Double emulsions: progress and applications Nissim Garti and Chris Bisperink Research work and progress on several major issues related to double emulsions have taken place recently. Such developments include the intrinsic thermodynamic instability derived from the droplets size of the double emulsion; how the release of active ingredients entrapped within the inner phase can be prolonged and controlled; new advanced methods for investigating the structure and nature of double emulsion droplets and their interfaces; new possible applications of double emulsions for sustained and prolonged release of active ingredients in cosmetics, food, agriculture and pharmaceuticals; double emulsions as intermediate systems in the preparation of microspheres or nanospheres; and the scope and limitations of possible new applications of multicompartment microspheres prepared from double emulsions.
Prima ry c .soerseo phase
Final ccntinuous p nase
Address Nestle Research Center-Lausanne, Nestec Ltd, Verz-Chez-Les-Blanc, 1000 Lausanne 26, Switzerland Current Opinion in Colloid & Interface Science 1998, 3:657-667
Diagram of single and multiple compartment double emulsions .
Electronic identifier: 1359-0294-003-00657 © Current Chemistry Ltd ISSN 1359-0294 Abbreviations BLG' p-Iactoglobulin bovine serum albumin BSA oil o polydimethylsiloxane PDMS polyglycerol polyricinoleate PGPR PHMS polyhydrogen methylsiloxanes microsphere s undecanoyl-polyethyleneglycol esters UPEG water w
Introduction The major developments that have taken place recently in stabilizing double emulsions are based on the use of amphiphilic macromolecules for steric stabilization. Synthetic block and graft amphiphilic copolymers as well as natural amphiphilic macromolecules (proteins and polysaccharides), in combination with low-molecular weight emulsifiers have been successfully used for the preparation of stable double emulsions. Solid fat particles (Pickering stabilizers) were also explored as possible mechanical barrier for controlling stability and transport across the oil lamellae and the interfaces. Significant progress has also been made in understanding the main transport and release mechanisms in double emulsions stabilized by macromolecular amphiphiles, Water-soluble entrapped substances appear to be control-released by the amount of reverse micelles present in the oil phase. Lag-time and release rates can be modified by adding variable amounts of selected low-molecular weight emulsifiers to the oil phase in the presence of the polymeric emulsifiers.
Most of recent papers are exploring the scope and limitations of release of drugs and other active matter from water-in-oil-in-water (w/o/w) double emulsion formulations. Kinetics, bioavailablity and stability of various double emulsions, in which different pharmaceuticals are entrapped, have been investigated. Special focusis directed towards examining the advantages of solid microspheres and nanoparticles prepared by a double-emulsions solvent-evaporation methods. Microsphercs are good 'reservoirs' for water-soluble drugs that can be control-released from the matrices upon reconstitution. Some interesting new formulations have been explored. In this review we will discuss all of the above developments in the hope that the readers understanding of this area will be improved.
Double emulsions - what are they? Multiple emulsions are complex systems, termed 'emulsions of emulsions' - the droplets of the dispersed phase themselves contain even smaller dispersed droplets. Some will view double emulsions as systems in which two liquids are separated by a third liquid which is not miscible with any of the original liquids. In a w/o/\\' double emulsion each dispersed globule is in the form of a vesicular structure with single or multiple aqueous compartments separated from the continuous aqueous phase by a layer of oil phase compartments [1-7]. Double emulsion droplets are in most cases very polydispersed. Some double emulsion droplets are rather big (15-50 urn) consisting of multiple compartments, containing 50 to 100 droplets of water in each double emulsion droplet, while others can be
Food colloids, emulsions, gels and foams
very small (2-5 Ilm) and will consist of a single or a few water droplets in each double emulsion droplet (Figure 1). Multiple emulsions were first described by Seifriz in 1925 but it has only been in the past 20 years that they have been studied in more detail. The two major types arc w/o/w and oil-in-water-in-oil (o/w/o) double emulsions. Double emulsions have shown significant promise in many technologies and for many applications, particularly in food, cosmetics, pharmacology and separation science. Their potential pharmaceutical applications include uses such as adjutant vaccines, prolonged and sustained drug delivery systems, sorbent reservoirs of drug overdose treatments, taste masking and immobilization of enzymes. In most studies double emulsions are prepared in a two-step emulsification process by two sets of emulsifiers; a hydrophobic emulsifier designed to stabilize the interface of the water-in-oil internal emulsion and a hydrophilic emulsifier for the external interface of the oil-in-water emulsion. The primary w/o emulsion is prepared under high-shear conditions while the secondary emulsification step is normally carried out without severe shear to avoid rupture of the internal droplets. The composition of the multiple emulsion components is of significant importance to the stability and release properties. Much work has been carried out on optimizing the nature of the oils, the weight fractions of the water and the oil, and the nature of the surfactants. Ionic and non-ionic low molecular weight surfacrants have traditionally been used as emulsifiers stabilizing double emulsions for food, cosmetic and pharmaceutical applications, in accordance with health restrictions. It was, however, well established that combinations of emulsifiers, at the outer phase, have a beneficial effect on stability, and that the inner hydrophobic emulsifiers must be used in great excess (10-30 wt% of the inner emulsion) while the hydrophilic emulsifiers must be used in low concentration (0.5-5 wt%). The inner emulsifier was found to migrate in part through the oil lamellae to the outer interface and to influence it. Most double emulsion formulations arc of multiple compartments. In a recent study  the authors claim to be able to formulate double emulsions of high stability with unidroplets or oligodroplcts of water-in-oil in the internal phase. A two-step emulsification process using classic nonionic emulsifiers together with a migrating cosurfacrant (cetyl alcohol) were employed. The migrating hydrophobic co-emulsifier facilitated transport of emulsifiers from the oil/water interface to the oil phase, leading to fast fusion of the internal droplets forming mostly uni-droplets within the oil phase.
