Vesicles polymerized

As a result of the potential utility and relatively low cost of vesicles, a great deal of effort has been applied to the development of polymerized surfactant and phospholipid systems. Covalently crosslinking the vesicle membrane after the encapsulation process should produce a system in which the basic nature of the vesicle as an encapsulating medium is retained while the stmctural integrity and increased stability of a crosslinked polymeric structure are added. 

Cell-sized giant liposomes [311-313] have been prepared and their mechanical properties have been extensively investigated [312-315]. Utilization of these systems as containers for advanced materials awaits the interested researcher. 

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. 

Dissymmetrical SUVs can be formed by limiting reactions to the outer surfaces of polymerized surfactant vesicles (Fig. 39). Chemical dissymmetry has been created, for example, in polymerized vesicles prepared from surfactants containing ester-linked viologen moieties in their headgroups. Cleavage of the 

Vesicle polymerized at both inner and outer surfaces 

The correct alignment of surfactants in some, but not all, SUVs is an essential requirement for polymerization. Polymerization of diacetylenes is topochemically controlled and only occurs below the phase transition temperature of the surfactant. In contrast, SUVs prepared from styrene-containing surfactants could be polymerized in their fluid states [55]. The degree of polymerization varied from very low (10-20 for SUVs prepared from styrene containing surfactants) to rather high (several hundred for SUVs prepared from diacetylene-containing surfactants). 

---

Copolymers of diallyl dimethyl ammonium chloride [7398-69-8] with acrylamide have been used in electroconductive coatings (155). Copolymers with acrylamide made in activated aqueous persulfate solution have flocculating activity increasing with molecular weight (156). DADM ammonium chloride can be grafted with cellulose from concentrated aqueous solution catalysis is by ammonium persulfate (157). Diallyl didodecylammonium bromide [96499-24-0] has been used for preparation of polymerized vesicles (158). 

A novel polymerized vesicular system for controlled release, which contains a cyclic a-alkoxyacrylate as the polymerizable group on the amphiphilic structure, has been developed. These lipids can be easily polymerized through a free radical process. It has been shown that polymerization improves the stabilities of the synthetic vesicles. In the aqueous system the cyclic acrylate group, which connects the polymerized chain and the amphiphilic structure, can be slowly hydrolyzed to separate the polymer chain and the vesicular system and generate a water-soluble biodegradable polymer. Furthermore, in order to retain the fluidity and to prepare the polymerized vesicles directly from prev lymerized lipids, a hydrophilic spacer has been introduced. 

In the 1980s, polymerization was introduced to overcome the limited stability of synthetic vesicles (2-4). It was found that the stability of the polymerized vesicles was improved dramatically compared to the unpolymerized vesicle and that entrapped substances are released to a much smaller extent from polymerized liposomes than from monomeric ones. 

BAILEY ZHOU Polymerized Vesicles with Hydrolyzable Linkages... 

The polymerized vesicles showed much greater stability over the unpolymerized ones. On standing at room temperature, the unpolymerized vesicles remained stable only for about 2 days, after which time precipitation of some of the lipid was noted substantial precipitation of the lipid was observed after 6 days of standing. By comparison, there was only a trace of precipitation in the polymerized vesicle system after standing 6 days at room temperature, and no substantial precipitation was observed after a 2-week period. The electron microscope pictures of Figure 4 showed that most of the polymerized vesicles retained their initial sizes and and shapes after 6 days. 

The UV absorption of the aqueous vesicular systems, which provides information on the relative concentration of lipids in aqueous system, also proved the enhanced stability of the polymerized vesicles. The absorptions at 238 nm of the unpolymerized vesicles showed a sharp decrease, as seen in Figure 5, as a result of the precipitation of the lipid in the system. However, the absorptions at the same wavelength of the polymerized system showed a relatively steady trend that meant the polymerized lipid had a longer suspension life in the aqueous system. 

Figure 4. Electron micrographs of the polymerized vesicles of Lipid 1 (stained by 1% uranyl acetate).

Figure 5. UV absorption at 238 nm of the vesicle systems of Lipid 1 ( polymerized vesicles o nonpolymerized vesicles).

The polymerized system gives similar results. The IR spectrum of the polymerized lipid shows two absorption peaks at 1805 cm-1 and 1735 cm"1 which correspond to the lactone carbonyl and ester groups, respectively. After the polymerized vesicle had been allowed to stand in an aqueous system for 2 weeks, the lactone carbonyl absorption peak at 1805 cm"1 disappeared as seen in Figure 3, which indicates the hydrolysis of the connecting acetal linkages has been completed. 

The fluidity is one of the most vital properties of biological membranes. It relates to many functions involved in biological system, and effective biomembrane mimetic chemistry depends on the combination of both stability and mobility of the model membranes. However, in the polymerized vesicles the polymer chain interferes with the motion of the side groups and usually causes a decrease or even the loss of the fluid phases inside the polymerized vesicle (72,13). 

