Oil-loaded monolinolein-based particles with confined inverse discontinuous cubic structure (Fd3m)

In our recent work, we reported on the effect of varying temperature and solubilizing tetradecane (TC) on the structural transitions observed in dispersed particles based on the monolinolein (MLO)-water-TC system. At a given temperature, the addition of TC induces a transition of the internal structure from the bicontinuous cubic phase, Pn3m, to the reversed hexagonal, H2, and to the isotropic liquid phase (water-in-oil (W/O) microemulsions). Our present study focuses on the discovery of a Fd3m phase (reversed discontinuous micellar cubic), which is formed in the MLO-water-TC system at a specific TC/MLO weight ratio. It is situated between the H2 and the isotropic liquid phase (W/O microemulsion). Remarkably, it is not found in the absence of TC by increasing the temperature. The Fd3m structure was investigated in detail by means of small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM). The present work proves that the structural transformation in the dispersed particles from H2 (hexosomes) to the W/O microemulsion system (emulsified microemulsion (EME)) is indirect and it occurs gradually via an emulsified intermediate phase. Specifically, in addition to the nanostructured aqueous dispersions described above, we present new TC-loaded aqueous dispersions with a confined intermediate phase, which is a discontinuous micellar cubic phase of the symmetry Fd3m. We denoted this type of emulsified particles as "micellar cubosomes".     

Emulsified microemulsions and oil-containing liquid crystalline phases

Self-assembled nanostructures, such as inverted type mesophases of the cubic or hexagonal geometry or reverse microemulsion phases, can be dispersed using a polymeric stabilizer, such as the PEO-PPO-PEO triblock copolymer Pluronic F127. The particles, which are described in the present study, are based on monolinolein (MLO)-water mixtures. When adding tetradecane (TC) to the MLO-water-F127 system at constant temperature, the internal nanostructure of the kinetically stabilized particles transforms from a Pn3m (cubosomes) to a H2 (hexosomes) and to a water-in-oil (W/O, L2) microemulsion phase (emulsified microemulsion (EME)). To our knowledge, this is the first time that the formation of stable emulsified microemulsion (EME) systems has been described and proven to exist even at room temperature. The same structural transitions can also be induced by increasing temperature at constant tetradecane content. The internal nanostructure of the emulsified particles is probed using small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). At each investigated composition and temperature, the internal structure of the dispersions is observed to be identical to the corresponding structure of the nondispersed, fully hydrated bulk phase. This is clear evidence for the fact that the self-assembled inner particle nanostructure is preserved during the dispersion procedure. In addition, the internal structure of the particles is in thermodynamic equilibrium with the surrounding water phase. The internal structure of the dispersed, kinetically stabilized particles is a "real" and stable self-assembled nanostructure. To emphasize this fact, we denoted this new family of colloidal particles (cubosomes, hexosomes, and EMEs) as "ISASOMES" (internally self-assembled particles or "somes").     

Understanding the interfacial properties of nanostructured liquid crystalline materials for surface-specific delivery applications

Nonlamellar liquid crystalline dispersions such as cubosomes and hexosomes have great potential as novel surface-targeted active delivery systems. In this study, the influence of internal nanostructure, chemical composition, and the presence of Pluronic F127 as a stabilizer, on the surface and interfacial properties of different liquid crystalline particles and surfaces, was investigated. The interfacial properties of the bulk liquid crystalline systems with coexisting excess water were dependent on the internal liquid crystalline nanostructure. In particular, the surfaces of the inverse cubic systems were more hydrophilic than that of the inverse hexagonal phase. The interaction between F127 and the bulk liquid crystalline systems depended on the internal liquid crystalline structure and chemical composition. For example, F127 adsorbed to the surface of the bulk phytantriol cubic phase, while for monoolein cubic phase, F127 was integrated into the liquid crystalline structure. Last, the interfacial adsorption behavior of the dispersed liquid crystalline particles also depended on both the internal nanostructure and the chemical composition, despite the dispersions all being stabilized using F127. The findings highlight the need to understand the specific surface characteristics and the nature of the interaction with colloidal stabilizer for understanding and optimizing the behavior of nonlamellar liquid crystalline systems in surface delivery applications.     

