Bactericial effect of a non-thermal plasma needle against \textit{Enterococcus faecalis} biofilms

Up to 3 cm long submillimeter-in-scale plasma needle was generated in ambient atmosphere for root canal disinfection. Powered with 1-2 kHz, multi-kilovolt nanosecond electric pulses, this He/(1{\%})O$_{2}$ plasma jet consists of ionization fronts propagating at speeds of the order of 10$^{7}$ cm/s. Plasma treatment of \textit{Enterococcus} \textit{faecalis} biofilms on hydroxyapatite (HA) discs for 5 min resulted in severe damage of the bacterial cells and sterilized HA surfaces of more than 3 mm in diameter, observed by the scanning electron microscopy. With a curing dielectric microtube placed 1 cm or less below the nozzle, the plasma jet entered even at a sharp angle and followed the curvature of the tube, and reached the bottom of the tube. The bactericidal effect of the plasma needle against \textit{E. faecalis} biofilm grown on the inner surfaces of the tube was demonstrated. However, the bactericidal effect weakens or diminishes for the bacteria grown deeper in the tube, indicating improvement of the plasma treatment scheme is needed. Mechanisms of the plasma bactericidal effects are discussed.