Simultaneous Achievement of Sterility Assurance Level (SAL) of 10-6 and Material and Functional Compatibility in Gas Plasma Sterilization Running Title: Simultaneous SAL and Compatibility
Faculty of Science and Engineering, Chuo University, Tokyo, Japan
- Corresponding Author:
- Hideharu Shintani
Faculty of Science and Engineering, Chuo University
1-13-27, Kasuga, Bunkyo, 112-8551, Tokyo, Japan
E-mail: [email protected]
Received December 22, 2014; Accepted January 23, 2015; Published January 27, 2015
Citation: Shintani H (2015) Simultaneous Achievement of Sterility Assurance Level (SAL) of 10-6 and Material and Functional Compatibility in Gas Plasma Sterilization Running Title: Simultaneous SAL and Compatibility. Pharmaceut Reg Affairs 4:131. doi:
Copyright: © 2015 Shintani H. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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In the existing sterilization procedures, it is quite hard or impossible to achieve sterility assurance level (SAL) of 10-6 and material/functional compatibility simultaneously. Simultaneous achievement of both is required in ISO 14161 and sterilization validation. As gas plasma sterilization penetration was quite shallow at around 10-20 nm level from the surface, so it can kill only one layer of bioburden and can maintain material and functional compatibility in success without any difficulties. Bioburden means sort and number of viable microorganisms in/or the products. It is so-called contaminant. Sterilization was finished in success but material was damaged and useless, such a phenomenon must be avoided. In the current sterilizations, gamma-ray irradiation, electron-beam irradiation, autoclaving, dry heating, hydrogen peroxide gas or ethylene oxide gas sterilization has inferiority not to obtain material and functional compatibility. If gas plasma sterilization will be applicable to the real healthcare products, simultaneous achievement of SAL of 10-6 and material/functional compatibility can attain without any difficulties, at that time simultaneous achievement is addressed to the existing sterilization procedures and sterilization validation. In that means gas plasma sterilization is the future promise sterilization procedure because only gas plasma sterilization can achieve both in success.
Plasma; Sterilization; Microorganisms; Irradiation
Gas plasma sterilization is popular among sterilization researchers and small number of commercial base gas plasma equipment is available from for example AST Products Inc (http://www.astp.com/plasma-equipment). SteradR from J & J is not exact H2O2 gas plasma. Plasma is used for aeration of residual H2O2. However, gas plasma sterilization is not popular due to narrower space of sterilization chamber. Sterilization is the toughest concept against microorganisms. Sterilization can kill all types of microorganisms including spores and vegetative cells [1-3]. Spores are the most tolerable organisms among microorganisms (Table 1). In addition according to ISO 14161 and sterilization validation, exact sterilization must attain sterility assurance level (SAL) of 10-6 and initial population of 106 CFU (Colony Forming Unit). From initial population down to SAL 10-6 is required 12 log reduction. It’s a mistake that from initial population of 106 CFU/carrier to 100 CFU/carrier, which is not correct requirement of 6 log reduction. The correct 6 log reduction is from 100 CFU/carrier to SAL of 10-6 in ISO 14161 and sterilization validation. For that purpose straight survivor curve must be indispensable. Initial population of 100 CFU/ carrier is the resemble population to bioburden and SAL of 10-6 is definitely required in ISO 11138-1 and ISO 14161 as well as sterilization validation by the authority. The six log reduction required to BI user is the absolute bioburden method in ISO 14161. This requ1irement is not addressed to BI manufacturer in ISO 11138-1.
||Scrapie, Creutzfedt-Jakob disease
chronic wasting disease
||Bacillus, Geobacillus, Clostridium
||Micobacterium tuberculosis, M terrae, M chelonea
|Small, Nonenveloped virus
||Poliovirus, Parvoviruses, Papillomaviruses
||Pseudomonas, Providencia, Escherichia
|Vegetative fungi and algae
||Aspergillus, Trichophyton, Candida. Chlamydomonas
|Vegetative Helminth and Protozoa
||Ascaris, Cryptosporidium, Giardia
|Lagre, Nonenveloped virus
||Staphylococcus, Streptococcus, Enterococcus
||Human immunodeficiency virus,
hepatitis B virus, Herpex simplex
Table 1: Tolerance order to sterilants among microorganisms.
