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Capsules of a Mythical God
November 3, 2014   
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Janus capsules are miniature, hollow structures that are in different parts composed of various micro- and nanoparticles. Theoreticians were able to design models of such capsules, but a real challenge was to produce them. Thanks to the use of an electric field, Janus capsules, named after the two-faced Roman god, can now be produced easily and at low cost.

Researchers say Janus capsules could be used to grow human skin or other body tissues, or to make porous tissues and composites. They can also be used to transport a variety of substances and release them in specific environments.

Janus, the old Roman god of beginnings and transitions, is symbolized by a figure with a double-faced head, with each face looking in the opposite direction. The ancient Romans paid tribute to Janus when seeds were planted and later when it was time to harvest. As the god of gates and doors, Janus was also revered as a sign of transition between primitive life and civilization, country living and city dwelling, peace and war, and as a symbol of young people growing into adulthood.

Janus capsules have been attracting researchers’ attention for some time. They see in the capsules an excellent tool for transporting drugs and a means of developing innovative materials. But to make Janus capsules generally accessible, efficient methods for their mass production must be developed.

An important step in this direction are the achievements of researchers from Norwegian and French research institutions and the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw, reported recently in Nature Communications.

At present, it is not a problem to produce Janus spheres—round, entirely filled micro- and nanoobjects with one part having different properties than the other. Such spheres can be, for instance, produced by sticking together two drops of different substances. After merging, the new drop requires a sufficiently fast fixation only, for example, by cooling it down or initiating polymerization of its materials. For instance, Janus spheres are particles with white and black halves, used for image generation in electrophoretic displays incorporated in e-book reading devices.

“Janus capsules differ from Janus spheres: the former are hollow structures, and their partially permeable shell is made of colloidal particles,” says ZbigniewRozynek, Ph.D., of the Institute of Physical Chemistry, who experimentally studied Janus capsules during his postdoctoral studies at the Norwegian University of Science and Technology in Trondheim. “How to make such a ‘two-faced bubble’ using micro- and nanoparticles? Many researchers are pondering the problem. We proposed a really uncomplicated solution.”

In their experiments, an international team of researchers produced Janus capsules with drops of single milliliters in volume. The drops were coated, for instance, with polystyrene or glass nanoparticles with diameters of about 500 nm (billionth parts of a meter) or 1,000 nm respectively. Also differently colored polyethylene particles were used.

The experiments were performed with oil drops suspended in another oil. Micro- or nanoparticles of one type were placed in an environment prepared in such a way and they were subsequently deposited on the surface of a selected drop. Then particles of another type were brought to the surface of the second drop. Due to the action of capillary forces, the particles were durably kept on the surfaces of both drops, being approximately uniformly distributed.

When an external electric field was turned on, microflows were induced inside and outside the drops. The microflows transported the particles toward the electric “equator.” In this step, the packing of colloidal particles could be controlled by shaking the drops in a slowly alternating electric field. The way in which the particles are packed is an important factor, as it determines the number and size of pores of the future capsule, and consequently capsule permeability.

The microflows around the electric equators of the drops resulted in the formation of a ring-shaped ribbon, composed of densely packed particles, whereas both electric poles became particle-free regions. At the same time, the poles of each drop started acquiring opposite electric charges.

Opposite electric charges attract one another, so the drops with charged poles headed towards each other. In this step, the only thing to do was to convince both drops to not only adjoin with their poles, but actually to merge. For that purpose the long-known electrocoalescence phenomenon was used: the drops were stimulated to merge faster by an electric field. Finally the drops electrocoalesced, resulting in the formation of a Janus capsule. Due to a dense packing of particles within the capsule, the particles of different types hardly mixed with each other at all.

The ultimate appearance of the capsules was determined by the number of particles deposited on the surfaces of the initial drops. If the particles covered both drops with a uniform film, extending almost to the poles, the coalescence resulted in a non-spherical structure. When empty areas around the poles were larger, the Janus capsules acquired a spherical shape. Finally, if the ribbons around the equators of the initial drops were narrow, the coalescence resulted in the formation of a structure that could be called a Janus ring.

The rings with two parts composed of two different types of particles provide interesting opportunities. They can be further melded to each other and produce more complex striped structures. The capsules could be then composed of alternately placed strips of particles, with each strip having different properties than its neighbors.

Janus capsules enable encapsulation of microobjects, nanoparticles or molecules that must be protected against the environment because of their sensitivity or reactivity. The different properties of both capsule parts make it easier to control the movement of the capsules and the release of their contents. In view of these factors, Janus capsules may find numerous applications. The proposed method for producing the Janus capsules is potentially of great importance to the pharmaceutical, dye and food industries as well as for the development of materials engineering and medicine—especially as this method is easy and cheap to use.
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