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Nanomaterials 2021, 11, 96 2 of 19 The properties of silver as a disinfectant have been known for centuries [3–5] but the wider use of silver as an antibiotic has only come about in recent decades, largely because of increasing concerns about the rise of infections caused by pathogenic bacteria that have, over time, become more resistant to commonly used antibiotics. In Western countries topical silver sulfadiazine creams, along with silver-based wound dressings such as ActicoatTM (Smith and Nephew) and Aquacel Ag® (Convatec), are often used for treatment of long-term skin wounds. As confirmed by Khundkar et al. [6], ActicoatTM is now widely used, although the clinical data for its use are not strong. However, it is generally accepted that this product provides an improved antibiotic effect when compared to other silver-containing dressings. Two versions of this product are available as ActicoatTM 7 and ActicoatTM Moisture Control, both of which contain SilcrystTM silver nanocrystals [5]. The efficacy of Aquacel Ag®, based on sodium carboxymethyl cellulose fibers integrated with ionic silver, which forms a gel on wound contact, has been demonstrated through both in vitro and in vivo studies [7]. In terms of their practical use, these expensive wound dressings are generally changed every three days, but ActicoatTM may be left on a wound area for as long as a week. This paper describes research on the development of an inexpensive silver-based hydrogel dressing material that might provide a moist comfortable covering and provide some level of antibacterial protection from the silver component. Poly(vinyl alcohol) (PVA) is a relatively inexpensive, biocompatible polymer that has been extensively studied for a wide range of medical applications and is used in commercial medical devices [8,9]. Due to solubility issues, for extended use, a cross-linked PVA gel would be preferable. Although there are reports in the literature on the use of borates and heat to cross-link PVA in a non-toxic manner [10,11], these methods do not provide long-term resistance to aqueous solubilization. A cross-linked water insoluble, yet hydrated, PVA film was considered a suitable formulation since its use could allow controlled release of an incorporated antibiotic over time rather than the immediate release of all available antibiotic that would likely occur from non-crosslinked PVA. We therefore pursued the idea of using silver both as an antibiotic and also as a hydrothermal cross- linking agent for PVA. Although their research was not aimed at a medical application, Luo et al. [11] demonstrated that silver-cross-linked PVA nanocables could be produced by a simple solution approach involving autoclaving at 160 ◦C for up to 72 h. We explored a similar approach that used lower temperatures and shorter times to facilitate both silver cross-linking of PVA films and in situ synthesis of silver nanoparticles. The antibiotic properties of silver nanoparticles have been widely reported [12–14] with recent evidence indicating that their antimicrobial efficiency is probably related to the slow release of silver ions rather than an intrinsic property of the nanoparticles per se [15]. It therefore seemed feasible that using cross-linked PVA films containing silver nanoparticles created in situ might meet the material needs of the burn wound dressing project. In recent years, a number of authors have reported methods for the preparation of PVA-silver nanocomposite films and have characterized the antibacterial and other properties of these films. Cencetti et al. [16] generated such films using a mixture of PVA with gellan, borax and silver nitrate in a hydroalcoholic formulation. The films showed sustained silver release and slow dehydration rates as well as antibacterial activity against Staphylococcus aureus and Pseudomonas aeruginosa. Another approach was adopted by Herron et al. [17] in which PVA-polyelectrolyte multilayer (PEM) films were formed by a layer-by-layer (LBL) technique to provide silver nanoparticle containing films with good antibacterial effects at low silver loadings in wound models. Oliveira et al. [18] specifically designed burn wound dressings by forming PVA-silver nanocomposite films in which silver nanoparticles were created by exposure to Co60 γ-radiation. The antibacterial activity of the films was confirmed by a disc diffusion method. Mahmoud [19] used sodium borohydride reduction of silver nitrate to create silver colloids that were then combined with PVA to form antibacterial nanocomposite films. Bhowmick and Koul [20] assessed the activity of antimicrobial PVA-silver nanocomposite hydrogel films as wound dressing scaffolds. The films were formed using a freeze-thaw technique and showed sustainedPDF Image | Hydrogel Forming Dressings Containing Silver Nanoparticles
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