A new coating could protect retainers, clear tooth aligners, and other plastic dental appliances from bacterial growth, which can cause bad odors and increase the chances of tooth decay and gum disease (ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.8b04433). Jinkee Hong of Yonsei University, Hyo-Wan Anh of Kyung Hee University, and colleagues wanted antibacterial films for plastics that would endure the hodgepodge of enzymes and pHs the mouth encounters, while being safe for oral use and made from readily available materials. They electrostatically deposited alternating layers of negatively charged carboxymethylcellulose, a common thickener used in food and pharmaceuticals, and positively charged chitosan, derived from crustacean shells, onto sheets of poly(ethylene terephthalate) (PET), the transparent plastic used to make most dental devices. The resulting coating attracted a smooth, ultrathin water film that stopped bacteria from sticking. After 24 hours of exposure to a common oral bacterium, a coated PET sheet had 75% less bacterial growth than an uncoated sheet. The coating resisted artificial saliva solutions with varying acidity levels and enzymes, and the coated plastic was slightly stronger and more fracture-resistant than uncoated plastic.