Surface-Retained Alginate Hydrogel Coating for Enamel Protection

Despite widespread adoption of oral hygiene routines, dental caries remains highly prevalent, affecting over 2.5 billion people globally alongside a range of associated oral diseases. As clinically established, the primary contributor to these dental conditions is the nighttime vulnerability, during which salivary flow and oral pH drop significantly, causing bacterial load and metabolism to peak uninterrupted for hours. Yet, conventional oral care relies on bulk environment-directed antimicrobial activity that is rapidly diluted and cleared from enamel, limiting sustained surface protection during this extended period. Here, we develop an enamel-retained hydrogel coating engineered specifically for routine overnight oral care. This system shifts oral care from transient bulk treatment to direct and sustained interfacial enamel protection. While many coating approaches require complex surface modification to generate stable immobilization, this platform leverages native calcium ions present on enamel and in saliva to enable in situ ionic crosslinking with alginate, forming a stable hydrogel coating without altering the substrate. The resulting coating remains stable on model enamel substrates for >24 h while remaining removable, as tested using a toothbrush. In growing bacterial biofilms directly on the gel surface, no stained biofilms were visibly detected on underlying enamel following coating removal. The coating further sustained encapsulated-xylitol release for up to 8 h and reduced metabolic activity of pre-existing bacteria by 70-90% in both planktonic and biofilm states. These effects were validated through in vitro staining and colorimetric assays using Lactobacillus acidophilus and Streptococcus mutans, two primary bacterial strains associated with dental caries. The platform itself is translationally feasible in its use of food-grade materials and a rinse-based application, both aligned with established FDA-regulated consumer oral care formats. This platform thereby has the potential to substantially reduce cumulative enamel caries while enabling sustained, localized delivery of additional oral-care agents.