The DSA701 system provided the possibility to perform plasma treatments of
three‐dimensonal objects. To investigate plasma sterilization under practical
conditions, vials made of cyclic olefin copolymer (Topas®) as used for
packaging of liquid pharmaceuticals were taken as substrates. These vials had
33
Results
an outer diameter of 20 mm and a total volume of 6 ml. The plasma system was
equipped with a roller mechanism allowing the vials to be rotated during
plasma treatment. Thus, like the experiments with glass slide substrates
described above, the organisms in the vials are also in direct contact with
plasma.
Plastic vials contaminated with cells of M. luteus, D. radiodurans, and R. glutinis,
conidia of A. niger and spores of B. licheniformis, B. subtilis or G. stearothermo‐
philus were subjected to plasma exposure to record their inactivation kinetics.
For these experiments, the plasmaʹs nitrogen‐oxygen mixture ratio had been
optimized for maximum UV emission (Fig. 3.1) at 80% nitrogen and 20%
oxygen which was found to obtain higher sterilization efficiency than the
previously used high‐oxygen plasma (Hägele 2005). Also, the plasma was
operated in pulsed mode to maintain the surface temperature of the vials at a
material compatible level. No deformations or visible material damages of the
vials were observed after plasma exposure.
For all organisms, the limit of detection (4 cfu per vial) for survivors was
reached within 120 s of pulsed plasma exposure (Fig. 3.3). None of the
inactivation curves followed a first order kinetics and therefore, no decimal
reduction values could be calculated. Notably, each kinetics exhibited a portion
of fast inactivation at its beginning followed by a portion of slower inactivation
in the further course of plasma exposure. The highest inactivation in this first
phase was observed for the spores of the bacilli (Fig. 3.3e) and cells of M. luteus
(a) while it was lowest for the conidia (c) and deinococcal cells (d). Regarding
this characteristic, cells and R. glutinis showed an intermediate resistance
against plasma. One can speculate a plateau, as observed with Deinococcus in
the results presented above (see 3.1.1), to be present from the reduced
inactivation rate in the phase between 10 s and 60 s of plasma exposure