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Monoglyceride self-assembly structures as delivery vehicles
Trends in Food Science & Technology 17 (2006) 204–214
Review
Monoglyceride selfassembly structures as delivery vehicles L. Sagalowicz*, M.E. Leser, H.J. Watzke and M. Michel
&
Food Science, Nestle´ Research Center, Vers-ChezLes-Blanc, CH-1000 Lausanne 26, Switzerland (Tel.: C41 217858079; fax: C41 217858554; e-mail: [email protected]) Monoglyceride molecules spontaneously self-assemble into various liquid crystalline structures when present in an aqueous environment. The various phases can be used to achieve different functionalities, e.g. to protect molecules from chemical degradation, to solubilize drugs and nutrients, to control release of flavours and drugs or to increase the yield in Maillard reactions. We will review (1) the typical characteristics of monoglyceride self-assembly structures, (2) the most common characterisation techniques, (3) how introduction of guest molecules influences the self-assembly structures, (4) their use for drug delivery and (5) how commercial food grade monoglycerides obtained from sunflower oil can be applied to achieve unique delivery functionalities in food systems.
Introduction Monodiglycerides and their derivatives represent 75% of the world production of food emulsifiers (Krog, 1997). Monoglycerides are used in many food applications such as in bread and cake production for improvement of shelf life (prevention of starch retrogradation due to formation of monoglyceride–amylose complexes) and flavour retention (Krog, 1997). Another important application of monoglycerides concerns dairy and oil based products for control of emulsion and foam stability. All these applications are based on the potential of monoglycerides to adsorb at interfaces, to crystallize and co-crystallize. * Corresponding author. 0924-2244/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tifs.2005.12.012
An intriguing characteristic of monoglycerides is their capacity to form various self-assembly structures, i.e. liquid crystalline phases, if contacted with water. As will be shown in the present work, liquid crystalline phases can be used for the delivery of drugs and aroma. Successful applications of self-assembly structures as delivery systems depend on the capacity to incorporate the potential guest molecules within the various phases in appropriate amounts and to disperse them within a complex food matrix without losing the expected functionality, e.g. delivery. This needs a thorough understanding of the phase behaviour of the bulk as well as the dispersed phases in presence of the guest molecules. In this review, we will describe the principles of surfactant self-assembly, their methods of characterization, the introduction of guest molecules and applications of monoglycerides in drug delivery and for food. We will emphasize that not only the understanding of the molecular properties, but also the characteristics of the formed selfassembly structures allow to create novel functionalities in drug delivery and food. It will be shown that monoglyceride delivery vehicles can be used to bring new or improved functionality to food products in terms of aromas, taste, health benefit and structuration. Self-assembly structure formation Fig. 1 shows the binary phase diagram of Dimodan U-water, a commercially available food grade monoglyceride. The phase sequence at room temperature when adding water is as follows: lamellar crystalline phase (Lc) in coexistence with a L2 phase, lamellar liquid crystalline phase (La phase) and the inverted bicontinuous cubic phase. When heating the inverted bicontinuous cubic, a transition to the reversed hexagonal phase is observed followed by the L2 phase. All these mesophases are in thermodynamic equilibrium, which means they are stable under given physicochemical conditions (temperature, water content, pH, etc.), unless the surfactant molecules react with the environment or degrade. A possible molecular degradation comes from hydrolysys of the monoglycerides and in that case the reversed bicontinuous cubic phase may transform to reversed hexagonal (Caboi et al., 2001). One of the most useful (and simple) concepts for a semiquantitative understanding of phase transitions in a surfactant system is based on molecular shape. Israelachvili, Mitchell, and Ninham (1976) and Tanford (1980) defined the shape of molecules with the dimensionless packing
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Fig. 1. Binary phase diagram of Dimodan U-water. Dimodan U is a commercial food grade emulsifier made of unsaturated monoglycerides. Coexistence of phases is not indicated in this diagram. Adapted with permission from Mezzenga, Meyer et al. (2005). Copyright (2005) American Chemical Society.
parameter P Z v=al
(1)
where v is the molecular volume, l the molecular length and a is the effective (or hydrated) cross-sectional area of the
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polar head group. It is important to note that the packing parameter (and therefore the self-assembly structure) can change with parameters such as temperature and solvent conditions. Cryo-TEM pictures and schematics of different self-assembly structures are given in Fig. 2 together with the packing parameter P. For PZ1, there is a natural tendency to form a flat bilayer with zero mean curvature. In that case, the lamellar liquid crystalline (La) structure is formed, giving rise to vesicles when dispersed (Fig. 2d). For P smaller than 1, (hydrophilic emulsifiers), oil in water selfassembly structures, such as normal micelles (1/3!P!1/2) (Fig. 2e) and normal hexagonal (Hi) phases are formed. Lipophilic emulsifiers (PO1) form reversed self-assembly structures (water in oil) such as the reversed hexagonal structure (Fig. 2b) or reversed micelles. The inverted bicontinuous cubic phase is appearing between the La and the reversed hexagonal phase (Fig. 2c). These bicontinuous cubic phases are formed at room temperature by unsaturated monoglycerides, such as monoolein (C18:1), monolinolein (C18:2) or their mixture, whereas with saturated monoglycerides they form only above 70 8C. In lipids, three different inverted bicontinuous cubic phases, corresponding
Fig. 2. (a) Schematic of some of the possible self-assembly structures and their corresponding packing factors, adapted from Jo¨nsson, Lindman, Holmberg, and Kronberg (1998). Copyright John Wiley and Sons Ltd; (b) Cryo-TEM for a dispersed reversed hexagonal phase. Reprinted with permission from Yaghmur et al. (2005). Copyright (2005) American Chemical Society; (c) Cryo-TEM for a dispersed reversed bicontinuous cubic phase of space group Im3m, adapted from Sagalowicz et al. (2006). Dispersion made from Dimodan U; (d) Cryo-TEM of a vesicle, which can be obtained by dispersion of a lamellar liquid crystalline phase (obtained from mixture of Dimodan U and sodium stearoyl lactylate); (e) Cryo-TEM of a micelle dispersion (obtained from a polysorbate 80 solution). See also Borne´ (2002) for a similar representation.
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to different periodic minimal surfaces of the lipid bilayer, could be identified experimentally; the gyroid (cG), the double diamond (cD) and the primitive (cP) one (Larsson & Tiberg, 2005). They are associated to the space groups Ia3d (cG), Pn3m (cD) and Im3m (cP), respectively. Liquid crystalline phases, such as the reversed bicontinuous cubic phase, can be rather viscous and difficult to handle. Therefore, for many practical applications these phases need to be dispersed into water in the form of small particles. Larsson and co-workers were the first ones to develop procedures to efficiently disperse reversed bicontinuous cubic phases (Gustafsson, Ljusberg-Wahren, Almgren, & Larsson, 1996; Gustafsson, Ljusberg-Wahren, Almgren, & Larsson, 1997). Dispersion processes used are similar to those used for liposome production, applying either high shear or hydrotropes (Friberg, 1997). A second surfactant is needed in order to stabilise the freshly formed particle surfaces and to prevent aggregation phenomena. Tri-block copolymers such as Pluronic F127 are most commonly used. The second surfactant can significantly interfere with the self-assembly structure formed inside the particles and may change the liquid crystalline character of the dispersed particles. Gustafsson et al. (1997) reported that using less than 2% Pluronic F127 relative to the total surfactant content resulted in fast particle aggregation. When the Pluronic concentration was 4 and 7.4 wt%, dispersed cubic phase particles were observed. At concentrations higher than 10%, an emulsified cubic phase (ECP) and a large number of vesicles were present. Those vesicles most probably correspond to a dispersed lamellar phase.
Landh (1994) studied the ternary system monoolein–water– Pluronic F127 and found in addition to the cubic phase a Lamellar La and L3 phase in the water rich region. Small addition of Pluronic F127 swells the inverted bicontinuous cubic phase with water. In all TEM images published so far ECPs have always been observed to coexist with vesicles (Gustafsson et al., 1997; Spicer, Hayden, Lynch, OforiBoateng, & Burns, 2001; Fig. 3). de Campo et al. (2004) studied the temperature dependent phase behaviour of dispersions made of glycerol monolinoleate and Pluronic F127 as a second emulsifier and compared it with the behaviour of the corresponding bulk phases in excess water. Upon heating, the internal particle structure changed from reversed cubic to reversed hexagonal at about 40 8C (Fig. 3) and to L2 (also called fluid isotropic phase) at about 87 8C. Upon cooling, the reversed transitions were observed at exactly the same temperatures. At each studied temperature, the structural information obtained was exactly the same for the dispersed particles and the bulk phase in excess of water.
Methods to study the formation of self-assembly structures The first monoglyceride–water binary phase diagrams were reported by Lutton (1965) using polarised light and polarised light microscopy in combination with qualitative viscosity measurements tilting the tubes. These phase diagrams are in good agreement with those reported more recently using more sophisticated methods such as small angle X-ray scattering (SAXS), which allows the identification of space groups. Other methods such as deuterium
Fig. 3. Transition from an ECP to an emulsified hexagonal phase (EHP) when heating from 25 8C (a and b) to 55 8C (c and d) as observed by CryoTEM. Image of the cubic phase dispersion at 25 8C showing several ECP particles and vesicles; (b) image of an ECP particle for which the observed reflections are in agreement with the Pn3m space group; (c) image of the hexagonal phase dispersion at 55 8C. Some particles show hexagonal motifs (arrowed) or/and curved striations (marked by a star); (d) image of a hexagonal phase particle showing curved striations. Reprinted with permission from de Campo et al. (2004). Copyright (2004) American Chemical Society.
