How Dunedin researchers are pioneering a new dental treatment

by Ruth Nichol / 13 March, 2018

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Using silver nanoparticles to treat dental decay makes a “magic bullet” treatment much better.

Silver has been used to fight infection for thousands of years. The ancient Greek physician Hippocrates is said to have used silver preparations to treat wounds, and silver continues to be used in medical products today.

Dentists are familiar with silver’s antimicrobial properties. For many years they put silver nitrate under fillings to help kill the bacteria that cause dental decay. The treatment eventually fell out of favour because it stained the teeth black.

More recently, a substance called silver diamine fluoride (SDF) has become a popular way of treating and preventing tooth decay in children without the need to drill and fill their teeth. SDF stops the decay from progressing, though it cannot restore lost tooth structure, making it useful for treating small cavities in teeth that will fall out in the next few years anyway.

It’s been available in many countries, including New Zealand, for decades, but it was approved by the US Food and Drug Administration only in 2014, where it’s now being hailed as a “magic bullet” for the treatment of dental decay in children.

But like silver nitrate, it turns the decayed area of the tooth black.

Carla Meledandri.

However, researchers at the University of Otago are using nanotechnology to develop a range of silver-based dental products that have the same antimicrobial qualities as SDF but don’t discolour teeth. The first of these products – developed by chemist Carla Meledandri and dentist Don Schwass – has been licensed to a multinational dental company for further development.

It’s a liquid that contains silver nanoparticles that can be used to disinfect cavities once the dentist has removed the decay. This helps kill any bacteria that remain after the tooth has been filled – reducing the likelihood of further decay under the filling.

“Every filling fails, and when it fails, the margins start to leak. And once the bacteria get a food source again, away they go,” says Schwass, a senior lecturer at the dental school. “Our product helps kill the remaining bacteria and it also provides a reservoir of silver nanoparticles in the dentine if bacteria start to attack the tooth later.”

Their product doesn’t discolour teeth because it contains tiny amounts of silver – just 10-20 micrograms per millilitre of liquid, compared with about 320,000 micrograms in silver diamine fluoride.

“It’s incredibly effective because you need to use so little to have the same antimicrobial effect,” says Meledandri, who on February 13 was awarded the Prime Minister’s MacDiarmid Emerging Scientist Prize for her work in nanotechnology.

Don Schwass.

That’s because nanoparticles – which can be as much as 100,000 times thinner than a human hair – have a huge surface area for their size.

“There are a lot of theories about how silver nanoparticles work, but they all involve the silver ions on the surface of the particles,” says Meledandri. “When you decrease something down to the nanoscale, you’re starting to make a material that has more surface area than it has volume, which means that for any reaction that happens on the surface of those particles, you have far more available surface.”

US-born Meledandri had never worked with silver – or with dentists – before she arrived to take up a lecturership at Otago eight years ago. A lucky meeting with Schwass soon set them on the path to using silver nanoparticles to treat dental decay.

Their biggest challenge wasn’t producing the nanoparticles, which is relatively straightforward. It was finding ways of stopping the nanoparticles from joining up, as this increase in size reduces their effectiveness and makes them more likely to discolour the teeth.

“All nanoparticles want to stick together, so we had to put molecules on the surface to keep them stabilised and separate.”

They also had to develop the right materials into which the nanoparticles could go. These include not just the liquid used in their first product but also a gel that could potentially help treat and prevent gum disease. They’re looking for an international partner to develop that further.

In the meantime, they’re working on a dental filling material that uses silver nanoparticles, and they’re exploring other uses for the particles, such as in veterinary health.

Says Schwass, “We’ve always understood that the technology could translate elsewhere – essentially anywhere where there’s infection.”

This article was first published in the February 24, 2018 issue of the New Zealand Listener.

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