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Using Salt Crystals to Create Self-Assembling “Origami” Shells That Encapsulate Liquids

This new “crystal capillary origami” technique could be used to improve precision in drug delivery and the development of nanoscale medical devices.

Mechanical engineers at the Korea Advanced Institute of Science and Technology have developed a novel process called the “crystal capillary origami” technique that allows them to spontaneously encapsulate microscopic droplets of water and oil emulsion in a tiny sphere made of salt crystals, akin to a tiny, self-constructing origami soccer ball filled with liquid.

Capillary action, or “capillarity,” is the process by which water or other liquids move up narrow tubes or other porous materials without the assistance of and seemingly in opposition to gravity. Often seen in vascular systems of plants, this effect is due to the forces of cohesion, which leads to surface tension and adhesion. The strength of capillarity greatly depends on the chemistry of the liquid, the chemistry of the porous material, and the other forces acting on both liquid and material. For instance, a liquid with lower surface tension than water would fail to hold up a water strider insect.

A lesser-known phenomenon related to capillarity is elasto-capillarity, which exploits the relationship between capillarity and the elasticity of a very thin flat sheet of solid material. Under special circumstances, the capillary forces can overcome the elastic bending resistance of the sheet, effectively causing it to deform. For example, when a liquid droplet is placed on the flat sheet, the sheet can spontaneously encapsulate the liquid due to surface tension.

By manipulating elasto-capillarity, various “capillary origami,” or three-dimensional structures can be created. The sheet can be directed to wrinkle, buckle, or self-fold into different kinds of 3D shapes depending on the chemistry of the flat sheet and liquid and by carefully designing the shape and size of the sheet.

However, Kwangseok Park, a lead researcher on the project, highlights one big issue with these small devices, “These conventional self-assembled origami structures cannot be completely spherical and will always have discontinuous boundaries, or what you might call ‘edges,’ as a result of the original two-dimensional shape of the sheet.” He added, “These edges could turn out to be future defects with the potential for failure in the face of increased stress.” Non-spherical particles are less advantageous than spherical particles in terms of cellular uptake.

“This is why researchers have long been on the hunt for substances that could produce a fully spherical capillary origami structure,” explained Professor Hyoungsoo Kim from the Department of Mechanical Engineering.

Now in their latest study, the authors have successfully created an origami sphere for the first time. Instead of using flat sheets, Park and Kim discovered that the growth of salt crystals can similarly perform capillary origami action. While their novel “crystal capillary origami” technique can spontaneously construct a smooth spherical capsule using the same surface tension effects, the elasto-capillary conditions of growing crystals are also critical factors to spontaneously encapsulate a liquid.

To envelop the water-oil emulsion, the researchers used four types of salts: calcium propionate, sodium salicylate, calcium nitrate tetrahydrate, and sodium bicarbonate. Unlike the cubical crystal structure of sodium chloride, these four salts form plate-like structures like crystallites or “grains”, which can self-assemble into perfect spheres.

Using scanning electron microscopy and X-ray diffraction analysis, the researchers examined the underlying mechanism of such formation. They concluded that the process by which crystallite plates cover emulsion surfaces was driven by “Laplace pressure.” Laplace pressure refers to the difference in pressure between the inside and outside of a curved surface. The pressure difference is caused by the surface tension of the interface between two substances, in this case between the saltwater and the oil.

Given these promising results, it is hoped that these self-assembling nanostructures can be used for encapsulation applications in various sectors such as the food and cosmetics industry, as well as drug delivery and nanoscale medical devices. [APBN]

Source: Park, K., & Kim, H. (2021) Crystal capillary origami capsule with self-assembled nanostructures. Nanoscale, 13(35), 14656–14665.