Requirement of Sterilization Procedure
This requirement exists in sterilization validation and ISO 14161 and 11138-1. To attain SAL of 10-6, survivor curve must be straight from the initial population (No) to half-cycle window (SAL 5 to SAL 10-1) must be straight (Figure 1) and from SAL 10-1 to SAL 10-2 can be confirmed experimentally straight and from SAL of 10-2 to SAL of 10-6 speculated to be straight because from SAL of 10-3 to SAL of 10-6 cannot be confirmed experimentally and only speculated from stochatics in ISO 11137-1. Up to SAL of 10-2 (1/100) it can be confirmed experimentally, but less than SAL 10-3 (1/1000), it has more possibility to be contaminated, thus exact SAL is uncertain below SAL of 10-3. Therefore, SAL of 10-6 is the speculation and this amount is defined as the closest amount to zero from stochastics and this concept is explained in ISO 11137-1. In this means that during 6 log reduction, any tailing curve due to clumping (Figure 4) (Curved survivor line, Figure 5A) is not approved. The reason why tailing curve can be observed and how to avoid this is also explained in the NOVA book [1-3], but curved survivor line less than 6 log can be observed in the papers and books from engineering researchers. All of them are invalid data.
Figure 1: Necessity to attain SAL of 10-6 and straight survivor curve
from initial population to SAL 10-6
Initial population of 106 CFU/carrier down to SAL of 10-6 is quite difficult to attain by gas plasma sterilization because the penetration depth of gas plasma is quite shallow at around 10 to 20 nm  (Figure 2). Figure 2 was the polystylene surface after (upper) and before (lower) gas plasma exposure and observed by Atomic Force Microscopy (AFM). From the upper figure, it can observe to be etched around the depth of 10 to 20 nm. From the presented scale the deepest etched depth can speculate to be around 20 nm. From this, the average etching scale is around 10-20 nm. The Geobacillus stearothermophilus ATCC 7953 spore has 1 m width and 3 μm length (Figure 3), indicating gas plasma cannot pass through even one spore. This indicates gas plasma can kill only one layer of spore and multi layers cannot kill sufficiently from the shallow penetration depth. Multi-layers (clumping, Figure 4) are the reason why survival curve presents curved line before SAL of 100 [1-3]. Multi-layers are called clumping among microbiologists (Figure 4) and same phenomenon as stacking among engineering researchers, must be avoided to obtain straight survival curve up to SAL of 10-6, not SAL of 100 (Figure 5).
Figure 2: Surface analysis of atomic force microscopy (AFM) before
and after gas plasma exposure to polystylene (PS) Upper is after
treatment for 7 min and lower is before treatment (control)
Figure 3: Scanning electron microscopy (SEM) of Geobacillus
stearothermophilus ATCC 7953, which spore is defined as
biological indicator (BI) of gas plasma sterilization
Figure 4: State of clumping of Geobacillus stearotheermophilus ATCC 7953
Figure 5: Straight line and tailing line of survivor curves
As gas plasma sterilization was quite shallow penetration depth, so products themselves are quite safe from damage, indicating simultaneous achievement of SAL of 10-6 and material/functional compatibility can easily obtain compared with the existing sterilization procedures such as gamma-ray irradiation, autoclaving, dry heating, hydrogen peroxide gas sterilization, ethylene oxide gas sterilization and so on [5-78].
