Researchers have developed a technique whereby they can spontaneously encapsulate microscopic droplets of water and oil emulsion into a tiny sphere made of salt crystals, a kind of self-constructed origami soccer ball filled with liquid. The process, which they call “crystal hair origami”, could be used in a range of fields from more precise drug delivery to nanoscale medical devices. The technique is described in an article published in the journal Nanoscale September 21.
Capillary action, or “capillarity,” will be familiar to most people as how water or other liquids can move up narrow tubes or other porous material apparently in defiance of gravity (for example in the vascular systems of plants, or even more simply, the drawing of paint between the hairs of a brush). This effect is due to cohesive forces (the tendency of molecules in a liquid to stick together), which results in surface tension, and adhesion (their tendency to stick to the surface of other substances) . The strength of the capillary action depends on the chemistry of the liquid, the chemistry of the porous material, and other forces acting on both. For example, a liquid with a surface tension lower than that of water would not be able to hold a water surveyor insect.
Less well known is a related phenomenon, elasto-capillarity, which takes advantage of the relationship between capillarity and elasticity of a very small flat sheet of a solid material. Under certain circumstances, capillary forces can overcome the elastic flexural strength of the sheet.
This relationship can be exploited to create “hair origami” or three-dimensional structures. When a droplet of liquid is deposited on the flat sheet, the latter may spontaneously encapsulate the former due to the surface tension. Hair origami can take other forms, including creasing, warping, or automatically folding into other shapes. The specific geometric shape that the structure of 3D hair origami eventually takes is determined by both the chemistry of the flat sheet and that of the liquid, and by carefully designing the shape and size of the sheet.
There is a big problem with these small devices, however. “These conventional self-assembled origami structures cannot be completely spherical and will always have discontinuous boundaries, or what you might call ‘edges’, due to the original two-dimensional shape of the sheet,” Kwangseok Park said. , principal investigator. about the project. He added: “These edges could turn out to be future flaws with the potential for failure in the face of increased stress.” Non-spherical particles are also known to be more disadvantageous than spherical particles in terms of cellular uptake.
Professor Hyoungsoo Kim from the Department of Mechanical Engineering explained, “This is why researchers have long been looking for substances that could produce a fully spherical capillary origami structure. ”
The authors of the study demonstrated for the first time such an origami sphere. They showed how instead of a flat sheet, growing salt crystals can perform origami capillary action in a similar way. What they call “crystalline capillary origami” spontaneously builds a smooth spherical shell capsule from these same surface tension effects, but now the spontaneous encapsulation of a liquid is determined by the elasto-capillary conditions of the crystals. growing.
Here, the term “salt” refers to a compound of one positively charged ion and another negatively charged. Table salt, or sodium chloride, is just one example of salt. The researchers used four other salts: calcium propionate, sodium salicylate, calcium nitrate tetrahydrate, and sodium bicarbonate to wrap a water-oil emulsion. Normally, a salt such as sodium chloride has a cubic crystal structure, but these four salts instead form plate-like structures in the form of crystallites or “grains” (the microscopic form that forms when a crystal begins to develop). These plates then self-assemble into perfect spheres.
Using scanning electron microscopy and X-ray directional analysis, they studied the mechanism of such a formation and concluded that it was the “Laplace pressure” that caused the crystallite plates to cover the surface. of the emulsion. Laplace pressure describes the difference in pressure between the inside and outside of a curved surface caused by surface tension at the interface between the two substances, in this case between salt water and oil.
The researchers hope that these self-assembled nanostructures can be used for encapsulation applications in various sectors, from the food and cosmetics industry to the delivery of drugs and even tiny medical devices.
Kwangseok Park, Hyoungsoo Kim “Crystal hair origami capsule with self-assembled nanostructure,” Nanoscale, 13 (35), 14656-14665 (DOI: 10.1039 / d1nr02456f)
Professor Hyoungsoo Kim
Fluids and Interfaces Laboratory
Department of Mechanical Engineering
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Crystal hair origami capsule with self-assembled nanostructure
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