3.1. Encapsulation of Active Ingredients Using O/W Emulsion Stabilized by Surfactants and Polymers
Petrovic et al. [34
] have demonstrated for the first time the potential of PS mixtures in microencapsulation. The influence of interactions between hydroxypropylmethyl cellulose (HPMC) and the anionic surfactant sodium dodecylsulfate (SDS) on the properties of 20% w/w
sunflower oil-in-water (O/W) emulsions and the microcapsules prepared on this basis by spray drying were investigated in [34
] (Figure 1
The amount of encapsulated oil depends on the SDS concentration, i.e., the HPMC–SDS interactions [34
]. The microcapsules prepared without SDS have low encapsulation efficiency, which is a consequence of weak intermolecular bonding between the HPMC molecules [34
]. The addition of SDS leads to the formation of HPMC/SDS complexes that have better emulsifying properties, and form more compact adsorption layers at the interface of oil droplets. The maximum amount of encapsulated oil (0.6 goil
) at 0.45% w/w
SDS is attributed to the very compact crosslinked interfacial layer formed at the interface of the oil droplets, resistant against breaking during the drying process, while after the saturation with SDS molecules, the interaction becomes more intramolecular in nature, hence, the ability of the interfacial layer formation decreases.
Highly charged microcapsules were obtained via an internal phase separation technique by using ionic dispersants such as poly(methacrylic acid), ionic amphiphilic block copolymers like poly(methyl methacrylate)-poly(sodium (meth)acrylate) and an oil-soluble anionic surfactant, sodium 1,5-dioxo-1,5-bis(3,5,5-trimethylhexyloxy)-3-((3,5,5-trimethylhexyloxy)carbonyl) pentane-2-sulfonate, in combination with a water-soluble polycation, poly(diallyldimethylammonium chloride) [42
]. All resulting microcapsules showed sufficient colloidal stabilities, especially for those made of block copolymers whose interaction remains intact even at high ionic strength due to the combination of electrostatic and steric stabilization. However, only weak polyacid- or block copolymer-based microcapsules possessed a core-shell morphology, whereas the microcapsules made with anionic surfactants and polycations had a multicore morphology [42
Multicore-shell particles consisting of a poly(methyl methacrylate) shell and multiple dodecane cores have been prepared via the internal phase separation method using an oil-soluble anionic surfactant sodium 1,5-dioxo-1,5-bis(3,5,5-trimethylhexylocy)-3-((3,5,5 trimethylhexyloxy)carbonyl)pentane-2-sulfonate (TC4) and the water-soluble polycation PDADMAC in combination as a dispersant pair and studied using SEM, light microscopy, microelectrophoresis, quartz crystal microbalance with dissipation monitoring (QCM-D), and different optical tensiometry methods [50
]. TC4 stabilizes, in particular, the polymer/water interface in the presence of PDADMAC instead of the O/W interface in contrast to water-soluble surfactants [50
]. In addition, the oil–polymer interface is stabilized by TC4 which prevents coalescence of the oil droplets and leads to a multicore-shell morphology rather than particles with a single core [50
]. In contrast, this new surfactant-inspired methodology uses TC4 as an oil-soluble dispersant to anchor the otherwise non-adsorbing water-soluble cationic polyelectrolyte PDADMAC on the O/W emulsion droplets by electrostatic interaction [42
] the encapsulation of coriander essential oil in alginate and alginate (Alg)/chitosan (Chi) microspheres by the emulsification external gelation method was performed. The optimum values of the corresponding parameters for the maximum encapsulation efficiency were: wall material 1.5%, Alg/Chi ratio 1:2, surfactant concentration 1%, oil concentration 30% and sonication time 15 min. The correlation between the swelling degree and the release rate was underlined and the dependence of different conditions of pH and temperature were studied. For pH 4 the release rate grows in close relation with the increase of the chitosan content while for pH 5 it grows together with the increase of the alginate content.
