Isabelle Martiel

Oil and drug control the release rate from lyotropic liquid crystals

The control of the diffusion coefficient by the dimensionality d of the structure appears as a most promising lever to efficiently tune the release rate from lyotropic liquid crystalline (LLC) phases and dispersed particles towards sustained, controlled and targeted release. By using phosphatidylcholine (PC)- and monolinoleine (MLO)-based mesophases with various apolar structural modifiers and water-soluble drugs, we present a comprehensive study of the dimensional structural control of hydrophilic drug release, including 3-d bicontinuous cubic, 2-d lamellar, 1-d hexagonal and 0-d micellar cubic phases in excess water. We investigate how the surfactant, the oil properties and the drug hydrophilicity mitigate or even cancel the effect of structure variation on the drug release rate. Unexpectedly, the observed behavior cannot be fully explained by the thermodynamic partition of the drug into the lipid matrix, which points out to previously overlooked kinetic effects. We therefore interpret our results by discussing the mechanism of structural control of the diffusion rate in terms of drug permeation through the lipid membrane, which includes exchange kinetics. A wide range of implications follow regarding formulation and future developments, both for dispersed LLC delivery systems and topical applications in bulk phase.

Direct visualization of dispersed lipid bicontinuous cubic phases by cryo-electron tomography

Bulk and dispersed cubic liquid crystalline phases (cubosomes), present in the body and in living cell membranes, are believed to play an essential role in biological phenomena. Moreover, their biocompatibility is attractive for nutrient or drug delivery system applications. Here the three-dimensional organization of dispersed cubic lipid self-assembled phases is fully revealed by cryo-electron tomography and compared with simulated structures. It is demonstrated that the interior is constituted of a perfect bicontinuous cubic phase, while the outside shows interlamellar attachments, which represent a transition state between the liquid crystalline interior phase and the outside vesicular structure. Therefore, compositional gradients within cubosomes are inferred, with a lipid bilayer separating at least one water channel set from the external aqueous phase. This is crucial to understand and enhance controlled release of target molecules and calls for a revision of postulated transport mechanisms from cubosomes to the aqueous phase.

Phospholipid-based nonlamellar mesophases for delivery systems: Bridging the gap between empirical and rational design

Phospholipids are ubiquitous cell membrane components and relatively well-accepted ingredients due to their natural origin. Phosphatidylcholine (PC) in particular offers a promising alternative to monoglycerides for lyotropic liquid crystalline (LLC) delivery system applications in the food, cosmetics and pharmaceutical industries, provided its strong tendency to form zero-mean curvature lamellar mesophases in water can be overcome. Higher negative curvatures are usually reached through the addition of a third lipid component, forming a ternary diagram phospholipid/water/oil. The initial part of this work summarizes the potential advantages and the challenges of phospholipid-based delivery system applications. In the next part, various ternary PC/water/oil systems are discussed, with a special emphasis on the PC/water/cyclohexane and PC/water/α-tocopherol systems. We report that R-(+)-limonene has a quantitatively similar effect as cyclohexane. The last part is devoted to the theoretical interpretation of the observed phase behaviors. A fruitful parallel is drawn with PC polymer-like reverse micelles, leading to a thermodynamic description in terms of interfacial bending energy. Investigations at the molecular level are reviewed to help in bridging the empirical and theoretical approaches. Predictive rules are finally derived from this wide-ranging overview, thereby opening the way to a future rational design of PC-based LLC delivery systems.

Facile dispersion and control of internal structure in lyotropic liquid crystalline particles by auxiliary solvent evaporation.