It was, and still is, very difficult to determine when the internal droplets coalesce, aggregate, or tend to rupture. Several modern techniques have been applied, including freeze-etching, sophisticated microscopic observations,
rheology measurements, self-diffusion Ni\IR and quantitarive estimation of entrapped active matter or markers (addenda) transported from the inner phase to the outer phase and vise versa. In addition, engulfment or shrinkage of the double emulsion droplets in the presence of water and migration of addenda in or our of the droplets have been studied and interpreted in terms of stability.
New approaches to improve stability Several approaches to improve stability of double emulsions have been considered recently. Somc of the techniques include irnproverncnt in the stability of the inner w/o emulsion by a more sophisticated selection of the type and the concentrations of classic emulsifiers . This is achieved by reducing droplets size, by forming w/o microernulsions, by forming microspheres, and by increasing the viscosity of the inner water phase. A second useful technique is the modification of the nature of the oil phase by increasing its viscosity, adding carriers and by adding complexing agents. Finally, by stabilizing the inner and/or outer emulsion using polymeric emulsifiers better stability can be achieved. In addition, some new approches have been investigated such as filtration of the double emulsion droplets, the use of colloidal solid panicles to form a strong and rigid film at the interface (Pickering stabilization), and so forth. This review will discuss only the more recent and the more significant contributions to the stability, coalescence and aggregation of the double emulsion droplets. Owusu-Apenten and Zhu  used classic non ionic low-molecular weight emulsifiers to stabilize double emulsions but introduced a new term 'effective minimum concentration' (E~IC) to describe the minimum amount of surfactant required to formulate a w/o/w double emulsion. They monitored the stability and yield of the double emulsions as a function of the emulsifier concentration required to form a 'calculated monolayer'. The emulsifier monolayer calculations are based on the emulsifier-specific surface area, the emulsifier surface excess and the emulsion interfacial size. It was concluded, based on the calculations, that the concentration of the external ernulsifier had no effect on the yield and stability of the double emulsion at the oil/water interface, This stresses the need for minimizing the hydrophilic emulsifier concentration and for modifying the well established concepts claiming that "good yields of double emulsion droplets arc obtained when the weight ratio of the hydrophobic to hydrophilic emulsifier is greater than ten". The practical meaning of this study is that double emulsions can be produced without the hydrophilic emulsifier. It is clear, however, that such emulsions prepared in the absence of a hydrophilic emulsifier, will be coarse and not very Stable. In spite of this study it is our recommendation to avoid total exclusion of the external emulsifier.
Rosano el at. [10°] re-examined, theoretically and experimentally, the stability of double emulsions emphasizing the role of the Ostwald ripening in countering stability in
Double emulsions: progress and applications Garti and Bisperink
the inner w/o emulsions. It was suggested that an oil-insoluble solute (electrolyte) be added to the inner phase into concentrated emulsions to prevent the ripening effect. It was also demonstrated that electrolytes contribute to the stability of the w/o and the w/o/w emulsions by counterbalancing the Laplace pressure differences between the inner water droplets and by playing a critical role in balancing osmotic pressure effects between the two water phases. The authors usc polyols as hydrophobic coemulsifiers. According to Rosano et 01. [10°], this reassessed the parameters affecting stability. Using the authors' [10°) terminology three possible factors affect the stability ofw/o/w emulsions: Laplace pressure and osmotic effects, interactions between the low and the high hydrophilic-lipophilic balanced emulsifiers, and polymeric-hydrophilic emulsifier interactions in the outer phase. \\'e must admit that there is not much new in these concepts except the usc of the different angle to examine stability. Kabalnov and Wennerstrom [II) have recently revised the 'oriented wedge theory' for rnacrocmulsions. They examined the correspondence between the equilibrium phase behavior of oil-water-surfactant mixtures and the macrocmulsion type and stability. The conclusion was that both phase behavior and emulsion stability are dependent on the bending elasticity of the nature of the surfactant monolayer at the oil/water interface. It seems, therefore, that o/w/o double emulsions with strong coalescence barriers will form (and be stable) when the surfactant dictates formation of films with large positive spontaneous curvatures. On the other hand, surfacrants favoring large negative spontaneous curvatures will induce formation of stable w/o/w emulsions. This theory can explain, in part, some of the stability/instability phenomena in double emulsions. For example, formation of double w/o/w emulsions, or multilarncllar stabilized systems, arc favored in the presence of surfactants forming rigid monolycrs (K - 100 k'T) while low molecular weight adsorbing surfactants form flexible rnonoluyers (K - 1 kT') will yield a non-stable double emulsions. The criteria and basic rules that govern the stability of double emulsions. prepared with low molecular weight emulsifiers were rc-explorcd in view of the 'general expected mode of the release' [12,13°). Two types of instabilities were identified (similar to observations made more than a decade ago): coalescence of the small inner droplets with the globule interface, and coalescence between the small inner droplets within the oil globules. The first type leads to a complete delivering of the small inner droplets toward the external phase, whereas the second type leads to internal coalescence and release of large w/o emulsion droplets. The kinetics associated with the release of the small inner droplets due to the first type of instability is clearly related to the hydrophilic surfactant concentration in the external phase while the second type of instability depends on the hydrophilic (internal) surfactant nature (since its concentration is, anyhow, in excess).
