Exposure to pathogenic organisms, and the potential for subsequent infectious disease acquisition, concerns everyone on a daily basis. The ever-increasing incidence of antibiotic resistance has resulted in renewed interest in products containing silver (Ag), and its innate antimicrobial activity is drastically increased at the nanoscale. Accordingly, Ag-nps have been incorporated into hundreds of personal and professional products ranging from surgical and food handling tools, water purifiers, textiles, cosmetics, contact lens cases, wound dressings, implantable devices and catheters, children’s toys, and most recently engineered scaffolds designed for tissue regeneration. Since skin is in constant, direct contact with the environment, it is often the site of unintentional or deliberate exposure to surfaces and products containing Ag-nps.

In this report, Ag-nps of various sizes, surface conditions, and synthesis methods were evaluated for their cytotoxic and inflammatory potential to skin cells, human epidermal keratinocytes (HEK), and their penetrating capacity into porcine skin dosed daily for 14d in vivo. Ag-nps were not toxic to HEK at concentrations up to 1024µg/ml, but residual solution contaminants present in freshly synthesized Ag-nps solutions were highly toxic to HEK. This revealed the importance of Ag-nps solution characterization and purity prior to their use. Ag-nps caused focal intercellular and intracellular edema to porcine skin, and were localized within intracytoplasmic vacuoles of HEK. Electron energy loss spectroscopy was used to examine the surface transformation of Ag-nps following HEK internalization. It was revealed that initially purely metallic Ag-nps acquired up to 8 oxide monolayers on their surface as a result of cellular internalization. The antibacterial efficacy of the Ag-nps was tested against a range of Gram-negative, Gram-positive, and antibiotic resistant bacterial strains. Ag-nps were capable of inhibiting the growth of all strains starting at 64µg/ml.

Elastomeric and bioresorbable skin scaffolds were successfully electrospun with a 50:​50 poly(L-lactide-co-epsilon-caprolactone) (PLCL) copolymer incorporated with 20nm Ag-nps. The scaffolds supported the confluent growth of HEK for up to 14d of culture. Lastly, the 10nm and 20nm Ag-nps were evaluated for their potential cellular uptake, toxicity, and affect on the differentiation of human adipose derived stem cells (hASC). Exposure of hASC to Ag-nps resulted in no significant cytotoxicity to hASC, and minimal dose-dependent toxicity to adipogenic and osteogenic cells at antimicrobial concentrations. Each of the hASC, adipogenic and osteogenic cells showed cellular uptake Ag-nps, without causing significant ultrastructural alterations, and exposure did not influence the intended differentiation of the cells. Therefore, the general biocompatibility and antibacterial efficacy of Ag-nps establishes their suitability for incorporation into tissue engineered graft scaffolds.