## 3 Electrospun (Nano)Materials Implementing Antibacterial Properties
Alternatively, the active material can be encapsulated inside the fiber either by blending the active agent in the polymer solution before electrospinning or by firstly encapsulating the active agent into nanostructures to be dispersed in the electrospinning solution. Additionally, precursor compounds can be blended in the solution to be electrospun, and conversion to their active form can be performed as a post-electrospinning treatment. These approaches are effective in providing a sustained release of the active agent, regulated by its diffusion through the polymer matrix, by the degradation of the matrix, or by a combination thereof. More sophisticated approaches also foresee the use of a multi-axial electrospinning setup, in order to confine the active agent in the core of the obtained fibers, enhancing the control over the achievable release profiles (see Figure 2). Many typologies of antibacterial nanofibers have been produced by the electrospinning technique, based on the incorporation of different biocides, as extensively reviewed in Reference [71], where a very comprehensive explanatory table is reported. As meaningful examples, we could focus on antibiotics incorporation (numerous hydrophilic and hydrophobic antibiotics have been used) [71] and cyclodextrins inclusion [72; 73; 74; 75] into a polymeric solution. Attention has to be taken to the drug concentration, as an excess will probably lead to changes to polymer electrospinningability, and as a consequence, on fibers morphology, due to changes to solution properties as conductivity, viscosity, and surface tension. Another relevant drawback comes from the burst release associated to the loading of high drug amounts. Indeed, as soon as fibers are submerged into an aqueous solution, antibiotics will tend to leach out rapidly. This behavior provokes a reduced functionality of the nanofiber membrane against bacteria by releasing all the antibiotics in a very short time frame without guaranteeing a prolonged coverage period. To avoid burst release, different set-ups have been used: Coaxial electrospinning technique allows to create a polymer (e.g., polycaprolactone [76] poly(lactic-co-glycolic acid) [77], poly(lactic acid) [78], poly(methyl methacrylate)/nylon [79], and poly(t-lactide-co-caprolactone) [80]) sheath that encapsulate the antibiotic (e.g., gentamycin, ampicillin, tetracycline hydrochloride) component into the core section of the nanofibers. Another common approach for a sustained release of the antibiotics is to adsorb or encapsulate the drug in a nanostructure such as hydroxyapatite nanoparticles before dispersing it in the polymer solution [81; 82; 83]. This approach does not allow a high drug payload, but still provides a sufficient antibacterial activity. Among the others, potent antimicrobial materials  
Figure 2: Various methods of incorporating biocides into electrospun nanofibers. (**1**) blending/ dispersion of the active agent in the polymer solution prior to electrospinningning; (**2**) confinement of the active agent in the core of the fiber through co-axial electrospinning; (**3**) encapsulation/adsorption of the active agent in nanostructures before dispersion in the electrospinning solution; (**4**) conversion of a precursor to active agent in the nanofibers after electrospinningning; and (**5**) attachment of the active agent onto the nanofibers after electrospinningning. Reprinted from Reference [71], Copyright 2014, with permission from [PERSON] and sons. that have been investigated in conjunction with polymers are the quaternary ammonium salts (QACs) and N-substituted halamines [84]. The strategy of preparing antimicrobial polymers based on QACs is very attractive, because tertiary or/and secondary amino groups provide convenient reactive moieties to be properly modified. One of the approaches is to permanently attach the active QACs agents by means of coupling agents, such as alkoxysilanes, to a variety of different substrates through covalent interactions [85; 86; 87; 88].