Flexible Bilayers Research Articles

Binding and unbinding of flexible surfactant bilayers

We have probed the influence of salt and temperature on the spatial periodicity (d) of the fluid lamellar (Lα) phase formed by two ionic amphiphiles, whose bilayer bending rigidity moduli are widely different. In the case of rigid 1,2-dimyristoyl-3-trimethylammonium-propane (chloride salt) (DMTAP) bilayers, d is found to decrease with increasing salt concentration (Cs) in the solution, in qualitative agreement with the DLVO theory. d depends only very weakly on temperature in this system. On the other hand, in the Lα phase formed by highly flexible didodecyldimethylammonium chloride (DDAC) bilayers, d is not set by the minimum of the DLVO potential over a wide range of Cs, due to inter-bilayer repulsion arising from thermal undulations of the bilayers. An abrupt transformation of this undulation stabilized swollen Lα phase into a collapsed Lα phase, where d is comparable to the bilayer thickness, is observed above a threshold value of Cs. At higher Cs an unbinding transition of the bilayers is observed on heating, where the collapsed Lα phase transforms into a dispersion of spatially uncorrelated bilayers. Our observations are consistent with this transition being first-order.

Construction and Properties of O/W Liquid Crystal Nanoemulsion.

Liquid crystal emulsion is a new type of emulsion, in which the emulsifier molecules are located at the oil/water (O/W) interface and form a long-range ordered and short-range disordered lamellar liquid crystal. The lamellar liquid crystal formed by the emulsifier is similar to the skin stratum corneum lipid structure, which enables it to have a broad application prospect in the fields of cosmetics, pharmaceuticals, etc. In this work, a liquid crystal nanoemulsion was obtained by passing a liquid crystal emulsion stabilized by hydrogenated lecithin and phytosterol combination through a microfluidizer. The microstructure of the prepared liquid crystal nanoemulsion was investigated experimentally by dynamic light scattering, transmission electron microscopy, and small-angle X-ray scattering. The results have shown that the nanoemulsion inherited the liquid crystal emulsion property, namely, the long-range ordered and short-range disordered lamellar structure still existed at the oil/water interface even though they underwent extrusion, friction, and acceleration. At the same time, the underlying mechanisms of the existence of lamellar liquid crystal between the oil phase and the water phase for the nanoemulsion were explored theoretically by molecular dynamics simulations. The simulation results elucidated that the hydrogenated lecithin and phytosterol combination improved the flexibility of the bilayer structure composed of emulsifiers. The bilayers were the basic structure units of lamellar liquid crystals, and thus, the improved flexibility of bilayers provided insurance for the existence of lamellar liquid crystals with larger curvature around the oil droplets. In addition, the applicable properties of liquid crystal nanoemulsion were studied, and the results have shown that the liquid crystal nanoemulsion presented better slow-release and moisturizing properties than traditional nanoemulsions due to the existence of multilayers between oil and water phases. This work not only provides necessary information for the development and effective application of liquid crystal emulsions but also is helpful for in-depth understanding the inner properties of lamellar liquid crystal at molecular level.

Insight into the liposomal encapsulation of mono and bis-naphthalimides.

Mitonafide-loaded liposomes are a promising strategy to overcome the neurotoxicity observed in clinical trials for this drug. This study investigates the influence of loaded mitonafide or a dimer analogue on different liposomal formulations and their therapeutic efficacy in vitro. Physicochemical properties of the liposomes were manipulated using different loading methods (namely bilayer or core loading) and varying the rigidity of the bilayer using distinct phospholipid compositions. Our results demonstrated that the mitonafide dimer analogue had a comparable encapsulation efficiency (EE%) into the liposomes when loaded into rigid or flexible bilayers in contrast to the low mitonafide monomer EE%. A pH gradient core loading method resulted in a more efficient mechanism to load the monomer into the liposomes. DOSY NMR and spectrofluorometric studies revealed key differences in the structure of the vesicles and the arrangement of the monomer or the dimer in the bilayer or the core of the liposomes. The in vitro assessment of the formulations using MDA-MB-231 and RT-112 cells revealed that a flexible lipid bilayer allows a faster drug release, which correlated well with the spectroscopy studies. This study investigated for the first time that the characteristics of the lipid bilayer and the loading method influence the encapsulation efficacy, colloidal properties, photoactivity and stability of mono and bis-naphthalimides loaded in a liposomal carrier, essential factors that will impact the performance of the formulation in a biological scenario.

Biomimetic Melanosomes Promote Orientation-Selective Delivery and Melanocyte Pigmentation in the H2O2-Induced Vitiligo Mouse Model.

