Article pubs.acs.org/Langmuir
Effects of High Pressure on Internally Self-Assembled Lipid Nanoparticles: A Synchrotron Small-Angle X‑ray Scattering (SAXS) Study Chandrashekhar V. Kulkarni,*,†,‡ Anan Yaghmur,§ Milos Steinhart,∥,⊥ Manfred Kriechbaum,# and Michael Rappolt#,∇ †
Biological and Soft Systems, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom ‡ Centre for Materials Science, School of Physical Sciences and Computing, University of Central Lancashire, Preston PR1 2HE, United Kingdom § Department of Pharmacy, Faculty of Health and Medical Sciences, University of Copenhagen, DK-2100 Copenhagen, Denmark ∥ Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague, Czech Republic # Institute of Inorganic Chemistry, Graz University of Technology, A-8010 Graz, Austria ∇ School of Food Science & Nutrition, University of Leeds, Leeds LS2 9JT, U.K. ABSTRACT: We present the first report on the effects of hydrostatic pressure on colloidally stabilized lipid nanoparticles enveloping inverse nonlamellar self-assemblies in their interiors. These internal self-assemblies were systematically tuned into bicontinuous cubic (Pn3m and Im3m), micellar cubic (Fd3m), hexagonal (H2), and inverse micellar (L2) phases by regulating the lipid/oil ratio as the hydrostatic pressure was varied from atmospheric pressure to 1200 bar and back to atmospheric pressure. The effects of pressure on these lipid nanoparticles were compared with those on their equilibrium bulk, nondispersed counterparts, namely, inverse nonlamellar liquidcrystalline phases and micellar solutions under excess-water conditions, using the synchrotron small-angle X-ray scattering (SAXS) technique. In the applied pressure range, induced phase transitions were observed solely in fully hydrated bulk samples, whereas the internal self-assemblies of the corresponding lipid nanoparticles displayed only pressure-modulated single phases. Interestingly, both the lattice parameters and the linear pressure expansion coefficients were larger for the self-assemblies enveloped inside the lipid nanoparticles as compared to the bulk states. This behavior can, in part, be attributed to enhanced lipid layer undulations in the lipid particles in addition to induced swelling effects in the presence of the triblock copolymer F127. The bicontinuous cubic phases both in the bulk state and inside lipid cubosome nanoparticles swell on compression, even as both keep swelling further upon decompression at relatively high pressures before shrinking again at ambient pressures. The pressure dependence of the phases is also modulated by the concentration of the solubilized oil (tetradecane). These studies demonstrate the tolerance of lipid nanoparticles [cubosomes, hexosomes, micellar cubosomes, and emulsified microemulsions (EMEs)] for high pressures, confirming their robustness for various technological applications. and Ia3d, respectively.4−6 Upon addition of polar and nonpolar components to certain binary lipid−water mixtures, it is possible to induce the formation of other phases including micellar (discontinuous) cubic Fd3m and sponge (L3) phases.4,5,7 The lyotropic nonlamellar liquid-crystalline phases have been exploited in various applications including crystallization of
1. INTRODUCTION Biogenic lipids have a general tendency to self-assemble into lamellar liquid-crystalline phases and/or inverse (type 2) lyotropic nonlamellar phases when mixed with water.1−3 Lamellar (fluid, Lα; gel, Lβ; or crystalline, Lc) phases, an inverse micellar solution (L2), and an inverse hexagonal (H2) phase are commonly observed; however, some lipids also form inverse bicontinuous cubic phases at ambient temperatures.3 The latter exhibit the geometrical basis of mathematically minimal surfaces of primitive (P), diamond (D), and gyroid (G) types with the corresponding space groups Im3m, Pn3m, © XXXX American Chemical Society
Received: September 7, 2016 Revised: October 25, 2016 Published: October 26, 2016 A
DOI: 10.1021/acs.langmuir.6b03300 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir membrane proteins,8,9 solubilization of biomolecules,10,11 development of drug-delivery matrixes with sustained drugrelease properties,12 and separation of biomolecules.13 For some of these applications, however, the nonlamellar phases need to be dispersed for some obvious reasons. For instance, the bicontinuous cubic phases are not easy to manipulate owing to their excessive viscosities.14 Moreover, the precise harvesting and delivery of the same quantities of bulk phases each time is highly difficult because of their nonfluid (viscoelastic) character.15 Therefore, it is advantageous to fragment these bulk phases into nanoparticles exhibiting sizes on the order of a few hundred nanometers by applying an external high-energy input. This is commonly achieved by means of ultrasonication or microfluidization in the presence of efficient stabilizers.16,17 The resulting nanostructured emulsions are similar to normal oil-in-water (O/W) emulsions, where the oil phase mainly consists of a lipid (or a lipid mixture) with a propensity to form inverse nonlamellar liquid-crystalline phases.17 These discrete lipid nanoparticles have unique features, as they envelop selfassemblies17 inside their cores while imparting water-like fluidity to the aqueous dispersion. Based on the type