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nuclear magnetic resonance (NMR) (Borne´, Nylander, & Kahn, 2000), differential scanning calorimetry (DSC) (Van Dijck, De Krujff, Van Deenen, De Gier, & Demel, 1976) or micro DSC (Raemy, Appolonia-Nouzille, Frossard, Sagalowicz, & Leser, 2005) were used to differentiate between different phases or study phase transitions. Freeze fracture electron microscopy was applied to discriminate between different space groups in bulk phases formed by dioleoylphosphatidylcholine–dioleoylglycerol (DOPC/DOG) (Delacroix, Gulik-Krzywicki, & Seddon, 1996). Measuring the rheological properties of the bulk phases as a function of temperature is another elegant way to distinguish between different self-assembly structures (Mezzenga, Meyer, et al., 2005). The storage moduli G 0 , loss moduli G 00 and relaxation time give enough information to discriminate between different crystal structures and allow to identify the Pn3m and Ia3d space group. The characterisation of the self-assembly structure inside the dispersed particles is more difficult than the structure of the bulk phases, since the dispersed particles are rather small and their mass fraction is very often below 5%. Simple methods like polarized light microscopy can usually not be used as the small particles do not show any birefringence. SAXS still can be applied and with a good equipment, structure identification can be made. However, in complex and diluted systems the signal may be difficult to interpret especially due to limited intensity and to peak broadening
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occurring because of the small size of the particles and crystallographic structural heterogeneity. In addition, presence of small particles may obscure part of the SAXS spectrum. Recently 13C NMR has been applied to identify the relaxation rate of the various monoolein carbons (Monduzzi, Ljusberg-Wahren, & Larsson, 2000; Nakano, Sugita, Matsuoka, & Handa, 2001). It was found that lipid organization and dynamics were similar in the dispersion and in the corresponding bulk phase (Monduzzi et al., 2000). Cryo-TEM is also a very powerful technique to characterize dispersed particles and was used by many groups (Barauskas, Johnsson, & Tiberg, 2005; Borne´, Nylander, & Khan, 2001; Gustafsson et al., 1997; Mezzenga, Schurtenberger, Burbidge, & Michel, 2005). It allows to obtain information on the microstructural characteristics such as size and shape of the single particles as well as on the space group using fast Fourier transform algorithms (Fig. 4). Sagalowicz et al. (2006) showed that it is possible to identify the nature of the different cubic particles (showing either Pn3m and Im3m space group) in a dispersion using tilting experiments. In addition, vesicles are often observed in Cryo-TEM images to coexist with ECPs. These vesicles are difficult to characterize by other techniques such as SAXS.
Fig. 4. Part of a Cryo-TEM tilting experiment. Showing the same particle under the [111] direction (a) and the [112] direction (b); (c) and (d) are fast Fourier transforms of (a) and (b), respectively. This experiment shows that the space group is Pn3m and not Im3m due to the presence of the {111} reflections. From Sagalowicz et al. (2006).
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Introduction of guest molecules in self-assembly structures Self-assembly structures formed by monoglycerides consist of nano-scaled hydrophilic and hydrophobic domains, which are separated by the surfactant selfassembled membrane. Consequently such structures contain an extremely large surface. Ericsson, Eriksson, Lo¨froth, and Engstro¨m (1991) reported that the gyroid reversed bicontinuous cubic structure forms a surface area of 400 m2 gK1 of cubic phase. The compartmentalization present in self-assembly structures can be used to introduce guest molecules of hydrophilic, lipophilic or amphiphilic nature (Fig. 5). Hydrophilic molecules will be located close to the emulsifier polar head or in the water domains while lipophilic molecules will be localized within the lipophilic domains and amphiphilic molecules in the interface. In this way lipophilic molecules can be protected from water or from hydrophilic molecules. In addition, amphiphilic molecules may expose only certain radicals to the water which may accelerate certain reaction paths and slow down others. Introduction of guest molecules generally influences the self-assembly structure properties. Addition of oil to a cubic phase induces a transition to reversed hexagonal (Hii). Diglycerol monooleate (Pitzalis et al., 2000) and sodium oleate (Borne´ et al., 2001) were reported to induce a transition from an inversed bicontinuous cubic to a lamellar liquid crystalline phase. Vauthey (1998) studied the influence of different flavours on the stability of an inverted bicontinuous cubic phases made with 80% of Dimodan U (unsaturated monoglycerides) and 20% of water. The more hydrophilic ones, 2,3 butanedione, methyl pyrazine, furfural, guaiacol, vanillin and 2-isobutyl-methoxypyrazine induced a transition to the lamellar phase while the more lipophilic one, limonene, induced a transition to the hexagonal phase. The situation is more complex for more hydrophilic compounds such as sugars. The addition of trehalose sugar to an inverted bicontinuous cubic phase does not induce the transformation to the lamellar liquid
Fig. 5. (a) Possible localization of (guest) molecules within the inverted bicontinuous cubic phase. For simplicity only part of the lattice is represented. The molecule marked 1 is hydrophilic, the one marked 2 is amphiphilic and the one marked 3 is lipophilic. Adapted from Leser, Michel, and Watzke (2003).
crystalline (La) phase, (Saturni, Rustichelli, Di Gregorio, Gordone, & Mariani, 2001). It could be shown that the addition of sugar, like glucose, induces the transformation of an inverted bicontinuous cubic to a reversed hexagonal phase (Mezzenga, Grigorov, et al., 2005; So¨derberg & Ljusberg-Wahren, 1990). Glucose does not follow the qualitative behaviour described previously by Vauthey (1998). The different behaviour has been explained (Mezzenga, Grigorov, et al., 2005) in terms of hydration changes of the monoglyceride polar head groups as the sugar molecules might compete for water molecules hydrating the polar head groups, thereby changing the packing parameter towards more reversed curvatures (P is increasing). In dispersed liquid crystalline monoglyceride particles, the effect of introducing lipophilic compounds is very similar to what happens in the corresponding bulk phase. Lipophilic additives such as triglycerides (Gustafsson et al., 1997), oleic acid at pH 7 (Nakano et al., 2002) (ratio of oleic acid to monoolein higher than 1:1) or tetradecane (Yaghmur, de Campo, Sagalowicz, Leser, & Glatter, 2005) (ratio of tetradecane to monolinolein higher than about 6:100) transform the internal structure of the particle from reversed bicontinuous cubic to hexagonal. At higher levels of oleic acid (when the ratio oleic acid: monololein is higher than 8:2) or tetradecane (when the ratio of tetradecane: monolinolein is higher than 1: 2.5 (Yaghmur, de Campo, Salentinig, Sagalowicz, Leser, & Glatter, 2006), the particle structure is the inverse micellar cubic phase and the Fd3m space group is observed. At even higher levels of tetradecane (when the ratio of tetradecane: monolinolein is higher than 1:1.5), the particle self-assembled structure is more disordered and a L2 structure is observed. For both tetradecane and oleic acid, the same sequence of phases (cubic, hexagonal, micellar cubic and L2) was observed in bulk samples. The effect of adding hydrophilic additives into monoglyceride dispersed liquid crystalline phases is probably more complex than adding lipophilic ones. As mentioned above, the addition of hydrophilic compounds to a bulk (non-dispersed) inverted bicontinuous cubic phase is likely to induce a change from reversed bicontinuous cubic phase to lamellar liquid crystalline (La) phase, decreasing the packing parameter P. This suggests that the presence of hydrophilic additives will induce a similar transformation in the corresponding dispersion, and ECP particles will transform to vesicles (which likely result from the dispersion of a La phase). It was observed that sodium oleate transforms ECP particles to vesicles (Borne´ et al., 2001). The use of the packing parameter can explain the influence of sodium oleate but not of all hydrophilic additives. For other hydrophilic guests, dispersions may have a phase behaviour very different from the bulk (nondispersed) structure, since hydrophilic additives will migrate out of the particles into the surrounding water matrix, strongly reducing the effect on particle liquid crystalline structure.
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Monoglyceride self-assembly structures used for drug delivery Protection of drugs Ericsson et al. (1991) showed that peptides incorporated into an inverted bulk bicontinuous cubic phase, formed in the system monoolein/water, can be protected from enzymatic degradation. The rate of enzymatic degradation of the renin inhibitor H214/O3 in the cubic phase, when exposed to the intestinal fluid, was 5.7% of that found in a homogeneous solution of the intestinal fluid. Since, the H214/O3 is lipophilic, it is protected by the inverted bicontinuous cubic phase from contact with the intestinal fluid. A similar protection was found for other peptidic drugs such as desmopressin, lysine–vasopressin, and somatostatin. Sadhale and Shah (1998) investigated the ability of the glycerol monooleate (also called monoolein or GMO) based structures to protect labile drugs from chemical instability reactions such as oxidation and hydrolysis. Oxidative stability of cezafolin incorporated in the inverted bicontinuous cubic phase was better than in solution at 22 and 37 8C, but was identical at 50 8C. Cefuroxime, which degrades by hydrolysis, was twice as stable when present in the inverted bicontinuous cubic phase compared to an aqueous solution. Monoglyceride self-assembly structures can also be used to prevent molecules from aggregating. In solution, insulin activity is reduced due to aggregation and formation of insoluble insulin aggregates during agitation. Sadhale and Shah (1999) demonstrated successful protection of insulin from agitation induced aggregation when incorporated into a cubic phase. It was also shown that agitation of insulin in the cubic phase had little deleterious effects on its biological activity. Controlled release of drugs One application associated with the inverted bicontinuous cubic phase is the controlled release of drugs. Shah, Sadhale, and Dakshina (2001) reviewed gel cubic phase use as drug delivery systems and in all cases, drug release from bulk phases followed a square root of time law. This indicates that the release is diffusion controlled. The release profile of bupivacaine (Shah et al., 2001) from GMO based structures was sustained and less than 40% of the drug was released within 24 h. Ericsson et al. (1991) studied in vivo and in vitro the release of some oligopeptidic drugs such as desmopressin and lysine–vasopressin. Self-diffusion NMR data indicate that the release kinetic is again diffusion controlled and release was sustained over a period of more than 6 h. The desmopressin diffusion coefficient in the cubic phase at 40 8C (DZ0.24!10K10 m sK1) is about a factor of 9 smaller than in 2H2O at 25 8C (DZ2.25!10K10 m sK1). An interaction between the peptides and the lipid matrix or membrane surface could be the reason for the significant release control achieved using the inverted bicontinuous cubic phase as the delivery system. Jeong, O’Brien, Ora¨dd, and Lindblom (2002) made a quantitative determination of the diffusion coefficient of a