In conclusion, it seems that most of the more recent studies related to stability of double emulsions in the presence of low molecular weight emulsifiers have contributed \'ery little corrections or modifications in the basic concepts that have been established during the past decade. Most of recent studies confirm the basic concepts that are in use for over 15 years.
Natural macromolecules as emulsifiers In the absence of sufficient thermodynamic stability in classic double emulsions it was essential to explore the use of polymeric amphiphiles as emulsifiers emulsions [5,6]. The stability of double emulsions can be improved by forming polymeric thick films or macromolecular complexes across the oil/water interfaces. A full coverage of multi-anchoring films, formed through an interfacial adsorption of the amphiphilic macromolecule or via an interfacial interaction between macromolecules such as proteins or hydrocolloids and non ionic surfacrants, are essential for such stabilization [12,13°,14-18]. It should be noted that polyrncrizable surfactants (consisting of polymerizable functional groups) can also be used. The polymerization process can be induced, after adsorption, to form an iu-sim cross-linked membrane. This concept was documented many years ago but was not tested either in pharmaceutical studies (for understandable regulation and health reasons) or in industrial applications (for cost reasons). The polymeric complex that is formed at thc interface is able to withstand extensive thinning and can result in swelling of the internal water droplets. The major route for steric stabilization that was selected by most researchers is to explore the usc of natural or synthetic amphiphilic polymers to enhance steric stabilization. Bovine serum albumin (RSA) is one possible example of a polymeric emulsifier (biopolymer amphiphile) which can adsorb onto the oil/water external interface during the emulsification process and provide additional stability to the emulsion. Other examples arc gelatin, casein and other proteins. BSA was also added to the inner phases in the presence of conventional low molecular weight emulsifiers (such as Span 80 and Tween 80) . The emulsions prepared with these blends of BSA and hydrophobic emulsifier at the inner interface and BSA and hydrophilic emulsifier at the external interface were found to have average droplets size below 10 urn (in comparison with 15-60 urn droplets in double emulsions stabilized with Span and Tween alone). The size reduction improved the double emulsion stability and long-term (weeks) stability was obtained. Ry using BSA a significant saving was made on the amount of the hydrophobic emulsifier necessary. It was suggested  that amphiphilic biopolymers form a complex with monomeric lipophilic surfacrants. The complex is probably a thick, strong, gel-like film that imparts elasticity and is resistant to rupture to the inner droplets.
Food colloids, emulsions, gels and foams
The film improves the mechanical and steric stability of the double emulsions and slows the coalescence rates. In addition, it appears that it depresses the formation of reverse micelles in the oil phase and thus slows the transport of electrolytes via the reverse micelle mechanism. The objective of most of these studies was to formulate a fine protein-stabilized vclo]»: emulsion (droplet diameter of 1-10 urn) with a high yield (over 80%), good stability with respect to gravity, creaming, shear-induced coalescence and time-dependent release. Such emulsions were made with a variety of amphiphilic proteins but mainly with low content of sodium caseinate as the hydrophilic emulsifier . Amphiphilic macromolecules were used for forming stable hemoglobin-in-oil-in-water multiple emulsions as an artificial red cell substitute [I8J. The emulsions have been formulated with blends of coplolymers (Plutonic) and monomeric emulsifiers at the inner interface and a blend of nSA, hydrophilic nonionic emulsifiers and Pluronics at the outer interface. The average diameter of the prepared multiple emulsions after homogenization and filtration was 2-3 urn with good hydrodynamic stability (sensitivity to shear). The formulation showed a very small release of hemoglobin from the primary emulsion to the outer aqueous, and good stability of the multiple emulsion during short-term storage.
