| Summary: | Capsules are employed in a range of industries as essential vehicles for the storage and delivery of drugs, biologically active species, surfactants, and personal care formulations. Key performance aspects are dictated by the capsule's overall shape and dimensions, porosity and internal microstructure. Various approaches are employed in the fabrication of capsules with precise size and shape control. This work focuses on the formation of capsules by the selective removal of solvent from monodisperse solution droplets - produced using microfluidics - upon immersion in an external solvent. The microfluidic generation of microporous polymer capsules using an ex-situ phase inversion process is presented and the effect of various process parameters on capsule structure and internal microstructure examined. These include polymer concentration, droplet size and non-solvent quality. Further, the effect of ternary solution thermodynamics on the microstructure of the capsules is explored, and precise control over pore size and distribution is demonstrated. Building on these findings and polymer-colloid phase behaviour, the fabrication of nanocomposite capsules, generated from polymer-nanoparticle mixtures, is reported and a well-defined external and internal morphology diagram established. These include nucleated and bicontinuous microstructures, as well as isotropic and non-isotropic external shapes. Upon dissolution, rapid and modulated, pulsed, release of the nanoparticle clusters over timescales ranging seconds to hours is demonstrated. The release profile is found to be dependent on capsule morphology, and a systematic study of the role of extraction solvent and kinetics on capsule gradient structure is presented; this provides an effective strategy to decouple demixing and coarsening timescales from the capsule solidification time, and is exploited to controllably design capsules with varied internal microstructures: hollow, core-shell, bicontinuous and compact domains. Overall, the work presents a robust and facile microfluidic approach for the design and fabrication of microcapsules, exhibiting a wide range of internal and external morphologies, by exploiting solution/mixture thermodynamics, solvent exchange kinetics, and phase inversion.
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