The anionic surfactant Aerosol OT (AOT) and polysaccharide polymer alginate were used for the PS nanoparticle formulation for the sustained release of water-soluble drugs [58
]. Due to the size of the nanoparticles in the range of 40–70 nm, weakly basic molecules could be encapsulated efficiently in AOT-alginate nanoparticles [58
]. It was shown a sustained release of 60–70% of the encapsulated drug over four weeks, with a nearly zero-order release during the first 15 days.
Microencapsulation of Vitamin E directly from oil-in-water (10% O/W) emulsions was carried out by means of a novel approach [24
] where a preformed polyelectrolyte-surfactant complex (sodium polystyrene sulfonate/dodecyl trimethyl ammonium bromide) was simultaneously used as an electrosteric emulsion stabilizer and as a charged precursor for the following build-up of further layers to form microcapsules. The primary emulsions were obtained by ultrasonication, which occurs via common mechanisms including cavitation and shear [59
]. Although microfluidization (MF) has been noted to be superior in size reduction and generating emulsions with more narrow size distributions, ultrasonication is deemed to be significantly easier to operate, clean, and maintain systems [60
]. With extended duration of processing, ultrasonication has been shown to be able to achieve comparably small emulsion droplets [59
]. Leong et al. [61
] demonstrated the capability of ultrasound to produce emulsions with particle sizes comparable to MF, provided that the energy density and the surfactant system were optimized.
Subsequently, a layer-by-layer technique was applied to emulsions leading to the formation of core-shell microcapsules with oily cores and polyelectrolyte shells (Figure 2
and Figure 3
The effect of the complexes on the process of emulsion formation and on the stability and characteristics of the resulting emulsions was investigated by measurements of dynamic and equilibrium interfacial tension, size distribution (DLS), and interfacial charge (zeta-potential) (Figure 4
It was shown that in each encapsulation step the capsules were recharged and the final capsules revealed the zeta potential value of −40mV. The relatively high values of the zeta potentials in each step of encapsulation (average absolute value of about 40 mV) ensure the good colloidal stability of all of the disperse systems studied (Figure 4
Via the kinetics of vitamin E a gradual slowing down of the vitamin E release was observed in each stage of the formation of the capsule shells. As can be seen from Figure 5
the final microcapsules showed a sustained release of vitamin E within at least 80 h [24
The microencapsulation approach developed is straightforward and economic and could be used for the encapsulation of a variety of bioactive ingredients for cosmetic and food applications
3.2. Encapsulation of Active Ingredients by Double W/O/W Emulsions
A second method of preparing microcapsules is the use of double emulsions, which are “emulsion within an emulsion”, i.e., droplets of one fluid encapsulated inside droplets of another fluid. The formation process consists of two steps: (1) the formation of W/O emulsion droplets, and (2) the formation of the final W/O/W emulsion [62
]. It is known that one surfactant has to have a low HLB to form the primary W/O emulsion, while the other has to have a higher HLB to facilitate the secondary emulsification [65
]. In W/O/W double emulsions, the internal and external aqueous phases are separated by an oil layer [63
Several reviews present results on advances in the investigation of many types of pharmaceutical and biopharmaceutical active components used for preparing microparticles by the double-emulsion technique and their controlled release [65
]. The shell fluid can serve as a barrier between the core droplets and the outer environment. In the double emulsion method, the core droplets can be loaded with many substances to achieve various functions, such as the encapsulation of pesticides, nutrients, drugs or antibodies, so, this method shows great potential in many applications [64
However, the formulation and stabilization of these structures is still a challenge and much more difficult than for simple emulsions. The choice of suitable emulsifiers for double emulsions is crucial for their stability and the emulsifier’s type, concentration, phase ratio, and homogenization method influence the properties of nano-capsules, such as size, encapsulation efficiency, and drug release. [63
The use of two different emulsifiers creates one of the most serious difficulties—the competition between the lipophilic emulsifier solved in the oil and covering the interface between oil droplets and the inner aqueous droplets, with the hydrophilic emulsifier present in the outer aqueous phase [64
Thus, one of the practical problems for the encapsulation of hydrophilic drugs is to get emulsions which provide stable polyelectrolyte complexes capable of maintaining stability over a wide range of external conditions (pH, ionic strength, temperature). At the same time, it should easily and reversibly react on very slight fluctuations leading to changes in the molecular characteristics and the phase state that promotes a controlled release [65
]. The restructuring of the complexes can be carried out in an environment that is suitable for the functioning of natural polyions (proteins, enzymes, nucleic acids). It was proposed in [79
] to use for encapsulation the polymeric drug delivery systems as carriers based on therapeutic proteins and other macromolecules. The use of biopolymers as hydrophilic emulsifiers in W/O/W double emulsions is often advantageous.