of selfassembly, the dispersions are called cubosomes for particles enveloping inverse bicontinuous cubic phases, hexosomes for particles enveloping inverse hexagonal (H2) phase, and so on.4,18 These nanostructured O/W emulsions are also recognized with the general term “isasomes”, meaning internally self-assembled particles (note that “soma” is the Greek word for “body”).19 The development of liquidcrystalline nanoparticulate formulations with low viscosities is desirable for their use in various pharmaceutical and food applications.5,20 Nanostructured lipid particles enveloping inverse nonlamellar liquid-crystalline phases differ largely in their features from the most investigated lamellar lipid dispersions, namely, vesicles. For instance, they exhibit a much larger surface-areato-particle-volume ratio.21 Moreover, vesicles are formed of one (unilamellar), a few (oligolamellar), or more (multilamellar) lipid bilayers enclosing water in their “hydrophilic” cores, whereas particles enveloping an inverse nonlamellar liquidcrystalline or micellar phase consist mainly of “hydrophobic” interiors (with little hydrophilic regions) fabricated from the self-assembled nanostructure of one or more of the following types: bicontinuous cubic (Pn3m, Im3m), micellar cubic (Fd3m), hexagonal (H2), inverse micellar (L2), and sponge (L3).4 It is interesting to note that similar particles, with hydrophobic cores, are also observed in biological entities and are known as lipid droplets, lipid particles, oily bodies, oleosomes, or spherosomes.22,23 Among these nano-objects, the lipid droplets are dissimilar from cellular membranes and play a major role in metabolic functions.22 Their most common function is the storage of lipids as an energy source.22 They also supply components required for membrane biogenesis.22 These lipid droplets are structurally related to other membranous organelles; a very good example is the evolution of the membrane of lipid droplets from the endoplasmic reticulum (ER) and the reported evidence of characteristic continuity among them.22 Similarly to biomembranes, these lipid droplets also undergo structural reorganization during their formation22 and functioning.24,25 In this context, nanostructured nanoparticles formed from simple lipid systems (as employed here) could work as appropriate model systems for gaining insight into some of the above-mentioned structural and morphological changes. Moreover, these investigations are beneficial for
the potential use of these nano-objects in a range of biotechnological applications where the stability of the original phase and intertransitions could play important roles, for example, for drug-delivery applications.26−28 In this work, we investigated the pressure-dependent phase behavior of nanostructured lipid particles and inverse bulk (nondispersed) self-assemblies prepared in an aqueous medium using two different lipids, namely, Dimodan U/J (DU, a commercial distilled unsaturated monoglyceride) and phytantriol (PT), together with varying amounts of the oil tetradecane (TC). The nanostructural characteristics of both dispersed and nondispersed samples at different pressures in the range of 1− 1200 bar at 25 °C were tested in a compression− decompression cycle by coupling pressure manipulations with synchrotron small-angle X-ray scattering (SAXS) measurements. This pressure range (up to about 1000 bar) is considered crucial for biological functioning, as, at higher pressures, many biomolecules start to denature or dissociate.29 It should be noted that most of the earlier studies in the literature were focused on the effects of pressure on equilibrium nonlamellar liquid-crystalline bulk phases. These studies involved the effects of hydrostatic30−33 and hydrodynamic34−37 pressure on pure lipids and their mixtures under dry (solventfree lipid), limited-hydration, or excess-water conditions. Most of these reports were focused on understanding isothermal pressure-induced structural alterations and dynamic phase transitions of the lipid crystalline and liquid-crystalline phases.30,31,38−42 As an advantage with respect to temperature, pressure equilibrates homogeneously and rapidly with the speed of sound within the sample, and therefore, great interest has been triggered in coupling rapid pressure induction experiments with synchrotron X-ray sources to capture the membrane dynamics and monitor in real time the kinetics of phase transitions within microseconds.36 This has allowed for the tracking of transient intermediate phases in a more precise manner and has led to an improved understanding of topological membrane transformations.35,37,43 The effects of pressure on the energetics and pivotal surface and its contribution in fine-tuning the dimensions of lipid bicontinuous cubic phases were also reported recently.44,45 Moreover, pressure has been applied to investigate the interactions of various biomolecules38,44,46−48 with lipid membranes, motivated by the development of biomimetic models of organisms (piezeophiles or barophiles) found under the high-pressure conditions of deep oceans.49 Despite these studies, to the best of our knowledge, there has been no report on the pressure dependence of internally self-assembled nanostructures in cubosomes, hexosomes, and other related nanostructured emulsions as presented here.
