## 1 Electrospinning: Generalities  
Electrospinning is an easy fabrication method in which electrostatic forces are used to produce fine fibers. For its versatility, simplicity, and low cost, electrospinning has attracted the interest of many researchers fascinated by the possibility to fabricate fibers with both micro and nano-sized structural features by finely tuning process parameters. For these reasons, electrospinning has been extensively reviewed with respect to its setup, applications, advantages, and prospective developments [1, 2]. Even though the origin of electrospinning can be traced back to the first years of the twentieth century [1, 3, 4, 5, 6, 7], the term \"electrospinning\" is rather recent. In fact, it was used for the first time in 1995 by [PERSON] and coworkers in two different scientific articles [8, 9]. The electrospinning fabrication process can be briefly summarized as follows. A high voltage (generally in the order of a few tens of kilovolts) is applied between an extrusion system (i.e., a needle or spinneret) and a collector. A polymeric solution is fed at a set flow rate to the spinneret where, under the force exerted b the electric field, it starts elongating forming a characteristic cone-shaped tip (Taylor's cone) [10, 11]. When surface tension is overcome by electrostatic forces, a jet of liquid is ejected from the Taylor cone, and reaches the collector in the form of a solid nonwoven web, with solvent evaporating during flight [12]. Lateral instabilities in the charged travelling jet result in a whipping motion that further contributes to fiber elongation [13] (see Figure 1). A thorough analysis of the physical phenomena behind the electrospinning process and a detailed description of the main experimental setups can be found in Reference [14]. The electrospinning process is affected by three classes of parameters, including solution properties (e.g., concentration, viscosity, surface tension, conductivity, and solvent vapor pressure), process parameters (e.g., solution feed rate, applied voltage, nozzle-to-collector distance), and ambient parameters (e.g., humidity and temperature). [PERSON] et al. have demonstrated the feasibility of applying statistical methods to gain a quantitative and systematic insight about the effects of process parameters on the resulting fiber properties [15; 16]. Electrospinning has been the subject of many different setup modifications that have been implemented over the years to accommodate different application needs. In fact, besides the more conventional randomly oriented fiber mats, a variety of morphologies and structures can be achieved by purposely changing the electrospinning conditions and by modifying the setup (e.g., the spinneret and the collector, among others) [17]. Mat morphology (i.e., fiber alignment and pattern) can be influenced by the collector geometry. In order to produce a randomly oriented non-woven mat, a static planar target is the conventional choice, whereas to obtain aligned fibers, collection is performed around a rotating cylindrical target [18; 19]. In different designs, a series of parallel electrodes have been used to generate aligned fibers [20; 21]. For more specific applications, collectors have been designed with customized geometries to obtain specific orientations or patterns, such as a knife-edge rotating disk to collect highly aligned fibers [22], or the use of grid-shaped collectors to create patterned nanofibrous mats [23]. The spinneret was also the subject of various modifications targeting specific applications. One of the major advances in electrospinning is represented by the introduction of the coaxial configuration in 2003 [24], enabling the production of core-shell microarchitectures with compartmented properties. Further incremental improvements (i.e., triaxial spinneret) have led to the fabrication of multilayered fiber structures [25] with the possibility to independently tune thickness, degradation rate, and diffusion properties of each layer for advanced drug delivery applications.