Capsules with submicron shells: production, stability, and leakage behavior

Capsules are often employed to prolong the shelf-life of active ingredients such as many drugs, food additives, or cosmetic substances because they delay their oxidation or prevent interactions with molecules contained in the surrounding. If designed properly, these capsules allow close control over the timing and location of the release of active ingredients. To take advantage of these features, capsules must possess shells whose thickness and composition are well-defined. A promising route to fabricate such capsules is the use of microfluidic devices to produce double emulsion drops that are used as templates to form the capsules. Additionnally, those double emulsions are also used as vehicles for active materials. However, current microfluidic techniques typically produce double emulsions with inhomogeneous shells, thereby negatively affecting the release kinetics of the encapsulants. A solution to this problem is to produce double emulsions with extremely thin shells to reduce their inhomogeneity. However a controllable production remains a challenge. This manuscript provides a novel strategy to controllably produce double emulsions that can be used as template to form capsules with submicron shell thicknesses. We introduce new microfluidics techniques to controllably reduce the shell thickness of double emulsions to values below a micrometer. We present a simple process that squeezes primary double emulsions through a constriction, thereby removing the vast majority of the oil initially contained in the shell. This technique allows the controllable production of double emulsions with shells as thin as 330 nm. To increase the throughput of the production of double emulsions with thin shells, we developed a second microfluidic device, the aspiration device. We show that we can produce shell thicknesses down to 240 nm with a 10 times higher throughput than obtained with the squeezing process. We also demonstrate that the resulting double emulsion shell thickness only depends on the fluid flow rates but not on the shell thickness of injected primary double emulsions such that this device enables processing polydisperse double emulsion drops into double emulsions with well defined thin shells. We characterize the permeability of double emulsions drops and show that the release rate of encapsulants decreases with decreasing shell thickness We demonstrate that the permeability of drops with submicron shells is decreased by at least one order of magnitude compared to that of primary double emulsions. Thus these thin shell double emulsions open up new opportunities to use them for high throughput screening assays that require a high precision. Finally we convert double emulsions into capsules with thin homogeneous solid shells and show that they display a low permeability and a high mechanical stability.