Hydroxyapatite is another name for Calcium Phosphate. It is the building block of your enamel and bone. This mineral creates hard and strong teeth and bones.
In the 1970s, NASA created a synthetic version of Hydroxyapatite. They discovered that Nano-sized particles re-mineralized enamel and bone. In zero gravity, astronauts would experience enamel and bone loss. Nano-hydroxyapatite corrected this problem.
Later that decade, a Japanese company created an enamel restoring Nano Hydroxyapatite toothpaste
When your saliva is acidic, it erodes your enamel and bone. Neutral to slightly alkaline saliva allows Nano-Hydroxyapatite to re-mineralize enamel and bone. It re-mineralizes by filling in the pores of enamel and bone. Nano-hydroxyapatite is safe to swallow, unlike fluoride toothpaste.