A brief spark in air produces a low-temperature plasma of partially ionized and dissociated oxygen and nitrogen that will diffuse into nearby liquids or skin, where they can kill microbes similar to the way some drugs and immune cells kill microbes by generating similar or identical reactive chemicals. Image: Steve Graves |
University of California, Berkeley, scientists have shown
that ionized plasmas like those in neon lights and plasma TVs not only can
sterilize water, but make it antimicrobial—able to kill bacteria—for as long as
a week after treatment.
Devices able to produce such plasmas are cheap, which means
they could be life savers in developing countries, disaster areas, or on the
battlefield where sterile water for medical use—whether delivering babies or
major surgery—is in short supply and expensive to produce.
“We know plasmas will kill bacteria in water, but there are
so many other possible applications, such as sterilizing medical instruments or
enhancing wound healing,” says chemical engineer David Graves, the Lam Research
Distinguished Professor in Semiconductor Processing at UC Berkeley. “We could
come up with a device to use in the home or in remote areas to replace bleach
or surgical antibiotics.”
Low-temperature plasmas as disinfectants are “an
extraordinary innovation with tremendous potential to improve health treatments
in developing and disaster-stricken regions,” says Phillip Denny, chief
administrative officer of UC Berkeley’s Blum
Center for Developing Economies, which
helped fund Graves’ research and has a mission
of addressing the needs of the poor worldwide.
“One of the most difficult problems associated with medical
facilities in low-resource countries is infection control,” adds Graves. “It is estimated that infections in these
countries are a factor of three-to-five times more widespread than in the
developed world.”
Graves and his UC Berkeley
colleagues published a paper in the Journal
of Physics D: Applied Physics, reporting that water treated with plasma
killed essentially all the E. coli
bacteria dumped in within a few hours of treatment and still killed 99.9% of
bacteria added after it sat for seven days. Mutant strains of E. coli have caused outbreaks of
intestinal upset and even death when they have contaminated meat, cheese, and
vegetables.
Based on other experiments, Graves and colleagues at the
University of Maryland in College Park reported at the annual meeting of the
American Vacuum Society that plasma can also “kill” dangerous proteins and
lipids—including prions, the infectious agents that cause mad cow disease—that
standard sterilization processes leave behind.
Diagram of dielectric barrier discharge, which generates a plasma (pink) that diffuses into a nearby liquid and kills bacterial contaminants. Source: Graves laboratory, UC Berkeley |
In 2009, one of Graves’
collaborators from the Max Planck Institute for Extraterrestrial Physics built
a device capable of safely disinfecting human skin within seconds, killing even
drug-resistant bacteria.
“The field of low-temperature plasmas is booming, and this
is not just hype. It’s real!” Graves says.
In the new study, Graves and his UC Berkeley colleagues
showed that plasmas generated by brief sparks in air next to a container of
water turned the water about as acidic as vinegar and created a cocktail of
highly reactive, ionized molecules—molecules that have lost one or more
electrons and thus are eager to react with other molecules. They identified the
reactive molecules as hydrogen peroxide and various nitrates and nitrites, all
well-known antimicrobials. Nitrates and nitrites have been used for millennia
to cure meat, for example.
Graves was puzzled to see,
however, that the water was still antimicrobial a week later, even though the
peroxide and nitrite concentrations had dropped to nil. This indicated that
some other reactive chemical—perhaps a nitrate—remained in the water to kill
microbes, he says.
Plasma discharges have been used since the late 1800s to
generate ozone for water purification, and some hospitals use low-pressure
plasmas to generate hydrogen peroxide to decontaminate surgical instruments.
Plasma devices also are used as surgical instruments to remove tissue or
coagulate blood. Only recently, however, have low-temperature plasmas been used
as disinfectants and for direct medical therapy, says Graves,
who recently focused on medical applications of plasmas after working for more
than 20 years on low-temperature plasmas of the kind used to etch
semiconductors.
“I’m a chemical engineer who applies physics and chemistry
to understanding plasmas,” Graves says. “It’s
exciting to now look for ways to apply plasmas in medicine.”