We have a data to indicate gas plasma sterilization does not cause serious damage to the material. Polystylene (PS) was sterilized by nitrogen gas plasma and determined the amount of CO, NOx, HCN, O3 and N2O before and after gas plasma exposure. These amounts were individually determined and all are less than safety level, indicating no significant deterioration of PS can be observed (Table 2). FT-IR data before and after gas plasma exposure to PS indicated that no significant change (for example no oxidation at 1670 cm-1 which is C=O functional group) after treatment for 7 min (Figure 6). In addition we have a data of nitrogen gas plasma exposure and autoclaving to scalpel (Figures 7 and 8). The result indicated that scalpel was unchanged before and after nitrogen gas plasma treatment for 8 min and 40 min (Figure 7), but significantly damage can be observed after autoclaving for 15 min at 121.1°C (Figure 8). From these, nitrogen gas plasma sterilization can be easily attained simultaneous achievement of SAL of 10-6 and material and functional compatibility in success [4,17,79].
Figure 6: FT-IR data of PS before and after gas plasma exposure for 7
min treatment AP: atmospheric pressure and LP: Low pressure
Figure 7: SEM observation of a scalpel before and after gas plasma
exposure (a0) is control and (a1) is 8 min treatment. (b1) is control and
(b1) is 40 min exposure.
Figure 8: SEM observation of a sclpel before and after autoclave treatment
for 15 min at 121.1 °C C1 is after treatment and c0 is control.
In addition, we have a tensile and elongation strength test of Latex rubber before and after nitrogen gas plasma exposure (Table 3) [4,80-88] and leaching test of latex rubber before and after nitrogen gas plasma exposure (Table 4) [4,89-91]. Using Table 3 data, it is statistically tested with Student t test (paired t test using Statt ViewR) and no significant difference was observed. In Table 4, statistical analysis cannot be done, but it can speculate that the significant difference may not exist.
Treatment to PS
||Detected Gases, ppm
|LowPressure Gas plasma
||Sort of gases
The data is cited from Shintani et al, Biocontrol Science (2007), 12, 131-143.
1 UV-absorbance method
2 chemical luminescence method
3 pyrazolone light absorption method
4 ozone detector
N.D. not detected, indicating less than limit of detection (LOD)
Table 2: Analysis of exhaust gas from polystylene (PS) treated with nitrogen gas
||Max Tensile test (N)
||Max elongation test (%)
||300% Elongation tensile test (N)
|Plsama 40 min
Table 3: Tensile test, elongation test and 300% elongation test of Latex rubber
before and after nitrogen gas plasma exposure
||Less than 2.0 ppm
||Less than 2.0 ppm
|Plasma 40 min exposure
||Less than 2.0 ppm
||Less than 2.0 ppm
Table 4: Leaching test of Latex rubber before and after of nitrogen gas plasma
As mentioned in the above, gas plasma sterilization can attain SAL of 10-6 and material/functional compatibility without any difficulties because penetration depth of the sterilization factors can be only 10-20 nm from the surface, which can kill only scattering bioburden in one layer on the products. Both achievement of SAL of 10-6 and material/ functional compatibility can be completed in success required in ISO 14161 and sterilization validation to the BI user by the authority. The existing sterilization procedures are all failed to attain material and functional compatibility in the exact status, therefore compatibility is not strictly applied to the existing sterilization procedures, and if strictly applied to the existing sterilization procedures, no sterilization procedures are available in the current status, so the use of the existing procedures was connived. But this kind of status is not correct and the real procedures to attain SAL of 10-6 and material/functional compatibility must be realized.
If gas plasma sterilization procedures can be applicable to the real healthcare materials, the present connivance to the existing sterilization procedures cannot be approved by the authority in future. It may be required to improve in order to attain SAL of 10-6 and material and functional compatibility to the existing sterilization procedures. In that meaning commercial base gas plasma sterilizer is anticipated as soon as possible. SteradR from J & J is not real H2O2 plasma sterilizer, but simply H2O2 sterilizer because chamber is too large when considering as H2O2 plasma sterilizer because sterilization factors are short-lived and small flight distance. Plasma in SteradR is used for H2O2 residue removal, not for sterilization.
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