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Material characteristics of PCDs

To assure sterility in steam sterilization processes it is necessary to monitor besides the physical data (pressure, temperature, time) which are recorded by the sterilizer itself, also the parameters “air removal” and “steam penetration”. The last parameters are not detected, if only temperature and pressure are recorded. Special test systems consisting of a process challenge device (PCD) and a suitable indicator strip can monitor air removal and steam penetration (so-called class 2 indicators). One of the oldest PCD constructions is the so-called Helix-PCD consisting of a deadend tube connected with a capsule. This capsule is provided with a screw cap to be able to insert an indicator strip into the PCD. After the screw cap is closed, the indicator is located at the place farthest away from the tube opening and is therefore most difficult to sterilize within the PCD. The first Helix-PCD has been developed 30 years ago to monitor formaldehyde (FO) sterilization processes. FO sterilization processes use temperatures between 60°C and 80°C. The PCDs require a much lower thermal stability of 80°C only in comparison to steam sterilization processes which must be stable up to 134°C. There are Helix-PCDs on the market which have been designed for FO processes but used in steam sterilization processes and are originally not designed for such high temperatures. Therefore the standard for the helix system EN 867-5 requests a
seal test at 134°C. Under such high temperatures polypropylene capsules may deform and loose their seal properties and volume dimensions. For steam sterilization processes gke uses stainless steel capsules which seal and remain stable under such temperature conditions. In comparison to polypropylene capsules gke PCDs don’t need to be exchanged producing reproducible results for a high number of cycles.

The following table provides an overview of the materials used in PCDs:

Characteristics of materials used PCDs

PCD
gke Compact-PCD
Original Helix-PCD
according EN 867-5
plastic Helix-PCD,
e.g. for FO sterilization
Material
Stainless steel V4A
PTFE
Polytetrafluoro-ethylene
PP
Polypropylene
Application
Hollow device construction
and screw cap in all gke
PCD according to standard,
indicator pin and tubes of
gke PCDs
Test capsule of Helix-PCD
models for low temperature
(e.g. FO) processes
Melting
temp.
ISO 11357
1.500°C
327°C
163°C
Max. temp.
temporary
800°C
300°C/
140°C Softening
at approx. 120-135°C
Max. temp.
permanently
450°C
260°C
100°C
Spec.
density at 20°C
7,9 g/cm³
2,16 g/cm³
0,905 g/cm³
specific
gravity g/ml
(much) heavier than
water
heavier than water
lighter than water
Colour
silver
white
lourized, fades yellowish
Application
routine monitoring
min. 10.000 cycles
only for laboratory
equivalent tests used
routine monitoring
100 – 400 cycles

This overview demonstrates that many of the Helix-PCDs on the market made of PP are only suitable to be used in processes with comparatively low temperature, e.g. in FO sterilization processes. Unfortunately such PCDs are also offered to be used in steam sterilization processes. These PCDs which are only limited or not suitable for steam sterilization are lighter than water (they float) and the material deforms, changing its internal volume just after a few cycles. If the PCD floats in water, it is mostly made of PP and not suitable for steam sterilization processes. More critical than the optical change is the risk that after a short period the screw cap does not seal anymore because the material has deformed, and therefore the steam does not enter through the tube but directly through the (leaky) screw cap into the indicator capsule. Also the deformation can change the inner capsule volume so that the sensitivity of the PCD can be changed. With such a PCD a reasonable test result concerning air removal and steam penetration is not possible anymore. Therefore the standard EN 867-5 describing the requirements on a Helix-PCD contains a description of a special test to check the leak tightness, see gke TI 730-091 EN.