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Light therapy . A portable plasma flashlight can kill bacteria in minutes. (Credit: X. Pei et al., Journal of Physics D: Applied Physics)

X. Pei et al., Journal of Physics D: Applied Physics (2012)

Plasma Flashlight Zaps Bacteria

Killing harmful bacteria in hospitals is difficult; out in the field, it can be an even bigger problem. Now, researchers may have a means for remote disinfection in a portable "flashlight" that shines a ray of cold plasma to kill bacteria in minutes.

Medical scientists have high hopes for plasmas. Produced in electrical discharges, these gases of free electrons and ions have already been shown to destroy pathogens, help heal wounds, and selectively kill cancer cells. No one is exactly sure how all of this works, but it seems that plasmas generate so-called reactive oxygen species in the air. These highly reactive molecules, which are present in our own immune system, oxidize cell membranes and damage DNA.

Plasma devices are already undergoing clinical testing to see whether they are safe to use. But these prototypes are limited: Either they need an external power source to generate the many kilovolts required for the electrical discharge, or they need an external gas supply and regulation to sustain the plasma. Such drawbacks make it difficult to use the devices in the field for emergency calls, natural disaster responses, or military operations.

A group led by engineer Xinpei Lu at the Huazhong University of Science and Technology in China believes it has a device with none of these drawbacks. Powered by a normal 12-volt battery and operating in open air without a gas supply, the prototype, which they call a plasma flashlight, should be portable enough to take anywhere. "It generates the plasma even being disconnected from wall power, even using very low power," says group member Kostya Ostrikov of CSIRO Materials Science and Engineering in Lindfield, Australia.

The flashlight's battery is far too small to create a plasma on its own, so the researchers use a common electronic device known as a DC booster to step up the voltage to 10 kilovolts. One output of the booster is wired to the device's shell—or "grounded," in technical speak—while the other goes to an array of 12 fine, stainless steel needles that create a rapidly pulsing electrical discharge. The circuit has several "ballast" resistors that limit the discharge's current so that the flashlight is safe to touch.

To test the device, Lu's group grew thick films of Enterococcus faecalis, bacteria that are well-known to infect root canals in the mouth and are highly resistant to both heat and antibiotics. The researchers used some of the so-called biofilms as control samples and subjected the others to the plasma flashlight for 5 minutes at a distance of 5 millimeters. Afterward, they marked all the samples with two fluorescent solutions: a green one that flagged living cells, and a red one that flagged dead cells.

The team found that the control samples stayed green, while the treated samples had turned almost completely red—even at the bottom of the biofilms, which were about 17 cells deep. The results, which are published online today in the Journal of Physics D: Applied Physics, were even better than a nonportable plasma device that Lu's group had tested previously.

Making the device truly portable is a big advance, says Michael Keidar, a plasma physicist at George Washington University in Washington, D.C. "Operating cold plasma in air is challenging, [and] it seems like they were able to make it work," he says. "This is a purely technical issue that was solved."

Engineer Miran Mozetič of the Jožef Stefan Institute in Ljubljana, Slovenia, points out another advantage of the plasma flashlight: It uses only a meager 60 milliwatts per discharge. "This is an important fact because it indicates that the battery [will] not have to be exchanged or refilled frequently," he says.

Like any other medical device, the plasma flashlight will have to go through rigorous clinical testing. But Ostrikov says that, besides making it smaller and optimizing its efficiency, the plasma flashlight is "pretty much" a commercial device already.

CORRECTION: This article originally gave Kostya Ostrikov's main affiliation as the University of Sydney, whereas it is in fact CSIRO Materials Science and Engineering. The article has now been corrected.