The authors in [63
] also describe that the droplets in W1/O/W2 double emulsions are thermodynamically unstable and the formation and stability of double emulsions remains an actual scientific problem because methods of improving the emulsion stability are key factors to yield the final microspheres. Unfortunately, the stability will be severely deteriorated with larger diameters of double-emulsion droplets during the encapsulation and solidification stages. Thus, the formation and stability of double emulsions have been the focus of numerous studies. Additionally, the structures of microcapsules should provide a sustainable release while protecting the material from degradation. However, as described in [87
], the conventional techniques such as solvent evaporation, coacervation and spray-drying methods, can be harmful to the substance being encapsulated. Thus, the use of biopolymers as hydrophilic emulsifiers in W/O/W double emulsions often allows to achieve more stable emulsions [64
Some works concluded that the combination of surfactants has an enormous importance to provide stability of double-emulsion droplets. [85
]. The mechanisms of interaction between surfactant and polymers chains used in the formation of emulsion were elucidated in [85
]. For example, sucrose as a native carbohydrate mixed with PVA chains, chitosan-alginate microcapsules, and other poly-complexes can serve as an oral delivery carrier for hydrophilic drugs [82
]. Large-sized W1/O/W2 double emulsion droplets with uniform wall thicknesses and diameters were prepared by adopting the emulsion microencapsulation method. By visually counting the number of double emulsion droplets online over the observation time, the stability of these droplets solidified in a constant rotating flow field can be explored. Combining emulsion kinetics of formation and destruction, the adsorption characteristics of polymers, surface tension, and rheological properties of the outer water phase (W2), the influence of surfactant concentration on the stability of double emulsion droplets was systematically investigated. Moreover, the results show that the stability of the double emulsion droplets can be improved when the content of surfactants was less than 1.0 wt%. This is probably due to the proper match between shear forces and surface tension of the W2 phase, and weaker forces were directed inwards caused by the osmotic pressure in dilute surfactant concentrations.
The amphiphilic character is also important for the stability of emulsion droplets stabilized by a surfactant in the W2 phase. It was shown by the authors in [85
] who selected polyvinyl alcohol (PVA) as a water-soluble polymer surfactant because of the good water solubility and its amphiphilic character to maintain the stability of double emulsion droplets. It is necessary to note that adding a second substance to the W2 phase increases the life-time of these droplets and gives a better understanding of the mechanism of the emulsion stability. Adjusting the different stripping velocity of the syringe pump used during emulsification influences the diameter and wall thickness of the droplets [63
Moreover, different emulsification methods were applied to study the influence of homogenization on the physicochemical characteristics of double emulsions. The obtained double emulsions were compared in terms of stability and droplet size [63
]. It was found that the homogenization method influences the physiochemical characteristics of the double emulsion and the most stable double emulsion with the smallest droplet size was obtained by the high-speed homogenization method. It was also found that the most stable emulsion was produced at high-speed homogenization because of its influence on the stability size distributions of the primary and secondary emulsion of the final double emulsion [63
]. In our earlier work, it was found that at an ultrasound amplitude of 35% the most stable double emulsions with the smallest droplet size were obtained [63
], and also the different volume ratios of the aqueous phase with insulin loaded into the oil phase were studied at different concentrations of surfactant, time as well as the regime of sonication (pulse or continuous).
