Influence of the second emulsification step during production of W/O/W multiple emulsions: Comparison of different methods to determine encapsulation efficiency in W/O/W emulsions

Glass Microdroplet Generator for Lipid-Based Double Emulsion Production

Microfluidics offers a highly controlled and reproducible route to synthesize lipid vesicles. In recent years, several microfluidic approaches have been introduced for this purpose, but double emulsions, such as Water-in-Oil-in-Water (W/O/W) droplets, are preferable to produce giant vesicles that are able to maximize material encapsulation. Flow focusing (FF) is a technique used to generate double emulsion droplets with high monodispersity, a controllable size, and good robustness. Many researchers use polydimethylsiloxane as a substrate material to fabricate microdroplet generators, but it has some limitations due to its hydrophobicity, incompatibility with organic solvents, and the molecular adsorption on the microchannel walls. Thus, specific surface modification and functionalization steps, which are uncomfortable to perform in closed microchannels, are required to overcome these shortcomings. Here, we propose glass as a material to produce a chip with a six-inlet junction geometry. The peculiar geometry and the glass physicochemical properties allow for W/O/W droplet formation without introducing microchannel wall functionalization and using a variety of reagents and organic solvents. The robust glass chip can be easily cleaned and used repeatedly, bringing advantages in terms of cost and reproducibility in emulsion preparation.

Determination of the Dominating Coalescence Pathways in Double Emulsion Formulations by Use of Microfluidic Emulsions

In water-in-oil-in-water (W1/O/W2) double emulsions several irreversible instability phenomena lead to changes. Besides diffusive processes, coalescence of droplets is the main cause of structural changes. In double emulsions, inner droplets can coalesce with each other (W1–W1 coalescence), inner droplets can be released via coalescence (W1–W2 coalescence) and oil droplets can coalesce with each other (O–O coalescence). Which of the coalescence pathways contributes most to the failure of the double emulsion structure cannot be determined by common measurement techniques. With monodisperse double emulsions produced with microfluidic techniques, each coalescence path can be observed and quantified simultaneously. By comparing the occurrence of all possible coalescence events, different hydrophilic surfactants in combination with PGPR are evaluated and discussed with regard to their applicability in double emulsion formulations. When variating the hydrophilic surfactant, the stability against all three coalescence mechanisms changes. This shows that measuring only one of the coalescence mechanisms is not sufficient to describe the stability of a double emulsion. While some surfactants are able to stabilize against all three possible coalescence mechanisms, some display mainly one of the coalescence mechanisms or in some cases all three mechanisms are observed simultaneously.

Novel glass capillary microfluidic devices for the flexible and simple production of multi-cored double emulsions

HYPOTHESIS

Double emulsions with many monodispersed internal droplets are required for the fabrication of multicompartment microcapsules and tissue-like synthetic materials. These double emulsions can also help to optically resolve different coalescence mechanisms contributing to double emulsion destabilization. Up to date microfluidic double emulsions are limited to either core-shell droplets or droplets with eight or less inner droplets. By applying a two-step jet break-up within one setup, double emulsion droplets filled with up to several hundred monodispersed inner droplets can be achieved.

EXPERIMENTS

Modular interconnected CNC-milled Lego®-inspired blocks were used to create two separated droplet break-up points within coaxial glass capillaries. Inner droplets were formed by countercurrent flow focusing within a small inner capillary, while outer droplets were formed by co-flow in an outer capillary. The size of inner and outer droplets was independently controlled since the two droplet break-up processes were decoupled.

FINDINGS

With the developed setup W/O/W and O/W/O double emulsions were produced with different surfactants, oils, and viscosity modifiers to encapsulate 25-400 inner droplets in each outer drop with a volume percentage of inner phase between 7% and 50%. From these emulsions monodispersed multicompartment microcapsules were obtained. The report offers insights on the relationship between the coalescence of internal droplets and their release.

Evaluating the Stability of Double Emulsions—A Review of the Measurement Techniques for the Systematic Investigation of Instability Mechanisms

Double emulsions are very promising for various applications in pharmaceutics, cosmetics, and food. Despite lots of published research, only a few products have successfully been marketed due to immense stability problems. This review describes approaches on how to characterize the stability of double emulsions. The measurement methods are used to investigate the influence of the ingredients or the process on the stability, as well as of the environmental conditions during storage. The described techniques are applied either to double emulsions themselves or to model systems. The presented analysis methods are based on microscopy, rheology, light scattering, marker detection, and differential scanning calorimetry. Many methods for the characterization of double emulsions focus only on the release of the inner water phase or of a marker encapsulated therein. Analysis methods for a specific application rarely give information on the actual mechanism, leading to double emulsion breakage. In contrast, model systems such as simple emulsions, microfluidic emulsions, or single-drop experiments allow for a systematic investigation of diffusion and coalescence between the individual phases. They also give information on the order of magnitude in which they contribute to the failure of the overall system. This review gives an overview of various methods for the characterization of double emulsion stability, describing the underlying assumptions and the information gained. With this review, we intend to assist in the development of stable double emulsion-based products.

The optimization of technological processes, stability and microbiological evaluation of innovative natural ingredients-based multiple emulsion

For the last couple of decades, multiple emulsions were prepared either by the re-emulsification of primary emulsion or they were produced by an emulsion inversion and their technological peculiarities were widely investigated. The aim of our study was to investigate and determine the optimal technological parameters of innovative multiple emulsion, prepared directly-by addition of ethanolic rosemary extract in the presence of polymeric emulsifier-and evaluate its stability by experimental surface response design approach. The results revealed that simplified W/O/W emulsification process is stirring time and stirring speed sensitive: the change of stirring time from 5 to 15 min at 600 rpm resulted in increased viscosity (from 1705.6 ± 62.2 to 3364.1 ± 112.5 mPA/s) and smaller oil droplet size (from 33.09 ± 1.51 to 17.81 ± 0.78 μm), though the conductivity increased from 800 ± 2 to 882 ± 2 μS/cm (p < .05). The second mixing stage (1000 rpm) had a negative effect on the conductivity of W/O/W emulsion because of the inner aqueous phase encapsulation efficiency. Ethanolic rosemary extract was used as multifunctional agent: not only to form multiple emulsion but also to preserve it; microbiological assay confirmed its effectiveness. A stable W/O/W type drug delivery system was successfully created without additional technological stages, phase inversion or surfactants.

Nanoparticles as drug carriers: current issues with in vitro testing

Incorporation of nanotechnology in the field of drug delivery has created exciting opportunities for the purposeful design of nanocarriers with potentials such as targeted delivery or controlled release of the incorporated cargo, improvement of bioavailability and reduction of therapeutic side-effects. Prior to in vivo administration, nanocarriers should undergo a set of in vitro evaluation procedures to ensure their stability, safety, conformity and ability to fulfill the desired mission. In this paper, current issues with in vitro evaluation techniques used for nanocarrier characterization (assessment of particle size, surface charge, drug release and toxicity) will be discussed. Furthermore, sufficiency of in vitro evaluation procedures for the prediction of in vivo scenarios and the necessary considerations to improve the correlation between the two settings will be debated.