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Nanomaterials 2021, 11, 96 3 of 19 release of silver for 96 h. Zhang et al. [21] investigated the electrospinning of PVA in the presence of silver nitrate to create a PVA-silver nanowire composite with antibacterial properties against. against E. coli and S. aureus. This paper outlines the novel, yet simple, inexpensive preparation of silver cross- linked PVA films that might provide a long-lasting protective hydrogel with a sustained release of anti-infective silver nanoparticles. The work characterizes PVA degradation, silver nanoparticle production, silver release, antibacterial activity and discusses how these films might be incorporated in a final dressing product for a burn survival kit intended for remote African villages. 2. Materials Poly(vinyl alcohol) (88 mole% hydrolyzed, Mw ~125,000) was obtained from Poly- sciences, Inc (Warrington, PA, USA). Silver nitrate (>99.0%) was purchased from Sigma- Aldrich (St. Louis, MO, USA). Silver nanoparticle suspensions (20, 40 and 60 nm particle sizes) at a concentration of 0.02 mg/mL were also obtained from Sigma-Aldrich. All chemi- cals were used as supplied and without further purification. Deionized water was used in the preparation of all experimental PVA-silver formulations. Antibacterial tests involved use of E. coli C1214, Staphylococcus Aureus. Newman and Pseudomonas Aeruginosa. Luria Bertani (LB) broth was obtained from Fisher Scientific. The S. aureus and P. aeruginosa bacterial species carry a reporter plasmid containing the lux gene, which is only expressed in viable bacteria, allowing for the quantification of bacteria via luminescence measurements. 3. Methods 3.1. Film Preparation A stock solution of PVA was prepared as a 10% w/w gel by slowly adding 20 g PVA powder to 180 mL of rapidly stirred deionized water, heating to 85–90 ◦C and continuing with magnetic stirring and heating for approximately two hours. When a clear gel had formed the vessel was removed from heating and stored at room temperature in the laboratory. The PVA solution was diluted four-fold to 2.5% prior to use by adding 75 mL of water to 25 mL of the 10% PVA solution. A stock silver nitrate solution was then prepared in deionized water at a concentration of 10 mg/mL and stored covered with aluminum foil in a dark cupboard until required. Solutions of PVA and silver nitrate were then mixed in defined volumes to give wt ratios (PVA:Silver nitrate)ranging from 99%:1% to 95%:5% in the dried film form. Films were manufactured using 10 or 5 mL of these solutions to allow 200 μm (10 mL) or 100 μm (5 mL) films to be cast in 60 × 15 mm disposable polystyrene Petri dishes (Sarstedt Inc., Montreal, PQ, Canada). For example: to manufacture 200 μm films, either 250 or 1250 μL of the 10% silver nitrate solution was added to 10 mL of the 2.5% PVA solution to give final PVA:silver ratios of 99%:1% or 95%:5% respectively. Film thickness was measured using an electronic micrometer. Formulations chosen on the basis of an initial screening contained 1%, 3% or 5% w/w silver nitrate based on PVA content. The PVA-silver nitrate solutions in Petri dishes were loosely covered with aluminum foil and left overnight under ambient conditions in order for water to evaporate. As a next step, the Petri dishes containing the cast films were placed in an oven set at 65 ◦C for a further overnight period. Finally, the dried films were removed from the Petri dishes, placed on aluminum foil and heat cured in a second oven set at temperatures ranging from 80 to 140 ◦C for 90 min. For some experiments, films were heat cured for different times at 140 ◦C for 15, 30, 45 or 90 min. The films dried at 65 ◦C or the dried films which had then been heat cured were stored in a dark cupboard before evaluation. The final heat curing process distorted the films somewhat but it is anticipated that if the films were produced commercially, they would not be removed from the casting chamber until all heat curing was complete.PDF Image | Hydrogel Forming Dressings Containing Silver Nanoparticles
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