Silver nanoparticles are useful as additives for polymeric materials in view of their antimicrobial properties. On the other hand, homogeneous distribution of the nanoparticles in the polymer matrix produces a lower effective concentration at the surface, where they matter most because that is where the material comes into contact with pathogens and microbes.
One way to improve this situation could be producing a material that is not homogeneous, for example with a surface layer that has nanoparticles and the bulk of the material which instead does not, or to a lesser degree. However, producing layered materials is in general complicated and expensive.
To address this issue in a simpler and more scalable fashion, Orlicky and co-workers at Army Research Lab, in a paper published in the Journal of Applied Polymer Science, have used hyperbranched functionalized polymer additives that automatically migrate to the surface of a polyurethane matrix, bringing with them the silver nanoparticles, which are tied to the hyperbranched polymers by ligand exchange, and thus follow them to the surface.
This generates a seven-fold increase of the concentration of silver nanoparticles at the surface, and the composites retain their antimicrobial properties.