Synthetic polymeric emulsifiers Attempts [19-21] have been made to prepare specific tailor-made synthetic polymeric surfacrants for agricultural and industrial applications based on polysiloxanc-grnfr-polytcthyleneglycol) copolymers. Hydrophobic comb-grafted copolymers based on polyhydrogcn methylsiloxanes (PH~IS) grafted by short-chain polyethylene glycols have been prepared and used at the inner interface to obtain small (-I urn droplet size) and stable water-in-oil emulsions. Hydrophilic comb-graft copolymers with high density grafting and longs polyethylene glycol chain lengths hooked to a copolymer with a PI-l~IS and polydimcthylsiloxane (PD~IS) backbone through a hydrophobic spacer of undecanoic acid (undccanoyl-polycthylcnglycol esters, UPEG), have been prepared and characterized. These new category of emulsifiers were used to stabilize the outer interface of the w/o/w emulsion. The hydrophilic UPEG-silicon based emulsifiers were named, in short, ·PH~IS-PD~IS-UPEG'. The efficiency of these emulsifiers was tested in three extreme sets of emulsions in which the inner wk» ernulsions were prepared using low-molecular weight classic non ionic emulsifiers such as sorbitan mono-oleate (Span 80), copolymers of polyglycerol polyricinolcatc (PGPR), and graft copolymers of PD~IS with polyethylene glycol. The outer interface consisted of the new tailor-made PH~IS-PD~IS-UPEG emulsifiers with differcnt hydrophilicities, All three sets of emulsions were very stable. The droplets were evenly distributed and
uniform. The droplet size distribution was narrow and the particles, in general, were smaller in size than double emulsion droplets prepared with monomeric emulsifiers. It is well documented that classic double emulsions should not be sheared during the second emulsification stage in order to prevent possible rupture of the double emulsion droplets which can lead to inversion into simple ol»: ernulsion and low yields of preparation. On the other hand, slow or gentle stirring form somewhat large multiple emulsion droplets, with limited thermodynamic stability and a strong tendency for coalescence. Polymeric type surfactams form a thick, rigid film at the oil/water interface somewhat resistant to shear. The siliconic emulsifiers (PH~IS-PD~IS-UPEG)are good examples of emulsifiers forming shear-resistant double emulsions with high yields of preparation. Furthermore, the average droplet size of the sheared emulsions were reduced to less than 2 urn (in comparison to 15-60 urn in classic double emulsions and I0 urn in protein-stabilized double emulsions) with a narrower size distribution in comparison to any previously known double emulsions. The mechanical strength of double emulsions stabilized with polymeric-silicone-based surfactants was surprising and "ery encouraging [21 J. Centrifugation usually leads to fast creaming that ends in the rupture of most of the droplets of the conventional double emulsions. The classic Span-Tween double emulsions separated into two phases after moderate centrifugation action. The silicone-based double emulsions, however, remained stable (with no change in the size distribution) even after 45 minutes of high-speed centrifugation. Only ultracentrifugation caused some damage to the double emulsion droplets. A patent [P I J describing a process for producing a w/o/w type multiple emulsion using PGPR has been filed. The PGPR is a poorly defined, hydrophobic, oligomeric, graft copolymeric emulsifier of triglycerol to tctraglyccrol entities and tricinoleate to pentaricinoleare groups. PGPR is added to the oil and is aimed to stabilize the inner interface of the w/o emulsions. It was claimed [pI J that PGPR increases the rate of formation of the w/o/w emulsion droplets and imparts lung-term stability. The emulsions formed have excellent heat stability.
Transport mechanisms The release rate from double emulsions conforms to first order kinetics [14,17,20]. The effective permeability coefficients for water, electrolytes and certain drugs. were calculated from the experimental apparent first-order rate constants. Surfactunrs with high hydrophilic-lipophilic halance values used as the secondary hydrophilic emulsifier increased the release rates, primarily by increasing the rate of diffusion of the marker or the drug through the oil (non-aqueous liquid membrane). Two possible mechanisms for the permeation of water and water-soluble materials through the oil phase have been suggested: the
Double emulsions: progress and applications Garti and Bisperink
first is 'via the reverse micellar transport' mechanism and the second is by 'diffusion across the very thin lamellae of surfactant formed', where the oil layer is very thin. It was clearly demonstrated that even if the osmotic pressure of the two phases was equilibrated and no visual, coalescence took place, (neither of the inner phase droplets nor of the outer phase droplets), electrolytes tend to be transported out mostly through a 'reverse micellar mechanism' controlled by the viscosity of the oil phase and the nature of the oil membrane. The mechanism is similar to the one described by Higuchi (with some modifications) for release from polymeric matrices - 'slab into the sink' of the oil phase. A system, stabilized by a two sets of polymeric amphiphiles, was designed, based on the kinetic studies. The emulsions were stable without any significant release during storage due to the formation of a thick mechanical barrier of polymeric film at the two interfaces. The absence of reverse micelles in the oil phase will retard any transport of oil-insoluble matter through the oil. Such stabilization will guarantee a long lag-time (sustained release). 'The transport can be controlled upon request by addition of calculated amounts of low-molecular weight emulsifier to the ready-made emulsion.