Extremely limited drug retention and depigmentation represent the greatest barriers against vitiligo treatment advancement. Here, inspired by biological melanosomes, the primary melanin transporter, we developed biomimetic melanosomes to combat reactive oxygen species (ROS)-mediated melanocyte damage and depigmentation. Briefly, methylprednisolone (MPS) and melanin-mimicking polydopamine (PDA) were encapsulated inside lysine-proline-valine (KPV)-modified deformable liposomes (KPV-Lipos). Owing to their phospholipid bilayer flexibility and the specific affinity for melanocortin 1 receptor (MC1R), KPV-Lipos exhibited 1.43-fold greater skin deposition than traditional liposomes. The binding of KPV and its receptor also contributed to activating the cAMP-tyrosinase (TYR) signaling pathway, improving the endogenous melanin content. In addition, PDA mimicked melanosomes as it effectively increased the exogenous melanin content and scavenged ROS. Meanwhile, MPS inhibited inflammatory cytokine secretion, limiting the depigmented area. Ultimately, the biomimetic melanosomes affected the skin color of mice with H2O2-induced vitiligo. These melanosomes show potential as a universal platform for the self-supply of melanin by self-driven melanin synthesis with exogenous supplementation. Furthermore, this study offers ideas for the production of artificial packed melanosome substitutes for melanocyte-related diseases.

On the Molecular Interactions in Lipid Bilayer-Water Assemblies of Different Curvatures.

This work concerns the importance of intermolecular interactions present in aqueous lipid assembly systems depending on the type of aggregates they form. We have studied aqueous mixtures of diglycerol monooleate, Capmul glycerol monoleate (GMO-50) and polyoxyethylene (20) sorbitan monooleate (Polysorbate 80, P80) using small-angle X-ray scattering (SAXS) measurements to reveal the structure of liquid crystalline phases. On the basis of the SAXS data, a phase diagram was constructed. We discuss the effect of curvature changes of the lipid-aqueous interface obtained by changing the water content and the temperature. The results are related to the intermolecular interactions, as revealed by Raman spectroscopy, with a focus on the bilayer type of system of different curvature and bilayer flexibility, namely, the lamellar phase, bicontinuous cubic phase, and sponge phase. All phases show large similarities in their chain conformation and head group interactions as revealed by the Raman spectra, arising from the fact that all three structures are formed by lipid bilayers. However, subtle differences in the molecular organization of the sponge phase were revealed by employing Raman difference spectroscopy and by analysis of key spectroscopic indicators, which show a less dense hydrocarbon chain packing compared to the inverse bicontinuous cubic or lamellar phase.

Self-Assembling Hybrid Linear-Dendritic Block Copolymers: The Design of Nano-Carriers for Lipophilic Antitumoral Drugs.

Two series of amphiphilic block copolymers with a hybrid linear-dendritic structure are presented. The compounds consisted of a hydrophilic poly (ethylene glycol) (PEG) block and a 2,2′-bis(hydroxymethyl)propionic acid (bis-MPA) dendron functionalized with stearic acid chains that impart a hydrophobic nature to the block. Different self-assembled nanostructures with a hydrophobic interior and a hydrophilic external part were obtained depending on the length of the PEG chain (Mn = 2000 and Mn = 5000) and the generation of the bis-MPA dendron. The materials were characterized by transmission electron microscopy (TEM). The shapes of the aggregates ranged from spherical or cylindrical micelles to flexible bilayers. The hydrophobic core enabled these nanostructures to encapsulate the water-insoluble drug plitidepsin. The efficacy of these new plitidepsin-containing carriers was evaluated in four cancer cell-lines and they showed similar anticancer activity to the current standard drug formulation.

Reliable Piezoelectricity in Bilayer WSe2 for Piezoelectric Nanogenerators.

Recently, piezoelectricity has been observed in 2D atomically thin materials, such as hexagonal-boron nitride, graphene, and transition metal dichalcogenides (TMDs). Specifically, exfoliated monolayer MoS2 exhibits a high piezoelectricity that is comparable to that of traditional piezoelectric materials. However, monolayer TMD materials are not regarded as suitable for actual piezoelectric devices due to their insufficient mechanical durability for sustained operation while Bernal-stacked bilayer TMD materials lose noncentrosymmetry and consequently piezoelectricity. Here, it is shown that WSe2 bilayers fabricated via turbostratic stacking have reliable piezoelectric properties that cannot be obtained from a mechanically exfoliated WSe2 bilayer with Bernal stacking. Turbostratic stacking refers to the transfer of each chemical vapor deposition (CVD)-grown WSe2 monolayer to allow for an increase in degrees of freedom in the bilayer symmetry, leading to noncentrosymmetry in the bilayers. In contrast, CVD-grown WSe2 bilayers exhibit very weak piezoelectricity because of the energetics and crystallographic orientation. The flexible piezoelectric WSe2 bilayers exhibit a prominent mechanical durability of up to 0.95% of strain as well as reliable energy harvesting performance, which is adequate to drive a small liquid crystal display without external energy sources, in contrast to monolayer WSe2 for which the device performance becomes degraded above a strain of 0.63%.

