Development of low-temperature sterilization device using atmospheric pressure air plasma with circulating flow

Abstract

Medical instruments are generally sterilized using high-pressure steam, ethylene oxide gas, gamma radiation, or electron beams. However, such conventional methods entail many shortcomings such as limitations of applicable materials, high temperature, long time, and high operating costs. Therefore, development of a safe, low-temperature, rapid, and inexpensive sterilization method has been anticipated. This study developed and assessed a plasma sterilization device that can generate effective chemical species necessary for sterilization. Its sterilization effectiveness was verified using biological indicators of Geobacillus stearothermophilus and Bacillus atrophaeus spores. The G. stearothermophilus and B. atrophaeus were sterilized, respectively, in 25 and 35 min with 5,400 ppm and in 35 min with 7,600 ppm of plasma-generated NOx, respectively, at around 25 °C. The main factor in sterilization of spores using the plasma was also assessed. In the conditions of atmospheric pressure air plasma, nitrogen oxide such as NO and NO2 were mainly generated. Morphological observations of spores exposed to the plasma and NO2 using scanning electron microscopy confirmed that nitrogen oxide perforated the spore coats. That spore coat perforation caused by nitrogen oxide is expected to be a main factor in sterilization. To remove the toxic gases used for sterilization, two types of methods were introduced into the developed devices. Bubbling in water could be reduced the concentration of residual NOx in the reaction chamber after sterilization to less than 50 ppm, but it remained higher than the limitation of environmental regulations. On the other hand, using the surface discharge system, we were able to reduce the residual NOx to less than 0.04 ppm, which is the limit of environmental regulations. This study presents the important techniques to improve infection prevention and public health.

Keywords

• atmospheric-pressure plasma
• low-temperature plasma
• surface discharge
• sterilization
• circulating flow
• nitrogen oxide