In a recent paper  a practical approach was taken to distinguish between the two possible pathways of release. The release kinetics of a water-soluble drug from two different w/o/w multiple emulsions, prepared with tWO lipophilie surfactants at different concentrations, were determined. The kinetics under both iso-osmoric and hypo-osmotic conditions allowed two possible release mechanisms to be distinguished from each other: swelling-breakdown or facilitated diffusion. The results indicate that water-soluble drug release occurs by a mechanism of swelling followed by a breakdown of the oil globules, in which the lipophilic surfactant is a decisive factor. It appears that the globule's swelling capacity is considerably increased when the lipophilic surfactant concentration increases, and the more the oil globule swells, the less the water-soluble drug releases. It seems that the stability could be improved by increasing the lipophilic surfactant concentration which could strengthen the interfacial film. In contrast, an excess of hydrophilic can surfactant destabilize the emulsion. Tb summarize, the main progress that was made in recent years on ill vitro and ill vivo studies is related to a better understanding of the release mechanisms, and better characterization of the release patterns of some specific water-soluble drugs from double emulsions.
New analytical techniques l\Iany new and interesting analytical tools have been employed to study and clarify certainstructural, stability aspects, and' release properties of double emulsions. Only the more recent and the most significant methods will be discussed in this review.
The microstructure of double emulsions is commonly investigated by a phase contrast microscope estimating the size distribution of the oil globules. The quality of the photomicrographs is constantly improving. In most of the recent photomicrographs one can clearly sec the droplet composition in the inner phase. The most significant conclusion observed by the advanced microscopic techniques is that the external oil globules mean diameter decreases with the increase of the w/o/w fraction. An interesting analytical tool was developed recently [22"] for the study of the formation and stability of w/o/w and o/w/o double emulsion globules using a microscopic examination inside 'microcapillarics' attached to a video camera and image analyzing system. A 200 11m internal diameter centrally pulled microcapillary and a 311m tip with 5 11m internal diameter micropipette were prepared. The capillary tubes were used as tunnels through which the emulsion droplets were forced to flow. Only droplets with sizes below the internal diameter of the capillary tube could pass and therefore the tubes served both as a double emulsion forming tool and as a cut-off filter for certain droplet sizes to obtain uniform droplets and to control the number of inner compartments in the emulsion. The droplers within the microcapillaries were examined under light microscope. The microscopic observations were correlated to stability parameters such as the nature of the surfactant and the surfactant concentration which were proved to be primary factors in the stability of the gloubules, whereas factors like pH and salinity were found to be less critical to the stability of the emulsions. The method provides high-resolution photomicrographs of the emulsion droplets (inner and outer). A short-time interval photomicrograph can also be recorded and used as a kinetic tool to study stability and size changes, The rheological behavior ofw/o/w emulsions was studied by means of steady-shear and oscillatory measurements . From the oscillatory results it appears that the two systems show similar values of the elastic (G') and the viscous (G") terms, confirming that the rheological behavior is of the same order of magnitude. A correlation of the rheological behavior to emulsion microstructure, , modeled in terms of possible interdroplet interactions, was made . It was concluded that there are no superstructural rheological units and viscosity depends only on the fraction of the single droplets, A detailed rheological study was done on rare o/w and o/w/o emulsions . It was found that o/w emulsions exhibit only marginal levels of shear-thinning at high values of dispersed phase concentrations. The double emulsions were found to be non-Newtonian and the shear-thinning depends on the primary o/w emulsion concentration. The oscillatory measurements indicate that multiple emulsions arc predominantly viscous and that the lossmodulus falls above the storage modulus over the entire frequency range investigated. Roth moduli (G' and G"), however, show an
Food colloids, emulsions, gels and foams
t'-Ooo \. 0
0,1 K--Bulk diffusion
Current Opinion in Colloid & Interface Science
Current Opinion in Colloid & Interface Science
111 0 (0000) versus k, [k = O(2)(tl.-O/3)] for the water in the w/o/w emulsion. The solid line through the open circles represent a least square fit to the data. For details regarding the different parameters the reader is referred to .