Effect of bilayer flexibility and medium viscosity on separation of liposomes upon stagnation

Liposomes are widely used as drug delivery systems in different forms including osmotic pumps, infusion and IV injection. In spite of these, there is no data available about their behavior under convective flow (e.g. infusion or osmotic pumps) and upon stagnation in such drug delivery systems. As a part of a series of investigations in this area, the present study investigates the effects of viscosity and flexibility on liposomes separation upon stagnation. Here, liposomes with different bilayer flexibility and medium viscosity were encountered gravity (separating force) in a designed sedimentation model and changes in their properties were monitored over time. Rigid liposomes in the low viscosity formulation showed significant phase separation (three times reduction in size) and decreased lipid content over time. Increasing the bilayer flexibility of large liposomes, prevented them from phase separation. Neither size reduction nor decreased lipid content was observed. Increasing viscosity of the liposomal formulation from 3.4 cP to 45.2 cP also prevented sedimentation of liposomes and phase separation in the system. These results indicate that bilayer flexibility and viscosity affect separation of large liposomes in pre-administrational steps and even stagnation during administration in systems such as infusion pumps.

Investigating the effects of size, charge, viscosity and bilayer flexibility on liposomal delivery under convective flow

Convective flow is one of the main mechanisms of mass transfer employed in drug delivery (e.g. osmotic pumps) and working in material transport in the body (e.g. blood circulation). Although convective flow has been investigated extensively, not much data is available on convective behavior of nanoparticles, the subject of the present investigation.Here, liposomes with different sizes, charges, bilayer flexibilities and medium viscosities were encountered convective flow and changes in their properties were monitored over 48h. For large particles (>350nm), neutral liposomes (NL) showed significant phase separation and decreased lipid content over time, while positively or negatively charged liposomes remained homogenous. Reduction of size in NLs to about 100nm resolved phase separation problem, but their lipid content still showed reduction; no such a problem was observed in charged small liposomes. When bilayer flexibility of large NLs was increased, neither phase separation nor decreased lipid content was observed. Increasing medium viscosity for large NLs from 3.4cP to 45.2cP again solved the problem and a uniform liposomal delivery was observed. These results indicate that size, charge, bilayer flexibility and viscosity affect convective liposomal delivery and that uniform delivery is possible even in large liposomes by optimizing such factors.

Loading of Vesicles into Soft Amphiphilic Nanotubes using Osmosis.

The facile assembly of higher-order nanoarchitectures from simple building blocks is demonstrated by the loading of vesicles into soft amphiphilic nanotubes using osmosis. The nanotubes are constructed from rigid interdigitated bilayers which are capped with vesicles comprising phospholipid-based flexible bilayers. When a hyperosmotic gradient is applied to these vesicle-capped nanotubes, the closed system loses water and the more flexible vesicle bilayer is pulled inwards. This leads to inclusion of vesicles inside the nanotubes without affecting the tube structure, showing controlled reorganization of the self-assembled multicomponent system upon a simple osmotic stimulus.

Interconvertible Self‐Assembly and Rheological Properties of Planar Bilayers and Vesicle Gels in Anionic/Nonionic (CF/CH) Surfactant Solutions

The surface and bulk properties of aqueous mixtures of nonionic tetraethylene glycol monododecyl ether (C12EO4) and anionic perfluorolauric acid (PFLA) were studied. These mixtures exhibited pronounced synergistic effects in terms of their mixed micellization and rheological properties. Only one type of mixed micelle is formed in dilute mixed solutions. At low PFLA content, with increasing the total surfactant concentration, short thread-like micelles grow to form larger discs. On increasing the amount of PFLA, owing to the fact that the fluctuant planar bilayers of nonionic C12EO4 become charged, vesicle gels that are comprised of densely packed unilamellar and multilamellar vesicles spontaneously form, in which the fluorocarbon chains in the bilayers are in a fluid state at room temperature. The elastic properties and the yield stress of the lamellar solutions largely increase upon the addition of small amounts of PFLA and they then pass through a maximum at a saturation of effective membrane charge density. Interestingly, increasing the pH value of the gels causes a backward transition from stiff vesicles to flexible planar bilayers. On increasing the amount of NaOH, the bilayers exhibit greater flexibility, owing to the decrease in membrane charges, along with a large decrease in the solution viscosity and their elastic properties. The combination of strong stability of the vesicle gels with the interconvertible transition of bilayers and vesicles is expected to be of practical use for controlled drug delivery and release.

