Now, a research team led by XinPei Lu from Huazhong University of Science and Technology in China has developed a portable plasma source that can generate stable, room-temperature, atmospheric-pressure plasma jets in open air without any external power or gas supply. The team has demonstrated the ability of this handheld source (dubbed the "Plasma Flashlight") to effectively inactivate a 25.5 µm thick biofilm (J. Phys. D: Appl. Phys. 45 165205).
"Portable jets can be used in remote areas, such as battlefields, natural disaster sites or ambulance emergency calls," explained co-author Kostya (Ken) Ostrikov from Australia's national science agency CSIRO and the University of Sydney.
Bacterial inactivation
The Plasma Flashlight is powered by a 12 V battery and uses an array of 12 stainless steel needles as an electrode. The plasma is produced as nanosecond discharges with a repetition rate of about 20 kHz. The current pulses have a FWHM of approximately 100 ns and a peak of 6 mA, with the power dissipated into the plasma estimated as approximately 60 mW. The temperature of the plasma plume is 20–28 °C – cool enough to prevent any skin damage.
The researchers used the Plasma Flashlight to inactivate a multilayered biofilm of Enterococcus faecalis – one of the most antibiotic- and heat-resistant bacteria. During treatment, the biofilms were placed 5 mm from the Plasma Flashlight nozzle and exposed for 5 min.
After treatment, the biofilms were stained and examined using confocal laser scanning microscopy (CLSM). A green fluorescence label mapped living bacteria cells while a red label indicated damaged cells. Three-dimensional CLSM images of a control sample appeared green, while those of a plasma-treated sample were completely red – indicating that the plasma has effectively inactivated at least the top surface of the biofilms.
Two-dimensional CLSM images, recorded using layer-by-layer scans in 1.5 µm steps, revealed almost no dead cells in the first, 7th, 13th and 17th layers of the control sample. Conversely, almost all cells were killed in all 17 layers of the plasma-treated sample, implying that the reactive species generated by the plasma penetrate through the 25.5 µm thick biofilm. This is an important finding, as self-organization of bacterial colonies into multilayer biofilms increases their treatment resistance.
The researchers also measured the optical emission of the plasma plume, and saw that the plasma was dominated by excited nitrogen and oxygen species. Reactive oxygen species in particular are known to play a crucial role in bacterial inactivation and can easily penetrate into the biofilms and kill bacteria deep inside. The researchers note that UV emission from the plasma jet was low and only played a very minor role in biofilm inactivation. From a radiation safety perspective, this low level of UV exposure is an added benefit of the Plasma Flashlight.
The researchers conclude that their simple and portable plasma jet demonstrated superior performance in inactivating a 25.5 µm biofilm, and note that this is the thickest reported biofilm inactivated using a room-temperature air plasma. The device, which can be easily made and costs less than $100 to produce, is now en route to commercialization. "We are developing the Plasma Flashlight for healthcare applications," said Ostrikov. "Size reduction will also be attempted in the future."