111 0 (0000) versus k [(k = o(3)(tl.-o/3)] for de cane in the w/olw emulsion. Parameters used were D= 20 ms, 9 =0.708 Tm- 1, T= 15 ms and Te = 100 ms. For comparison, the results for neat decane (straight line) and for decane in w/o emulsion (dashed line) are included as well .
increase with the ol»: concentration (phase volume of the primary emulsion is 0.23-0.58). Upon aging the viscosity increase is marginal while the storage and loss moduli show a significant increase on the increase of the primary olw emulsion concentration.
double emulsion. The decay ofthc oil phase was also measured as a function of k (10-6 5 3 ) . The attenuation of the decanc diffusion signal (Figure 3) is quite small under the conditions used. These results arc also very surprising and will need further interpretation since the explanations that the authors provided are not experimentally well documented. In addition, the different phenomena that govern the oil diffusion in both systems was identified and discussed. The double emulsions that were chosen for this study arc from old literature, of poor quality, and very polydispcrsed, It would be of significant value if the technique was applied to emulsions stabilized with polymeric emulsifiers, composed of smaller droplets and more monodispcrscd.
A pulsed-field-gradient Nl\IR technique was used to determine the diffusion behavior of different components of the emulsion [25 The information obtained pertains to the size distribution of the water droplets in the inner wlo emulsion and the mobility of the water in the inner phase. The logarithm of the attenuation of the water signal in the wlolw emulsion is shown in Figure 2. Free diffusion would result in a straight line; the water signal is indicative of restricted diffusion. These results arc surprising in view of the fact that 68% of the water is in the continuous phase. Therefore, one would expect to find a pattern of free water. The authors explain these strange results as being caused by instrumental limitations. The fact that water within the droplets can be discriminated from water in the continuous phase by this technique, however, is an important acheivmcnt since it allows the individual study of the two different classes of water. It is also worth mentioning that the size distribution of the water droplets is very similar in the starting emulsion and in the double emulsion. The authors also address the question of the yield of formation of the double emulsion, in other words, the volume fraction of the starting emulsion that remains as individual droplets in the wlolw emulsion. 0 0
Other questions that have been answered arc related to the dynamics of the water molecules in the droplets of the
Some new fluorescent probes have been tested for the characterization ofwlolw emulsions to study release mech-. anisrns from double emulsions and to improve its stability . The probes allow the transport rates through the oil membrane to be measured more accurately.
Applications Double emulsions were always considered to have a high potential for sustained and prolonged release of active matter, mainly in the field of water-soluble drugs. Much work has been devoted to the usc of different formulations of double emulsions for various drugs [27-l9]. Several other important applications arc in cosmetics [50,S 1], agriculture [19-21], food [..J9-52], photography [1>2], leather , and others [1>3]. Most of the reports arc case studies using a large range of water soluble active compounds. It is important to stress that practically all of the papers emphasize the prolonged release advantages of the double
Double emulsions: progress and app lications Garti and Bisperink
- - - - -- - - -
Preparation of microsphere-cont aining pept ides by the multiple emul sion solvent evaporat ion technique [58"] _
lr terna' aqu eo us pnase ...·at r+ d'ug
Ho rr.oqemzauo n
O rganic phase (pol. me /sclvent)
1) Formal on of the ' ,rst ( , 10) emulsion
External aqueous chase w ater-ssurtactant
2 Formation 0 :he rnulup e emulsion W /OIW
3) So.v nt ev coration
Curr ent Op mon ,
emulsions over other formul ation s. Most studies arc based on previous formulation knowledge and use mostly nonionic emulsifiers. In some cosmetic applications cationic and zwittcrionic emulsifiers have been also mentioned. No sign ificant or surprising findings have been reported. In spite of the great progress made in understanding the mechanism of stabilizing double emulsions and controlling the relea se of active matter, and in spite of the fact that most of the studies have demonstrated prolonged release rate s, no pharmaceutical double emulsion products exist in the market place because the double emulsion droplets arc rather large for intra -venous application s. Food and cosmeti c applications do not require dropl et sizes bel ow 5 urn and therfore it is easy to forsce a possible use of double emulsions in the market place . i\l any of the researchers continue to explore the double emulsion s for controlling the relea se of pharmaceuticals such as anticancer drugs, anti-inflammarory drugs , antibiotics and vaccines administration. The main variations (apart from the entrapped dru g) between th e studies arc the nature of the oil, the nature of the emulsifiers and their ratios to improve yield of preparations (entrapments), stability and release.
Double emulsion solvent-extraction techniques for preparation of microspheres Drugs, cosmetic-ingredients and food additives are microencapsulated for variety of reasons, which include
ee. Ole & lnte rt
reducing local side-effects, controlled release, site-specific (dru g) delivery and drug targeting. A tremendous amount of rese arch work has been carried out in a search of suitable methods to achieve a good encapsulation of water-soluble active matter, The ph ysical characteristics of the microspheres produced largely determine their suitability for use for different objectives. Microsphercs are prepared from both natural and synthetic polymers. Among microencapsulation techniques, the double emulsion solvent-evaporation method, also known as 'the in-water drying method', is one of the most useful methods for _entrapping water-soluble compounds [54,55·,56,57,58\59,60·,61---65,66·,67-70). Figure 4 show s schematically the solvent evaporation preparation tech- nique. This technique allows formation of powdered double emulsion preparations that can be easily stored and reconstituted at need. Over on e hundred papers and many patents have been published in the course of the past five YC3rs on the usc of this te chnique, It is beyond the scope of this review to screen or categorize them. We have selectcd onl y some examples that we think arc of a greater significance than the others. Thc preparation of microspheres using ol»: emulsions is not efficient for the cntruppmcnt of water-soluble drugs be cause the compounds rapidly dissolve into the aqueou s continuous phase and arc lost. The problem of inefficient encapsulation of water-soluble drugs can be overcome by using the double emulsion solve nt-evaporation technique.
