperatures above T under an inert atmosphere such as nitrogen or argon for 1 h to allow the annealing process to come to completion. Mean size and polydispersity index of vesicles is influenced by lipid composition and concentration, temperature, sonication time and power, sample volume, and sonicator tuning. Since sonication process is difficult to reproduce, size variation between batches produced at different times is not uncommon. 

Theoretically, to achieve long-circulating properties and avoid extensive uptake by the liver and spleen, injected colloids should have a size between 70 nm and 200 nm (16). Atypical diameter of 100-200 nm is obtained for the pH-sensitive PEGylated liposomes presented here. The number of times the formulation is extruded through the membrane regulates size distribution, the latter becoming more uniform as the number increases. In DLS, the polydispersity index (Pdl) is calculated from the square of the normalized standard deviation (Pdl = (o/Z ) ). Acceptable Pdl values will depend on the desired properties of the vesicles, but values under 0.1 can be usually attained. 

The authors also reported on the supramolecular self-assembly from rod—coil—rod triblock copolymers prepared by copolymerization of 5-acetyl-2-aminob-ezophenone with diacetyl functionalized polystyrene with low polydispersity (Scheme 12).110 In contrast to the rod—coil diblock copolymers which exhibit multiple morphologies, the triblock copolymers were found to spontaneously form only microcapsules or spherical vesicles in solution as evidenced by optical polarized, fluorescence optical, and scanning electron microscopies (Figure 33). 

The cosolvent method is known to result in a dispersion of vesicles of a rather broad size distribution. The polydispersity can be reduced by repeated extrusion [83], A drawback of the method is that organic solvent could remain in the vesicles and the surrounding liquid. To exclude residual solvent, the vesicle suspension is dialyzed or ultrafiltered. 

---