Core-sheath electrospun nanofibrous systems for controlled gene delivery. (A) Two representative methods to form core-sheath electrospun nanofibers: coaxial (left) and emulsion (right) electrospinning. (B) Transmission electron microscopy (TEM) image of an individual core-sheath nanofiber fabricated using coaxial electrospinning. Core and sheath are composed of viruses dispersed in Minimal Essential Medium and PCL, respectively. Scale bar is 2 μm. Reprinted from , Copyright 2009, with permission from Elsevier. (C) A scheme depicting gene vector encapsulation within the core layer for controlled release. The core-sheath fibrous formulations contribute (D) to preventing the direct contact of gene vectors in the core layer with organic solvents in the sheath layer, (E) to regulating delivery modes by producing porous sheath layers, and (F) to enhancing delivery efficiencies by modifying the sheath layers with polycationic polymers.