The polymerized vesicles from the prepolymerized lipid showed enhanced stability as expected. In a 6-day period there was little precipitate of the lipid. Even after 18 days, as seen in Figure 7, some of the vesicles could be seen in the electron micrograph. 

The DSC spectra confirm that the fluid phase of the polymerized vesicles remains and the phase transitions are retained with the introduction of the spacer group. As can been seen in Figure 8 of the DSC spectrum of the monomeric lipid, there is a peak around 28°C which corresponds to the phase transition of monomeric lipid. As the result of the presence of the spacer group, a similar phase transition can also be observed clearly in the spectrum of the polymerized lipid as shown in Figure 9, but the transition temperature is increased to 36°C by the presence of the polymer chains. 

In this paper two new polymerized vesicle systems have been presented. The first lipid can be polymerized in vesicle through UV irradiation. Because the second lipid contains a flexible spacer group it can be prepolymerized in benzene and then converted to vesicles by ultrasonication in water. The polymerization improves the stabilities of the synthetic liposomes. Since there is a acetal linkage between the... 

In order to determine whether these surfactant vesicles were of polymerized vesicle forms, a 25% V/V ethanol (standard grade) was added to the three year old sample solution. Alcohols are known (34) to destroy surfactant vesicles derived from natural phospholipids, however, synthetically prepared polymerized vesicles are stable in as much as 25% (V/V) alcohol addition. Photomicrographs shown in Figures 7c and 7d indicate that these vesicles partially retain their stability (being mesomorphic) and therefore are suspected to be polymerized surfactants. Whether surfactant molecules of these vesicles are single or multipla bonds in tail, or in head groups remains to be seen. 

Several workers have introduced polymerizable groups into twin-tailed amphiphiles and formed vesicles by sonication. They then link the amphi-philes by initiating polymerization, either chemically or photochemically. The polymerized vesicles which are so generated show little tendency to fuse, and are much more stable than the vesicles formed by sonication or vaporization. They therefore have considerable potential for compartmentalizing reagents, although as with normal vesicles there is always the... 

Polymer formation from monomeric butadiene lipid vesicles is demonstrated by the decrease of the monomer absorption at 260 nm as well as by GPC of the residue of a freeze dried polymer vesicle dispersion. The latter method was also used for proving the formation of polymeric vesicles from the methacryloyl lipids. 

The great stability of polymerized vesicles can be demonstrated by several experiments (9) dilution of polymer vesicle solutions with 50% of ethanol does not result in a precipitation. Turbidity measured at 300 nm remains the same (28), whereas monomer vesiclesare destroyed under these conditions followed by a considerable decrease of turbidity. A precipitation of stabilized vesicles can, however, be achieved by the addition of salts (KC1), but again it has to be pointed out that the polymeric vesicles... 

Figure 13. VIS spectrum of polymerized vesicles from (5) after 15 min of UV irradiation with multichromatic UV light at 0°C. c = 0.5 mg/mL I cm cuvet...

Polymeric vesicles could be of better use for such an antitumor therapy on a cellular level, since they have at least one of the properties required, namely an extraordinary membrane stability. For a successful application, however, the simple systems prepared so far must be varied to a great extent, because the stability of a model cell membrane is not the only condition to be fulfilled. Besides stability and possibilities for cell recognition as discussed above the presence of cell membrane destructing substances such as lysophospholipids is necessary. These could e.g. be incorporated into the membrane of stabilized liposomes without destruction of the polymeric vesicles. There have already been reports about thekilling of tumor cells by synthetic alkyl lysophospholipids (72). 

Ma G, Cheng Q (2006) Manipulating fret with polymeric vesicles development of a mix-and-detect type fluorescence sensor for bacterial toxin. Langmuir 22 6743-6745... 

Okada S, Jelinek R, Charych D (1999) Induced color change of conjugated polymeric vesicles by interfacial catalysis of phospholipase A(2). Angew Chem Int Ed 38 655-659... 

In addition to the micelle-type assemblies described above, there has been significant interest in developing conditions for forming vesicle-type assemblies from amphiphilic polymers. Polymeric vesicles are formed by bolamphiphilic block... 

Biesalski M, Tu R, Tirrell MV. Polymerized vesicles containing molecular recognition sites. Langmuir 2005 21 5663-5666. 

Surfactants provide several types of well-organized self-assembhes, which can be used to control the physical parameters of synthesized nanoparticles, such as size, geometry and stability within liquid media. Estabhshed surfactant assembles that are commonly employed for nanoparticie fabrication are aqueous micelles, reversed micelles, microemulsions, vesicles [15,16], polymerized vesicles, monolayers, deposited organized multilayers (Langmuir-Blodgett (LB) films) [17,18] and bilayer Upid membranes [19](Fig. 2). 

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. 

These molecular assemblies are unfortunately not stable enough to construct practical solar energy conversion systems. Vesicles composed of polymerizable monomers (e.g., 4, 5) were polymerized to give polymeric vesicles having enhanced stability 25 26). 

---