The formulation of double emulsions to be used in food and medical systems were studied and the impact of the W1/O ratios of primary emulsions, and W1/O/W2 ratios of the double emulsions were investigated [63
]. Particle size distribution, stability, encapsulation efficiency (EE), rheological properties, and morphology of the double-emulsion droplets were studied. Additionally, the optimum ratio of the liquid phases where double emulsions showed the highest stability were determined. In all cases, high-speed homogenization was used to produce stable double emulsions. The formulation parameters (type of biopolymer, type of oil and stabilizer, and its concentration, volume fraction, and content of internal aqueous phase, time, and regime of mixing), and oil concentration, surfactant concentration, ratio of W1/O/W, etc., were also considered in [63
As described above, poly-complexes of chitosan (CS) and xanthan gum showed the potential to be used for drug delivery systems [63
]. Chitosan solutions were investigated at various pH (2.31; 3.97; 5.01; 6.62; 11.42), and the morphology of the resulting emulsions analysed. Stabilization was possible due to electrostatic interaction with xanthan gum solutions (NH3+
group of chitosan and COO−
group of xanthan). The results confirm that chitosan mixed with xanthan gum can form double emulsions with a considerably long-term stability. These results testified that the addition of xanthan gum to chitosan solutions allows to reach process yield efficiencies of 65.6–77.5%.
Rheological and release properties of double microemulsions (containing active ingredients) prepared with various polyelectrolytes such as angum gum (AG), arabic gum (GA), whey protein, chitosan, xanthan gum, etc., were investigated in [63
]. For example, crocin (a bioactive of saffron) is a highly water-soluble carotenoid with several physiological benefits which is sensitive to environmental conditions, such as light, oxygen, and pH. In this study, firstly a W1/O microemulsion containing crocin in the W1 phase was prepared using a spontaneous emulsification method, and then the double emulsion (W1/O/W2) was prepared with AG in the outer aqueous phase (W2). The resulting double emulsions were compared with those obtained by means of whey protein concentrate (WPC) and GA [93
]. Emulsions containing AG showed highest viscosity (about 10 times higher) and a gel-like behavior comparable to GA with an average droplet size of a W1/O microemulsions of approximately 10 nm. In contrast, WPC and GA produced double emulsions with droplet diameters of 429 and 695 nm, respectively, for 5% added biopolymers. Although the highest droplet size was observed for AG stabilized emulsions, they showed the lowest creaming and highest stability which could be attributed to their high viscosity. In terms of a dynamic behavior, for all samples the storage modulus G’ was higher than the loss modulus G”. The sizes of droplets correlated with the results described in [63
] where we determined nano-particle formulations that showed a low polydispersity index (PdI) ranging from 0.02 to 0.142, and droplet sizes from 202 to 393 d.nm. This indicates the stability of the obtained emulsions. The outer droplet diameter of the double emulsions was 695–1250 d.nm.
The effect of mixtures of CS and poly(d
-lactic-co-glycolic acid) (PLGA) on the preparation of double emulsions and microcapsules (MC) was investigated in [94
]. MCs were prepared by the W/O/W double emulsion method and characterized in terms of morphology, size, encapsulation efficiency, and physicochemical and thermal properties. The resulting MCs had a spherical shape and different morphology, with sizes ranging from 11 to 20 nm, and encapsulation efficiencies of 40–52%, depending on the CS concentration. In summary, CS/PLGA MCs can be proposed as an attractive delivery system to control the release and long-term protection of resveratrol (RSV).
All the works mentioned here were devoted to microencapsulation and study of release of active ingredients from microcapsules and demonstrated the efficiency of the encapsulation methodology based on the formation of double emulsions. The cumulative release kinetics of insulin from double emulsion microdroplets stabilized by CS is presented in Figure 6
. A saline phosphate buffer (PBS) solution at pH 7.4 was chosen as the release medium for insulin to allow the establishment of sink conditions for it [63
From the release profile of insulin, it is possible to determine that 15% is released during the first 30 min, 40% within 1 h, and 70% within 1.5 h. Thus, the slow kinetics of the release of the active substances demonstrated the efficiency of the chosen methodology for obtaining double emulsions. Hence, compositions of surfactants with polyelectrolytes provide more stable emulsions and good formulation parameters for microencapsulation of active hydrophilic drugs.