Microcapsules: from encapsulation to artificial cells

In the fields of cosmetics and pharmacology, it is essential to protect water-soluble active ingredients from the outside environment by encapsulating them. The microcapsules used are generally micrometric spherical objects, composed of an aqueous core containing the active ingredients, covered by a more or less solid layer (or membrane) providing protection. Similarly, to synthesize an artificial cell, biological material (DNA, proteins, etc.) is encapsulated within microcapsules, mimicking a cell such as a bacterium. This approach represents a major challenge for understanding the origins of life, for determining the minimum conditions for life to exist, and for creating bioreactors capable of synthesizing molecules of interest on demand.

A classic example of microcapsules is liposomes (or vesicles), whose protective layer is composed of a lipid membrane. However, there are also microcapsules whose membrane is made of particles or polymers, forming a relatively solid structure around the aqueous core. The methods of preparing such microcapsules described below make it difficult to encapsulate water-soluble active ingredients or biological material within them.

One emerging method involves the use of water-in-water emulsions. These result from microscopic phase separation in aqueous mixtures of polymers, surfactants or even polyelectrolytes. Micrometric aqueous droplets, enriched with one or more of these compounds, are dispersed in the aqueous medium, hence the term water-in-water emulsion, by analogy with oil-in-water emulsions in which oil droplets are dispersed in the aqueous medium.

The major advantage of these water-in-water emulsions is that they are capable of spontaneously sequestering water-soluble compounds such as DNA or proteins, as well as particles and small molecules. However, these aqueous water-in-water droplets have no protective layer on their surface and can therefore coalesce, leading to macroscopic phase separation and the formation of two distinct phases one on top of the other.

This article shows how, by adding various compounds, it is possible to stabilize these water-in-water emulsions, forming a layer or membrane at the droplet interface. The result is microcapsules in which active ingredients or biological material initially sequestered spontaneously in water-in-water droplets can be encapsulated.

A glossary and table of acronyms are provided at the end of the article.