Submicron sized, structured lyotropic liquid crystalline (LLC) particles, so-called hexosomes and cubosomes, are generally obtained by high energy input dispersion methods, notably ultrasonication and high-pressure emulsification. We present a method to obtain dispersions of such LLC particles with a significantly reduced energy input, by evaporation of an auxiliary volatile solvent immiscible with water, e.g. cyclohexane or limonene. The inner structure of the particles can be precisely controlled by the addition of a nonvolatile oil, such as α-tocopherol or tetradecane consistently with bulk phase diagrams,. Two different lyotropic surfactants were employed, industrial grade monolinoleine (MLO) and soy bean phosphatidylcholine (PC). The lyotropic surfactant and oil phase modifier were first dissolved in the volatile solvent to give a liquid reverse micellar (L2) phase, which requires significantly less energy input to be dispersed in an aqueous solution of secondary emulsifier compared to the corresponding gel-like bulk mesophase. The auxiliary volatile solvent was then removed from the emulsion by evaporation at room temperature, yielding LLC particles of the desired inner structure, Pn3̅m, H2, or Fd3̅m. The obtained particles were characterized by small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). Our method enables fine-tuning of the final particle size through the volatile-to-nonvolatile volume ratio and processing conditions.

A Reverse Micellar Mesophase of Face-Centered Cubic Fm3̅m Symmetry in Phosphatidylcholine/Water/Organic Solvent Ternary Systems

We report the formation of a reverse micellar cubic mesophase of symmetry Fm3m (Q(225)) in ternary mixtures of soy bean phosphatidylcholine (PC), water, and an organic solvent, including cyclohexane, (R)-(+)-limonene, and isooctane, studied by small-angle X-ray scattering (SAXS) and oscillatory shear rheology at room temperature. The mesophase structure consists of a compact packing of remarkably large reverse micelles in a face-centered cubic (fcc) lattice, a type of micellar packing not yet reported for reverse micellar mesophases. Form factor fitting in the pure L2 phase and in the Fm3m-L2 coexistence region yields quantitative estimations of the PC interface rigidity. The compact Fm3m structure results mainly from release of lipid tail frustration and hard-sphere interactions between monodisperse micelles, as suggested by a comparison with the Fd3m structure found in the PC/water/α-tocopherol system.

Fast two-dimensional grid and transmission X-ray microscopy scanning methods for visualizing and characterizing protein crystals

This article reports the incorporation of a fast continuous grid scan with both still and oscillation images into the Swiss Light Source macromolecular crystallography beamlines and its application in visualization of protein crystals with scanning transmission X-ray microscopy.

Oil transfer converts phosphatidylcholine vesicles into nonlamellar lyotropic liquid crystalline particles.

There is a need for the development of low-energy dispersion methods tailored to the formation of phospholipid-based nonlamellar lyotropic liquid crystalline (LLC) particles for delivery system applications. Here, facile formation of nonlamellar LLC particles was obtained by simple mixing of a phosphatidylcholine (PC) liposome solution and an oil-in-water emulsion, with limonene or isooctane as an oil. The internal structure of the particles was controlled by the PC-to-oil ratio, consistently with the sequence observed in bulk phase. For the first time, reverse micellar cubosomes with Fm3̅m inner structure were produced. The size, morphology, and inner structure of the particles were characterized by small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and freeze-fracture cryo scanning electron microscopy (cryo-SEM). These findings pave the way to new strategies in low-energy formulation of LLC delivery systems.

Viscoelasticity and Interface Bending Properties of Lecithin Reverse Wormlike Micelles Studied by Diffusive Wave Spectroscopy in Hydrophobic Environment

Upon the addition of minute quantities of water into a phosphatidylcholine (PC) solution in certain organic solvents, PC micelles elongate into giant reverse wormlike micelles that entangle and form highly viscous microemulsions, called lecithin organogels. We investigated the microrheological properties of concentrated PC-cyclohexane reverse wormlike micellar systems by diffusive wave spectroscopy (DWS) in apolar medium, combined with bulk shear rheology. We applied DWS to our oil-continuous system by using hydrophobic poly(hydroxystearic acid)-grafted PMMA particles as monodisperse tracer particles. Relevant parameters such as the micellar scission energy and persistence length were extracted from the microrheology data and interpreted according to the sphere-to-rod-to-sphere structural transition. On the basis of these quantities, we calculated the bending and saddle-splay moduli of the PC-covered water-cyclohexane interface. This approach represents a new method for the quantitative estimation of these fundamental parameters, which are thought to underpin the self-assembly of surfactants.