What nano-hydroxyapatite is, and why particle size decides what it can do.

Nano-hydroxyapatite is a manufactured copy of the calcium phosphate mineral that tooth enamel and the hard part of dentine are built from, made as crystals under 100 nm long. Particle size decides where one of those crystals can physically go: into a dentine tubule, across the enamel surface, or neither. That is a claim about geometry, and geometry is not relief. The channel it is aimed at is measured in micrometres, tens of times wider than the crystal itself¹. S3 Sensitivity Science™ carries two of them, nano-hydroxyapatite under 100 nm and biomimetic hydroxyapatite around two microns, and its ingredient list names them as separate entries, one of the two followed by "(nano)" as the labelling rule requires.

What was checked11 peer-reviewed studies, two European safety opinions on hydroxyapatite (nano), the Great Britain cosmetics labelling rule, and the S3 ingredient list as published

What is hydroxyapatite, and what makes it "nano"?

Hydroxyapatite is the crystalline calcium phosphate that makes a tooth hard. A 2023 review of the mineral in dentistry puts enamel at roughly 96% inorganic material by weight and dentine at roughly 65%, and the inorganic part is largely this one mineral². A toothpaste carrying hydroxyapatite is therefore carrying the same substance as the surface it is brushed against. That is the genuinely interesting fact about the ingredient, and it is also the fact most easily oversold.

It is not harvested from anything. The hydroxyapatite in oral care products is synthesised, and the standard review of how it is made and characterised, written in 2018 by a university chemist and a senior scientist employed by a German maker of hydroxyapatite toothpastes, treats it throughout as a manufactured agent rather than a natural extract⁶. That is more impressive than the alternative, not less. The point of the ingredient is that a chemist can build the mineral to a specification, which is not something anyone can do with a plant.

"Nano" describes size and nothing else. A nanometre is a billionth of a metre, and in the cosmetics regulation Great Britain retained, Article 19 requires that all ingredients present in the form of nanomaterials "shall be clearly indicated in the list of ingredients" and that their names "shall be followed by the word 'nano' in brackets"³. The word on the pack is a declaration, not a boast. What it is a declaration of, and how to read the rest of the label around it, is the subject of reading the label correctly.

How small is nano-hydroxyapatite, actually?

Small enough that the honest answer is a distribution rather than a number. In an in vitro characterisation of three batches of one commercial oral-care nano-hydroxyapatite, measured by dynamic light scattering and transmission electron microscopy, the particles were rod-like with an average length of about 20 to 40 nm; two of the paper's authors work for the manufacturer of the material, which is worth knowing when reading it⁴. An earlier in vitro characterisation of a commercial oral-care grade, by a group that also includes the manufacturer, found the same rod-like morphology and the chemical and phase composition expected of hydroxyapatite⁵. The dossier behind the 2023 European opinion, SCCS/1648/22, is more precise still: the material submitted had a median particle length of 28 nm and a median width of 15 nm, with the longest particle measured at 178 nm⁷.

Put those numbers beside the thing they are meant to enter. Measured in vitro by scanning electron microscopy on extracted human adult molars, dentinal tubules were about 2.4 micrometres across in the superficial layer of coronal dentine, widening to about 4.28 micrometres in the layer nearest the pulp¹. A micrometre is a thousand nanometres, so a tubule of the width measured in that in vitro study is something like eighty times wider than a crystal of 30 nm¹. That is a loose fit rather than a tight one, which is why the geometry works at all, and why the geometry is not the whole story.

What does particle size decide, and what does it not decide?

It decides where a particle can go, and one study has tested whether that difference shows up on a tooth⁹. Dentine discs cut from 40 extracted premolars and 40 molars were brushed twice a day, under 100 g of force, for two minutes, over seven consecutive days, with one of four preparations: distilled water, an ordinary dentifrice, a dentifrice carrying 80 nm hydroxyapatite, and the same base carrying 300 nm hydroxyapatite⁹. Both hydroxyapatite preparations plugged significantly more tubules than the ordinary dentifrice under the electron microscope, with plugging rates across the hydroxyapatite groups running from 90.31% to 98.81%, and the 80 nm group ranked above the 300 nm group⁹. Each disc was then halved and one half brushed with distilled water alone for another seven days; the 80 nm group's plugging rate fell but stayed above 90%, and stayed ahead of the 300 nm group⁹.

