Penetration and growth of DPPC/DHPC bicelles inside the stratum corneum of the skin

Pathways of Membrane Solubilization: A Structural Study of Model Lipid Vesicles Exposed to Classical Detergents

Understanding the pathways of solubilization of lipid membranes is of high importance for their use in biotechnology and industrial applications. Although lipid vesicle solubilization by classical detergents has been widely investigated, there are few systematic structural and kinetic studies where different detergents are compared under varying conditions. This study used small-angle X-ray scattering to determine the structures of lipid/detergent aggregates at different ratios and temperatures and studied the solubilization in time using the stopped-flow technique. Membranes composed of either of two zwitterionic lipids, DMPC or DPPC, and their interactions with three different detergents, sodium dodecyl sulfate (SDS), n-dodecyl-beta-maltoside (DDM), and Triton X-100 (TX-100), were tested. The detergent TX-100 can cause the formation of collapsed vesicles with a rippled bilayer structure that is highly resistant to TX-100 insertion at low temperatures, while at higher temperatures, it partitions and leads to the restructuring of vesicles. DDM also causes this restructuring into multilamellar structures at subsolubilizing concentrations. In contrast, partitioning of SDS does not alter the vesicle structure below the saturation limit. Solubilization is more efficient in the gel phase for TX-100 but only if the cohesive energy of the bilayer does not prevent sufficient partitioning of the detergent. DDM and SDS show less temperature dependence compared to TX-100. Kinetic measurements reveal that solubilization of DPPC largely occurs through a slow extraction of lipids, whereas DMPC solubilization is dominated by fast and burst-like solubilization of the vesicles. The final structures obtained seem to preferentially be discoidal micelles where the detergent can distribute in excess along the rim of the disc, although we do observe the formation of worm- and rodlike micelles in the case of solubilization of DDM. Our results are in line with the suggested theory that bilayer rigidity is the main factor influencing which aggregate is formed.

Transformation of Lipid Vesicles into Micelles by Adding Nonionic Surfactants: Elucidating the Structural Pathway and the Intermediate Structures

The phospholipid lecithin (L) and the nonionic surfactant Tween 80 (T) are used together in various contexts, including in drug delivery and oil spill remediation. There is hence a need to elucidate the nanostructures in LT mixtures, which is the focus of this paper. We study these mixtures using cryogenic transmission electron microscopy (cryo-TEM), coupled with dynamic light scattering and small-angle neutron scattering. As the concentration of Tween 80 is increased, the vesicles formed by lecithin are transformed into spherical micelles. We identify bicelles (i.e., disc-like micelles) as well as cylindrical micelles as the key stable nanostructures formed at intermediate L/T ratios. The bicelles have diameters ∼13-26 nm, and the bicelle size decreases as the Tween 80 content increases. We propose that the lecithin lipids form the body of the discs, while the Tween 80 surfactants occupy the rims. This hypothesis is consistent with geometric arguments because lecithin is double-tailed and favors minimal curvature, whereas the single-tailed Tween 80 molecules prefer curved interfaces. In the case of cylindrical micelles, cryo-TEM reveals that the micelles are short (length < 22 nm) and flexible. We are able to directly visualize the microstructure of the aggregates formed by lecithin-Tween 80 mixtures, thereby enhancing the understanding of morphological changes in the lecithin-Tween 80 system.

Physical Enhancement? Nanocarrier? Current Progress in Transdermal Drug Delivery

A transdermal drug delivery system (TDDS) is a method that provides drug adsorption via the skin. TDDS could replace conventional oral administration and blood administration because it is easily accessible. However, it is still difficult to design efficient TDDS due to the high barrier property of skin covered with stratum corneum, which inhibits the permeation of drug molecules. Thus far, TDDS methods by applying physical stimuli such as microneedles and chemical stimuli such as surfactants have been actively developed. However, it has been hard to avoid inflammation at the administration site because these methods partially destroy the skin tissue. On the other hand, TDDS with nanocarriers minimizing damage to the skin tissues has emerged together with the development of nanotechnology in recent years. This review focuses on current trends in TDDS.

