High-tech dentistry

St Elmo's frier

Jun 17th 2009
From The Economist print edition

Using a plasma torch to clean your teeth

Science Photo Library Getting to the root of it

INFECTIONS in the roots of teeth are hell to treat. The tooth needs to be drilled into, right down to the bottom of the nerve-carrying canal that runs through the root. The infected material must then be cleaned out completely and the drilled section filled in. Although the procedure is routine, it is common for some of the bacteria to survive it and therefore for infections to re-emerge shortly after treatment.

The surviving bacteria are often gathered in the form of what is known as a biofilm. Bacteria in such a film are embedded in a polymer matrix, which makes them harder to kill than isolated individuals. High temperatures can destroy biofilms, as can some chemicals, but neither approach is safe to use inside the delicate interior of a human tooth. However Chunqi Jiang, a physicist at the University of Southern California, Los Angeles, and her colleagues have come up with a possible alternative: a dental plasma torch.

Plasmas are gases in which the molecules have been stripped of some or all of their electrons, to create positive ions. One way to do this is to heat the gas up. Conventional plasma torches employ such hot plasma to cut metal. But cold plasmas can be made using high electrical voltages. St Elmo’s fire—violet and blue “flames” that appear around ships’ masts during thunderstorms—is a good example. Dr Jiang reckoned that a cold plasma, particularly one rich in oxygen ions (which are notoriously destructive of organic materials), would be enough to do the job of breaking up a biofilm without harming the patient.

To test this idea, she and her colleagues designed a device that uses short pulses of electricity to ionise the surrounding air, creating a purple plume of plasma rich in oxygen ions. And it worked. The team report in Plasma Processes and Polymers that when the plume was directed into the infected interiors of teeth, it succeeded in clearing up well-established infections completely.

That may just be the beginning. Bacteria in biofilms are also more resistant to antibiotics than their isolated confreres, so the new device could have other medical applications. Wound infections, for example, often form biofilms. If they cannot be treated successfully, the result may be gangrene. And if Dr Jiang’s version of St Elmo’s fire can deal with that problem, the saint may become patron of a lot more people than just sailors.

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St. Elmo's fire (also St. Elmo's light^[1]) is an electrical weather phenomenon in which luminous plasma is created by a coronal discharge originating from a grounded object in an atmospheric electric field (such as those generated by thunderstorms or thunderstorms created by a volcanic explosion).

St. Elmo's fire is named after St. Erasmus of Formiae (also called St. Elmo), the patron saint of sailors. The phenomenon sometimes appeared on ships at sea during thunderstorms, and was regarded by sailors with religious awe, accounting for the name.

Ball lightning is often erroneously identified as St. Elmo's fire. They are separate and distinct meteorological phenomena.^[2]