Abstract. Potential sterilization/disinfection of medical devices (MDs) is investigated using a specific plasma process developed at the Université de Montréal over the last decade. The inactivating medium of the microorganisms is the flowing afterglow of a reduced-pressure N2-O2 discharge, which provides, as the main biocidal agent, photons over a broad ultraviolet (UV) wavelength range. The flowing afterglow is considered less damaging to MDs than the discharge itself. Working at gas pressures in the 400-700 Pa range (a few torr) ensures, through species diffusion, the uniform filling of large volume chambers with the species outflowing from the discharge, possibly allowing batch processing within them. As a rule, bacterial endospores are used as bio-indicators (BI) to validate sterilization processes. Under the present operating conditions, Bacillus atrophaeus is found to be the most resistant one and is therefore utilized as BI. The current paper reviews the main experimental results concerning the operation and characterization of this sterilizer/disinfector, updating and completing some of our previously published papers. It uses modeling results as guidelines, which are particularly useful when the corresponding experimental data are not (yet) available, hopefully leading to more insight into this plasma afterglow system. The species flowing out of the N 2-O2 discharge can be divided into two groups, depending on the time elapsed after they left the discharge zone as they move toward the chamber, namely the early afterglow and the late afterglow. The early flowing afterglow from a pure N2 discharge (also called pink afterglow) is known to be comprised of N + 2 and N + 4 ions. In the present N2-O2 mixture discharge, NO + ions are additionally generated, with a lifetime that extends over a longer period than that of the nitrogen molecular ions. We shall suppose that the disappearance of the NO + ions marks the end of the early afterglow regime, thereby stressing our intent to work in an ion-free process chamber to minimize damage to MDs. Therefore, operating conditions should be set such that the sterilizer/disinfector chamber is predominantly filled by N and O atoms, possibly together with long-lived metastable-state O2( 1 Δg) (singlet-delta) molecules. Various aspects related to the observed survival curves are examined: the actual existence of two "phases" in the inactivation rate, the notion of UV irradiation dose (fluence) and its implications, the UV photon best wavelength range in terms of inactivation efficiency, the influence of substrate temperature and the reduction of UV intensity through surface recombination of N and O atoms on the object/packaging being processed. To preserve their on-shelf sterility, MDs are sealed/wrapped in packaging material. Porous packaging materials utilized in conventional sterilization systems (where MDs are packaged before being subjected to sterilization) were tested and found inadequate for the N 2-O2 afterglow system in contrast to a (non-porous) polyolefin polym...
Microwave plasmas sustained at atmospheric pressure, for instance by electromagnetic surface waves, can be efficiently used to abate greenhouse-effect gases such as perfluorinated compounds. As a working example, we study the destruction and removal efficiency (DRE) of SF6 at concentrations ranging from 0.1% to 2.4% of the total gas flow where N2, utilized as a purge gas, is the carrier gas. O2 is added to the mixture at a fixed ratio of 1.2–1.5 times the concentration of SF6 to ensure full oxidation of the SF6 fragments, providing thereby scrubbable by-products. Fourier-transform infrared spectroscopy has been utilized for identification of the by-products and quantification of the residual concentration of SF6. Optical emission spectroscopy was employed to determine the gas temperature of the nitrogen plasma. In terms of operating parameters, the DRE is found to increase with increasing microwave power and decrease with increasing gas flow rate and discharge tube radius. Increasing the microwave power, in the case of a surface-wave discharge, or decreasing the gas flow rate increases the residence time of the molecules to be processed, hence, the observed DRE increase. In contrast, increasing the tube radius or the gas-flow rate increases the degree of radial contraction of the discharge and, therefore, the plasma-free space close to the tube wall: this comparatively colder region favors the reformation of the fragmented SF6 molecules, and enlarging it lowers the destruction rate. DRE values higher than 95% have been achieved at a microwave power of 6 kW with 2.4% SF6 in N2 flow rates up to 30 standard l/min.
This paper introduces a new type of high-frequency (HF) sustained discharge where the HF field applicator is a planar transmission line that allows us to fill with plasma a long chamber of rectangular cross-section (typically 1 m × 15 cm × 5 cm). Peculiar interesting features of this plasma source are a low gas temperature (typically below 40 °C in the 1 Torr range in argon), broadband impedance matching with no need for retuning, stability and reproducibility of the discharge (non-resonant behaviour). This type of plasma source could be useful for web processing; nonetheless, it is applied here to plasma sterilization, taking advantage of its low gas temperature to inactivate microorganisms on polymer-made medical devices to avoid damaging them. The predominant biocide species are the UV photons emitted by the discharge whereas most plasma sterilization techniques call for reactive species such as O atoms and OH molecules, which induce significant erosion damage on polymers. Polystyrene microspheres are actually observed to be erosion-free under the current plasma sterilization conditions (scanning electron micrographs have been examined). Moreover, inactivation is quite fast: 106 B. atrophaeus spores deposited on a Petri dish are inactivated in less than 1 min. Correlation of the UV radiation with the spore inactivation rate is examined by (i) considering the emitted light intensity integrated over the 112–180 nm vacuum UV (VUV) range with a photomultiplier; (ii) looking with an optical spectrometer at the emission spectrum over the 200–400 nm UV range; (iii) using absorption spectroscopy to determine the role of the VUV argon resonant lines (105 and 107 nm) on spore inactivation. It is found that the test-reference spores are mainly inactivated by VUV photons (112–180 nm) that are primarily emitted by impurities present in the argon plasma.
Catheters, which comprise small diameter (≤4 mm), long (typically more than a meter), thermosensitive polymer tubings, are generally used only once. This is because conventional low temperature sterilisation techniques are considered inadequate for used catheters. The plasma sterilisation method proposed in the current paper should allow for the achievement of re‐sterilisation of catheters according to accepted regulations. The plasma process described enables one to sterilise, in less than 10 min, the inner part of a long 4 mm i.d. Teflon tube contaminated initially with 106 Bacillus atrophaeus spores. This result was obtained by achieving an argon discharge at reduced pressure (750 mTorr) within the hollow (dielectric) tube itself. The discharge was sustained using a microwave field‐applicator called a stripline, fully enclosing the tube to be treated. This linear field‐applicator yields a uniform plasma all along the tube, hence the uniform biocide action. The biocide agents are the vacuum ultra‐violet (VUV) photons, which include oxygen and nitrogen atomic lines, the N2 Lyman‐Birge‐Hopfield (LBH) bands, the UV photons emitted by the NOβ and NOγ molecular systems resulting from the contamination, even though at a very low‐level, of the argon gas (high purity argon is used) by air. Scanning electron microscopy (SEM) revealed no apparent damage to the external structure of the spores and to polystyrene microspheres exposed to plasma during the time required for reaching sterility. To check for sterility in such narrow bore tubes without having to cut them into two pieces, a procedure was developed to introduce and afterwards collect the bacterial spores used as bio‐indicators. This diagnostic procedure allowed, at the same time, the imaging of the microorganisms relatively efficiently with SEM, showing the eventual stacking of bacterial spores, a possible source of sterilisation failure.
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