“…Nanoparticles' good antibacterial properties have been attributed to their large surface area to volume ratios [9]. Antibacterial activity of quaternary ammonium polyethylenimine (PEI) nanoparticles embedded in resins has been tested both in vitro and in vivo [118,119,122,123] as well in temporary restorative materials [124]. In vitro, materials embedded showed antibacterial activity [118,124], with effects lasting for a month [118].…”