## 3 Electrospun (Nano)Materials Implementing Antibacterial Properties
### Inclusion of Metal Nanophases as Biocides  
Silver has long been known as an effective antibacterial agent and it has been used in various applications thanks to its chemical versatility. Copper and copper oxide, as well, possess antimicrobial properties that have been known for centuries on the macroscale [91]. With the progress of technological applications, the cost control remains a key factor when considering the scale up and the wide range of applications on a large scale. For this reason, cheaper metals (copper, zinc, iron, titanium) have increasingly become the selected metals to confer antimicrobial properties, in place of or in combination with silver. Among others, electrospun nanofiber materials have been produced incorporating metal nanostructures by different strategies and with various polymers, according to the final application. One of the widespread methods consists of a salt solution mixed with a polymer one to obtain the final electrospinning mixture (_blending_). In this conditions the metal species are stabilized in the nanoparticle form by means of the polymer itself, acting both as reductant and as capping agent. Following this approach, [PERSON] at al [92] obtained Ag-, Zn- and Cu-poly(vinylpyrrolidone) (PVP) composite mats at different copper loadings, with good antibacterial performances against _E. coli_ and _S. aureus_. The same approach has been reported using poly(vinyl alcohol) (PVA), as one of the most used and desirable material for biomedical applications. Its highly favorable properties, such as biocompatibility, nontoxicity, and bioadhesive characteristics make it a good candidate to be modified with biocidal nanophases inclusion. As the incorporation of Ag salts into easily electrospinning polymers matrices produces an even particles distribution, many papers have been published on the AgNPs/PVA system [93; 94; 95]. Typically, the polymer acts as stabilizer of the nanocomposite fibers and also has good reductant capabilities to yield Ag nanoparticles directly formed during the electrospinning process [96]. In some cases, _post-treatment_ processes are required, like adding a second reductant [97] or a combination of UV irradiation and/or heat treatment/calcination [98; 99]. This approach could be also beneficial from the application point of view. For instance, [PERSON] et al. [97] reported on the influence of AgNPs size and/or a second component on the antimicrobial effect (see Figure 3). While [PERSON] et al. [96] studied how different reducing conditions can affect the morphology of Ag-PVA nanofibers (Figure 4). Figure 4: On the (**left**): TEM of PVA nanofibers electrospun from 12 wt% PVA solutions containing 1 wt% of AgNO3: (**a**) Not refluxed, (**b**) refluxed at 80 \({}^{\circ}\)C for 1 h, and (**c**) refluxed at 80 \({}^{\circ}\)C for 2 h. On the (**right**): TEM of PVA (polyvinyl alcohol) nanofibers elect rospun from 12 wt% PVA solutions containing 1 wt% of AgNO3 after being annealed at (**a**) 50 \({}^{\circ}\)C, (**b**) 80 \({}^{\circ}\)C, and (**c**) 110 \({}^{\circ}\)C for 14 h. Reprinted from [96], Copyright 2007, with permission from Elsevier. Figure 3: The photographs of antibacterial testing of nanofibrous mats that were performed against (**a**) _E. coli_ and (**b**) _S. aureus_. Reprinted from [97], Copyright 2014, with permission from Elsevier. Electrospun Cu-PVA fibers have been produced in conjunction with CuCl\({}_{2}\)[100] or copper acetate [101] to obtain PVA-protected copper nanoparticles. Additionally, mixed metal nanostructures have been produced, like CuO-ZnO, and have been included in PVA. The combination of two metals was demonstrated to be effective as enhanced adsorption and antimicrobial efficiency were obtained with these nanocomposite fibers [102]. Along with the antimicrobial results, the authors also proposed a mechanism responsible for the bactericidal action of those nanofibers (see Figure 5).