Effects of fluidity and charge density on the morphology of a bicellar mixture - A SANS study

The spontaneously formed structures of physiologically relevant lipid model membranes made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and 1,2-hexanoyl-sn-glycero-3-phosphocholine have been evaluated in depth using small angle neutron scattering. Although a common molar ratio of long- to short- chain phospholipids (~4) as reported in many bicellar mixtures was used, discoidal bicelles were not found as the major phase throughout the range of lipid concentration and temperature studied, indicating that the required condition for the formation of bicelle is the immiscibility between the long- and short- chain lipids, which were in the gel and L_α phases, respectively, in previous reports. In this study, all lipids are in the L_α phase. The characterization outcome suggests that the spontaneous structures tie strongly with the physical parameters of the system such as melting transition temperature of the long-chain lipid, total lipid concentration and charge density of the system. Multilamellar vesicles, unilamellar vesicles, ribbons and perforated lamellae can be obtained based on the analysis of the small angle neutron scattering results, leading to the construction of structural diagrams. This report provides the important map to choose suitable lipid systems for the structural study of membrane-associated proteins, design of theranostic nanocarriers or other related research fields.

Evaluation of Molecular Ordering in Bicelle Bilayer Membranes Based on Induced Circular Dichroism Spectra

Bicelles are submicrometer-sized disc-shaped molecular self-assemblies that can be obtained in aqueous solution by dispersing mixtures of certain amphiphiles. Although phospholipid bicelle and phospholipid vesicle assemblies adopt similar lipid bilayer structures, the differences in bilayer characteristics, especially physicochemical properties such as bilayer fluidity, are not clearly understood. Herein, we report the lipid ordering properties of bicelle bilayer membranes based on induced circular dichroism (ICD) and fluorescence polarization analyses using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe. Bicelles were prepared by using 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), while pure DMPC vesicles and pure DHPC micelles were used as references. At temperatures below the phase transition temperature of DMPC, the bicelles showed lower membrane fluidities, whereas DHPC micelles showed higher membrane fluidity, suggesting no significant differences in bilayer fluidity between the bicelle and vesicle assemblies. The ICD signals of DPH were induced only when the membrane was in ordered (solid-ordered or ripple-gel) phases. In the bicelle systems, the ICD of DPH was more significant than that of the DMPC vesicle. The induced chirality of DPH was dependent on the chirality of the bilayer lipid. Compared to that of the DMPC/DHPC bicelle, the ICD of the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/DHPC bicelle was higher, while that of the bovine sphingomyelin/DHPC bicelle was lower. Because the lipids are tightly packed in the ordered phase, the ICD intensity reflects the molecular ordering state of the lipids in the bicelle bilayer.

Small-Angle X-ray Scattering Studies on the Structure of Disc-Shaped Bicelles Incorporated with Neutral PEGylated Lipids

In this study, small-angle X-ray scattering (SAXS) is successfully employed to investigate the structure of the DPPC/diC7PC disc-shaped bicelles incorporated with different amounts of C16-PEG2000-Ceramide lipids. The incorporation of the C16-PEG2000-Ceramide lipids could provide an antifouling capability to the bicelle for biomedical applications. However, traditionally it is believed that most of the incorporated PEGlylated lipids should lie in the rim of the disc-shaped bicelle. In this study, high sensitivity SAXS reveals the distribution of the added C16-PEG2000-Ceramide lipids in both the planar region and in the rim of the bicelle. The PEG brushes of C16-PEG2000-Ceramide lipids form a second shell outside the lipid headgroup shell of the bicelle. A double shell disc bicelle model is used in analyzing the SAXS data. The lipid density of C16-PEG2000-Ceramide in the rim is found to be about 1.7 times the C16-PEG2000-Ceramide lipid density in the planar region for all three C16-PEG2000-Ceramide concentrations, 1, 2, and 3 mM. Moreover, the bicelle core radius can be predicted well using the actual molecular ratio of lipids in the planar region to the lipids in the rim of the bicelles in the model calculation.