That is the whole of the evidence on size as a variable: one comparison, on extracted-tooth discs, not in anybody's mouth. Its authors say plainly that they watched the plugs for one week only, and that saliva, chewing force and acid challenge were not modelled⁹. "Smaller works better" is a reasonable expectation from geometry. Nobody has yet shown it in a mouth, and a page that says otherwise is guessing.

Size decides less than the marketing implies, in one measurable respect. A laboratory took sixteen toothpastes, eight sodium fluoride, three hydroxyapatite, one chitosan and three stannous, put human enamel through ten days of citric acid six times a day, and measured the tissue lost with a profilometer. Applied as slurries most of the pastes protected the enamel. Applied with brushing, they mostly did not: brushing increased tissue loss in almost all of them, and only five formulations reduced loss significantly against erosion alone. The association between abrasiveness and the quantity of particles in a formulation was non-linear, and particle size had no impact¹⁰.

So the size of the crystal tells you about occlusion geometry and nothing about how gently the tube brushes. It also tells you nothing about how much mineral the tube carries, which is a separate question with its own literature: see what percentage a toothpaste needs.

Why does shape matter more than the marketing suggests?

Because shape, not size, is what the European safety assessment specifies. The European Scientific Committee on Consumer Safety (SCCS), which advises the European Commission on the levels at which cosmetic ingredients may be used, has published two opinions on hydroxyapatite (nano) in oral products, and each of them describes the particle by shape before anything else. The 2023 opinion, SCCS/1648/22, adopted at the plenary meeting of 21 to 22 March 2023, states that "this safety evaluation only applies to the hydroxyapatite (nano) with the following characteristics: composed of rod-shaped particles of which at least 95.8% (in particle number) have an aspect ratio less than 3, and the remaining 4.2% have an aspect ratio not exceeding 4.9; the particles are not coated or surface modified"⁷. The 2025 opinion on submission IV, SCCS/1677/25, adopted 26 June 2025, keeps the same architecture with wider tolerances, at least 87 per cent of particles by number at or below an aspect ratio of three and the remainder not exceeding nine, again uncoated, and adds a maximum particle length of 122 plus or minus 43 nm⁸.

Then each opinion says what it does not cover, in almost the same words. The 2023 text: "This Opinion is not applicable to hydroxyapatite (nano) composed of needle-shaped particles"⁷. The 2025 text is broader: "not applicable to any hydroxyapatite (nano) material that is composed of, or contains, needle-shaped particles"⁸.

Aspect ratio is length divided by width. A particle 28 nm long and 15 nm wide has an aspect ratio a little under two, which is a stubby rod rather than a fibre⁷. A reader who has taken in "rod, not needle, aspect ratio under three" has understood more about this ingredient than a reader who has learnt the word nanometre. The reasoning behind the shape condition is a genotoxicity argument set out at length inside the opinions, and it belongs on the safety page rather than this one.

Why does S3 use two sizes of hydroxyapatite?

Because there are two places to put mineral and one particle cannot be in both. The nano-hydroxyapatite in S3 is under 100 nm, rod-shaped and crystalline; the biomimetic hydroxyapatite beside it is around two microns and less crystalline, and the two are sized for the tubule and for the surface respectively. Two microns is two thousand nanometres, so the pair are not two grades of one idea but two different tools.

On the pack the two are given as 10% nano-hydroxyapatite and 5% biomimetic hydroxyapatite, and both figures are inclusion levels of the ingredient as supplied rather than the weight of mineral in the finished paste; the active hydroxyapatite content is lower, and S3 states both. The ingredient list carries them as two separate entries, "Hydroxyapatite" and "Hydroxyapatite (nano)", which is the labelling rule from the top of this page doing its work on a real tube.

Sealing a channel and quieting a nerve are different problems, and mineral only addresses the first; the potassium nitrate in the same tube addresses the second. Fluoride stays in at full adult strength, so the mineral is an addition to decay protection and not a substitute for it.

