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Silver-Impregnated Thin Film Takes New Approach to Antimicrobial Activity
Proper wound management requires a delicate balancing act, according to Ankit Agarwal, a postdoctoral researcher in the department of chemical and biological engineering at the University of Wisconsin-Madison
). The use of antimicrobial silver at the wound site can help stave off infection. But the accumulation of large quantities of silver in a wound bed could actually prolong healing by killing not just bacteria, but also critical cells that facilitate healing such as fibroblasts, he says. To master this balancing act, Agarwal, along with a multidisciplinary team of researchers, has engineered a polymer-based thin film that demonstrates silver-based antibacterial activity without affecting helpful cells.
“There are new dressings called active dressings that contain large quantities of silver,” Agarwal explains. “When you put those dressings on top of a wound, silver has to diffuse from these dressings across the wound fluid onto the surface of the wound bed. To allow the mass transport of silver from the dressing to the wound bed, the dressing has to be loaded with large quantities of silver that end up in the wound bed.” This high concentration of silver that ends up in the wound bed, according to Agarwal, can cause tissue toxicity, thereby destroying fibroblasts.
To avoid tissue toxicity, Agarwal, his departmental advisor, Nicholas Abbott, and colleagues from the university’s School of Veterinary Medicine and School of Medicine and Public Health joined forces to develop a new approach that modifies only the surface of the wound bed. At the root of this technology is a nanometer-thick polyelectrolyte multilayer film composed of alternating layers of two different, oppositely charged polymers held together by electrostatic interactions.
When applied directly to the wound, the team’s technology would immobilize and localize silver on the surface of the wound bed where antimicrobial protection is needed. Because immobilization permits precise delivery and minimal loading of silver onto the wound bed, the result is effective wound treatment without cytotoxic consequences.
This technique requires a significantly smaller concentration of silver to yield such results, Agarwal notes. Through his team’s method, the amount of silver impregnated in the films can be precisely monitored by controlling the assembly conditions of the films. Thus, these films can also be integrated into existing wound-care products as antibacterial coatings on their surface.
“We have shown that films that contain per square inch just 0.4% of the silver that is found in currently available commercial wound dressings killed 99.9999% of the bacteria incubated on them, but did not damage the mammalian cells like fibroblasts that are involved in wound repair,” he says. “We have also shown that silver content in the films is very sensitive to the fibroblasts, and increasing the silver dose from 0.4 to 1% of the level used in commercial dressings severely damaged the fibroblasts.”
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