Evolution of core-shell structure: From emulsions to ultrafine emulsion electrospun fibers

Due to the relative ease for fabricating fibers with a core-shell structure, emulsion electrospinning has been investigated intensively for making nanofibrous delivery vehicles for local and sustained release of biomolecules such as growth factors. In this investigation, water-in-oil (w/o) emulsions consisting of deionized (DI) water or phosphate buffer saline (PBS), poly(lactic-co-glycolic acid) (PLGA) solution and a surfactant were electrospun into fibers for studying the evolution of core-shell structures in emulsion electrospun fibers. Optical microscopy, high speed imaging, fluorescence microscopy and transmission electron microscopy were employed to examine the morphology of the water phase in fibers at different locations of the jet (or fiber) trajectory during emulsion electrospinning. The results indicated that the water phase in emulsion jets underwent multi-level stretching and breakup. When the jet (or fiber) diameter was large (i.e., for the jet near the Taylor cone), the water phase within the jet (or fiber) exhibited a discrete morphology whereas in ultrafine fibers collected beyond certain distance from the Taylor cone, a mostly continuous water phase core was formed. The Rayleigh/capillary instability determined the breakup point of the deformed water phase. When the water phase in emulsions for electrospinning was changed from DI water to PBS, it changed from a continuous state to a discontinuous state in electrospun nanofibers.