Structural analysis of “flexible” liposome formulations: new insights into the skin-penetrating ability of soft nanostructures

Self-assembled nanocontainers that can penetrate the protective barrier of skin could prove useful for the needle-free delivery of chemicals, drugs or vaccines into the skin. The main examples of these are the so-called ‘‘flexible liposomes’’, which are mixtures of lipids and detergents. In vitro experiments by numerous researchers have confirmed the skin-penetrating ability of these structures, which is believed to involve the squeezing of flexible bilayers through the pores in the stratum corneum. Here, we reexamine the structure of ‘‘flexible’’ liposome formulations and show that these are actually mixtures of liposomes and micelles. These findings are reinforced through a combination of small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). We believe that our new findings necessitate a new mechanism for the penetration of soft assemblies into the skin. Several possibilities in this regard are considered, including one in which micelles facilitate the dynamic exchange of amphiphiles between liposomes and/or lipid bilayers.

Salamander retina phospholipids and their localization by MALDI imaging mass spectrometry at cellular size resolution

Salamander large cells facilitated identification and localization of lipids by MALDI imaging mass spectrometry. Salamander retina lipid extract showed similarity with rodent retina lipid extract in phospholipid content and composition. Like rodent retina section, distinct layer distributions of phospholipids were observed in the salamander retina section. Phosphatidylcholines (PCs) composing saturated and monounsaturated fatty acids (PC 32:0, PC 32:1, and PC 34:1) were detected mainly in the outer and inner plexiform layers (OPL and IPL), whereas PCs containing polyunsaturated fatty acids (PC 36:4, PC 38:6, and PC 40:6) composed the inner segment (IS) and outer segment (OS). The presence of PCs containing polyunsaturated fatty acids in the OS layer implied that these phospholipids form flexible lipid bilayers, which facilitate phototransduction process occurring in the rhodopsin rich OS layer. Distinct distributions and relative signal intensities of phospholipids also indicated their relative abundance in a particular cell or a cell part. Using salamander large cells, a single cell level localization and identification of biomolecules could be achieved by MALDI imaging mass spectrometry.

Morphological transformation of self-assembled nanostructures prepared from cholesteryl acyl didanosine and the optimal formulation of nanoparticulate systems: Effects of solvents, acyl chain length and poloxamer 188

Self-assembled nanostructures were obtained through injecting cholesteryl-succinyl didanosine (CSD) solutions in tetrahydrofuran (THF) into water. The incorporation of THF leads to CSD flexible bilayers. Spherical vesicles of CSD initially formed based on hydrophobic interaction and transformed to short nanotubes with THF leaving. A large proportion of THF led to formation of long flexible nanoribbons, also transforming to short nanotubes after removing THF because of rigid bilayers recovering. Cholesteryl-adipoyl didanosine (CAD) only formed spherical vesicles due to its long and flexible tails. A triblock copolymer, poloxamer 188 (P188) could insert into CSD monolayers at the air/water interface. P188 stabilized CSD nanoparticulate systems by incorporation and adsorption. The optimal formulation of nanoparticulate systems was identified and the appropriate ratio of CSD/P188 and CSD concentration in injected solutions were the keys. A highly concentrated system had a narrow size distribution, allowing heat sterilization at 100 °C, and resisting aggregation under high gravity accelerations in spite of the low zeta potential of −18.5 mV. A gel–liquid crystalline phase transition at 47 °C occurred. The system may become promising self-assembled drug delivery systems (SADDS) for anti-HIV therapy.

Retraction for Coldren et al., Flexible bilayers with spontaneous curvature lead to lamellar gels and spontaneous vesicles

CHEMISTRY.For the articles “Flexible bilayers with spontaneous curvature lead to lamellar gels and spontaneous vesicles,” by Bret A. Coldren Heidi Warriner Ryan van Zanten Joseph A. Zasadzinski and Eric B. Sirota, which appeared in issue 8, February 21, 2006, of Proc Natl Acad Sci USA (103:2524–2529; first published February 8, 2006; 10.1073/pnas.0507024103), and “Lamellar gels and spontaneous vesicles in catanionic surfactant mixtures,” by Bret A. Coldren, Heidi Warriner, Ryan van Zanten, Joseph A. Zasadzinski, and Eric B. Sirota, which appeared in issue 6, March 14, 2006, of Langmuir (22:2465–2473), the editors of both journals retract these papers because they constitute duplicate publication.

