**4. Discussion**

This study focused on the development of antimicrobial fibrous networks consisting of PET materials through the electrospinning technique. With the various applications of electrospun networks, the addition of silver nanoparticles onto the surface of the fibers could potentially be implemented for antimicrobial purposes.

The mechanism of toxicity could be associated with the surface oxidation of the silver nanoparticles and the subsequent release of silver ions, or with the generation of reactive oxygen species and the consequent destabilization of the bacterial membrane [51]. Therefore, an advantage of this study is represented by the formation of the silver nanoparticles on the surface, compared to their direct encapsulation, which could considerably reduce their release. While the material characterization through the GIXRD and FT-IR techniques confirmed the formation of silver nanoparticles on the surface of the PET fibers, a tendency of nanoparticle agglomeration at the nodes was observed in the SEM images. Similar results were previously reported in the literature, either by grafting the nanoparticles on the plasma-treated PET [52] or by using the reduction method [53], and it was stated these clusters could be responsible for a slightly reduced antibacterial property [35,54]. In this regard, there are several solutions reported for enhancing the homogeneous distribution of the silver nanoparticles onto the surface of the fibers, such as sonochemical coating [55], plasma treatment of the silver nitrate-containing polymer solution [56], or chemically reducing the silver nitrate in the polymer matrix prior to the fiber fabrication. Moreover, it was previously reported in the literature

that concentrations of silver nanoparticles as low as 0.05% can considerably reduce the incidence of surgery-related infections [57], with demonstrated bacteriostatic effects [58]. Studies reported that the effects of silver nanoparticles are strongly related to the size of the nanoparticles, with 10 nm being related to both the formation of silver ions during dissolution and the penetration of nanoparticles through the bacterial wall [59].

Furthermore, green synthesis methods could also be applied for the production of silver nanoparticles, thus avoiding the use of potentially toxic reducing agents. Examples of such agents can be found in the literature, most commonly involving chitosan [60] and other polysaccharides, plant extracts [61–63], or microbial extracts [64], all exhibiting enhanced antibacterial properties. In the present study, the investigated biomaterials proved to be efficient against the three types of bacteria, both in the planktonic and attached states, thus confirming their potential in anti-infective applications.

Despite their wide use as antimicrobial agents, silver nanoparticles are often associated with cytotoxic effects on the normal cells and have a tendency to accumulate in vital organs, such as the spleen, liver, kidney, and brain. Therefore, they must be considered "double-edged swords" for biomedical applications [65]. In this respect, silver nanoparticles are usually embedded in a composite matrix that has a significant role in controlling the release of the silver ions. In this study, the formation of silver nanoparticles on the surface of the PET biomaterials proved to reduce the cytotoxic effects of the uncoated materials determined by the MTT assay. Similarly, the in vivo biocompatibility tests demonstrated the anti-inflammatory potential of the NanoAg samples as compared to the PET samples, as fewer inflammatory cells were present at the implantation site in the former case. It could be hypothesized that the composite material could delay the release of the silver ions and, subsequently, the release of the pro-inflammatory factors, thus improving the potential for the long-term use of these biomaterials. This is in accordance with previous studies, which demonstrated foreign body reactions of PET materials, including commercial sutures for orthopedic purposes [66]. Additionally, there are studies suggesting the influence of the fiber size, mesh porosity, or contact surface, with one group reporting a minimum foreign body reaction of a PET mesh compared to the highly biocompatible bulk PET material [67].

This study could, therefore, represent a starting point for the development of antimicrobial membranes, which could be used as wound dressings or biomedical coatings, as they exhibit good antimicrobial properties and no inflammatory or cytotoxic effects.