Shear-Induced Transformation of Polymer-Rich Lamellar Phases to Micron-Sized Vesicles

In the present work, we study the shear-induced transformation of polymer-rich lamellar phases into vesicles. The evolution of vesicle size is studied by different scattering techniques, rheology, and microscopy methods. The lamellar phase found in the system D₂O/ o-xylene/ Pluronic PE9400/C₈TAB can be fully transformed to multilamellar vesicles (MLVs) by applying shear. The size of the MLVs is proportional to the inverse square root of the shear rate. Hence, the polymer-based quaternary system behaves similar to lamellar phases based on small surfactant molecules. Additionally, we found a growth effect leading to a size increase of the vesicles after shearing was stopped.

Lipid Nanodisc-Templated Self-Assembly of Gold Nanoparticles into Strings and Rings

Gold nanoparticles (AuNPs) exhibit strong fluorescent and electromagnetic properties, which can be enhanced upon clustering and used in therapeutic, imaging, and sensing applications. A combination of gold nanoparticles with lipid nanodiscs can be attractive for AuNP self-assembly and useful in biomedical applications. Using molecular dynamics simulations we show that lipid nanodiscs can serve as templates for AuNP clustering into rings and string-like structures. We demonstrate that equilibrium encapsulation of 1 nm hydrophobically modified AuNPs into lipid nanodiscs composed of a mixture of dipalmitoylphosphatidylcholine (DPPC) and dihexanoylphosphatidylcholine (DHPC) lipids occurs at the rim and results in formation of a ring of gold. The interior of the nanodisc is inaccessible to AuNPs due to the DPPC liquid crystalline order. With temperature increase the lipid order diminishes, initiating the nanodisc transformation into a vesicle, upon which encapsulated AuNPs cluster into a close-packed string or nanoring, thereby stalling the vesiculation process at a "round vase" or cup-like stage depending on the AuNP concentration. In contrast, encapsulation of AuNPs by an equilibrium lipid vesicle results in its deformation with randomly clustered AuNPs, in agreement with experimental observations. We characterize the AuNP cluster size and surface-to-surface pair distribution, both of which impact the AuNP luminescent properties. We investigate the effect of alkane tether length on the nanodisc stability and AuNP clustering inside the nanodiscs and vesicles. Our results show that lipid nanodiscs can enhance gold cluster formation, which can be further exploited in imaging applications.

Synthesis and characterization of pHLIP® coated gold nanoparticles

Novel approaches in synthesis of spherical and multispiked gold nanoparticles coated with polyethylene glycol (PEG) and pH Low Insertion Peptide (pHLIP^®) were introduced. The presence of a tumor-targeting pHLIP^® peptide in the nanoparticle coating enhances the stability of particles in solution and promotes a pH-dependent cellular uptake. The spherical particles were prepared with sodium citrate as a gold reducing agent to form particles of 7.0±2.5 nm in mean metallic core diameter and ∼43 nm in mean hydrodynamic diameter. The particles that were injected into tumors in mice (21 µg of gold) were homogeneously distributed within a tumor mass with no staining of the muscle tissue adjacent to the tumor. Up to 30% of the injected gold dose remained within the tumor one hour post-injection. The multispiked gold nanoparticles with a mean metallic core diameter of 146.0±50.4 nm and a mean hydrodynamic size of ~161 nm were prepared using ascorbic acid as a reducing agent and disk-like bicelles as a template. Only the presence of a soft template, like bicelles, ensured the appearance of spiked nanoparticles with resonance in the near infrared region. The irradiation of spiked gold nanoparticles by an 805 nm laser led to the time- and concentration-dependent increase of temperature. Both pHLIP^® and PEG coated gold spherical and multispiked nanoparticles might find application in radiation and thermal therapies of tumors.