Food colloids, emulsions, gels and foams
using a range of polyhydroxybutyrare molecular weights was examined.
Couvreur elol. [55·] reviewed the preparation and characterization of many of the different types of the solventevaporation microspheres and discusses mostly small polytlacric-co-glycolic acid) microspheres (mean size lower than 10 11m) containing small pcptides (Figure 5). Three main evaporation strategies have been utilized in order to increase the encapsulation capacity: an interrupted process, continuous process and the rotary evaporation procedure.
Much work has been devoted in recent years to prepare rnicroparticles of a narrow size distribution with different biodegradable polymers. Sizes of common micorcapsules arc 40-50 11m [55·,56,57,58·,59,60·,61-65,66·,67-69].
90 80 -..
~ "C .,..,
60 50 40
Time (h) Release kinetics of BLG microspheres prepared with the external phase at pH 5.2. Loading amounts of BLG in terms of mg/1 00 mg of polymer. Closed circles = 4.1 ; closed squares = 8.3; closed triangles = 12.6. Reproduced with permission from [61).
Water-insoluble drugs are usually adequately encapsulated by the o/w emulsion technique [54,55·,56,57]. The w/o/w emulsions are generally used for encapsulating proteins or pcptides. These highly water-soluble molecules are quantitatively introduced in the internal aqueous phase of multiple emulsions .which result in microcapsules with increased encapsulation efficiency in comparison to particles produced by the single emulsionsolvent evaporation method. The particular location of the proteins induce a stabilizing effect on the two emulsions which, in turn, contributes to a successful stabilization of the double emulsion and loading. The double emulsion solvent-evaporation technique is commonly used to prepare biodegradable hydrophobic microspheres containing hydrophilic pharmaceuticals, proteins and polypeptides for sustained. release applications [55·,56,57,58·,59,60·,61-65,66·,t./,68]. In most cases the microspheres are in the size range of 10-100 11m. Recently, however, Blanco-Prieto el al. [60·] managed to reduce the microcapsules sizes to less than 5 11m. The microspheres, consisnng of poly(L)laetide, poly(DL)lactide, poly(DL)lactide-eo-glycolide, polyhydroxybutyrate of various molecular weights and polyhydroxybutyrate-co-valerate, were characterized for their structure size' distribution, drug loading, release kinetics, surface morphology and hydrophobicity [58·,68]. The influence of these properties on the dynamics of the immune response, following topical administration, was studied. The hydrophobic nature of polyhydroxybutyrate microspheres, compared with those formed using polylactides, was confirmed and the generation of a significant immune response was delayed using these preparations. Also, the time course of immune responses generated
Liquid-liquid emulsification is a critical step in the double emulsion micro-encapsulation process (w/o/w or o/w/o). It was found that the. size of these droplets decreases with increasing homogenization intensity and duration. As expected, the emulsion droplet size depends on viscosity, total volume size, volume ratio of the continuous phase to the dispersed phase; the rotor/stator design (of the homogenizer used to shear the droplets and reduce their sizes) was also investigated. All these physical parameters influence the structure of the microspheres obtained by this technique. The incorporation of a protein-based drug in microspheres can be made from a hydrophobic polymer via double liquid-liquid emulsification (w/o/w) or by dispersing a powdered protein in a polymer solution followed by liquid-liquid emulsification (microsphere (s)/o/w). BSA was used as the model protein and polymethyl methacrylate was used as the model polymer. The droplet sizes of the w/o emulsion were controlled using rotor/stator homogenization. The s/o emulsion was characterized based on protein powder size and shape. The size of the microspheres thus prepared was found to increase with increasing size of the protein powder in the slolv: system but increase with decreasing size of the liquid emulsion droplets in the wlol»: system. Empirical correlation can accurately predict the size of the microspheres. Protein loading in the microspheres decreased with respect to an increase in w/o emulsion droplet size or in protein powder size. These phenomena are attributed to two possible mechanisms: fragmentation along the weak routes in· the w/o/\\' system and particle redistribution as the result of terminal velocity in the s/o/w system. The role. of protein powder shape was not significant until the protein powder size exceeded 5 11m. In a very recent paper [58·] it was demonstrated that a milk model protein, p-Iactoglobulin (BLG), was encapsulated into mierospheres prepared by the solvent-evaporation technique. The effect of the pH on the outer aqueous phase of the protein encapsulation and release as well as on the microsphere morphology has been investigated. It was demonstrated that as the amount of BLG increases the
Double emulsions: progress and app lications Garti and B isperink