None of that is a performance claim, and this page is not making one. Two sizes is a description of a formula. A search of PubMed for a clinical trial setting a two-size hydroxyapatite formula against a one-size formula returns nothing, for any brand. Until something does, the two-scale argument stays where it belongs, in the geometry.

What can a hydroxyapatite crystal do once it is on the tooth?

Two things: lodge inside an open tubule and narrow it, or settle on the surface above and add mineral there. Both arguments are handed on from here. How a crystal reaches a tubule and builds into a plug is the subject of how nano-hydroxyapatite seals dentine tubules; why a crystal of this particular chemistry sticks to a surface of the same chemistry is the subject of rod-shaped crystals and the enamel lattice. Neither argument is re-run here. What the tubules are, and why an open one hurts, is set out in the Journal's piece on what dentine tubules are.

On the outcome that brings people to this ingredient, the evidence is real and narrower than the advertising. A 2019 systematic review and meta-analysis pooled six four-week randomised trials of nano-hydroxyapatite and found it ahead of its comparators on evaporative and tactile stimuli, with the pooled result rated high quality on GRADE, and no difference at all on cold¹¹. A 2023 systematic review and meta-analysis of 44 clinical trials of hydroxyapatite oral care products in general, whose authors include two senior scientists employed by a German maker of hydroxyapatite toothpastes, reported a 39.5% reduction in dentine hypersensitivity against placebo¹². Read the second alongside the first rather than instead of it, and note that neither separates its results by particle size.

It is worth knowing who pays for this literature. A 2022 systematic review of nano-hydroxyapatite and caries prevention found that six of the ten studies it could include had been funded or published by the makers of the products tested, and rated the certainty of the evidence very low¹⁴. And a 2021 review from a university dental school with no declared industry tie puts the state of play in its own abstract: clinical data on hydroxyapatite oral care products "is limited with varied results", and the effectiveness of these compounds against fluoride "has not been established"¹³. Both sentences belong on a page about particle size, because a size argument is a mechanism argument, and mechanism is the cheapest evidence there is.

Is a nanomaterial in toothpaste safe?

Not this page's question, and three sentences is all it should get here. The nano-hydroxyapatite raw material in S3, nanoXIM CarePaste, is tested to the SCCS guidance for nanomaterials, and the Committee has separately assessed hydroxyapatite (nano) as an ingredient class in oral products. The published in vitro cell-culture work on commercial oral-care grades reports cytocompatibility with human gingival fibroblasts, no irritation on the HET-CAM assay and no genotoxicity in the OECD 487 micronucleus test, and comes from groups that include the material's manufacturer among their authors⁵⁴. The argument, the caveats and what the bracketed word actually tells a shopper are set out in can you use it every day.

Frequently asked questions

Is nano-hydroxyapatite the same mineral as tooth enamel?

Chemically it is the same calcium phosphate. A 2023 review of the mineral in dentistry gives enamel as roughly 96% inorganic material by weight and dentine as roughly 65%, hydroxyapatite being the bulk of that inorganic part². What differs is the making of it: enamel crystals are grown by cells during tooth development, while the crystals in a tube are synthesised to a specification and checked batch by batch, in work published by chemists including the manufacturer's own⁶⁴.

Does a smaller particle always work better?

No, and the honest answer is that nobody has shown it in a mouth. The one study that compared two particle sizes of the same mineral in the same toothpaste base found the smaller size plugged more tubules on extracted-tooth discs after seven days⁹. On abrasiveness, the property that decides how harshly a paste treats enamel, a separate laboratory found particle size had no impact at all¹⁰.

Is the hydroxyapatite in S3 a nanomaterial?

One of the two is. The ingredient list carries two separate entries, "Hydroxyapatite" and "Hydroxyapatite (nano)", and the bracketed word is there because the retained cosmetics regulation requires a nanomaterial ingredient to be labelled that way³. The other entry is the larger biomimetic form, around two microns, which is not a nanomaterial. What that means for daily use is covered in can you use it every day.

Why do some toothpastes use micro-hydroxyapatite instead?

Because a larger particle is doing a different job. A crystal too wide to enter a tubule can still deposit across the surface and over the opening, which is the surface half of the argument rather than the inside-the-tubule half. Neither size wins in the abstract; the sensible question is which places a formula is trying to reach, and whether it says so on the pack.