Afterglows in N2 Gas Mixtures for Sterilization

Abstract

The sterilizing properties of cold plasmas have been demonstrated in the last decades and have raised a wide interest. The employ of N₂ microwave afterglows provides a mild platform for sterilization of thermosensitive and delicate objects, and has allowed the transfer of this technology to the industry by the development of an industrial reactor. It is the aim of this work to compile and summarize the most relevant findings regarding this topic and provide the reader of the most suitable conditions for sterilization of contaminated material, and the outreach of this technique.

In particular, the sterilisation of E-coli bacteria by N₂ microwave flowing afterglows was obtained at 5 Torr, 1 slpm, 100 w in a treatment time of 40 minutes by heating the bacteria holder at 60°C, and following this promising result, an industrial reactor (Plasmalyse) of 100 litres built up by the Satelec company has allowed the sterilization of B-Stearo spores in 30 minutes at 80°C with N₂ at 4 Torr, 1 slpm, 300 W.

Testing a more complex configuration, the transmission of N-atoms though hollow tubes, interesting the sterilization of endoscopes, was obtained in polyamide tubes of int.dia. 1.5 mm and 50 cm length, by pulsing the plasma gas: pulse 1 s - period 2s, pressure 0.9-2.3 Torr, Q=1slpm and 200 W: T_N= 3.5%.

When increasing the int.dia.to 3mm and the length to 80 cm, the transmission of N-atoms was T_N= 8.5% in the continuous discharge (4 Torr in the 5 litre reactor, 1 slpm and 150 W). It is deduced a destruction probability of the N-atoms on the tube wall:  = 1.6 10^-3.With stainless tubes of int.dia.1.5mm and length 6.5 cm, it was obtained T_N= 1.3% and = 1.6 10^-2.

Besides, in the development of conditioning materials for the sterilization process, the  N-atom transmission was tested through different membranes for sterilisation pouches where in polypropylene ones it was measured to be T_N= 0.65 at 0.1 slpm.