Revisiting β-casein as a stabilizer for lipid liquid crystalline nanostructured particles

Lipid liquid crystalline nanoparticles such as cubosomes and hexosomes have unique internal nanostructures that have shown great potential in drug and nutrient delivery applications. The triblock copolymer, Pluronic F127, is usually employed as a steric stabilizer in dispersions of lipid nanostructured particles. In this study, we investigated the formation, colloidal stability and internal nanostructure and morphology of glyceryl monooleate (GMO) and phytantriol (PHYT) cubosome dispersions on substituting β-casein with F127 in increasing proportion as the stabilizer. Internal structure and particle morphology were evaluated using small-angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (cryo-TEM), while protein secondary structure was studied using synchrotron radiation circular dichroism (SRCD). The GMO cubosome dispersion stabilized by β-casein alone displayed a V(2) (Pn3m) phase structure and a V(2) to H(2) phase transition at 60 °C. In comparison, F127-stabilized GMO dispersion had a V(2) (Im3m) phase structure and the H(2) phase only appeared at higher temperature, that is, 70 °C. In the case of PHYT dispersions, only the V(2) (Pn3m) phase structure was observed irrespective of the type and concentration of stabilizers. However, β-casein-stabilized PHYT dispersion displayed a V(2) to H(2) to L(2) transition behavior upon heating, whereas F127-stabilized PHYT dispersion displayed only a direct V(2) to L(2) transition. The protein secondary structure was not disturbed by interaction with GMO or PHYT cubosomes. The results demonstrate that β-casein provides steric stabilization to dispersions of lipid nanostructured particles and avoids the transition to Im3m structure in GMO cubosomes, but also favors the formation of the H(2) phase, which has implications in drug formulation and delivery applications.     

Bulk and dispersed aqueous phase behavior of phytantriol: effect of vitamin E acetate and F127 polymer on liquid crystal nanostructure

Phytantriol (3,7,11,15-tetramethylhexadecane-1,2,3-triol, PHYT) is a cosmetic ingredient that exhibits similar lyotropic phase behavior to monoolein (GMO), forming bicontinuous cubic liquid crystalline structures (Q(II)) at low temperatures and reversed hexagonal phase (H(II)) at higher temperatures in excess water. Despite these similarities, phytantriol has received little attention in the scientific community. In this study, the thermal phase behavior of the binary PHYT-water and ternary PHYT-vitamin E acetate (VitEA)-water systems have been studied and compared with the behavior of the dispersed cubosomes and hexosomes formed with the aid of a stabilizer (Pluronic F127). The phase behavior and nanostructure were studied using crossed polarized light microscopy (CPLM), differential scanning calorimetry (DSC), and small-angle X-ray scattering (SAXS) techniques. The presence of lipophilic VitEA in the PHYT-water system suppressed the temperature of the Q(II)-to-H(II)-to-L2 transitions, indicating that lipophilic compounds, in relatively small amounts, may have a significant impact on the phase behavior. Increasing the F127 concentration in the phytantriol-based cubosome system did not induce the Q(II)(Pn3m) to Q(II)(Im3m) transition known for the GMO-water system. This indicates a different mode of interaction between F127 and the lipid domains of phytantriol-water systems. Taken together, these results indicate that phytantriol may not only provide an alternative lipid for preparation of liquid crystalline systems in excess water but may also provide access to properties not available when using GMO.     

Transfer of lipid and phase reorganisation in self-assembled liquid crystal nanostructured particles based on phytantriol

