"[Our work] demonstrates the potential of making template polymer networks for metal nanoparticles via readily available and relatively cheap starting materials, and might have applications in coatings for medical devices to keep them sterile," Joerg Tiller told nanotechweb.org. "There is also the potential for catalytic or optical applications."

The scientists based their material on the fact that it's possible to form silver nanoparticles within amphiphilically modified poly(ethylene imine) (PEI) derivatives in solution.

"As found in previous work, the approach of coating with silver nanoparticles instead of plain silver was very promising," said Tiller. "However, adhesion of bacteria dramatically changes the microbes' defence system, making them less vulnerable. So we decided to add an additional modification to the coatings: the microbe-repelling polymer poly(ethylene glycol) (PEG)."

To make the material the team copolymerized PEI, derivatized with double bonds, with 2-hydroxyethyl acrylate (HEA). The result was a polymer network containing nanoseparated PEI phases. The material can bind silver ions within these PEI phases as the silver links to the PEI's nitrogen groups. The HEA component, meanwhile, contained a hydroxyl group that can covalently attach to PEG.

"Most significant about our findings is that the silver nanoparticles can be formed within a coating simply by immersing it in aqueous silver nitrate solution, followed by treatment with aqueous ascorbic-acid solution," said Tiller. "Furthermore, the nanoparticle content can be perfectly controlled by the composition of the network."

The team tested the anti-microbial properties of the films by trying to grow Staphylococcus aureus bacteria on them. These bacteria are a major source of nosocomial infections - diseases acquired in hospital. After 12 hours of incubation at 37° C, the silver-loaded polymer networks did not support any bacterial colonies.

In contrast, colonies were visible after 12 hours on polymer networks that didn't contain silver nanoparticles, both on a PEG-modified and an unmodified version. The PEG-modified network supported four to five times fewer colonies than its sister material without PEG. The scientists say that this demonstrates the microbe-repelling properties of the "PEGylated" network.

Silver-loaded polymer networks did support bacterial colonies after longer periods, however. This indicates that the films only inhibited bacterial growth, i.e. that they were bacteriostatic.

The researchers reported their work in Advanced Materials.