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water soluble polyamidoamine dendrimer derivative labelled with fluorine inserted in an inverted bicontinuous cubic phase. The hydrated diameter of this dendrimer was 3.2 nm. The cubic phase with space group Ia3d was obtained by hydration of a 9:1 molar mixture of polymerisable monoacylglycerol and 1,2-diacyglycerol. The diffusion coefficient was found to be about 100 times lower in the cubic phase than in water (1.10K12 m sK1 instead of 1.4! 10K10 m2 sK1 in water), but it still shows that globular molecules, such as proteins, can diffuse rapidly enough in the cubic phase for having enough molecules released. Similar diffusion results were obtained with the dendrimer diffusing in the ‘classical’ inverted bicontinuous cubic phase made with monoolein and water. The low diffusion coefficient of the dendrimer is likely associated with the tortuosity of the water channel in the cubic phase or/and the interaction of the guest molecules with the interface. Puvvada, Naciri, and Ratna (1994) showed that the release of 1% bovine serum albumin (BSA) in a monoolein– water cubic phase is about 10 times lower after 200 h if the water channels contain an alginate gel. Due to their associated sustained release and their bioadhesive properties to the mucus, bulk cubic phases are of interest for topical applications (skin, mouth, vaginal.). However, since the inverted bicontinuous cubic phase is very stiff, insertion at a given place can be rather difficult. A nice way to overcome this problem has been described by Engstro¨m, Lindahl, Wallin, and Engblom (1992). The idea is that the product itself is a La phase which can be easily handled due to its relatively low viscosity. Upon heating from room temperature to body temperature or swelling with water, it transforms to the reversed bicontinuous cubic phase. The presence of the cubic phase induces the expected sustained release of the drug at the place of action. The release from emulsified cubic phase (ECP) particles has also been studied. Hydrophilic molecules, if not having a strong interaction with the interface are most likely lost and released into the aqueous media. Boyd (2003) used ECP particles loaded with more lipophilic molecules: diazepam, rifampicin, and propofol. In vitro experiments showed that with these drugs a burst release is measured, but no sustained release. If diffusion controls the release, release is expected to be several orders of magnitude faster for ECP particles than for bulk samples because of the small size of the ECP particle and the large surface area between the particles and the matrix (Boyd, 2005). However, sustained actions of molecules loaded into ECP particles were observed in in vivo studies. Chung, Kim, Um, Kwon, and Jeong (2002) reported that nanocubiles (similar to ECP particle) loaded with insulin enable to control the serum glucose concentration in diabetic rats for more than 6 h after oral administration. When insulin was administrated in an aqueous solution, the serum glucose concentration was not controlled and no hypoglycaemic effect could be detected. Engstro¨m, Ericsson, and Landh (1996) found sustained plasma levels of stomatostatin from ECP particles in a rabbit model. These
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data suggest that diffusion may not be the only factor controlling sustained action of the cubic phase particles. It was shown that the ECPs, which were made of monoolein and stabilized by Pluronic F127, were not stable in the plasma (Leesajakul, Nakano, Taniguchi, & Handa, 2004). In the same study it was suggested that the long circulation time of the incorporated substances, analyzed by Engstro¨m et al. (1996), was due to the presence of particles, which were made of degradation products of the used ECP particles. The influence of the ECP particles on the observed sustained plasma level of the drugs needs to be further studied. Monoglyceride self-assembly structures in foods Commercial monoglycerides used in food applications, are always complex mixtures of monoglycerides having various fatty acid compositions. The monoglyceride (Dimodan U from Danisco) used for the determination of the binary phase diagram in Fig. 1 has the following fatty acid composition of C18-2:62 wt%, C18-1:25 wt%, C16:7 wt% and C18:4%. In addition about 1.5 wt% diglycerides and 0.4 wt% free fatty acids are present. The phase sequence (transition between Lc, La, cubic Ia3d, cubic Pn3m and Hii) when changing water content and temperature is the same as what is determined in the phase diagram of (pure) monoolein–water (Qiu & Caffrey, 2000). Therefore emulsifiers generally used in food can be used equally well as the pure surfactant molecule for obtaining many desired self-assembly structures. The major differences observed between the phase behaviour of commercial food emulsifiers and pure substances are the shift of transition temperatures and phase changes as function of water contents. The amount of saturated monoglycerides in the sample primarily affects the location of the lamellar crystal region. Batches of Dimodan U with higher amount of saturated monoglycerides show a more extended region of the lamellar crystalline (Lc) phase. In that case, crystals can coexist with the inverted bicontinuous cubic phase. In addition, diglycerides also strongly influence the phase behaviour of commercial monoglyceride samples. Addition of 5 wt% diolein to the mixture Rylo Mg 90 glycerol monooleate (Danisco)–water results in the formation of a reversed hexagonal phase at room temperature instead of the reversed bicontinuous cubic phase (Borne´ et al., 2000). Delivery of functionality of self-assembly structures in food Self-assembly structures as reactors The fact that monoglycerides form self-assembly structures can be used to create also novel functionalities in food. One possibility is to use the liquid crystalline phase as a reactor for the creation of flavour compounds. Vauthey, Milo, et al. (2000) used structured fluids (i.e. the L2 and the reversed bicontinuous cubic phase) for the enhancement of flavour formation in a model Maillard reaction. The experiments were conducted at 100 8C. The L2 (at 100 8C) phase was made by mixing 80% Dimodan U
and 20% water while the cubic phase (at 100 8C) was made by mixing 80% Dimodan HR (saturated monoglyceride, mainly C18:0) and 20% water. The formation of 2-furfurylthiol (FFT) from the model reaction between L-cysteine and furfural was particularly efficient in the L2 microemulsion and inverted bicontinuous cubic phase compared to the respective aqueous systems (Fig. 6). The production of FFT was about five times higher in the L2 than in the aqueous phase and about seven times higher in the cubic phase than in water. The reaction also led to the formation of two new sulfur compounds: 2-(2-furanyl)-thiazolidine and N-(2mercaptovinyl)-2-(2-furanyl)-thiazolidine. The high efficiency of the Maillard reaction was attributed to the capacity of the water–monoglyceride interface to serve as a host for the reaction precursors and thereby enhancing the reaction yield. It is also possible that the presence of oleic domains in the cubic phase serves as a host for some reaction compounds, such as FFT, shifting the reaction equilibrium in favour of the reaction products. Aroma controlled release Vauthey, Visani, et al. (2000) studied the release of eight model aroma compounds using semiconductor gas sensors (‘electronic nose’). They found that the release (at pseudo equilibrium) is significantly influenced by the phase in which the aroma compounds were dissolved. Using a liquid crystalline phase (i.e. reversed bicontinuous cubic phase containing 20 or 30 wt% water) gave a different pattern than using a water in oil emulsion containing 20% water, oil, or water as a matrix. Fig. 7 shows the result of the principal component analysis of the obtained release data. Since, about 98% of the information is contained in the first principal component, the 2D plot given in Fig. 7 can practically be reduced to a 1D graph (x-axis). The structure on the right part (pure water) gave the highest concentration of volatiles in the headspace. The lowest concentrations of molecules in the gaseous phase are observed for pure sunflower oil and pure monoglycerides (without water). More interesting, the headspace concentration of the volatiles released from the cubic phase (20% water) is higher than the headspace concentration of the volatiles from the w/o emulsion (20% water). We explained this, in the part concerning drug delivery, by assuming that the controlled release of drugs is associated with the low diffusion coefficient within the cubic phase. The result of the electronic-nose experiments cannot be interpreted the same way. In the electronic nose experiment, the system is at a quasi-equilibrium and measured concentrations are associated with partitioning between head space and the matrix (cubic phase or emulsion). Concentrations are not associated with diffusion. The electronic nose results suggest that the structure of the matrix determines the partition of the volatiles. The higher equilibrium volatile headspace concentration obtained from the cubic phase (compared to the water in oil emulsion) could be related to the presence of the large internal interface.
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Fig. 6. Gas chromatography (GC)–flame photometric detector (FPD) chromatogram of volatile compounds from processed furfural/cysteine mixture in different matrix (inverted bicontinuous cubic phase, microemulsion and aqueous phase) and generated during the Maillard reaction. The FFT peaks are indicated and the two new compounds (see text) are marked by n. Adapted with permission of Vauthey, Milo et al. (2000). Copyright (2000) American Chemical Society.
Self-assembly structures for structuring food products Heertje, Roijers, and Hendtrickx (1998) studied the use of saturated monoglyceride lamellar liquid crystalline phases to structure food products. When cooling down the La structure formed by monostearate to room temperature, a
metastable a-gel (Fig. 8) is formed which can incorporate between 20 and 95% water. Maximum swelling appears for samples, which contain small amounts of anionic surfactants. The effect of charge is to produce electrostatic repulsion between the monoglyceride head groups and to
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water. a-gels have excellent foaming properties. The foam using 40 mg g-1 of monoglycerides which are present in the a-gel form, and 40 mg gK1 of sucrose at pH 7 was shown to give a specific volume of 6.4 mL g-1. Applying the coagel structure allowed to formulate a margarine with 3% monoglyceride which was ‘fat free’, i.e. having low calories. It was also suggested (Heertje et al., 1998), that a-gel formation in aerated deserts can be associated with a full fat impression by the consumer.
Fig. 7. Electronic nose headspace data obtained after the release of a model aroma mixture from different matrices (inverted bicontinuous cubic phase, water in oil emulsion, water and oil); Adapted from Vauthey, Visani et al. (2000). Reproduced by permission of Taylor and Francis. Copyright (2000).
increase water swelling. Due to undercooling, the a-gel can be maintained at room temperature for several months. The a-gel can also transform to a coagel, consisting of a network of b crystals. This phase can incorporate large amounts of
Concluding remarks Monoglyceride based self-assembly structures are promising systems for many applications in the drug or food industry. The possibility of solubilizing active ingredients in these structures can be used for many different purposes. The application of bicontinuous cubic phases has led so far to the most intriguing new functionalities such as molecular protection, controlled release, prevention of aggregation and changes in chemical reaction. In this work we summarized a few potential applications. However, we are convinced that learning more about the self-assembly structures themselves and how they interact with their
Fig. 8. (a) Binary phase diagram saturated monoglyceride–water. Dimodan P, the monoglyceride contains 65% glycerol monostearate and 30% monopalmitate; (b) schematic of b crystals, after heating them above Tc (55 8C) and exposing them to water, they transform to; (c) a lamellar liquid crystalline (La) phase which when cooled below Tc transforms to an a-gel; (d) d is the periodicity of the structure, dw is the thickness of the water domain and da is the thickness of the bilayer. Adapted from Krog (1997).
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environment will significantly trigger the development of new areas of applications. In addition, since monoglycerides are already present in many food products there is no need to use other additives in order to form self-assembly structures as delivery vehicles.
