Pearly whites

Nanotechnology has been the subject of science fiction-inspired hype for almost a decade. Touted as the next revolution in science, it has been promoted as capable of almost anything, including tiny robots that would swim in the bloodstream and cure diseases.

The truth is, of course, more prosiac - nanotechnology is simply any technology involving substances of around a thousandth of a millimetre. But at such small scales - around the size of a few molecules - many unusual things are still possible. One of these may be a toothpaste that repairs your teeth.

Tooth enamel is made from a form of calcium phosphate called hydroxyapatite (HAP). It's one of the hardest materials known, but it's also naturally porous. The holes in the surface of teeth allow pigments from food and tobacco smoke to adsorb onto the surface of the enamel.

Moreover, the pores provide footholds for bacteria, often leading to decay. All toothpastes contain abrasives, and some contain optical brighteners to whiten the teeth. But what nanotechnology offers, explains Rolf Norenberg of BASF's surfactants technology division, is the opportunity to deposit a fresh layer of HAP onto the tooth surface, sealing the pores and providing a new, smooth surface, whitening the teeth by adding a layer of enamel, rather than removing one.

Adding HAP to toothpaste has been tried before, Norenberg says. Trials were conducted using a precipitated form of the material, but this process produced particles of an uneven size and shape, which didn't form a uniform layer on the teeth.

Nanotechnology solves this problem by exploiting some of the physical properties of extremely small particles, Norenberg explains. 'The surface, which accounts for almost all of the particle, is arranged in such a way that it has the lowest energy in relation to the environment - this is not the crystal arrangement of a solid, but rather of a swollen gel,' he says. 'The high-energy content of the crystallite surface can be exploited in special reactions, such as the fusing of nanoparticles to form a film.'

The HAP nanoparticles used are needle-shaped and around 0.1µm (100nm) long. The key to this shape lies in the production process, which is a proprietary secret.

However, Norenberg can reveal that it is a hydrothermal reaction, performed under pressure using calcium hydroxide, phosphoric acid and water as starting materials, in a similar reactor to those used to make chromium dioxide and iron oxide magnetic pigments.

Testing the substances involved immersing teeth in a concentrated suspension of the HAP nanoparticles. The crystallites stuck firmly to the tooth's surface, showing that they had the expected affinity for the enamel surface. Nörenberg then diluted the suspension to five percent solids, and wiped it over the tooth surface with a cloth. This time, the nanoparticles organised themselves into a non-porous continuous film across the enamel. The film formation is a very fast process, which is good news for toothpaste formulations, because most people spend less than three minutes brushing their teeth.

It's the crystallites' energetic properties that are the key to this self-organisation, Norenberg says. The particles contain positive and negative charges, distributed across their surfaces. 'The greatest energy gain is produced by a parallel arrangement of the surfaces, so that positives and negatives always interact with each other,' he explains. Precipitated crystals will not form films, Norenberg explains, because there are so many reaction paths involved in the precipitation reactions that the crystals do not have the functionality that the nanoparticles display.

There is still a great deal of research to be done, Norenberg says. But commercial toothpaste manufacturers are interested in the technology, he adds, and have begun making trial formulations. If consumers can be convinced of the benefits of these materials - and the inevitable cost that would go with them - there could be smiles all around in BASF's nanotechnology labs. And no doubt, they'll be dazzlingly white ones.