The internal structure of dispersed liquid crystal nanostructured particles of the V(2) and H(2) phases, termed cubosomes and hexosomes respectively, is integral to their application in the pharmaceutical, agricultural and food industries. However the nanostructure is susceptible to change upon incorporation of other lipids and hence it is important to understand the potential for interparticle lipid transfer for such particles when they encounter a particle of dissimilar lipid content. Using time resolved synchrotron small angle X-ray scattering, we have investigated the transfer of material between cubosomes composed of phytantriol with three different particle types of dissimilar composition, (i) hexosomes and (ii) emulsified microemulsion composed of phytantriol and vitamin E acetate, and (iii) cubosomes prepared from glycerol monooleate. It was found that material was transferred between the different dispersed nanostructured particles, with the transfer being caused by compositional ripening. Somewhat counter-intuitively the transfer was bidirectional with phytantriol being more rapidly transferred than the minor component vitamin E acetate. The greater lipophilicity of vitamin E acetate supports previous studies suggesting greater mobility for the less lipophilic components, regardless of the more efficient transfer route to achieve uniform composition. When particles comprising lipids with similar lipophilicities were mixed, the transfer was limited and did not achieve completion; a phase change between cubic nanostructures required to achieve complete mixing provides an apparent barrier to further compositional ripening. The conclusions from this study provide additional support to lipid transfer mechanisms, and highlight some subtleties in using dissimilar lipid mixtures in e.g. food applications.     

Self-assembled geometric liquid-crystalline nanoparticles imaged in three dimensions: hexosomes are not necessarily flat hexagonal prisms

Attempts to understand the complex 3D morphology of non-lamellar liquid-crystalline nanostructured particles, formed by the dispersion of a reversed hexagonal phase (hexosomes) and bicontinuous cubic phase (cubosomes) in water, have been limited by the lack of suitable 3D imaging techniques. Using cryo-field emission scanning electron microscopy, we show that whereas the structure of cubosomes generally reflects that anticipated from modeling approaches, hexosomes, which were previously proposed to be flat hexagonal prisms, in fact often possess a "spinning-top-like" structure, which is likely to influence their interactions with surfaces.     

High-throughput discovery of novel steric stabilizers for cubic lyotropic liquid crystal nanoparticle dispersions

High-throughput methodologies have been employed to establish structure-property relationships and assess the effectiveness of nonionic steric stabilizers for inverse bicontinuous cubic lyotropic liquid crystalline nanoparticulate dispersions of monoolein and phytantriol. The ability of the stabilizers to disperse the lipids was compared with that of the commonly employed triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer Pluronic F127, which was used as a positive control. The poly(ethylene oxide) stearate class of stabilizers (commercially known as Myrj) were discovered to be effective as steric stabilizers for cubosomes, while retaining the internal nanostructure of the "parent" bulk phase. In particular, Myrj 59, with an average of 100 poly(ethylene oxide) units, was more effective than F127 at dispersing phytantriol, forming stable phytantriol cubosome dispersions at a concentration of 0.1 wt %, 5-fold lower than that achievable with Pluronic F127. The discovery of this new effective class of stabilizers for cubosomes, specifically enabled by high-throughput approaches, broadens the versatility of components from which to construct these interesting potential drug delivery and medical imaging nanoparticles.     

Spontaneous formation of vesicles and dispersed cubic and hexagonal particles in amino acid-based catanionic surfactant systems

Mixed catanionic surfactant systems based on amino acids were investigated with respect to the formation of liquid crystal dispersions and the stability of the dispersions. The surfactants used were arginine-N-lauroyl amide dihydrochloride (ALA) and N(alpha)-lauroyl-arginine-methyl ester hydrochloride (LAM), which are arginine-based cationic surfactants; sodium hydrogenated tallow glutamate (HS), a glutamic-based anionic surfactant; and the anionic surfactants sodium octyl sulfate (SOS) and sodium cetyl sulfate (SCS). It is demonstrated that in certain ranges of composition there is a spontaneous formation of vesicular, cubic, and hexagonal structures. The solutions were characterized with respect to internal structure and size by cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and turbidity measurements. Vesicles formed spontaneously and were found for all systems studied; their size distribution is presented for the systems ALA/SCS/W and ALA/SOS/W; they are all markedly polydisperse. The aging process for the system ALA/SOS/W was monitored both by turbidity and by cryo-TEM imaging; the size distribution profile for the system becomes narrower and the number average radius decreases with time. The presence of dispersed particles with internal cubic structure (cubosomes) and internal hexagonal structure (hexosomes) was documented for the systems containing ALA and HS. The particles formed spontaneously and remained stably dispersed in solution; no stabilizer was required. (Cubosome and hexosome are USPTO registered trademarks of Camurus AB, Sweden.) The spontaneous formation of particles and their stability, together with favorable biological responses, suggests a number of applications.     