References Barauskas, J., Johnsson, M., & Tiberg, F. (2005). Self-assembled lipid superstructures: Beyond vesicles and liposomes. Nano Letters, 5(8), 1615–1619. Borne´ J. (2002). Lipid self-assembly and lipase action. PhD dissertation, Lund University. Borne´, J., Nylander, T., & Khan, A. (2000). Microscopy, SAXD, and NMR studies of phase behavior of the monoolein–diolein–water system. Langmuir, 16, 10044–10054. Borne´, J., Nylander, T., & Khan, A. (2001). Vesicle formation and other structures in the aqueous dispersions of monolein and sodium oleate. Journal of Colloid and Interface Science, 257(2), 310–320. Boyd, B. J. (2003). Characterisation of drug release from cubosomes using the pressure ultrafiltration method. International Journal of Pharmaceutics, 260, 239–247. Boyd, B. J. (2005). Controlled release from cubic liquid–crystalline particles (cubosomes). Bicontinuous Liquid Crystals. Surfactant Science Series, 127, 285–305. Caboi, F., Amico, G. S., Pitzalis, P., Monduzzi, M., Nylander, T., & Larson, K. (2001). Addition of hydrophilic and lipophilic compounds of biological relevance to the monoolein/water system I. Phase behavior. Chemistry and Physics of Lipids, 109, 47–62. Chung, H., Kim, J., Um, J. Y., Kwon, I. C., & Jeong, S. Y. (2002). Selfassembled ‘nanocubicle’ as a carrier for peroral insulin delivery. Diabetologia, 45, 448–451. de Campo, L., Yaghmur, A., Sagalowicz, L., Leser, M. E., Watzke, H., & Glatter, O. (2004). Reversible phase transitions in emulsified nanostructured lipid systems. Langmuir, 20(13), 5254–5261. Delacroix, H., Gulik-Krzywicki, T., & Seddon, J. M. (1996). Freeze fracture electronmicroscopy of lyotropic system lipid systems: Quantitative analysis of the inverse micellar cubic phase of space group Fd3m (Q227). Journal of Molecular Biology, 258, 88–103. Engstro¨m, S., Ericsson, B., & Landh, T. (1996). A cubosome formulation for intravenous administration of somatostatin. Proceeding of the International Symposium of Controlled Release Bioactive Materials, 23, 382–383. Engstro¨m, S., Lindahl, L., Wallin, R., & Engblom, J. (1992). A study of polar lipid drug carrier systems undergoing a thermoreversible lamellar-to-cubic phase transition. International Journal of Pharmaceutics, 86, 137–145. Ericsson, B., Eriksson, P. O., Lo¨froth, J. E., & Engstro¨m, S. (1991). Cubic phases as delivery systems for peptide drugs Polymeric drugs and drug delivery system. Washington, DC: American Chemical Society pp. 251–265. Friberg, S. E. (1997). Hydrotropes. Current Opinion in Colloid and Interface Sciences, 2, 490–494. Gustafsson, J., Ljusberg-Wahren, H., Almgren, M., & Larsson, K. (1996). Cubic lipid–water phase dispersed into submicron particles. Langmuir, 12(20), 4611–4613. Gustafsson, J., Ljusberg-Wahren, H., Almgren, M., & Larsson, K. (1997). Submicron particles of reversed lipid phases in water stabilized by a nonionic amphiphilic polymer. Langmuir, 13, 6964–6971. Heertje, I., Roijers, E. C., & Hendtrickx, C. M. (1998). Liquid crystalline phases in the structuring of food products. Lebensmittel-Wissenschaft und-Technologie, 31, 387–396.
213
Israelachvili, J. N., Mitchell, D. J., & Ninhan, B. W. (1976). Theory of self assembly of hydrocarbon amphiphiles into micelles and bilayers. Journal of the Chemical Society, Faraday Transactions II, 72, 1525–1568. Jeong, S. W., O’Brien, D. F., Ora¨dd, G., & Lindblom, G. (2002). Encapsulation and diffusion of water-soluble dendrimers in bicontinuous cubic phase. Langmuir, 18, 1073–1076. Jo¨nsson, B., Lindman, B., Holmberg, K., & Kronberg, B. (1998). Surfactants and polymers in aqueous solution. Chichester: Wiley. Krog, N. J. (1997). In S. E. Friberg, & K. Larson (Eds.), Food emulsifiers and their chemical and physical properties (pp. 141– 188). New York: Marcel Dekker. Landh, T. (1994). Phase behavior in the system pine oil monoglycerides–poloxamer 407-water ar 20 8C. Journal of Physical Chemistry, 98, 8453–8467. Larsson, K., & Tiberg, F. (2005). Periodic minimal surface structures in bicontinuous lipid–water phases and nanoparticles. Current Opinion in Colloid and Interface Science, 9, 365–369. Leesajakul, W., Nakano, M., Taniguchi, A., & Handa, T. (2004). Interaction of cubosomes with plasma components resulting in the destabilization of cubosomes in plasma. Colloids and Surfaces B—Biointerfaces, 34(4), 253–258. Leser, M. E., Michel, M., & Watzke, H. J. (2003). In E. Dickinson, & T. Van Vliet (Eds.), ‘Food goes nano’—new horizons for food structure research (pp. 3–13). Cambridge: The Royal Society of Chemistry. Lutton, E. S. (1965). Phase behavior of aqueous systems of monoglycerides. Journal of the American Oil Chemists Society, 42, 1068–1070. Mezzenga, R., Grigorov, M., Zhang, Z. D., Servais, C., Sagalowicz, L., Romoscanu, A., et al. (2005). Polysaccharide-induced orderto-order transitions in lyotropic liquid crystals. Langmuir, 21(14), 6165–6169. Mezzenga, R., Meyer, C., Servais, C., Romoscanu, A., Sagalowicz, L., & Hayward, R. (2005). Shear rheology of lyotropic liquid crystals: A case study. Langmuir, 21(8), 3322–3333. Mezzenga, R., Schurtenberger, P., Burbidge, A., & Michel, M. (2005). Understanding foods as soft materials. Nature Materials, 4, 729–740. Monduzzi, M., Ljusberg-Wahren, H., & Larsson, H. (2000). A 13CNMR study of aqueous dispersions of reversed lipid phases. Langmuir, 16, 7355–7358. Nakano, M., Sugita, A., Matsuoka, H., & Handa, T. (2001). Small angle X-ray scattering and 13C NMR investigation on the internal structure of ‘cubosomes’. Langmuir, 17, 3917–3922. Nakano, M., Teshigawara, T., Sugita, A., Leesajakul, W., Taniguchi, A., Kamo, T., et al. (2002). Dispersions of liquid crystalline phases of the monoolein/oleic acid/pluronic F127 system. Langmuir, 18, 9283–9288. Pitzalis, P., Monduzzi, M., Krog, N., Larsson, H., Ljusberg-Wahren, H., & Nylander, T. (2000). Characterization of the liquid– crystalline phases in the glycerol monooleate/diglycerol monooleate/water system. Langmuir, 16, 6358–6365. Puvvada, S., Naciri, J., & Ratna, B. R. (1994). Ionotropically gelled bicontinuous cubic phase as a matrix for controlled release. Materials Research Society Symposium Proceedings, 331, 217–222. Qiu, H., & Caffrey, M. (2000). The phase diagram of the monolein/water system: Metastability and equilibrium aspects. Biomaterials, 21, 223–234. Raemy, A., Appolonia Nouzille, C., Frossard, P., Sagalowicz, L., & Leser, M. E. (2005). Thermal behaviour of emulsifier–water systems studied by micro-DSC. Journal of Thermal Analysis and Calorimetry, 80(2), 430–443. Sadhale, Y., & Shah, J. C. (1998). Glyceryl monooleate cubic phase gel as chemical stability enhancer of cefazolin and cefuroxime. Pharmaceutical Development and Technology, 3(4), 549–556. Sadhale, Y., & Shah, J. C. (1999). Stabilization of insulin against agitation-induced aggregation by the GMO cubic phase gel. International Journal of Pharmaceutics, 191(1), 51–64.
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Sagalowicz, L., Michel, M., Adrian, M., Frossard, P., Rouvet, M., Watzke, H. J., et al. (2006). Crystallography of dispersed selfassembly structures. Journal of Microscopy 221, 110-121. Saturni, L., Rustichelli, F., Di Gregorio, G. M., Cordone, L., & Mariani, P. (2001). Sugar-induced stabilization of the monoolein Pn3m bicontinuous cubic phase during dehydration. Physical Review E, 64040902. Shah, J. C., Sadhale, Y., & Dakshina, M. C. (2001). Cubic phase gels as drug delivery systems. Advanced Drug Delivery Reviews, 47, 229–250. So¨derberg, I., & Ljusberg-Wahren, H. (1990). Phase properties and structure of a monoglyceride/sucrose/water system. Chemistry and Physics of Lipids, 55, 97–101. Spicer, P. T., Hayden, K. L., Lynch, M. L., Ofori-Boateng, A., & Burns, J. L. (2001). Novel process for producing cubic liquid crystalline nanoparticles (cubosomes). Langmuir, 17, 5748–5756. Tanford, C. (1980). The hydrophobic effects: Formation of micelles and biological membranes. New York: Wiley. Van Dijck, P. W. M., De Krujff, B., Van Deenen, L. L. M., De Gier, J., & Demel, R. A. (1976). The preference of cholesterol for
phosphatidylcholine in mixed phosphatidylcholine–phosphatidylethanolamine bilayers. Biochimica et Biophysica Acta, 455, 576–587. Vauthey, S. (1998). Solubilization of flavour compounds in structured fluids. PhD dissertation, Universite´ de Lausanne. Vauthey, S., Milo, C., Frossard, P., Garti, N., Leser, M. E., & Watzke, H. J. (2000a). Structured fluids as microreactors for flavor formation by the Maillard reaction. Journal of Agricultural and Food Chemistry, 48(10), 4808–4816. Vauthey, S., Visani, P., Frossard, P., Garti, N., Leser, M. E., & Watzke, H. J. (2000b). Release of volatiles from cubic phases monitoring by gas sensors. Journal of Dispersion Science and Technology, 21(3), 263–278. Yaghmur, A., de Campo, L., Sagalowicz, L., Leser, M. E., & Glatter, O. (2005). Emulsified microemulsions and oil-containing liquid crystalline phases. Langmuir, 21, 569–577. Yaghmur, A., de Campo, L., Salentinig, S., Sagalowicz, L., Leser, M.E., & Glatter, O. (2006). Oil - loaded monolinolein - based particles with confined inverse discontinuous cubic structure (Fd3m). Langmuir, 22, 517–521.
