Schematic overview showing the modes of action of HAP in the remineralization and protection of enamel. (A) HAP-particles from oral care products are introduced into the oral cavity. (B) HAP particles from oral care formulations adhere to enamel surfaces as a protective layer [23, 48]. Bacterial adhesion is reduced. Free HAP-particles act as “magnet” for oral bacteria and are spit out. (C) Bacteria forming a biofilm on top of the HAP-layer after a certain time will produce acids when sugar/starch is provided, and acids are produced. These acids might lead to dissolution of the protective HAP-layer. The tooth will be protected, because of free calcium ions, and phosphate buffer mechanisms from the HAP-layer.