Designing osmotically-inflated microcapsules with ultra-thin shell for mechanical-stress-triggered-release

Abstract

Microencapsulation technology has enabled the stable storage and controlled release of active materials for various purpose. For example, oppositely charged two different pigments are encapsulated to serve as electronic inks for reflection-mode display. Ideally, these microcapsules are impermeable to encapsulants during storage and when used. In particular, capsules that ruptures or breaks under certain mechanical stress has drawn attention for cosmetics application, especially for topical formulation of cosmetic active materials. Furthermore, submillimeter-sized capsules with ultra-thin membrane are highly demanding as they can show high visibility before release yet no tactile sensation after release. In addition, by control over their intrinsic mechanical property, capsules can be tuned for new kind of release. By droplet microfluidics, monodisperse microcapsules with controlled size, membrane properties, and membrane thickness can be produced. However, directly produced microcapsules have a low size upper limit and low frequency generation. Herein, two types of submillimeter-sized microcapsules with distinct mechanical property are produced: plastic microcapsules and elastomeric microcapsules. On one hand, submillimeter-sized biodegradable plastic microcapsules are produced by osmotic swelling of a water-in-oil-in-water (W/O/W) double-emulsion drops in a strong hypotonic condition and subsequently consolidated by solvent evaporation. The double-emulsion drops are incubated in aqueous solutions with various osmolarities to have a strong hypotonic condition, where an oil layer is introduced at the interface between air and the incubation solution to prevent the rupture of double-emulsion drops and the evaporation of solvents. The resulting microcapsules can be swollen up to nine times their original volume. On the other hand, the elastomeric microcapsules were produced by fully curing the double-emulsion drops containing high osmolarity core and subsequently osmotic swelling of the cured microcapsules in a strong hypotonic condition. The microcapsules were incubated in aqueous solution with various osmolarities to have a strong hypotonic condition. The elastic microcapsules store elastic energy due to osmotic inflation and thus allow fast core release, such as in popping balloons. Both swollen microcapsules are perceivable with sight and touch when they are intact, but lose both sensations under mechanical stress. To characterize the unique mechanical property, compression test was done to show that as osmotic pressure difference increases, the stiffness of capsule decreases and larger capsules with thinner shell ruptures at lower strain.