stabili ty o f the inner emulsion was de creased and th e entrapm ent was less efficient. Therefore, adjusting th e com bine d pl] effect and the stability of the inner emulsion ma y lead to better entrapment. As to the release, it wa s demonstrated that the 'burst effect', attributed to a morphology change in the microcapsules characterized by the pre sence of pores or channel s able to accelerate the release of the BLG, is the mo st significant release factor . These pores were attributed to the pre sence of large amount of BLG on the surface, that aggregates during microsphcrc formation at pll 5.2. It was concluded th at it is beneficial to lower the solubility of the protein in the outer phase in order to improve the encapsulation efficiency, although thi s benefit is provided by a strong ad sorption of the protein on microsphcrc surface. Mua and Hsu [(6 reported the form ation of nanoparticlcs by the double emulsion method (\\'/0/\\'), using methylene chl oride as an organic solve nt and polyvinyl alcohol or human serum albumin as a surfactant. Experimental parameters such as the preparation temperatu re, the solve nt evapor ati on method, the internal aqueou s pha se volume, the surfactant concentration and the polymer molecular weight-were invcstigatcd for particle size. The zeta potential, the residual surfactan t percentage and the polvdi spcrsity index were also investigated for calculations. Preparation parameters leading to particles with well-defined characteristics such as an average size around 200 nm and a polydi spcrsity index lower th an 0.1 wer e identified. 0
There arc some more intere sting papers on the protein entrapme nt s for variou s oral and ot her intakes [69-72).
Conclusions Double emulsions of w/o/w have many potential applications, hut only very few real commercial products (mostly in cosmetics) exist in the market place. The main reason is the inherent instability of the preparation, the relatively large droplet size, and the uncontrolled release of the entrapped matter (at storage and during applicat ion),
entrapped in relatively small droplets with improved polydisp ersibility, It was demon strated that the thermodynamic stab ility, e ntrap me nt yields and capacities were significantly improved and the release was prol onged. In addition, the micellar transport via re ver se micell es was reduced and therefor e storage migration was very lim ited. The release can be triggered at need. Th e usc of well design ed and cha racter ized synt he tic pol ymeric s u rfacra n rs wa s ve ry helpful in reducing uncontrolled leak age of addend a, improving shea r and heat resistance and obtaining small and/or unicompartmcnt double emulsions droplets.
It seem s that for agriculturul and industrial applications mo st of the technological problems have been solved. Good quality formulations ca n be made. In foods and cosmeti cs, some legal restrictions on the usc of macromolecu lar amphiphilcs may still pre vent commercial double e mul sion p rep arations-. ln ph arm aceuticals, the situation is mor c com plicate d. The restri cti on s on intravenous applica tions, in view of the size of droplets and the nature of emulsifier s in usc, arc understandabl e. Oral, rrunsdcrmul and other drug intakes many have less legal and safe ty restrictions and therefore man y of the available formulation s arc ready for usc . Polymeric surfacrants, in combination with the con vention al sma ll molecular weight emulsifiers, arc suggested as the future e m ulsifie rs for double emul sion s, Microsp hc rcs made by the d ou ble emulsion evaporation technique arc excellent reservoirs for entrapment of watersoluble pepridcs, proteins and hydrophilic substance s. ~Iicro spheres offer high cap acities, sta bility upon storage and ea sy and instant reconstitution and controlled release. Much progress has been done in recent years but more is needed before double emul sion s and microsphcrc or nanosphcrcs fonnulations can be offered to manufacturers.
Acknowledgemeit-ts The usc of .macromolcculcs as stc ric stabilizers or as 'mechanical gel-like harriers ' for both the inner and the outer int e rfaces have opened new options and possibilities to prepare stable double emulsion s. Naturally occurring macromolecules such as sel ec te d proteins (BSA, human serum albumin , casein, BLG and gel atin) and hydrocolloid s (gum urubic) hav e been used with great succes s to improve the film formation oyer the water and the oil interface s (better anchoring, full coverage , thick layer, low desorption and no interfacial migration). The polymeric arnphiphilc can he used together with monomeric classical hydrophobic and hydrophilic emulsifiers. The polymer-polymer or pol ymer-surfactant complex is an ideal interfacial barrier for diffusion controlled transport. Hydrophobic (non-ionized, lipid-like molecules) and hydrophilic (ionized molecules, hydrophilic organic molecules, pcptidcs, proteins, vitamins and electrolytes) substances have been successfully
;-': i ~~ i lll (; 311i i, on leave rrom the Cas li I nsritutc of ,\ pplied Chemistr y, Sdwul of Appli ed Scie nce an d ' Ic d m olo;:y. The Hebrew Univcrsi tv of . j er usalem, 'J!'J04 j eru sale m, Israel ,
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