Review
Monoglyceride selfassembly structures as delivery vehicles L. Sagalowicz*, M.E. Leser, H.J. Watzke and M. Michel
&
Food Science, Nestle´ Research Center, Vers-ChezLes-Blanc, CH-1000 Lausanne 26, Switzerland (Tel.: C41 217858079; fax: C41 217858554; e-mail: [email protected]) Monoglyceride molecules spontaneously self-assemble into various liquid crystalline structures when present in an aqueous environment. The various phases can be used to achieve different functionalities, e.g. to protect molecules from chemical degradation, to solubilize drugs and nutrients, to control release of flavours and drugs or to increase the yield in Maillard reactions. We will review (1) the typical characteristics of monoglyceride self-assembly structures, (2) the most common characterisation techniques, (3) how introduction of guest molecules influences the self-assembly structures, (4) their use for drug delivery and (5) how commercial food grade monoglycerides obtained from sunflower oil can be applied to achieve unique delivery functionalities in food systems.
Introduction Monodiglycerides and their derivatives represent 75% of the world production of food emulsifiers (Krog, 1997). Monoglycerides are used in many food applications such as in bread and cake production for improvement of shelf life (prevention of starch retrogradation due to formation of monoglyceride–amylose complexes) and flavour retention (Krog, 1997). Another important application of monoglycerides concerns dairy and oil based products for control of emulsion and foam stability. All these applications are based on the potential of monoglycerides to adsorb at interfaces, to crystallize and co-crystallize. * Corresponding author. 0924-2244/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tifs.2005.12.012
An intriguing characteristic of monoglycerides is their capacity to form various self-assembly structures, i.e. liquid crystalline phases, if contacted with water. As will be shown in the present work, liquid crystalline phases can be used for the delivery of drugs and aroma. Successful applications of self-assembly structures as delivery systems depend on the capacity to incorporate the potential guest molecules within the various phases in appropriate amounts and to disperse them within a complex food matrix without losing the expected functionality, e.g. delivery. This needs a thorough understanding of the phase behaviour of the bulk as well as the dispersed phases in presence of the guest molecules. In this review, we will describe the principles of surfactant self-assembly, their methods of characterization, the introduction of guest molecules and applications of monoglycerides in drug delivery and for food. We will emphasize that not only the understanding of the molecular properties, but also the characteristics of the formed selfassembly structures allow to create novel functionalities in drug delivery and food. It will be shown that monoglyceride delivery vehicles can be used to bring new or improved functionality to food products in terms of aromas, taste, health benefit and structuration. Self-assembly structure formation Fig. 1 shows the binary phase diagram of Dimodan U-water, a commercially available food grade monoglyceride. The phase sequence at room temperature when adding water is as follows: lamellar crystalline phase (Lc) in coexistence with a L2 phase, lamellar liquid crystalline phase (La phase) and the inverted bicontinuous cubic phase. When heating the inverted bicontinuous cubic, a transition to the reversed hexagonal phase is observed followed by the L2 phase. All these mesophases are in thermodynamic equilibrium, which means they are stable under given physicochemical conditions (temperature, water content, pH, etc.), unless the surfactant molecules react with the environment or degrade. A possible molecular degradation comes from hydrolysys of the monoglycerides and in that case the reversed bicontinuous cubic phase may transform to reversed hexagonal (Caboi et al., 2001). One of the most useful (and simple) concepts for a semiquantitative understanding of phase transitions in a surfactant system is based on molecular shape. Israelachvili, Mitchell, and Ninham (1976) and Tanford (1980) defined the shape of molecules with the dimensionless packing
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Fig. 1. Binary phase diagram of Dimodan U-water. Dimodan U is a commercial food grade emulsifier made of unsaturated monoglycerides. Coexistence of phases is not indicated in this diagram. Adapted with permission from Mezzenga, Meyer et al. (2005). Copyright (2005) American Chemical Society.
parameter P Z v=al
(1)
where v is the molecular volume, l the molecular length and a is the effective (or hydrated) cross-sectional area of the
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polar head group. It is important to note that the packing parameter (and therefore the self-assembly structure) can change with parameters such as temperature and solvent conditions. Cryo-TEM pictures and schematics of different self-assembly structures are given in Fig. 2 together with the packing parameter P. For PZ1, there is a natural tendency to form a flat bilayer with zero mean curvature. In that case, the lamellar liquid crystalline (La) structure is formed, giving rise to vesicles when dispersed (Fig. 2d). For P smaller than 1, (hydrophilic emulsifiers), oil in water selfassembly structures, such as normal micelles (1/3!P!1/2) (Fig. 2e) and normal hexagonal (Hi) phases are formed. Lipophilic emulsifiers (PO1) form reversed self-assembly structures (water in oil) such as the reversed hexagonal structure (Fig. 2b) or reversed micelles. The inverted bicontinuous cubic phase is appearing between the La and the reversed hexagonal phase (Fig. 2c). These bicontinuous cubic phases are formed at room temperature by unsaturated monoglycerides, such as monoolein (C18:1), monolinolein (C18:2) or their mixture, whereas with saturated monoglycerides they form only above 70 8C. In lipids, three different inverted bicontinuous cubic phases, corresponding
Fig. 2. (a) Schematic of some of the possible self-assembly structures and their corresponding packing factors, adapted from Jo¨nsson, Lindman, Holmberg, and Kronberg (1998). Copyright John Wiley and Sons Ltd; (b) Cryo-TEM for a dispersed reversed hexagonal phase. Reprinted with permission from Yaghmur et al. (2005). Copyright (2005) American Chemical Society; (c) Cryo-TEM for a dispersed reversed bicontinuous cubic phase of space group Im3m, adapted from Sagalowicz et al. (2006). Dispersion made from Dimodan U; (d) Cryo-TEM of a vesicle, which can be obtained by dispersion of a lamellar liquid crystalline phase (obtained from mixture of Dimodan U and sodium stearoyl lactylate); (e) Cryo-TEM of a micelle dispersion (obtained from a polysorbate 80 solution). See also Borne´ (2002) for a similar representation.
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to different periodic minimal surfaces of the lipid bilayer, could be identified experimentally; the gyroid (cG), the double diamond (cD) and the primitive (cP) one (Larsson & Tiberg, 2005). They are associated to the space groups Ia3d (cG), Pn3m (cD) and Im3m (cP), respectively. Liquid crystalline phases, such as the reversed bicontinuous cubic phase, can be rather viscous and difficult to handle. Therefore, for many practical applications these phases need to be dispersed into water in the form of small particles. Larsson and co-workers were the first ones to develop procedures to efficiently disperse reversed bicontinuous cubic phases (Gustafsson, Ljusberg-Wahren, Almgren, & Larsson, 1996; Gustafsson, Ljusberg-Wahren, Almgren, & Larsson, 1997). Dispersion processes used are similar to those used for liposome production, applying either high shear or hydrotropes (Friberg, 1997). A second surfactant is needed in order to stabilise the freshly formed particle surfaces and to prevent aggregation phenomena. Tri-block copolymers such as Pluronic F127 are most commonly used. The second surfactant can significantly interfere with the self-assembly structure formed inside the particles and may change the liquid crystalline character of the dispersed particles. Gustafsson et al. (1997) reported that using less than 2% Pluronic F127 relative to the total surfactant content resulted in fast particle aggregation. When the Pluronic concentration was 4 and 7.4 wt%, dispersed cubic phase particles were observed. At concentrations higher than 10%, an emulsified cubic phase (ECP) and a large number of vesicles were present. Those vesicles most probably correspond to a dispersed lamellar phase.
Landh (1994) studied the ternary system monoolein–water– Pluronic F127 and found in addition to the cubic phase a Lamellar La and L3 phase in the water rich region. Small addition of Pluronic F127 swells the inverted bicontinuous cubic phase with water. In all TEM images published so far ECPs have always been observed to coexist with vesicles (Gustafsson et al., 1997; Spicer, Hayden, Lynch, OforiBoateng, & Burns, 2001; Fig. 3). de Campo et al. (2004) studied the temperature dependent phase behaviour of dispersions made of glycerol monolinoleate and Pluronic F127 as a second emulsifier and compared it with the behaviour of the corresponding bulk phases in excess water. Upon heating, the internal particle structure changed from reversed cubic to reversed hexagonal at about 40 8C (Fig. 3) and to L2 (also called fluid isotropic phase) at about 87 8C. Upon cooling, the reversed transitions were observed at exactly the same temperatures. At each studied temperature, the structural information obtained was exactly the same for the dispersed particles and the bulk phase in excess of water.
Methods to study the formation of self-assembly structures The first monoglyceride–water binary phase diagrams were reported by Lutton (1965) using polarised light and polarised light microscopy in combination with qualitative viscosity measurements tilting the tubes. These phase diagrams are in good agreement with those reported more recently using more sophisticated methods such as small angle X-ray scattering (SAXS), which allows the identification of space groups. Other methods such as deuterium
Fig. 3. Transition from an ECP to an emulsified hexagonal phase (EHP) when heating from 25 8C (a and b) to 55 8C (c and d) as observed by CryoTEM. Image of the cubic phase dispersion at 25 8C showing several ECP particles and vesicles; (b) image of an ECP particle for which the observed reflections are in agreement with the Pn3m space group; (c) image of the hexagonal phase dispersion at 55 8C. Some particles show hexagonal motifs (arrowed) or/and curved striations (marked by a star); (d) image of a hexagonal phase particle showing curved striations. Reprinted with permission from de Campo et al. (2004). Copyright (2004) American Chemical Society.