Flexible bilayers with spontaneous curvature lead to lamellar gels and spontaneous vesicles.

Flexible bilayers with spontaneous curvature lead to lamellar gels and spontaneous vesicles.

RETRACTED: Flexible bilayers with spontaneous curvature lead to lamellar gels and spontaneous vesicles

Mixtures of cetyltrimethylammonium tosylate (CTAT) and sodium dodecylbenzene sulfonate (SDBS) in water form a fluid lamellar phase at < or = 40 wt % water but surprisingly turn into viscous gels at higher water fractions. The gels are characterized by spherulite and other bilayer defects consistent with a low bending elasticity, kappa approximately k(B)T, and a nonzero spontaneous curvature. Caillé analysis of the small-angle x-ray line shape confirms that for 7:3 wt:wt CTAT:SDBS bilayers at 50% water, kappa = 0.62 +/- 0.09 k(B)T and kappa = -0.9 +/- 0.2 k(B)T. For 13:7 wt:wt CTAT:SDBS bilayers, the measured bending elasticity decreases with increasing water dilution in good agreement with predictions based on renormalization theory, giving kappa(o) = 0.28 k(B)T. These results show that surfactant mixing is sufficient to make kappa approximately k(B)T, which promotes strong, Helfrich-type repulsion between bilayers that can dominate the van der Waals attraction. These are necessary conditions for spontaneous vesicles formed at even higher water fractions to be equilibrium structures.

Enhancement of the transdermal delivery of catechins by liposomes incorporating anionic surfactants and ethanol

The aim of this study was to develop and evaluate liposomal formulations encapsulating tea catechins, which possess antioxidant and chemopreventive activities. Liposomes were characterized for size, zeta potential, and entrapment efficiency. Both in vitro and in vivo skin permeation were examined using nude mouse skin as a model. The results suggested that the liposomal composition plays an important role in affecting the efficiency of transdermal catechin delivery. Incorporation of anionic surfactants such as deoxycholic acid (DA) and dicetyl phosphate (DP) in the liposomes in the presence of 15% ethanol increased the (+)-catechin permeation by five to seven-fold as compared to the control. The flexibility of bilayers is suggested as an important factor governing the enhancing effect of liposomes. Intercellular spaces within the stratum corneum but not shunt routes are the major pathways for catechin delivery from liposomes. (+)-Catechin and (−)-epicatechin are isomers which showed similar encapsulation efficiencies and skin permeation in liposomes. (−)-Epigallocatechin-3-gallate showed the highest encapsulation rate and in vivo skin deposition level in liposomes among all catechins tested. The stability and in vitro tranepidermal water loss test indicated the safety of the practical use of liposomes developed in this study.

Flexible lipid bilayers in implicit solvent

A minimalist simulation model for lipid bilayers is presented. Each lipid is represented by a flexible chain of beads in implicit solvent. The hydrophobic effect is mimicked through an intermolecular pair potential localized at the "water"/hydrocarbon tail interface. This potential guarantees realistic interfacial tensions for lipids in a bilayer geometry. Lipids self-assemble into bilayer structures that display fluidity and elastic properties consistent with experimental model membrane systems. Varying molecular flexibility allows for tuning of elastic moduli and area per molecule over a range of values seen in experimental systems.

Structure and order in lamellar phases determined by small-angle scattering

Multilamellar phases can be identified and characterized by small-angle scattering of X-rays (SAXS) or neutrons (SANS). Equidistant peaks are the typical signature and their spacing allows the fast determination of the repeat distance,i.e.the mean distance between the midplane of neighbouring bilayers. The scattering function can be described as the product of a structure factor and a form factor. The structure factor is related to the ordering of the bilayers and is responsible for the typical equidistant peaks, but it also contains information about the bilayer flexibility and the number of coherently scattering bilayers. The form factor depends on the thickness and the internal structure (scattering length density distribution) of a single bilayer. The recently developed generalized indirect Fourier transformation (GIFT) method is extended to such systems. This method allows the simultaneous determination of the structure factor and the form factor of the system, including the correction of instrumental broadening effects. A few-parameter model is used for the structure factor, while the determination of the form factor is completely model-free. The method has been tested successfully with simulated scattering data and by application to experimental data sets.