"Take Me To Your Cleaner: Plasma Could Soon Be The New Household Bleach And It Might Save Us From Martian Invasion, Too"
The Guardian
April 19, 2001
by David Hambling
New cleaning products sometimes come with claims so exaggerated that they border on the paradoxical. What is whiter than white? Now researchers at the University of Tennessee have found a method of killing germs which has yet to receive any hyperbole. But if it lives up to its promise, a luminous blue gas called cold plasma will be the bactericide of the 21st century.
A plasma is a gas in which the atoms have had outer electrons stripped away. One way of creating a plasma is to heat a gas to a very high temperature, generally over 3,000 C (5,432 F). The surface of the sun is hot plasma, and so is the glowing core of lightning. Plasma can also be made at lower temperatures with electricity. Fluorescent lights and the lightning globes sold as novelties use an electric current to ionise gas. Both of these work at low pressure; crack a strip light and it will implode with a popping sound. A relatively recent discovery is One Atmosphere Uniform Glow Discharge Plasma. In other words, a means of steadily creating plasma at room temperature and pressure. The device itself is essentially quite simple and can consist of little more than a pair of insulated metal terminals powered by a high-voltage, high-frequency alternating current. The electric field ionises the air between the terminals, creating plasma.
Professor Kimberly Kelly- Wintenburg of the University of Tennessee believes that the highly reactive components of cold plasma make it an ideal sterilising agent. Her company, Atmospheric Glow Technologies, is researching this application.
Existing methods of killing bacteria all have disadvantages. Bacteria are tough. Some can go into suspended animation, covering themselves with a thick membrane to form bacterial spores, which are tougher still. They can survive boiling temperatures for hours. The standard methods of killing them are with extremes of chemicals, heat and radiation. Chemicals can be highly effective: strong household bleach really will kill all known germs.
However, the action which makes chemicals so effective also makes even mild chemical agents hazardous to humans. A friend once demonstrated this by putting her contact lenses in without rinsing them after cleaning; she was blinded for days. Sterilising agents like chlorine, formaldehyde and hydrogen peroxide are all noxious substances.
Surgical instruments are sterilised by heat treatment, which leaves no dangerous chemical residues. Hospitals commonly use an autoclave for this purpose, a steam chamber which maintains a temperature of 121 C (250 F) at high pressure for 20 minutes. This is more efficient than dry sterilisation which takes two hours at temperatures of over 160 C (320 F).
Radiation is a more modern development. Ultraviolet light kills bacteria, effectively sunburning them into extinction. However, it is only useful for smooth surfaces without cracks which could harbour microbes. Hard gamma radiation is far more powerful, but it is controversial and can damage some materials. Cold plasma offers a gentler alternative. It contains a mixture of reactive components, particularly monatomic oxygen and nitrogen oxides. These act by disrupting the cellular membrane of bacteria. Prof Kelly-Wintenberg has carried out tests showing that high concentrations of E coli bacteria could be killed in five to 15 seconds. Even when the bacteria were embedded in a gel the time taken was only two minutes.
Like an autoclave, the prototype cold plasma sterilisation unit is the size of a microwave oven, but with the added ability to treat plastic containers and other items that would melt with heat sterilisation. Another proposed use is to pipe away the monatomic oxygen from cold plasma and for hosing down vehicles and equipment. This is less effective than using the plasma directly, but would have obvious uses in situations like the current foot and mouth outbreak. Viruses, like foot and mouth, are much more vulnerable than bacterial spores.
Air conditioning systems are notorious for harbouring large numbers of microbes. Legionnaire's disease is caused by a pneumonia bacterium which can thrive on damp surfaces, and sick building syndrome is believed by some to be caused by a cocktail of bacteria and fungi infesting air vents. It is also a feature of air travel that passengers are exposed to air which is repeatedly recirculated. If anyone on the aircraft has a sniffle, everyone is exposed to it. Current methods of air filtration are necessarily limited; cold plasma provides an efficient way of ensuring that the air we breathe is free from dangerous microorganisms.
One agency which has a particular interest in cold plasma sterilisation is Nasa. The quest to discover conclusively whether there is life on Mars is approaching a critical phase. Bacteria's toughness makes them natural space travellers.
A Surveyor probe which landed on the moon in 1967 was brought back by Apollo astronauts, and astonishingly was found to be harbouring streptococci bacteria which had hitched a ride to the moon and back - and survived. Any space probe sent to Mars should not itself be carrying any bacterial contamination. Otherwise the exciting discovery of Martian bacteria may be followed a few months later by the revelation that the microbes were actually of earthly origin.
A space probe is a mass of sensitive electronic equipment. The outside may be able to survive the rigours of space, but Nasa would sooner avoid having its circuit boards baked, irradiated or dunked in corrosive bleach to ensure they are free from bacteria. Cold plasma looks like an attractive option, as it has been shown to sterilise microelectronic components without harm.
The Mars bacteria issue goes both ways. When a Mars mission does finally bring back rock samples to earth, scientists would love to find signs of bacteria. Live bacteria, however, would be a different matter. They might not be harmful. But there is a risk that, arriving in the warm, moist atmosphere of earth these alien microbes would multiply rapidly. Nobody could predict the impact of such bacteria in an ecosystem with no natural resistance to them. In HG Wells' War Of the Worlds, the Martian invaders were killed off by earthly diseases; it would be unfortunate if we were wiped out by a Martian plague.
Prof Kelly-Wintenburg will continue this autumn with toxicity studies to ensure that cold plasma is harmless. If these are successful, then the technology should soon be available for general use.