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nuclear magnetic resonance (NMR) (Borne´, Nylander, & Kahn, 2000), differential scanning calorimetry (DSC) (Van Dijck, De Krujff, Van Deenen, De Gier, & Demel, 1976) or micro DSC (Raemy, Appolonia-Nouzille, Frossard, Sagalowicz, & Leser, 2005) were used to differentiate between different phases or study phase transitions. Freeze fracture electron microscopy was applied to discriminate between different space groups in bulk phases formed by dioleoylphosphatidylcholine–dioleoylglycerol (DOPC/DOG) (Delacroix, Gulik-Krzywicki, & Seddon, 1996). Measuring the rheological properties of the bulk phases as a function of temperature is another elegant way to distinguish between different self-assembly structures (Mezzenga, Meyer, et al., 2005). The storage moduli G 0 , loss moduli G 00 and relaxation time give enough information to discriminate between different crystal structures and allow to identify the Pn3m and Ia3d space group. The characterisation of the self-assembly structure inside the dispersed particles is more difficult than the structure of the bulk phases, since the dispersed particles are rather small and their mass fraction is very often below 5%. Simple methods like polarized light microscopy can usually not be used as the small particles do not show any birefringence. SAXS still can be applied and with a good equipment, structure identification can be made. However, in complex and diluted systems the signal may be difficult to interpret especially due to limited intensity and to peak broadening
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occurring because of the small size of the particles and crystallographic structural heterogeneity. In addition, presence of small particles may obscure part of the SAXS spectrum. Recently 13C NMR has been applied to identify the relaxation rate of the various monoolein carbons (Monduzzi, Ljusberg-Wahren, & Larsson, 2000; Nakano, Sugita, Matsuoka, & Handa, 2001). It was found that lipid organization and dynamics were similar in the dispersion and in the corresponding bulk phase (Monduzzi et al., 2000). Cryo-TEM is also a very powerful technique to characterize dispersed particles and was used by many groups (Barauskas, Johnsson, & Tiberg, 2005; Borne´, Nylander, & Khan, 2001; Gustafsson et al., 1997; Mezzenga, Schurtenberger, Burbidge, & Michel, 2005). It allows to obtain information on the microstructural characteristics such as size and shape of the single particles as well as on the space group using fast Fourier transform algorithms (Fig. 4). Sagalowicz et al. (2006) showed that it is possible to identify the nature of the different cubic particles (showing either Pn3m and Im3m space group) in a dispersion using tilting experiments. In addition, vesicles are often observed in Cryo-TEM images to coexist with ECPs. These vesicles are difficult to characterize by other techniques such as SAXS.
Fig. 4. Part of a Cryo-TEM tilting experiment. Showing the same particle under the [111] direction (a) and the [112] direction (b); (c) and (d) are fast Fourier transforms of (a) and (b), respectively. This experiment shows that the space group is Pn3m and not Im3m due to the presence of the {111} reflections. From Sagalowicz et al. (2006).
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Introduction of guest molecules in self-assembly structures Self-assembly structures formed by monoglycerides consist of nano-scaled hydrophilic and hydrophobic domains, which are separated by the surfactant selfassembled membrane. Consequently such structures contain an extremely large surface. Ericsson, Eriksson, Lo¨froth, and Engstro¨m (1991) reported that the gyroid reversed bicontinuous cubic structure forms a surface area of 400 m2 gK1 of cubic phase. The compartmentalization present in self-assembly structures can be used to introduce guest molecules of hydrophilic, lipophilic or amphiphilic nature (Fig. 5). Hydrophilic molecules will be located close to the emulsifier polar head or in the water domains while lipophilic molecules will be localized within the lipophilic domains and amphiphilic molecules in the interface. In this way lipophilic molecules can be protected from water or from hydrophilic molecules. In addition, amphiphilic molecules may expose only certain radicals to the water which may accelerate certain reaction paths and slow down others. Introduction of guest molecules generally influences the self-assembly structure properties. Addition of oil to a cubic phase induces a transition to reversed hexagonal (Hii). Diglycerol monooleate (Pitzalis et al., 2000) and sodium oleate (Borne´ et al., 2001) were reported to induce a transition from an inversed bicontinuous cubic to a lamellar liquid crystalline phase. Vauthey (1998) studied the influence of different flavours on the stability of an inverted bicontinuous cubic phases made with 80% of Dimodan U (unsaturated monoglycerides) and 20% of water. The more hydrophilic ones, 2,3 butanedione, methyl pyrazine, furfural, guaiacol, vanillin and 2-isobutyl-methoxypyrazine induced a transition to the lamellar phase while the more lipophilic one, limonene, induced a transition to the hexagonal phase. The situation is more complex for more hydrophilic compounds such as sugars. The addition of trehalose sugar to an inverted bicontinuous cubic phase does not induce the transformation to the lamellar liquid
Fig. 5. (a) Possible localization of (guest) molecules within the inverted bicontinuous cubic phase. For simplicity only part of the lattice is represented. The molecule marked 1 is hydrophilic, the one marked 2 is amphiphilic and the one marked 3 is lipophilic. Adapted from Leser, Michel, and Watzke (2003).
crystalline (La) phase, (Saturni, Rustichelli, Di Gregorio, Gordone, & Mariani, 2001). It could be shown that the addition of sugar, like glucose, induces the transformation of an inverted bicontinuous cubic to a reversed hexagonal phase (Mezzenga, Grigorov, et al., 2005; So¨derberg & Ljusberg-Wahren, 1990). Glucose does not follow the qualitative behaviour described previously by Vauthey (1998). The different behaviour has been explained (Mezzenga, Grigorov, et al., 2005) in terms of hydration changes of the monoglyceride polar head groups as the sugar molecules might compete for water molecules hydrating the polar head groups, thereby changing the packing parameter towards more reversed curvatures (P is increasing). In dispersed liquid crystalline monoglyceride particles, the effect of introducing lipophilic compounds is very similar to what happens in the corresponding bulk phase. Lipophilic additives such as triglycerides (Gustafsson et al., 1997), oleic acid at pH 7 (Nakano et al., 2002) (ratio of oleic acid to monoolein higher than 1:1) or tetradecane (Yaghmur, de Campo, Sagalowicz, Leser, & Glatter, 2005) (ratio of tetradecane to monolinolein higher than about 6:100) transform the internal structure of the particle from reversed bicontinuous cubic to hexagonal. At higher levels of oleic acid (when the ratio oleic acid: monololein is higher than 8:2) or tetradecane (when the ratio of tetradecane: monolinolein is higher than 1: 2.5 (Yaghmur, de Campo, Salentinig, Sagalowicz, Leser, & Glatter, 2006), the particle structure is the inverse micellar cubic phase and the Fd3m space group is observed. At even higher levels of tetradecane (when the ratio of tetradecane: monolinolein is higher than 1:1.5), the particle self-assembled structure is more disordered and a L2 structure is observed. For both tetradecane and oleic acid, the same sequence of phases (cubic, hexagonal, micellar cubic and L2) was observed in bulk samples. The effect of adding hydrophilic additives into monoglyceride dispersed liquid crystalline phases is probably more complex than adding lipophilic ones. As mentioned above, the addition of hydrophilic compounds to a bulk (non-dispersed) inverted bicontinuous cubic phase is likely to induce a change from reversed bicontinuous cubic phase to lamellar liquid crystalline (La) phase, decreasing the packing parameter P. This suggests that the presence of hydrophilic additives will induce a similar transformation in the corresponding dispersion, and ECP particles will transform to vesicles (which likely result from the dispersion of a La phase). It was observed that sodium oleate transforms ECP particles to vesicles (Borne´ et al., 2001). The use of the packing parameter can explain the influence of sodium oleate but not of all hydrophilic additives. For other hydrophilic guests, dispersions may have a phase behaviour very different from the bulk (nondispersed) structure, since hydrophilic additives will migrate out of the particles into the surrounding water matrix, strongly reducing the effect on particle liquid crystalline structure.
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Monoglyceride self-assembly structures used for drug delivery Protection of drugs Ericsson et al. (1991) showed that peptides incorporated into an inverted bulk bicontinuous cubic phase, formed in the system monoolein/water, can be protected from enzymatic degradation. The rate of enzymatic degradation of the renin inhibitor H214/O3 in the cubic phase, when exposed to the intestinal fluid, was 5.7% of that found in a homogeneous solution of the intestinal fluid. Since, the H214/O3 is lipophilic, it is protected by the inverted bicontinuous cubic phase from contact with the intestinal fluid. A similar protection was found for other peptidic drugs such as desmopressin, lysine–vasopressin, and somatostatin. Sadhale and Shah (1998) investigated the ability of the glycerol monooleate (also called monoolein or GMO) based structures to protect labile drugs from chemical instability reactions such as oxidation and hydrolysis. Oxidative stability of cezafolin incorporated in the inverted bicontinuous cubic phase was better than in solution at 22 and 37 8C, but was identical at 50 8C. Cefuroxime, which degrades by hydrolysis, was twice as stable when present in the inverted bicontinuous cubic phase compared to an aqueous solution. Monoglyceride self-assembly structures can also be used to prevent molecules from aggregating. In solution, insulin activity is reduced due to aggregation and formation of insoluble insulin aggregates during agitation. Sadhale and Shah (1999) demonstrated successful protection of insulin from agitation induced aggregation when incorporated into a cubic phase. It was also shown that agitation of insulin in the cubic phase had little deleterious effects on its biological activity. Controlled release of drugs One application associated with the inverted bicontinuous cubic phase is the controlled release of drugs. Shah, Sadhale, and Dakshina (2001) reviewed gel cubic phase use as drug delivery systems and in all cases, drug release from bulk phases followed a square root of time law. This indicates that the release is diffusion controlled. The release profile of bupivacaine (Shah et al., 2001) from GMO based structures was sustained and less than 40% of the drug was released within 24 h. Ericsson et al. (1991) studied in vivo and in vitro the release of some oligopeptidic drugs such as desmopressin and lysine–vasopressin. Self-diffusion NMR data indicate that the release kinetic is again diffusion controlled and release was sustained over a period of more than 6 h. The desmopressin diffusion coefficient in the cubic phase at 40 8C (DZ0.24!10K10 m sK1) is about a factor of 9 smaller than in 2H2O at 25 8C (DZ2.25!10K10 m sK1). An interaction between the peptides and the lipid matrix or membrane surface could be the reason for the significant release control achieved using the inverted bicontinuous cubic phase as the delivery system. Jeong, O’Brien, Ora¨dd, and Lindblom (2002) made a quantitative determination of the diffusion coefficient of a