糖和盐类物质对生物膜超分子结构稳定性影响的研究

用原子力显微镜(AFM)和小角X射线(SAXS)技术, 研究了NaCl、KCl、胆固醇、葡萄糖和蔗糖等与膜脂的相互作用. 研究发现它们能引起脂质膜超分子体系液晶态结构的变化. 葡萄糖和蔗糖对脂双层膜结构有稳定作用. 在NaCl溶液中制成的脂质膜, 随着NaCl浓度的增加, 它们的双层膜更稳定. 在KCl溶液中结果恰好相反. AFM研究发现液晶态脂双层膜结构与双亲性分子的结构、浓度以及介质的组分和pH等因素有关. 在1,2-反十八碳-3-磷脂酰乙醇胺(DEPE)液晶态中, 钠盐诱导形成Q²²⁹(Im3m)立方相. 油酸的含量对DEPE-PVP(聚乙烯吡咯烷酮)超分子结构也有一定的影响, 当油酸含量达到某一临界值时, 则发生从Im3m(Q²²⁹)到Pn3m(Q²²⁴)的转变. 胆固醇能促使形成Pn3m(Q²²⁴)和六角相H_II共存相. 研究结果表明, 生物膜超分子聚集体的氢键、分子van der Waals力、离子的静电力等这些弱相互作用的协同性、方向性和选择性, 可能决定着生物膜的结构和功能.     

Dispersions of internally liquid crystalline systems stabilized by charged disklike particles as pickering emulsions: basic properties and time-resolved behavior

The present paper reports on dispersions of internally liquid crystalline particles, formed from monoglyceride and oil mixtures, stabilized with discrete disklike particles of Laponite clay. Small-angle X-ray scattering (SAXS) was used to probe the presence of dispersed particles as well as their internal liquid crystalline structure. The data were compared to scattering results of reference systems, namely, from the bulk as well as from well-defined particles formed with a polymer as the emulsifier. The submicrometer sizes of the various particles could be derived using dynamic light scattering (DLS). The possible mechanisms involved in the stabilization of each of the different phases by the Laponite platelets, including the role of the residual salt, are discussed. Time-resolved experiments were performed over 60 days in order to follow the evolution of both the internal structure and size of the particles. In particular, we discuss the peculiar behavior of the sample without added oil, where the cubosomes transform into hexosomes over time. The effect of the high pH induced by the Laponite platelets in water, which could result in a hydrolysis of the monoglycerides, was shown to be responsible for the observed cubosome-to-hexasome transition, as well as for the decrease in the lattice parameters.     

Characterization of oil-free and oil-loaded liquid-crystalline particles stabilized by negatively charged stabilizer citrem

The present study was designed to evaluate the effect of the negatively charged food-grade emulsifier citrem on the internal nanostructures of oil-free and oil-loaded aqueous dispersions of phytantriol (PHYT) and glyceryl monooleate (GMO). To our knowledge, this is the first report in the literature on the utilization of this charged stabilizing agent in the formation of aqueous dispersions consisting of well-ordered interiors (either inverted-type hexagonal (H(2)) phases or inverted-type microemulsion systems). Synchrotron small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) were used to characterize the dispersed and the corresponding nondispersed phases of inverted-type nonlamellar liquid-crystalline phases and microemulsions. The results suggest a transition between different internal nanostructures of the aqueous dispersions after the addition of the stabilizer. In addition to the main function of citrem as a stabilizer that adheres to the surface of the dispersed particles, it has a significant impact on the internal nanostructures, which is governed by the following factors: (1) its penetration between the hydrophobic tails of the lipid molecules and (2) its degree of incorporation into the lipid-water interfacial area. In the presence of citrem, the formation of aqueous dispersions with functionalized hydrophilic domains by the enlargement of the hydrophilic nanochannels of the internal H(2) phase in hexosomes and the hydrophilic core of the L(2) phase in emulsified microemulsions (EMEs) could be particularly attractive for solubilizing and controlling the release of positively charged drugs.     

