Hydroxyapatite, explained.

A tooth is a composite. Enamel is the outer shell—the hardest tissue in the body. Dentin sits underneath, softer and living. Pulp is the nerve and blood supply in the core.

Hydroxyapatite is not a coating you add on top of that anatomy. It is the mineral those hard tissues are built from. Understanding the ingredient starts here, at actual scale, before anyone says nano.

Enamel covers the crown in a layer a couple of millimeters thick at its heaviest, thinning toward the gumline. By weight it is about 96% mineral, mostly hydroxyapatite, with a little water and protein holding the crystals in place.

It has no cells and no blood supply. The body cannot rebuild a missing chunk of enamel the way it can knit a bone. What it can do, every day, is exchange mineral at the surface—losing some when the mouth is acidic, gaining some when calcium and phosphate are available again.

Those crystals are not a jumble. They stack into rods, or prisms, that run from the dentin toward the surface, like fibers in a composite. Between the rods is a slightly more soluble mineral that acid finds first.

Early decay is often that in-between mineral dissolving, leaving a chalky, more porous surface. The rods are still there. The lattice has vacancies. That is the scale hydroxyapatite toothpaste is trying to work at—not the tooth you see in the mirror, the crystal packing you cannot.

The formula is Ca₁₀(PO₄)₆(OH)₂. Calcium and phosphate make the scaffold; hydroxyl ions sit on the axis of a hexagonal crystal. Bone and dentin use the same mineral family, wrapped in more protein. Enamel is the dense, almost ceramic version.

Toothpaste hydroxyapatite is a manufactured form of that mineral, not a living extract. It is not fluoride, and it is not bleach. It is a calcium phosphate you can put on the same surface it came from. How it behaves in a tube depends on particle size, concentration, and everything else in the formula.

Demineralization is mineral leaving the lattice when plaque acids drop the pH. Remineralization is mineral returning when calcium and phosphate are available and the pH allows crystals to grow again. Saliva does this all day. Hydroxyapatite toothpastes supply the same ions, already in mineral form, directly to the surface.

Published studies of other hydroxyapatite pastes have measured surface changes, dentin-tubule interaction, stain in the lab, and caries endpoints in people. Particle size changes where researchers look: smaller particles at microscopic defects and tubules, larger particles nearer the surface. None of those studies are clinical trials of Klyyr One+. Oral-care products are not a substitute for a dentist.

Nano-hydroxyapatite and micro-hydroxyapatite are the same mineral at different particle-size ranges. One+ describes nano as 20–100 nm and micro as 5–10 microns, in a combined 10% blend. Those numbers are formulation descriptions, not a measurement of your enamel after you brush.

A dentinal tubule is a few microns across. A human hair is about 70. Drawn to linear size, a nano particle next to a micro particle would nearly vanish—so this figure uses a log scale on purpose. Read nano vs micro hydroxyapatite when you want the comparison in a table instead of a picture.