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water soluble polyamidoamine dendrimer derivative labelled with fluorine inserted in an inverted bicontinuous cubic phase. The hydrated diameter of this dendrimer was 3.2 nm. The cubic phase with space group Ia3d was obtained by hydration of a 9:1 molar mixture of polymerisable monoacylglycerol and 1,2-diacyglycerol. The diffusion coefficient was found to be about 100 times lower in the cubic phase than in water (1.10K12 m sK1 instead of 1.4! 10K10 m2 sK1 in water), but it still shows that globular molecules, such as proteins, can diffuse rapidly enough in the cubic phase for having enough molecules released. Similar diffusion results were obtained with the dendrimer diffusing in the ‘classical’ inverted bicontinuous cubic phase made with monoolein and water. The low diffusion coefficient of the dendrimer is likely associated with the tortuosity of the water channel in the cubic phase or/and the interaction of the guest molecules with the interface. Puvvada, Naciri, and Ratna (1994) showed that the release of 1% bovine serum albumin (BSA) in a monoolein– water cubic phase is about 10 times lower after 200 h if the water channels contain an alginate gel. Due to their associated sustained release and their bioadhesive properties to the mucus, bulk cubic phases are of interest for topical applications (skin, mouth, vaginal.). However, since the inverted bicontinuous cubic phase is very stiff, insertion at a given place can be rather difficult. A nice way to overcome this problem has been described by Engstro¨m, Lindahl, Wallin, and Engblom (1992). The idea is that the product itself is a La phase which can be easily handled due to its relatively low viscosity. Upon heating from room temperature to body temperature or swelling with water, it transforms to the reversed bicontinuous cubic phase. The presence of the cubic phase induces the expected sustained release of the drug at the place of action. The release from emulsified cubic phase (ECP) particles has also been studied. Hydrophilic molecules, if not having a strong interaction with the interface are most likely lost and released into the aqueous media. Boyd (2003) used ECP particles loaded with more lipophilic molecules: diazepam, rifampicin, and propofol. In vitro experiments showed that with these drugs a burst release is measured, but no sustained release. If diffusion controls the release, release is expected to be several orders of magnitude faster for ECP particles than for bulk samples because of the small size of the ECP particle and the large surface area between the particles and the matrix (Boyd, 2005). However, sustained actions of molecules loaded into ECP particles were observed in in vivo studies. Chung, Kim, Um, Kwon, and Jeong (2002) reported that nanocubiles (similar to ECP particle) loaded with insulin enable to control the serum glucose concentration in diabetic rats for more than 6 h after oral administration. When insulin was administrated in an aqueous solution, the serum glucose concentration was not controlled and no hypoglycaemic effect could be detected. Engstro¨m, Ericsson, and Landh (1996) found sustained plasma levels of stomatostatin from ECP particles in a rabbit model. These
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data suggest that diffusion may not be the only factor controlling sustained action of the cubic phase particles. It was shown that the ECPs, which were made of monoolein and stabilized by Pluronic F127, were not stable in the plasma (Leesajakul, Nakano, Taniguchi, & Handa, 2004). In the same study it was suggested that the long circulation time of the incorporated substances, analyzed by Engstro¨m et al. (1996), was due to the presence of particles, which were made of degradation products of the used ECP particles. The influence of the ECP particles on the observed sustained plasma level of the drugs needs to be further studied. Monoglyceride self-assembly structures in foods Commercial monoglycerides used in food applications, are always complex mixtures of monoglycerides having various fatty acid compositions. The monoglyceride (Dimodan U from Danisco) used for the determination of the binary phase diagram in Fig. 1 has the following fatty acid composition of C18-2:62 wt%, C18-1:25 wt%, C16:7 wt% and C18:4%. In addition about 1.5 wt% diglycerides and 0.4 wt% free fatty acids are present. The phase sequence (transition between Lc, La, cubic Ia3d, cubic Pn3m and Hii) when changing water content and temperature is the same as what is determined in the phase diagram of (pure) monoolein–water (Qiu & Caffrey, 2000). Therefore emulsifiers generally used in food can be used equally well as the pure surfactant molecule for obtaining many desired self-assembly structures. The major differences observed between the phase behaviour of commercial food emulsifiers and pure substances are the shift of transition temperatures and phase changes as function of water contents. The amount of saturated monoglycerides in the sample primarily affects the location of the lamellar crystal region. Batches of Dimodan U with higher amount of saturated monoglycerides show a more extended region of the lamellar crystalline (Lc) phase. In that case, crystals can coexist with the inverted bicontinuous cubic phase. In addition, diglycerides also strongly influence the phase behaviour of commercial monoglyceride samples. Addition of 5 wt% diolein to the mixture Rylo Mg 90 glycerol monooleate (Danisco)–water results in the formation of a reversed hexagonal phase at room temperature instead of the reversed bicontinuous cubic phase (Borne´ et al., 2000). Delivery of functionality of self-assembly structures in food Self-assembly structures as reactors The fact that monoglycerides form self-assembly structures can be used to create also novel functionalities in food. One possibility is to use the liquid crystalline phase as a reactor for the creation of flavour compounds. Vauthey, Milo, et al. (2000) used structured fluids (i.e. the L2 and the reversed bicontinuous cubic phase) for the enhancement of flavour formation in a model Maillard reaction. The experiments were conducted at 100 8C. The L2 (at 100 8C) phase was made by mixing 80% Dimodan U
and 20% water while the cubic phase (at 100 8C) was made by mixing 80% Dimodan HR (saturated monoglyceride, mainly C18:0) and 20% water. The formation of 2-furfurylthiol (FFT) from the model reaction between L-cysteine and furfural was particularly efficient in the L2 microemulsion and inverted bicontinuous cubic phase compared to the respective aqueous systems (Fig. 6). The production of FFT was about five times higher in the L2 than in the aqueous phase and about seven times higher in the cubic phase than in water. The reaction also led to the formation of two new sulfur compounds: 2-(2-furanyl)-thiazolidine and N-(2mercaptovinyl)-2-(2-furanyl)-thiazolidine. The high efficiency of the Maillard reaction was attributed to the capacity of the water–monoglyceride interface to serve as a host for the reaction precursors and thereby enhancing the reaction yield. It is also possible that the presence of oleic domains in the cubic phase serves as a host for some reaction compounds, such as FFT, shifting the reaction equilibrium in favour of the reaction products. Aroma controlled release Vauthey, Visani, et al. (2000) studied the release of eight model aroma compounds using semiconductor gas sensors (‘electronic nose’). They found that the release (at pseudo equilibrium) is significantly influenced by the phase in which the aroma compounds were dissolved. Using a liquid crystalline phase (i.e. reversed bicontinuous cubic phase containing 20 or 30 wt% water) gave a different pattern than using a water in oil emulsion containing 20% water, oil, or water as a matrix. Fig. 7 shows the result of the principal component analysis of the obtained release data. Since, about 98% of the information is contained in the first principal component, the 2D plot given in Fig. 7 can practically be reduced to a 1D graph (x-axis). The structure on the right part (pure water) gave the highest concentration of volatiles in the headspace. The lowest concentrations of molecules in the gaseous phase are observed for pure sunflower oil and pure monoglycerides (without water). More interesting, the headspace concentration of the volatiles released from the cubic phase (20% water) is higher than the headspace concentration of the volatiles from the w/o emulsion (20% water). We explained this, in the part concerning drug delivery, by assuming that the controlled release of drugs is associated with the low diffusion coefficient within the cubic phase. The result of the electronic-nose experiments cannot be interpreted the same way. In the electronic nose experiment, the system is at a quasi-equilibrium and measured concentrations are associated with partitioning between head space and the matrix (cubic phase or emulsion). Concentrations are not associated with diffusion. The electronic nose results suggest that the structure of the matrix determines the partition of the volatiles. The higher equilibrium volatile headspace concentration obtained from the cubic phase (compared to the water in oil emulsion) could be related to the presence of the large internal interface.
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Fig. 6. Gas chromatography (GC)–flame photometric detector (FPD) chromatogram of volatile compounds from processed furfural/cysteine mixture in different matrix (inverted bicontinuous cubic phase, microemulsion and aqueous phase) and generated during the Maillard reaction. The FFT peaks are indicated and the two new compounds (see text) are marked by n. Adapted with permission of Vauthey, Milo et al. (2000). Copyright (2000) American Chemical Society.
Self-assembly structures for structuring food products Heertje, Roijers, and Hendtrickx (1998) studied the use of saturated monoglyceride lamellar liquid crystalline phases to structure food products. When cooling down the La structure formed by monostearate to room temperature, a
metastable a-gel (Fig. 8) is formed which can incorporate between 20 and 95% water. Maximum swelling appears for samples, which contain small amounts of anionic surfactants. The effect of charge is to produce electrostatic repulsion between the monoglyceride head groups and to
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water. a-gels have excellent foaming properties. The foam using 40 mg g-1 of monoglycerides which are present in the a-gel form, and 40 mg gK1 of sucrose at pH 7 was shown to give a specific volume of 6.4 mL g-1. Applying the coagel structure allowed to formulate a margarine with 3% monoglyceride which was ‘fat free’, i.e. having low calories. It was also suggested (Heertje et al., 1998), that a-gel formation in aerated deserts can be associated with a full fat impression by the consumer.
Fig. 7. Electronic nose headspace data obtained after the release of a model aroma mixture from different matrices (inverted bicontinuous cubic phase, water in oil emulsion, water and oil); Adapted from Vauthey, Visani et al. (2000). Reproduced by permission of Taylor and Francis. Copyright (2000).
increase water swelling. Due to undercooling, the a-gel can be maintained at room temperature for several months. The a-gel can also transform to a coagel, consisting of a network of b crystals. This phase can incorporate large amounts of
Concluding remarks Monoglyceride based self-assembly structures are promising systems for many applications in the drug or food industry. The possibility of solubilizing active ingredients in these structures can be used for many different purposes. The application of bicontinuous cubic phases has led so far to the most intriguing new functionalities such as molecular protection, controlled release, prevention of aggregation and changes in chemical reaction. In this work we summarized a few potential applications. However, we are convinced that learning more about the self-assembly structures themselves and how they interact with their
Fig. 8. (a) Binary phase diagram saturated monoglyceride–water. Dimodan P, the monoglyceride contains 65% glycerol monostearate and 30% monopalmitate; (b) schematic of b crystals, after heating them above Tc (55 8C) and exposing them to water, they transform to; (c) a lamellar liquid crystalline (La) phase which when cooled below Tc transforms to an a-gel; (d) d is the periodicity of the structure, dw is the thickness of the water domain and da is the thickness of the bilayer. Adapted from Krog (1997).
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environment will significantly trigger the development of new areas of applications. In addition, since monoglycerides are already present in many food products there is no need to use other additives in order to form self-assembly structures as delivery vehicles.
