**2. General process of electrospinning**

The basic setup for electrospinning consists of three major components: a high voltage power supply, a spinneret, and a collector (a counter electrode) (Fig. 1). In the process of electrospin‐ ning, the applied voltage causes a cone-shaped deformation of the drop of polymer solution (Taylor cone). Once the strength of electric field exceeds a threshold value, the electrostatic force on the deformed polymer drop can overcome the surface tension and thus a liquid jet is formed. This electrified jet then moves toward counter electrode, leading to the formation of a long and thin thread. As the liquid jet is continuously elongated and the solvent is evaporated, solid fibers with diameters as small as tens nanometers are deposited on the colletor.

**Figure 1.** Schematic illustration of the basic setup of electrospinning.

In spite of the simple setup, there are a number of parameters that can greatly affect the morphology and diameter of electrospun fibers, including: (1) the intrinsic properties of solution such as the type of polymer, concentration, conductivity, and solvent volatility; and (2) the processing parameters such as the strength of the applied electric field, solution flow rate, and the distance between spinneret and collector [2, 10, 11]. In addition, the humidity and temperature of the surroundings may also play an important role in determining the mor‐ phology and diameter of electrospun fibers. Numerous experimental investigations and theoretical models have drawn some general relationships between these parameters and fiber morphology. For example, the higher applied voltage will lead to a larger fiber diameter, but this trend is not monotonic; the higher polymer concentration (higher viscosity) or faster flow rate usually results in the larger nanofiber diameters. In contrast, the increase of solution conductivity can significantly reduce the fiber diameter. These results are instructive to some extent in experiment design and predicting the resultant fiber morphology. However, empirical knowledge is crucial because the ideal values of these parameters vary considerably with the polymer/solvent system.
