Membranes polymersomes

Recently, unique vesicle-forming (spherical bUayers that offer a hydrophilic reservoir, suitable for incorporation of water-soluble molecules, as well as hydrophobic wall that protects the loaded molecules from the external solution) setf-assembUng peptide-based amphiphilic block copolymers that mimic biological membranes have attracted great interest as polymersomes or functional polymersomes due to their new and promising applications in dmg delivery and artificial cells [ 122]. However, in all the cases the block copolymers formed are chemically dispersed and are often contaminated with homopolymer. 

Fig. 10 (a) Chemical structure of PEG-6-PCL copolymer, (b) CLSM image of PEG-6-PCL polymersomes containing membrane-encapsulated Nile Red (2 mol%) and aqueous entrapped Calcein dyes. Scale bar 5 pm. (c) Cryo-TEM image of PEG-6-PCL polymersomes. Scale bar 100 nm. Reprinted from [228] with permission... 

Hammer and coworkers prepared PEG-h-PCL polymersomes entrapping DXR (Fig. 11a). The release of DXR from the polymersomes was in a sustained manner over 14 days at 37 °C in PBS via drug permeation through the PCL membrane, and hydrolytic degradation of the PCL membrane [228]. The release rate of encapsulated molecules from polymersomes can be tuned by blending with another type of block copolymer [229]. Indeed, the release rate of encapsulated DXR from polymersomes prepared from mixtures of PEG- -PLA with PEG- -PBD copolymers increased linearly with the molar ratio of PEG- -PLA in acidic media (Fig. lib). Under acidic conditions, the PLA first underwent hydrolysis and, hours later, pores formed in the membrane followed by final membrane... 

Polymeric vesicles, or polymersomes, are of interest for the encapsulation and delivery of active ingredients. They offer enhanced stability and lower permeability compared to lipid vesicles, and the versatility of synthetic polymer chemistry provides the ability to tune properties such as membrane thickness, surface... 

The formation of the spherocylindrical shapes is not well understood. They are observed not only on lipid vesicles but also on polymersomes (vesicles made of diblock copolymers [123, 124]) [107]. Therefore, lipid-specific effects, for example partial head group charge and membrane thickness, as a possible cause for the observed cylindrical deformations are to be excluded. One possible explanation could be that ions flatten the equatorial zone of the deformed vesicle. During the pulse there is an inhomogeneity in the membrane tension due to the fact that the... 

Lee JCM, Santore M, Bates FS, Discher DE (2002) From membranes to melts, rouse to reptation diffusion in polymersome versus lipid bilayers. Macromolecules 35 323-326... 

Fig. 6 Schematic scaling of polymersome membrane thickness with copolymer molecular weight. Ahmed F, Photos PJ, Discher DE (2006) Drug Dev Res 67 4. Copyright Wiley. Reproduced with permission [130]...

In polymersomes with thicker membranes the limited mobility of high MW polymer chains is the reason for the better resistance to dissolution. Thus, while liposomes would normally be destroyed when exposed to SDS (sodium dodecyl sulfate) or other strong detergents, polymer vesicles would be more resistant to detergents since this process is obstructed by the restricted chain mobility. Combination of experimental data and theoretical modeling suggests that polymer vesicle dissolution is mediated by the diffusion of detergent molecules across the membrane [132],... 

The membrane elastic behavior of PEO-PEE giant polymersomes has been studied by a micropipette aspiration method [5], The results showed that the polymer membrane elasticity is comparable to fluid-state lipid membranes however the vesicles could sustain a greater strain before rupture, proving an enhanced polymersome toughness, which originates from membrane thickness. 

Fig. 9 Confocal LSM images of micron-size TAT-conjugated NIR polymersomes. a Fluorescein-Tat functionalized vesicles, b FITC-Tat functionalized vesicle surface with near-infrared fluorophores (NIRF) leaded within the membrane. Green = FITC, red = NIRF, yellow = fluorophore colocalization scale bar = 10 pm. Reprinted with permission from [181]. Copyright (2007) American Chemical Society...

Fig. 16 a-d Schematic representation of polymer nanoreactors, a Cross section of triblock copolymer vesicle, b Polymersome with encapsulated enzyme and membrane-embedded channel protein. In the case described in the text, the substrate entering the vesicle is ampicillin, and the product of the hydrolysis is ampicillinoic acid, c Polymersome with embedded ionophores allowing Ca2+ ions to enter the vesicle ere they react with phosphate ions to form calcium phosphate crystals, d The LamB protein serves as a receptor to the 1 phage virus which can inject its DNA through the channel into the polymersome [259]. Reproduced with permission of The Royal Society of Chemistry... 

The potential of polymersomes in biomedical applications have been extensively discussed in several reviews [19,22-26], so they are mentioned here only briefly. Mainly due to the high molecular weight of their amphiphiles they differ from liposomes in several aspects, which makes them beneficial for certain purposes. (1) Typically, they have a much thicker shell. For the vesicles shown in Fig. 2c the hydrophobic core thickness is d = 21 nm, while for lipid membranes typically dm 3 nm. (2) Due to the larger thickness, polymeric membranes are much less susceptible to fluctuations and defects, and they can withstand larger deformations than lipid systems. It is remarkable that, while lipid bilayers can be stretched only 5%... 

Fig. 3 Left Schematic representation of a DNA-loaded triblock-based polymersome. The virus, a X phage, binds a LamB protein and the DNA is transferred across the block copolymer membrane. Right An electron micrograph of negatively stained complexes formed between X phage and vesicles bearing LamB proteins at 37° C. The X phage (large structure on the top left comer) is attached to one vesicle via its tail. Ref. [27]. Copyright (2002) National Academy of Sciences, U.S.A...

Fig. 6 Plot of membrane tension t as a function of dilation for a wide range of copolymer amphiphiles as extracted from MD simulations. The computational models, derived from systematic coarse-graining (black symbols), show nearly the same dilational behavior marked by the solid line. The slope of the line, ka, is very close to experimental measurements performed on giant vesicles 0colored symbols). Experimental data for a dimyristoyl phosphatidylcholine lipid membrane are also shown. The point of membrane lysis as observed experimentally for selected lipid and polymersome systems is also shown in the plot with green and red stars, respectively. Reprinted by permission from Macmillan Publishers Ltd Nature Materials, Ref. [85], copyright (2004)...

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