Silver has long been used for its antibacterial properties on burns, diabetes wounds, decubitus ulcers and other serious wounds in danger of infection. But there is one major side effect: the same silver can kill fibroblasts that are necessary for wound repair. Now Ankit Agarwal, a postdoc at the University of Wisconsin-Madison, has developed a very thin material impregnated with a small and precise amount of silver. So far in lab tests, this silver concentration seems to have the necessary antibacterial properties while preserving the healthy cells that infiltrate the wound site.
Agarwal builds the experimental material from polyelectrolyte multilayers — a sandwich of ultra-thin polymers that adhere through electrical attraction. To make the sandwich, Agarwal alternately dips a glass plate in two solutions of oppositely charged polymers and finally adds a precise dose of silver.
“This architecture is very easily tuned to different applications,” Agarwal says, because it allows exact control of such factors as thickness, porosity and silver content. The final sandwich may range from a few nanometers to several hundred nanometers in thickness.
Although both mammalian cells and bacteria are sensitive to silver, bacteria are much more sensitive, leaving a sweet spot — a concentration of silver that can kill bacteria without harming cells needed for healing.
In tests using mouse cells and sample bacteria, Agarwal has tuned the dose to find the sweet spot where the silver bullet destroys 99.9999 percent of the bacteria, but does not harm fibroblasts.
Indeed, the system is so sensitive that increasing the silver dose from 0.4 percent to 1 percent of the level used in a commercial dressing severely damaged the fibroblasts.
To kill bacteria, silver must take the form of charged particles, or ions, and the tiny silver nanoparticles that Agarwal embeds in the sandwich can be designed to release ions for days or weeks as needed. In contrast, Agarwal says, commercial wound dressings contain a large dose of silver ions, which are released faster and with less control.
Press release: New approach to wound healing may be easy on skin, but hard on bacteria …
Images: Top: Healthy fibroblast cells (green) in a low dose of silver Bottom: Cells die (red) in a slightly higher dose of silver.