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pHLIP^®-PEG coated pH-sensitive gold spherical nanoparticles were synthesized.

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30% of the injected gold dose remained within the tumor one hour post-injection.

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pHLIP^®-PEG coated pH-sensitive gold multispiked nanoparticles were synthesized.

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Bicelles were used as a soft template to obtain multispiked nanoparticles.

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Temperature increases after 805 nm irradiation of spiked gold nanoparticles.

Hyper-cell-permeable micelles as a drug delivery carrier for effective cancer therapy

Although PEGylated liposomes (PEG-LS) have been intensively studied as drug-delivery vehicles, the rigidity and the hydrophilic PEG corona of liposomal membranes often limits cellular uptake, resulting in insufficient drug delivery to target cells. Thus, it is necessary to develop a new type of lipid-based self-assembled nanoparticles capable of enhanced cellular uptake, tissue penetration, and drug release than conventional PEGylated liposomes. Herein, we describe a simple modification of bicellar formulation in which the addition of a PEGylated phospholipid produced a dramatic physicochemical change in morphology, i.e., the disc-shaped bicelle became a uniformly distributed ultra-small (∼12 nm) spherical micelle. The transformed lipid-based nanoparticles, which we termed hyper-cell-permeable micelles (HCPMi), demonstrated not only prolonged stability in serum but also superior cellular and tumoral uptake compared to a conventional PEGylated liposomal system (PEG-LS). In addition, HCPMi showed rapid cellular uptake and subsequent cargo release into the cytoplasm of cancer cells. Cells treated with HCPMi loaded with docetaxel (DTX) had an IC₅₀ value of 0.16 μM, compared with 0.78 μM with PEG-LS loaded with DTX, a nearly five-fold decrease in cell viability, indicating excellent efficiency in HCPMi uptake and release. In vivo tumor imaging analysis indicated that HCPMi penetrated deep into the tumor core and achieved greater uptake than PEG-LS. Results of HCPMi (DTX) treatment of allograft and xenograft mice in vivo showed high tumoral uptake and appreciable tumor retardation, with ∼70% tumor weight reduction in the SCC-7 allograft model. Taken together, these findings indicate that HCPMi could be developed further as a highly competent lipid-based drug-delivery system.

Advanced lipid systems containing β-carotene: stability under UV-vis radiation and application on porcine skin in vitro

Phospholipid-based nanostructures, bicelles and bicosomes, are proposed as carriers of the antioxidant β-carotene.

Bicelles: lipid nanostructured platforms with potential dermal applications

Bicelles emerge as promising membrane models, and because of their attractive combination of lipid composition, small size and morphological versatility, they become new targets in skin research. Bicelles are able to modify skin biophysical parameters and modulate the skin's barrier function, acting to enhance drug penetration. Because of their nanostructured assemblies, bicelles have the ability to penetrate through the narrow intercellular spaces of the stratum corneum of the skin to reinforce its lipid lamellae. The bicelle structure also allows for the incorporation of different molecules that can be carried through the skin layers. All of these characteristics can be modulated by varying the lipid composition and experimental conditions. The remarkable versatility of bicelles is their most important characteristic, which makes their use possible in various fields. This system represents a platform for dermal applications. In this review, an overview of the main properties of bicelles and their effects on the skin are presented.

Structural Versatility of Bicellar Systems and Their Possibilities as Colloidal Carriers

