O/W-emulsion technique

Protein-containing PEG/PBT microspheres were prepared according to the well-known double-emulsion method (52,53). In contrast to what is often observed for hydrophobic polymers like PLGA, dense poly(ether ester) microspheres were obtained. This outcome was explained by the amphiphilic character of the polymers, which created very stable emulsions. The latter is considered a prerequisite for the formation of non-porous microspheres. A drawback of applying the w/o/w emulsion technique for the production of protein-... 

O/W emulsion technique is suitable for the encapsulation of hydrophobic (poorly water soluble) compounds since they partition favorably into the oil phase, and the technique has been employed for the encapsulation of various drugs, such as steroids, thioridazine, dexamethasone, chlorproma-zine, methadone, and anticancer agents—aclarubicin, lomustine, " " and paclitaxel." " ... 

The ABA type block copolymer, PLA-PEG-PLA, has been most extensively studied since late 80 s in regard to its synthesis (22-27), properties 28,29), and degradability 30-34). In the ABA system, PEG works as an intermolecular plasticizer in processing implant pastes, films, and scaffolds. Its hydrogels have also been studied mainly for their biomedical application 35,36). Vert et al. have recently reported the utilization of the hydrogels to the protein release 37). Microspheres have been prepared from the ABA block copolymers by W/0 or W/O/W emulsion technique 38-40), and utilized for... 

Phase inversion technique the external phase is added to the internal phase. For example, if an O/W emulsion is to be prepared, the aqueous phase is added to the oil phase. First a W/O emulsion is formed. At the inversion point, the... 

Bibette has used this method to study the effect of osmotic pressure on the stability of thin films in concentrated o/w emulsions [96], by means of an osmotic stress technique. The emulsion is contained in a dialysis bag, which is immersed in an aqueous solution of surfactant and dextran, a water-soluble polymer. The bag is permeable to water and surfactant, but impermeable to oil and polymer. The presence of the polymer causes water to be drawn out of the emulsion, increasing the phase volume ratio and the deformation of the dispersed droplets (Fig. 10). 

Aoki, T., Decker, E.A., McClements, D.J. (2005). Influence of environmental stresses on stability of O/W emulsions containing droplets stabilized by multilayered membranes produced by a layer-by-layer electrostatic deposition technique. Food Hydrocolloids, 19, 209-220. 

A similar technique can be used to study the rheological properties of liquid films. Figure 4 shows the formation of a W/O/W emulsion film with two, identical aqueous phases (such as in water-in-oil emulsions) at the tip of the capillary. A pre-requisite of the experiment is that the surface of the capillary must be well wetted by the film phase, i.e., it should be hydrophobic in this case. First, an aqueous drop is formed inside the oil (film liquid) and the aqueous phase is in the bottom of the cuvette. Then, the level of the aqueous phase is slowly increased. As the oil/water interface passes the drop, a cap shaped oil film, bordered by a circular meniscus, covers the drop. This film can be studied in equilibrium and in dynamic conditions, similar to the single interfaces (See above). The technique can be used to study films from oil or aqueous phase which can be sandwiched between identical or different liquid or gas phases. 

The pulsed field gradient NMR technique can be readily used for the determination of the water droplet size distribution in W/O emulsions or the oil droplet size distributions in O/W emulsions. Important advantages are the non-invasive nature, the ease of sample preparation, and the feet that pulsed field gradient NMR measures the droplet size distribution of the bulk in contrast with microscopic methods which estimate the size distribution of the surface. Both the proposed matrix method and the iterative curve fitting procedure can be successfully applied in a factory environment. The method can be implemented on a high as well as on a low resolution NMR spectrometer. 

A number of techniques are used commercially to accelerate emulsion breakdown. Mechanical methods indude centrifugal separation, freezing, distillation and filtration. Another method is based on the principle of antagonistic action - i.e. the addition of O/W-promoting emulsifiers tends to break W/O emulsions, and vice-versa. Emulsions can also be broken by the application of intense electrical fields, the principal factors involved being electrophoresis in the case of O/W emulsions and droplet deformation in the case of W/O emulsions. 

Figure 2.3 shows an optical micrograph obtained with reflected fluorescent light. This technique clearly shows the presence of a multiple, O/W/O/W, emulsion which would have been extremely difficult to detect by other means. Combined techniques can be used to observe different dispersed species. For example, reflected white light can make dispersed particles easy to observe (Figure 2.4a) whereas reflected light on... 

Alumina/hydrophilic Tubular = 35% 0.1-0.8 0.5-0.2 0.5-0.8 Westfalia Separator Membraflow (Germany) Societe des Ceramiques Techniques (France) Pall-Exekia (France) o/w, w/o/w emulsions o/w emulsions [5, 26-28]... 

Rouzes, C., R. Gref, et al. (2000). Surface modification of poly(lactic acid) nanospheres using hydrophobically modified dextrans as stabilizers in an o/w emulsion/evaporation technique. J Biomed Mater Res 50(4) 557-65. 

JEV-loaded poly(lactide) (PLA) lamellar and PLG microspheres were successfully prepared with low-molecular-weight PLA by the precipitate method and with 6% w/v PLG in the organic phase, 10% w/v PVP, and 5% w/v NaCl in the continuous phase by using a w/o/w emulsion/solvent extraction technique, respectively [233], The JEV incorporation, physicochemical characterization data, and animal results obtained in this study may be relevant in optimizing the vaccine incorporation and delivery properties of these potential vaccine targeting carriers. 

In this section, surface imprinting which is carried out in O/W emulsion will be reviewed [22-26]. Maeda et al. presented the concept of the technique as illustrated in Scheme 9.7. This technique relies on a cooperative interaction of multiple func-... 

Water-soluble drugs cannot be encapsulated with the 0/W-emulsion technique because the drug would be lost to the external aqueous phase. Two methods have been described for the encapsulation of hydrophilic drugs one is based on using an external oil phase and the other one on the formation of the microparticles by a w/o/w-melt dispersion technique. 

This technique was originally applied by Herbert (1 ) in the comparative studies on the role of antigen adjuvants. Engel et al. (2 ) also tried to prepare a W/O/W emulsion using the two-step procedure for examining the intestinal absorption of insulin. Then, the similar works have followed in some fields, especially in the pharmaceutical science (22). 

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