Summary of Lumen Sterilization Testing:

Comparison of the Noxilizer and STERRAD Cycles

Summary

The ability of the Noxilizer sterilization process to sterilize rigid and flexible lumens was

compared to the ASP STERRAD 100NX hydrogen peroxide process. For this test, three

different lumen geometries were used. These were: stainless steel, 300 mm long and 2 mm

diameter; stainless steel, 500 mm long and 2 mm diameter; and, Teflon, 4000 mm long and

1 mm diameter. Lumens were inoculated with more than 106 spores of Geobacillus

stearothermophilus prior to exposure to the sterilization processes. The NO2 processed

lumens were treated in a prototype hospital sterilizer, called the ARTS unit. The lumens

processed by hydrogen peroxide were taken to a hospital for processing in a STERRAD 100

NX system. The results show that, for rigid and flexible lumens, the Noxilizer cycle (with the

cycle parameters chosen) was at least equivalent to the H2O2 cycle in the STERRAD 100 NX.

Testing Outline

For this head-to-head comparison, rigid lumens were prepared for exposure by inoculation

with a liquid spore suspension. Each cycle had 8 stainless steel lumens, as shown in Table

1, which were packaged in a Tyvek/mylar pouches (2 lumens per package). The NO2

exposure cycle for the rigid lumens was 10 mg/L NO2 concentration, 5 minutes per

exposure pulse, four pulses, and > 75% relative humidity. This cycle is represented by the

graph shown in Figure 1. The Noxilizer cycle conditions were chosen so that some number

of spores would survive and could be enumerated. A total of 3 cycles were completed in the

Noxilizer sterilizer (ARTS 4 unit) and 3 cycles were completed in the STERRAD unit, 8

lumens per cycle, for a total of 24 lumens tested in each unit.

The lumens were inoculated with either a clean spore suspension or with spores mixed

with artificial soils. The artificial soils are comprised of synthetic hard water (≥ 500 ppm

salts) and fetal bovine serum (FBS, 10%). The purpose of the artificial soil (hard water and

FBS) was to determine the impact on lethality that might be posed by the salt and protein

soil mixture.

The flexible lumens were Teflon, with a 1 mm diameter and were 4 meters in length. The

inoculum was a clean spore suspension (no artificial soils were used). These lumens were

coiled and packaged in Tyvek/mylar pouches. Six lumens were included in each NO2 load

and 4 lumens tested in the STERRAD load. The Noxilizer exposure cycle consisted of 10

mg/L NO2 concentration, 2 exposure pulses, >75% relative humidity. This NO2 exposure

cycle corresponded to a half-cycle, as shown in Figure 1.

Table 1. Lumens included in the NO2 and H2O2 exposure cycle loads. Number of

Lumens per Cycle

Material Diameter Length Minimum Inoculum

2 Stainless Steel 2 mm 300 mm 106 Spores

2 Stainless Steel 2 mm 300 mm 106 Spores and Soils

2 Stainless Steel 2 mm 500 mm 106 Spores

2 Stainless Steel 2 mm 500 mm 106 Spores and Soils

4 or 6 Teflon 1 mm 4000 mm 106 Spores

Figure 1. This graph shows a typical exposure cycle, consisting of four exposure pulses,

each with a 5 minute dwell. The temperature sensor in the chamber registers fluctuations

due to the addition and removal (compression and decompression) of the gas.

Results

The rigid lumen testing demonstrated nearly equivalent lethality between the NO2 and

STERRAD processes. The results are shown in Figure 2. The graphs in this figure show the

results of the three exposures in the Noxilizer and STERRAD units, plotted as the measured

reduction in spore population for each lumen configuration. Each graph shows the average

value for the two replicate samples of each configuration that were included in each run.

The testing of the flexible lumen is compiled in a Test Report (TR.0153). This test report

shows the results from 24 inoculated lumens that were exposed to the NO2 process. It is

important to note that these NO2 exposure cycles only included two 5 minute dwell pulses,

rather than the 4 pulse cycles used with the rigid lumens. The number of recovered viable

spores averaged less than one per lumen, while the average inoculum was 2.4 x 106 CFU

per lumen. The method of inoculation and results of the study are included in TR.0153.

The STERRAD exposure of the flexible lumens was a full cycle in the 100 NX. This testing

was not part of the TR.0153 study. Four inoculated lumens were included in each of the

two cycles. Similarly, 4 lumens were run in the Noxilizer unit (half cycle) concurrently with

the STERRAD (full) cycles. There were no surviving spores recovered in either the

STERRAD or the NO2 exposures.

Figure 2. These three graphs show the results of the three rounds of rigid lumen testing.

Each graph shows the results of 8 exposed lumens. These results show that the lethality

observed with the Noxilizer process is equivalent to that observed with the hydrogen

peroxide process.

Conclusion

The first conclusion is that the Noxilizer sterilization unit and the STERRAD 100 NX

provide similar results on stainless steel and Teflon lumens. However, the Noxilizer

sterilizer could be programmed for a more aggressive cycle, while the Sterrad unit is

approaching its maximum cycle parameters. With regard to the flexible lumen tests and

based on the comparison of the NO2 half cycle with the H2O2 full cycle, further studies are

needed to determine whether the NO2 process has a measureable advantage over the H2O2

process with the flexible lumens. Those studies would include head-to-head half and full

cycles.

A second conclusion is that polymeric lumens pose less of a sterilization challenge than do

the stainless steel lumens. This is likely due to the hydrophobic nature of Teflon, as

compared to stainless steel.