Study of liquid crystal space groups using controlled tilting with cryogenic transmission electron microscopy

We developed a method that enables differentiation between liquid crystalline-phase particles corresponding to different space groups. It consists of controlled tilting of the specimen to observe different orientations of the same particle using cryogenic transmission electron microscopy. This leads to the visualization of lattice planes (or reflections) that are present for a given structure and absent for the other one(s) and that give information on liquid crystalline structures and their space groups. In particular, we show that we can unambiguously distinguish among particles having the inverted micellar cubic (space group Fd(3)m, 227), the inverted bicontinuous gyroid (space group Ia(3)d, 230), the inverted bicontinuous diamond (space group Pn(3)m, 224), and the inverted bicontinuous primitive cubic structure (space group Im(3)m, 229).     

Progress in liquid crystalline dispersions: Cubosomes

Dispersed particles of bicontinuous cubic liquid crystalline phase, cubosomes, are self-assembled nanostructured particles that can be formed in aqueous lipid and surfactant systems. Contributions to cubosome research have come from the fields of biology, material science, medicine, and mathematics and much is known about their formation and properties. At the center of much of the discovery and innovation is the technique of cryo-transmission electron microscopy. Most of the research into cubosomes is motivated by potential applications in drug delivery and material synthesis although no commercialized product based on cubosomes is known. Recent advances in understanding and use of cubosomes are discussed in the context of some of the more promising application areas and the opportunities for microscopy techniques to make unique contributions to these areas.     

Characterization of bupivacaine-loaded formulations based on liquid crystalline phases and microemulsions: the effect of lipid composition

This report details the structural characterization and the in vitro drug-release properties of different local anesthetic bupivacaine (BUP)-loaded inverted-type liquid crystalline phases and microemulsions. The effects of variations in the lipid composition and/or BUP concentration on the self-assembled nanostructures were investigated in the presence of the commercial distilled glycerol monooleate Myverol 18-99K (GMO) and medium-chain triglycerides (MCT). Synchrotron small-angle X-ray scattering (SAXS) and rotating dialysis cell model were used to characterize the BUP formulations and to investigate the in vitro BUP release profiles, respectively. The evaluation of SAXS data for the BUP-loaded GMO/MCT formulations indicates the structural transition of inverted-type bicontinuous cubic phase of the symmetry Pn3m → inverted-type hexagonal (H(2)) phase → inverted-type microemulsion (L(2)) with increasing MCT content (0-40 wt %). In the absence of MCT, the solubilization of BUP induces the transition of Pn3m → H(2) at pH 7.4; whereas a transition of Pn3m → (Pn3m + H(2)) is detected as the hydration is achieved at pH 6.0. To mimic the drug release and transport from in situ formed self-assembled systems after subcutaneous administration, the release experiments were performed by injecting low viscous stimulus-responsive precursors to a buffer in the dialysis cell leaving the surface area between the self-assembled system and the release medium variable. Our results suggest that the pH-dependent variations in the lipidic partition coefficient, K(l/w), between the liquid crystalline nanostructures and the surrounding buffer solution are significantly affecting BUP release rates. Thus, a first step toward understanding of the drug-release mechanism of this drug-delivery class has been undertaken tackling the influence of drug ionization as well as the type of the self-assembled nanostructure and its release kinetics under pharmaceutically relevant conditions.     

