Time-Modulated LF-Microwave Hybrid Plasma for Surface Sterilization

The plasma surface sterilization method has emerged as a new sterilization method for medical instruments with many advantages, including nontoxicity, short treatment time, and low thermal damage to the sterilized material, over conventional sterilization methods. In this study, low-frequency (LF)-microwave hybrid plasma is produced by irradiation of microwave to an LF plasma jet. This hybrid plasma is characterized by atmospheric-pressure nonthermal plasma with a large amount of reactive species and has been applied for plasma surface sterilization. To reduce the treatment temperature while maintaining the effect of surface sterilization for applicability to heat-sensitive materials, the LF-microwave hybrid plasma was modulated in time by using pulsed microwaves. It was found that the gas temperature of the LF-microwave hybrid plasma produced by pulsed microwaves can be much lower than that of the LF-microwave hybrid plasma produced by continuous microwaves. Moreover, the gas temperature of the plasma was found to depend on the pulsewidth and duty ratio of microwaves used to produce it even if the time-averaged microwave power remained constant. Analyses of the optical emission spectra and chemical indicator tests indicated that the number of reactive oxygen radicals produced in the LF-microwave hybrid plasma also depends on the pulsewidth and duty ratio of the microwaves. In addition, it was found that the dependence of the number of oxygen atoms produced in the LF-microwave hybrid plasma on the microwave pulsewidth is opposite to that of the gas temperature. Spore-forming bacteria (Geobacillus stearothermophilus) with a population of $2.4 \times 106$ CFU/$0.25{''} \times 1.5$ ” in a paper strip were sterilized for 1 min at a treatment temperature of 82.43 °C using pulsed LF-microwave hybrid plasma. The temperature and time of sterilization with the pulsed LF-microwave hybrid plasma were found to be lower than those with continuous LF-microwave hybrid plasma.