Bicellar systems are lipid nanostructures formed by long- and short-chained phospholipids dispersed in aqueous solution. The morphological transitions of bicellar aggregates due to temperature, composition and time variations have been revised in this work. To this end, two bicellar systems have been considered; one formed by dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl- phosphatidylcholine (DHPC) and another formed by dipalmitoyl-phosphatidylcholine (DPPC) and DHPC. The relationship between the magnetic alignment, the morphology of the aggregates and the phase transition temperature (T_m) of lipids is discussed. In general terms, the non-alignable samples present rounded objects at temperature below the T_m. Above this temperature, an increase of viscosity is followed by the formation of large elongated aggregates. Alignable samples presented discoidal objects below the T_m. The best alignment was achieved above this temperature with large areas of lamellar stacked bilayers and some multilamellar vesicles. The effect of the inclusion of ceramides with different chain lengths in the structure of bicelles is also revised in the present article. A number of physical techniques show that the bicellar structures are affected by both the concentration and the type of ceramide. Systems are able to incorporate 10% mol of ceramides that probably are organized forming domains. The addition of 20% mol of ceramides promotes destabilization of bicelles, promoting the formation of mixed systems that include large structures. Bicellar systems have demonstrated to be morphologically stable with time, able to encapsulate different actives and to induce specific effects on the skin. These facts make bicellar systems good candidates as colloidal carriers for dermal delivery. However, water dilution induces structural changes and formation of vesicular structures in the systems; stabilization strategies have been been explored in recent works and are also updated here.

Bicosomes: bicelles in dilute systems

Bicelles are discoidal phospholipid nanostructures at high lipid concentrations. Under dilute conditions, bicelles become larger and adopt a variety of morphologies. This work proposes a strategy to preserve the discoidal morphology of bicelles in environments with high water content. Bicelles were formed in concentrated conditions and subsequently encapsulated in liposomes. Later dilution of these new structures, called bicosomes, demonstrated that lipid vesicles were able to isolate and protect bicelles entrapped inside them from the medium. Characterization of systems before and after dilution by dynamic light-scattering spectroscopy and cryo-transmission electron microscopy showed that free bicelles changed in size and morphology, whereas encapsulated bicelles remained unaltered by the effect of dilution. Free and entrapped bicelles (containing the paramagnetic contrast agent gadodiamide) were injected into rat brain lateral ventricles. Coronal and sagittal visualization was performed by magnetic resonance imaging. Whereas rats injected with free bicelles did not survive the surgery, those injected with bicosomes did, and a hyperintensity effect due to gadodiamide was observed in the cerebrospinal fluid. These results indicate that bicosomes are a good means of preserving the morphology of bicelles under dilution conditions.

Application of bicellar systems on skin: diffusion and molecular organization effects

The effect of bicelles formed by dipalmitoylphosphatidylcholine (DPPC)/dihexanoylphosphatidylcholine (DHPC) on stratum corneum (SC) lipids was studied by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy at different temperatures. Analysis of the lipid organization in terms of chain conformational order and lateral packing shows that the use of bicelles hampers the fluidification of SC lipids with temperature and leads to a lateral packing corresponding to a stable hexagonal phase. Grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS) techniques confirm these results and give evidence of higher lamellar order after treatment with these bicelles. Additionally, the effects of DPPC/DHPC and dimyristoylphosphatidylcholine (DMPC)/DHPC bicelles at different SC depths were compared. The combination of ATR-FTIR spectroscopy and the tape-stripping method was very useful for this purpose.

Conformational changes in stratum corneum lipids by effect of bicellar systems

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was applied to study the effects of the bicelles formed by dimyristoyl-glycero-phosphocholine (DMPC) and dihexanoyl-glycero-phosphocholine (DHPC) in porcine stratum corneum (SC) in vitro. A comparison of skin samples treated and untreated with bicelles at different temperatures was carried out. The analysis of variations after treatment in the position of the symmetric CH2 stretching, CH2 scissoring, and CH2 rocking vibrations reported important information about the effect of bicelles on the skin. Bicellar systems caused a phase transition from the gel or solid state to the liquid crystalline state in the lipid conformation of SC, reflecting the major order-disorder transition from hexagonally packed to disordered chains. Grazing incidence small and wide X-ray scattering (GISAXS and GIWAXS) techniques confirmed this effect of bicelles on the SC. These results are probably related to with the permeabilizing effect previously described for the DMPC/DHPC bicelles.