Hexosome and hexagonal phases mediated by hydration and polymeric stabilizer

In this research, we studied the factors that control formation of GMO/tricaprylin/water hexosomes and affect their inner structure. As a stabilizer of the soft particles dispersed in the aqueous phase, we used the hydrophilic nonionic triblock polymer Pluronic 127. We demonstrate how properties of the hexosomes, such as size, structure, and stability, can be tuned by their internal composition, polymer concentration, and processing conditions. The morphology and inner structure of the hexosomes were characterized by small-angle X-ray scattering, cryo-transmission electron microscope, and dynamic light scattering. The physical stability (to creaming, aggregation, and coalescence) of the hexosomes was further examined by the LUMiFuge technique. Two competing processes are presumed to take place during the formation of hexosomes: penetration of water from the continuous phase during dispersion, resulting in enhanced hydration of the head groups, and incorporation of the polymer chains into the hexosome structure while providing a stabilizing surface coating for the dispersed particles. Hydration is an essential stage in lyotropic liquid crystal (LLC) formation. The polymer, on the other hand, dehydrates the lipid heads, thereby introducing disorder into the LLC and reducing the domain size. Yet, a critical minimum polymer concentration is necessary in order to form stable nanosized hexosomes. These competing effects require the attention of those preparing hexosomes. The competition between these two processes can be controlled. At relatively high polymer concentrations (1-1.6 wt % of the total formulation of the soft particles), the hydration process seems to occur more rapidly than polymer adsorption. As a result, smaller and more stable soft particles with high symmetry were formed. On the other hand, when the polymer concentration is fixed at lower levels (<1.0 wt %), the homogenization process encourages only partial polymer adsorption during the dispersion process. This adsorption is insufficient; hence, maximum hydration of the surfactant head group is reached prior to obtaining full adsorption, resulting in the formation of less ordered hexosomes of larger size and lower stability.     

Aqueous phase behavior and dispersed nanoparticles of diglycerol monooleate/glycerol dioleate mixtures

The first part of this study concerns the aqueous phase behavior of mixtures of diglycerol monooleate (DGMO) and glycerol dioleate (GDO) examined by X-ray diffraction (XRD). The ternary phase diagram displays a multitude of liquid crystalline phases (polymorphism). With increasing GDO content the following phase sequence was observed: lamellar (L(alpha)); two reversed bicontinuous cubic phases (Q(230) and Q(224)); reversed hexagonal (H(II)); the reversed micellar (L(2)) phase. The second part deals with the preparation and characterization of aqueous dispersions of the reversed hexagonal phase in the presence of the nonionic triblock copolymer Pluronic F127. Submicrometer-sized monocrystalline H(II) phase particles were obtained, as evidenced by cryo-transmission electron microscopy (cryo-TEM), laser diffraction, and XRD, by use of a simple and reproducible preparation method including a heat-treatment step. Moreover, the particle size distributions of the H(II) phase nanoparticle dispersions were narrow as determined by laser diffraction measurements. Using XRD, we show that the polymeric stabilizer is depleted from the core of the hexagonal particles and preferentially located at the surface. It is concluded that the preferential distribution of stabilizing agents at particle surfaces is a prerequisite for the formation of structurally well-defined and kinetically stable H(II) phase particles (Hexosome).     

Reversible phase transitions in emulsified nanostructured lipid systems

Aqueous submicron-sized dispersions of the binary monolinolein/water system, which are stabilized by means of a polymer, internally possess a distinct nanostructure. Taking this as our starting point, we were able to demonstrate for the first time that the internal structure of the dispersed particles can be tuned by temperature in a reversible way. Upon increasing the temperature, the internal structure undergoes a transition from cubic via hexagonal to fluid isotropic, the so-called L2 phase, and vice versa. Intriguingly, in addition to the structural changes in topology, the particles expel (take up) water to (from) the aqueous continuous phase when increasing (decreasing) the temperature in a reversible way. At each temperature, the internal structure of the dispersed particles corresponds very well to the structure observed in nondispersed binary monolinolein with excess water. This agreement is independent of any thermal history (including phase transitions), which proves that the structures in the dispersed particles actually are in thermodynamic equilibrium with the surrounding water phase.     

In vitro skin permeation of cubosomes containing water soluble extracts of Korean barberry

The monoolein (MO) cubic phases containing water soluble extract (WSE) from Berberis koreana (Korean barberry) were prepared by hydrating the molten MO with aqueous solutions of WSE (0.5, 1.0, and 1.5%). The phase transition temperature of cubic phase containing WSE (∼70°C) was almost the same as that of WSE-free MO cubic phase that indicates that WSE was immobilized in the water channels of the cubic phase and did not affect its structure. The release of WSE from the cubic phase fits the first order process. The cubosomes were obtained by micronizing the cubic phase in a sonicator using Pluronic F127 as a dispersant. The cubosomes were stable in size at the ethanol concentration ≲16%. When compared with WSE solution in phosphate-buffered saline (10 mM, pH 7.4), in vitro skin permeation of WSE in the cubosomes was enhanced by about two times.