2. EXPERIMENTAL METHODS 2.1. Materials. The used lipids were as follows: Dimodan U/J (DU) was a generous gift from Danisco, Copenhagen, Denmark, and phytantriol (PT) was kindly provided by Adina Cosmetic Ingredients, Kent, U.K. (local distributor of DSM Nutritional Products AG, Kaiseraugst, Switzerland). DU is a commercial lipid containing distilled glycerides, of which 96% are monoglycerides and the rest are diglycerides and free fatty acids. Glycerol monolinoleate (62%) and glycerol monooleate (25%) constitute its main monoglycerides. The hydrophobic part of DU is thus made up mainly of C18 chains (91%). DU was chosen as a main lipid because its phase behavior upon exposure to water is very similar to that reported for the most investigated pure lipid, glycerol monooleate, also known as monoolein (MO),14,50 and it has been extensively used for the fabrication of B
DOI: 10.1021/acs.langmuir.6b03300 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir various nanostructured lipid particles.17 Pluronic F127 was used as a stabilizer to stabilize nanostructured emulsions. It is a triblock copolymer made of poly(ethylene oxide) and poly(propylene oxide) blocks (PEO99−PPO67−PEO99). Tetradecane (TC) was used as an oil, and its ratio with DU or PT was altered to control the type of lipid self-assembled nanostructure, as shown in a previously reported phase diagram.51 F127 and TC were purchased from Sigma-Aldrich, Dorset, U.K. All ingredients were stored in a refrigerator and used without further purification. The water was purified using a Barnstead Nanopure system (Thermo Scientific, Waltham, MA). 2.2. Preparation of Bulk Nondispersed Lipid Phases. Bulk nondispersed phases were prepared using 1 g of a lipid (DU or PT) or a binary lipid mixture (lipid + oil) and an equivalent weight of aqueous phase, ensuring that the lipid phases were in the excess-water region according to the corresponding phase diagrams of DU and PT.14,52 Both lipids usually form the cubic Pn3m phase in excess water at ambient temperatures;14,52 however, for the current investigations, we used 0.5 wt % aqueous solutions of F127 instead of pure water. This aqueous concentration was chosen according to the studies by Salonen et al., who demonstrated the “excess-water boundary” for bulk phases (with oil) resides well below 50% water and decreases further with increasing oil content.53 The molecular structure of the used lipids and the polymeric stabilizer F127 are presented in Figure 1. The ratio between the lipid (DU or PT) and oil (TC) was controlled by the δ value, which is defined by54 mlipid δ= × 100 mlipid + moil (1) where mlipid and moil are the masses of lipid (DU or PT) and solubilized oil (TC), respectively. The lipid compositions (i.e., δ values) were chosen carefully to obtain a range of different phases,51 as shown in Figure 1b. (Note that δ = 100 means an oil-free sample: 100% lipid and 0% oil.) After the components (a lipid or a lipid + oil binary mixture and F127 solution) had been mixed in an Eppendorf tube, the samples were subjected to 20 freeze−thaw cycles with intermittent mixing with a needle spatula. The needle spatula was prepared in-house by cutting the sharp part of a dispensing syringe needle and flattening the rest of the tip. This spatula was small enough to reach the bottom of an Eppendorf tube for efficient mixing, especially of viscous lipid liquidcrystalline phases. These samples were then allowed to stand at room temperature for at least 24 h prior to the pressure studies. 2.3. Preparation of Nanostructured Lipid Particles. In the present work, the nanostructured lipid particles were prepared using similar lipid-phase compositions as above (eq 1),54 but the proportion of the aqueous phase (aqueous medium containing 0.5% F127) was considerably higher than that used for preparing the fully hydrated bulk nondispersed phases: The value of ϕ = 10 as defined by the equation ϕ=
mlipid + moil m water + mlipid + moil
Figure 1. Lipid composition and types of lipid self-assemblies: (a) Chemical structures of lipids (DU and PT), an oil (TC), and a stabilizer (F127). Blue and yellow colors indicate hydrophilic and hydrophobic moieties, respectively, of the amphiphilic molecules (not to scale), which are supposed to aid in understanding the self-assembly process and the emulsion steric stabilization mechanism. (b) By tuning the δ value (i.e., the composition of the lipid phase), the type of selfassembly can be controlled, as shown by illustrations for inverse nondispersed (equilibrium) bulk liquid-crystalline phases and the inverse micellar phase in their equilibrium states. (c) Self-assembled nanostructures shown in panel were kinetically stabilized using the polymeric stabilizer F127 to form O/W emulsions (structured aqueous dispersions) comprising lipid nanoparticles. (d) Schematics of the inverse bicontinuous cubic Im3m and Ia3d phases; these phases were experimentally detected in the bulk states of DU and PT upon the incorporation of F127 into the nonlamellar liquid-crystalline phase and the application of high pressure, respectively.
× 100
For bulk phases, a 120-s exposure time was adequate. SAXS patterns obtained at the synchrotron source were analyzed using the AXcess software package (based on the IDL programming language) developed by the Membrane Biophysics research group at Imperial College London, U.K. Two-dimensional patterns were integrated into one-dimensional plots of intensity (arbitrary units) versus scattering vector (q in Å−1) and calibrated with silver behenate having a standard lattice spacing of 58.38 Å. The scattering vector q is defined as 4π sin(θ/λ), where 2θ is the scattering angle and λ is the wavelength of radiation. Each phase displayed a number of peaks positioned at characteristic ratios (Figure 2). These ratios are well-defined for each space group5 (indicated in Figure 2) and were used to calculate the lattice parameters of each self-assembled nanostructure. The lattice parameters were determined within an error of
