## 3 Electrospun (Nano)Materials Implementing Antibacterial Properties  
The advantage of using antimicrobial polymers to fight the increasing resistance against classical antibiotics arises from the possibility of synergistically combining these materials and applying them in different physical forms (e.g., coatings, films, emulsions and fibers). Polymer fibers produced via electrospinning are among the most efficient and cost effective systems for two main reasons: In the first place, because of their huge surface area-to-volume ratio; and secondly, the polymers can be implemented with antimicrobial functionalities before or after electrospinning. This provides the opportunity to design and modify the fibers in the smartest way to specifically target different types of microorganisms. Antimicrobial polymers can be classified into two categories. The first one includes all those polymers with inherent antimicrobial or antiseptic properties, that can be directly applied or used in a variety of substrates, namely textiles, metal materials or plastics ones. The second category contains inert polymers incorporating additional antimicrobial phases/agents that are released over time. The first class comprises natural and synthetic polymers, including polypeptides. Antimicrobial peptides are naturally associated to the defense system of microorganisms and humans [63], and are generally cationic species with an overall net positive charge. Based on those structures, synthetic amphiphilic polymers have been designed to mimic the antimicrobial peptides [64; 65]. While many antimicrobial peptides have been described, their large-scale production is expensive, their in vivo stability is generally poor and relatively little investigated [66; 67]. Electrospun nanofibrous membranes of natural polymers have recently drawn much attention as novel materials in biomedical applications, such as wound dressing, tissue engineering, and drug delivery. While the use of bio-based materials as precursors for electrospinning seemed to be an excellent solution to address all the raised concerns, many difficulties arose on the actual practical side. In fact, natural polymers are usually difficult to be electrospun, therefore the starting solution is frequently composed by a synthetic polymer in blend with the natural one. Another possible approach is to chemically modify the natural polymer, chitosan processing represents a classic example of this approach. Chitosan is an inherently antimicrobial natural polymer being highly active against both gram-positive and gram-negative bacteria. Thus, it would be an excellent candidate for the production of antimicrobial nanofibers [68]. Anyhow, pure chitosan cannot be merely electrospun into nanofibers, and this lead to the need of blending chitosan into the starting solution with other polymers (e.g., polyethylene oxide, polyvinyl alcohol, polycaprolactone) even though this dilution could lead to lowering the biocidal properties of the as made nanofibrous membranes. An interesting overview of different natural electrospun nanofibers for antibacterial uses, including a useful list of pure polymers and their blends successfully processed by electrospinning, is given by [PERSON] et al. and [PERSON] et al. [69; 70]. The second class of antimicrobial nanofibers is by far the most investigated, since different active agents as well as different strategies have been explored. Most commonly, biocides (e.g., antibiotics, ammonium salts, cyclodext, chlorhexidine, metal, or metal oxide nanoparticles, etc.) are attached to or encapsulated into supporting fibers, by means of different preparation approaches. The most straightforward one consists in anchoring the active agent onto the surface of pre-synthesized fibers. This approach, however, provides little control over the release rate, that is usually characterized by a burst.