Microfluidic Emulsion Templating, Directional Solidification and Controlled Release

Microcapsules and particles offer efficient means of carrying and delivering active ingredients, including medical drugs and cosmetics, with precise spatio-temporal release profiles. Mechanistic understanding of the encapsulation, solidification and release processes is crucial in designing future microcapsules, combining thermodynamics, mass transfer and non-equilibrium phenomena, including kinetic arrest or phase changes. Emulsion droplets, generated within microfluidic devices, which can form a barrier between the cargo and external environment can be used to template microcapsules from, by directional solidification processes, and are being investigated to tackle this challenge.
Rapidly prototyped microfluidic devices, produced by frontal photopolymerization of thiol-ene resists, with flow-focussing geometries, and 200-600 micrometers dimensions, were used to generate single and double emulsion droplets from several polymeric, amphiphilic and hydrophobic species. Directional solidification processes, e.g. solvent extraction and ionic crosslinking, were exploited to form solid capsules from the droplets. The effects of varying process parameters, such as multivalent salt concentration or non-solvent quality, as well as the materials used on the solidification process and particle/capsule morphology was observed primarily by optical microscopy. Hydrogel particles (100-500 micrometers) were formed from single emulsion droplets of sodium carboxy methylcellulose (NaCMC) and solidified in multivalent salts. Gel particle formation was thought to be a result of aggregation of polymer chains; dictated by the interplay of shielding, ion bridging and charge inversion occurring upon salt addition. Solid polymer particles were also produced by a solvent extraction process; immersion of aqueous poly (vinyl alcohol) droplets, from a microfluidic single emulsion generator, into a non-solvent bath. Polymer concentration and non-solvent quality affected the morphology of the resultant particles and the kinetics of the directional solidification process. This versatility of this approach was shown by also solidifying aqueous NaCMC droplets into solid particles.
Further understanding of these directional solidification processes, their response to changes in molecular structure, processing parameters and flow, will enable them to be applied to encapsulation. Double emulsion droplets with an inner core of hydrophobic material, can be solidified by these methods to produce solid shells. Their potential application in a wide range of industries, from pharmaceuticals to personal care, will be demonstrated by studying their controlled release properties.