Microfluidic Tracking of the Growth of Polymeric Vesicles in Hydrodynamic Flow

Polymeric vesicles (PVs) have proven to be a promising container for various agents because of the benefit of core-shell structures. The formation mechanism of PVs has been investigated in the hydrostatic media numerically and experimentally. However, it has been hardly reported in the hydrodynamic media of the microfluidic channel. This paper provides the visual evidence of the hydrodynamic formation mechanism of polystyrene-block-poly(ethylene glycol) vesicles. The PVs were prepared in a multiple lamination flow formed with a double flow-focusing microchannel (DFFM). To visualize the formation mechanism, the PV synthesis at each stage was characterized by scanning electron microscopy (SEM) taken along the microchannel length. Time-evolution of PV structure reveals that the formation of the PVs undergoes three distinctive morphological intermediates (micelles, disklike micelles, and semivesicles) before eventually reaching PVs, which can be tracked not only along the flow direction but also in its transverse direction. This mechanistic study for PV formation via microfluidic self-assembly provides an essential guideline for fabricating PVs with programmable morphologies that can be used in a variety of applications.