Rational design and direct fabrication of multi-walled hollow electrospun fibers with controllable structure and surface properties

Multi-walled hollow fibers with a novel architecture are fabricated through utilizing a direct, one-step tri-axial electrospinning process with a manufacturing methodology which does not require any post-treatments for the removal of core material for creating hollowness in the fiber structure. The hydrophilicity of both inner and outer layers' solution needs to be dissimilar and carefully controlled for creating a two-walled/layered hollow fiber structure with a sharp interface. To this end, Hansen solubility parameters are used as an index of layer solution affinity hence allowing for control of diffusion across the layers and the surface porosity whereby an ideal multi-walled hollow electrospun fiber is shown to be producible by tri-axial electrospinning process. Multi-walled hollow electrospun fibers with different inner and outer diameters and different surface morphology are successfully produced by using dissimilar material combinations for inner and outer layers (i.e., hydrophobic polymers as outer layer and hydrophilic polymer as inner layer). Upon using different material combinations for inner and outer layers, it is shown that one may control both the outer and inner diameters of the fiber. The inner layer not only acts as a barrier and thus provides an ease in the encapsulation of functional core materials of interest with different viscosities but also adds stiffness to the fiber. The structure and the surface morphology of fibers are controlled by changing applied voltage, polymer types, polymer concentration, and the evaporation rate of solvents. It is demonstrated that if the vapor pressure of the solvent for a given outer layer polymer is low, the fiber diameter decreases down to 100 nm whereas solvents with higher vapor pressure result in fibers with the outer diameter of up to 1 μm. The influence of electric field strength on the shape of Taylor cone is also monitored during the production process and the manufactured fibers are structurally investigated by relevant surface characterization techniques.