References Barauskas, J., Johnsson, M., & Tiberg, F. (2005). Self-assembled lipid superstructures: Beyond vesicles and liposomes. Nano Letters, 5(8), 1615–1619. Borne´ J. (2002). Lipid self-assembly and lipase action. PhD dissertation, Lund University. Borne´, J., Nylander, T., & Khan, A. (2000). Microscopy, SAXD, and NMR studies of phase behavior of the monoolein–diolein–water system. Langmuir, 16, 10044–10054. Borne´, J., Nylander, T., & Khan, A. (2001). Vesicle formation and other structures in the aqueous dispersions of monolein and sodium oleate. Journal of Colloid and Interface Science, 257(2), 310–320. Boyd, B. J. (2003). Characterisation of drug release from cubosomes using the pressure ultrafiltration method. International Journal of Pharmaceutics, 260, 239–247. Boyd, B. J. (2005). Controlled release from cubic liquid–crystalline particles (cubosomes). Bicontinuous Liquid Crystals. Surfactant Science Series, 127, 285–305. Caboi, F., Amico, G. S., Pitzalis, P., Monduzzi, M., Nylander, T., & Larson, K. (2001). Addition of hydrophilic and lipophilic compounds of biological relevance to the monoolein/water system I. Phase behavior. Chemistry and Physics of Lipids, 109, 47–62. Chung, H., Kim, J., Um, J. Y., Kwon, I. C., & Jeong, S. Y. (2002). Selfassembled ‘nanocubicle’ as a carrier for peroral insulin delivery. Diabetologia, 45, 448–451. de Campo, L., Yaghmur, A., Sagalowicz, L., Leser, M. E., Watzke, H., & Glatter, O. (2004). Reversible phase transitions in emulsified nanostructured lipid systems. Langmuir, 20(13), 5254–5261. Delacroix, H., Gulik-Krzywicki, T., & Seddon, J. M. (1996). Freeze fracture electronmicroscopy of lyotropic system lipid systems: Quantitative analysis of the inverse micellar cubic phase of space group Fd3m (Q227). Journal of Molecular Biology, 258, 88–103. Engstro¨m, S., Ericsson, B., & Landh, T. (1996). A cubosome formulation for intravenous administration of somatostatin. Proceeding of the International Symposium of Controlled Release Bioactive Materials, 23, 382–383. Engstro¨m, S., Lindahl, L., Wallin, R., & Engblom, J. (1992). A study of polar lipid drug carrier systems undergoing a thermoreversible lamellar-to-cubic phase transition. International Journal of Pharmaceutics, 86, 137–145. Ericsson, B., Eriksson, P. O., Lo¨froth, J. E., & Engstro¨m, S. (1991). Cubic phases as delivery systems for peptide drugs Polymeric drugs and drug delivery system. Washington, DC: American Chemical Society pp. 251–265. Friberg, S. E. (1997). Hydrotropes. Current Opinion in Colloid and Interface Sciences, 2, 490–494. Gustafsson, J., Ljusberg-Wahren, H., Almgren, M., & Larsson, K. (1996). Cubic lipid–water phase dispersed into submicron particles. Langmuir, 12(20), 4611–4613. Gustafsson, J., Ljusberg-Wahren, H., Almgren, M., & Larsson, K. (1997). Submicron particles of reversed lipid phases in water stabilized by a nonionic amphiphilic polymer. Langmuir, 13, 6964–6971. Heertje, I., Roijers, E. C., & Hendtrickx, C. M. (1998). Liquid crystalline phases in the structuring of food products. Lebensmittel-Wissenschaft und-Technologie, 31, 387–396.
213
Israelachvili, J. N., Mitchell, D. J., & Ninhan, B. W. (1976). Theory of self assembly of hydrocarbon amphiphiles into micelles and bilayers. Journal of the Chemical Society, Faraday Transactions II, 72, 1525–1568. Jeong, S. W., O’Brien, D. F., Ora¨dd, G., & Lindblom, G. (2002). Encapsulation and diffusion of water-soluble dendrimers in bicontinuous cubic phase. Langmuir, 18, 1073–1076. Jo¨nsson, B., Lindman, B., Holmberg, K., & Kronberg, B. (1998). Surfactants and polymers in aqueous solution. Chichester: Wiley. Krog, N. J. (1997). In S. E. Friberg, & K. Larson (Eds.), Food emulsifiers and their chemical and physical properties (pp. 141– 188). New York: Marcel Dekker. Landh, T. (1994). Phase behavior in the system pine oil monoglycerides–poloxamer 407-water ar 20 8C. Journal of Physical Chemistry, 98, 8453–8467. Larsson, K., & Tiberg, F. (2005). Periodic minimal surface structures in bicontinuous lipid–water phases and nanoparticles. Current Opinion in Colloid and Interface Science, 9, 365–369. Leesajakul, W., Nakano, M., Taniguchi, A., & Handa, T. (2004). Interaction of cubosomes with plasma components resulting in the destabilization of cubosomes in plasma. Colloids and Surfaces B—Biointerfaces, 34(4), 253–258. Leser, M. E., Michel, M., & Watzke, H. J. (2003). In E. Dickinson, & T. Van Vliet (Eds.), ‘Food goes nano’—new horizons for food structure research (pp. 3–13). Cambridge: The Royal Society of Chemistry. Lutton, E. S. (1965). Phase behavior of aqueous systems of monoglycerides. Journal of the American Oil Chemists Society, 42, 1068–1070. Mezzenga, R., Grigorov, M., Zhang, Z. D., Servais, C., Sagalowicz, L., Romoscanu, A., et al. (2005). Polysaccharide-induced orderto-order transitions in lyotropic liquid crystals. Langmuir, 21(14), 6165–6169. Mezzenga, R., Meyer, C., Servais, C., Romoscanu, A., Sagalowicz, L., & Hayward, R. (2005). Shear rheology of lyotropic liquid crystals: A case study. Langmuir, 21(8), 3322–3333. Mezzenga, R., Schurtenberger, P., Burbidge, A., & Michel, M. (2005). Understanding foods as soft materials. Nature Materials, 4, 729–740. Monduzzi, M., Ljusberg-Wahren, H., & Larsson, H. (2000). A 13CNMR study of aqueous dispersions of reversed lipid phases. Langmuir, 16, 7355–7358. Nakano, M., Sugita, A., Matsuoka, H., & Handa, T. (2001). Small angle X-ray scattering and 13C NMR investigation on the internal structure of ‘cubosomes’. Langmuir, 17, 3917–3922. Nakano, M., Teshigawara, T., Sugita, A., Leesajakul, W., Taniguchi, A., Kamo, T., et al. (2002). Dispersions of liquid crystalline phases of the monoolein/oleic acid/pluronic F127 system. Langmuir, 18, 9283–9288. Pitzalis, P., Monduzzi, M., Krog, N., Larsson, H., Ljusberg-Wahren, H., & Nylander, T. (2000). Characterization of the liquid– crystalline phases in the glycerol monooleate/diglycerol monooleate/water system. Langmuir, 16, 6358–6365. Puvvada, S., Naciri, J., & Ratna, B. R. (1994). Ionotropically gelled bicontinuous cubic phase as a matrix for controlled release. Materials Research Society Symposium Proceedings, 331, 217–222. Qiu, H., & Caffrey, M. (2000). The phase diagram of the monolein/water system: Metastability and equilibrium aspects. Biomaterials, 21, 223–234. Raemy, A., Appolonia Nouzille, C., Frossard, P., Sagalowicz, L., & Leser, M. E. (2005). Thermal behaviour of emulsifier–water systems studied by micro-DSC. Journal of Thermal Analysis and Calorimetry, 80(2), 430–443. Sadhale, Y., & Shah, J. C. (1998). Glyceryl monooleate cubic phase gel as chemical stability enhancer of cefazolin and cefuroxime. Pharmaceutical Development and Technology, 3(4), 549–556. Sadhale, Y., & Shah, J. C. (1999). Stabilization of insulin against agitation-induced aggregation by the GMO cubic phase gel. International Journal of Pharmaceutics, 191(1), 51–64.
214
L. Sagalowicz et al. / Trends in Food Science & Technology 17 (2006) 204–214
Sagalowicz, L., Michel, M., Adrian, M., Frossard, P., Rouvet, M., Watzke, H. J., et al. (2006). Crystallography of dispersed selfassembly structures. Journal of Microscopy 221, 110-121. Saturni, L., Rustichelli, F., Di Gregorio, G. M., Cordone, L., & Mariani, P. (2001). Sugar-induced stabilization of the monoolein Pn3m bicontinuous cubic phase during dehydration. Physical Review E, 64040902. Shah, J. C., Sadhale, Y., & Dakshina, M. C. (2001). Cubic phase gels as drug delivery systems. Advanced Drug Delivery Reviews, 47, 229–250. So¨derberg, I., & Ljusberg-Wahren, H. (1990). Phase properties and structure of a monoglyceride/sucrose/water system. Chemistry and Physics of Lipids, 55, 97–101. Spicer, P. T., Hayden, K. L., Lynch, M. L., Ofori-Boateng, A., & Burns, J. L. (2001). Novel process for producing cubic liquid crystalline nanoparticles (cubosomes). Langmuir, 17, 5748–5756. Tanford, C. (1980). The hydrophobic effects: Formation of micelles and biological membranes. New York: Wiley. Van Dijck, P. W. M., De Krujff, B., Van Deenen, L. L. M., De Gier, J., & Demel, R. A. (1976). The preference of cholesterol for
phosphatidylcholine in mixed phosphatidylcholine–phosphatidylethanolamine bilayers. Biochimica et Biophysica Acta, 455, 576–587. Vauthey, S. (1998). Solubilization of flavour compounds in structured fluids. PhD dissertation, Universite´ de Lausanne. Vauthey, S., Milo, C., Frossard, P., Garti, N., Leser, M. E., & Watzke, H. J. (2000a). Structured fluids as microreactors for flavor formation by the Maillard reaction. Journal of Agricultural and Food Chemistry, 48(10), 4808–4816. Vauthey, S., Visani, P., Frossard, P., Garti, N., Leser, M. E., & Watzke, H. J. (2000b). Release of volatiles from cubic phases monitoring by gas sensors. Journal of Dispersion Science and Technology, 21(3), 263–278. Yaghmur, A., de Campo, L., Sagalowicz, L., Leser, M. E., & Glatter, O. (2005). Emulsified microemulsions and oil-containing liquid crystalline phases. Langmuir, 21, 569–577. Yaghmur, A., de Campo, L., Salentinig, S., Sagalowicz, L., Leser, M.E., & Glatter, O. (2006). Oil - loaded monolinolein - based particles with confined inverse discontinuous cubic structure (Fd3m). Langmuir, 22, 517–521.