New Use for Plasma Pencil Reported by ODU's Laroussi

The plasma pencil that Mounir Laroussi created more than five years ago was hailed in publications throughout the world, including National Geographic, as an easy-to-use, miniature light saber for germ-killing applications.

Now, in an article published by AIP (American Institute of Physics) Advances, the Old Dominion University electrical engineering professor is reporting an innovative new use for the pencil: the ignition of a large-volume cold plasma.

Laroussi's specialty is "cold" plasma, which can be created in regular atmospheric conditions and can be used - in dental or wound-healing treatments, for example - without burning normal human tissue. Conventional plasma, like that present in lightning and in television sets, is created in the absence of atmospheric pressure and is radically hot.

Both plasmas have been shown to kill germs, but the cold version has gotten more attention for bio-medical applications because it is safer and easier to use.

Laroussi's new application uses the plasma pencil, which is about the size of an electric toothbrush, to ignite a much larger volume of cold plasma. Previously, applications proposed for the hand-held plasma pencil were limited by the small size of the 2-4-inch plume, which is composed of tiny bullets of a highly energized gas such as helium.

In recent laboratory experiments, Laroussi and graduate student Arda Akman shot the plume from the pencil into a Pyrex chamber. A vacuum tube connected to the chamber allowed the pressure to be regulated inside. By lowering the pressure, fewer air molecules would interact with the helium gas in the plasma jet, theoretically producing a longer plume.

But strange things happened as they took the pressure down. From normal atmospheric pressure of about 760 Torr to about 200 Torr, the plume changed very little. From 200 to 70 they observed a rapid increase in the length of the plume to more than 10 inches. However, below 70 Torr the length of the plume began to decrease rapidly and simultaneously cold plasma expanded in all directions inside the Pyrex chamber. Below 2 Torr, the entire volume of the chamber filled up with a uniform, diffuse cold plasma.

"The sudden shortening of the plume, immediately followed by the ignition of a large-volume, uniform plasma is an intriguing new observation and constitutes an interesting transition mode (from jet) that beckons further detailed investigations, which can lead to various interesting applications," Laroussi and Akman wrote in the article, which was given highlight treatment this month in AIP Advances.

Laroussi said in an interview that the "neat" innovation he and Akman are reporting is the way such a large volume of plasma can be created so easily and cheaply. "Another way to generate a comparable large volume of plasma would be to launch high-power microwaves or radio-frequency power to ignite a gas in an evacuated chamber, but RF and microwave circuitry are quite complicated, expensive and require special skills to operate. In our case, the igniter is a little plasma plume generated by simple means."

Plasma-filled chambers have been proposed to sterilize all sorts of containers and products headed for market.

For example, Laroussi has collaborated with bacteriologist Wayne Hynes, chair of ODU's Department of Biological Sciences, to conduct tests to see if cold plasma could kill E. coli bacteria on green beans without affecting the looks or taste of the beans. Laroussi called the results "encouraging."

Laroussi and Michele Darby and Gayle McCombs of ODU's School of Dental Hygiene have used the plasma pencil to kill oral-borne bacteria that cause periodontal diseases. Darby is an Eminent Scholar, University Professor and the dental hygiene chair. McCombs is a University Professor and director of ODU's Dental Hygiene Research Center.

Plasmas are a super-excited "soup" that have been called the fourth state of matter - quite different from solids, liquids and gases. Cold plasmas are cool to the touch because only the very lightweight electrons in them are energized, and not the heavier nuclei. But for reasons not yet fully understood, these non-thermal plasmas can kill or inactivate bacterial cells while at the same time enhancing the proliferation of healthy cells. Experiments have shown them to be useful for sterilization, treatment of disease and wound healing.

Last year at the International Conference on Plasma Medicine in Germany, Laroussi was one of three researchers worldwide to receive the inaugural International Society for Plasma Medicine Award.