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Various means of sterilization and their impact on medical electronics

Posted: 19 Jul 2011     Print Version  Bookmark and Share

Keywords:sterilization  medical devices  Autoclave 

Chlorine Dioxide (CD) Gas Sterilization: It was discovered in 1811 or 1814 (both years are listed), and it gained widespread commercial use as a bleaching agent in the paper industry. In 1988, the EPA registered chlorine dioxide as a sterilant. This opened the door for applications in the medical field.

How it works3, 4: The CD sterilizer is a container that is first filled with the objects to be sterilized. The basic CD sterilization cycle consists of five stages (i.e., preconditioning with humidification, conditioning, generation and delivery of chlorine dioxide gas, exposure, and aeration) and takes approximately 2 ½ hours, including aeration time (removal of CD). After the aeration is complete, the sterilized objects are removed (batch processing).

Chlorine dioxide (ClO2) acts as an oxidizing agent and reacts with several cellular constituents, including the cell membrane of microbes. By "stealing" electrons from them (oxidation), CD breaks their molecular bonds, resulting in the death of the organism by the break up of the cell. Since CD alters the proteins involved in the structure of microorganisms, the enzymatic function is broken, causing very rapid bacterial kills.

The potency of CD is attributable to the simultaneous, oxidative attack on many proteins, thereby preventing the cells from mutating to a resistant form. Additionally, because of the lower reactivity of chlorine dioxide, its antimicrobial action is retained longer in the presence of organic matter.

Issues: CD sterilization is suited for objects that cannot sustain the high temperature and moisture necessary for steam (autoclave) sterilization. Due to the low temperature of +15° to +40°C, the CD sterilization process is well suited for medical devices with embedded electronics. CD gas is nonflammable at the concentrations used for this method, and it is not carcinogenic. It does not require high concentrations to achieve sporicidal effects.

Hydrogen Peroxide sterilization: Hydrogen peroxide was first isolated in 1818. It has a long usage history in the pharmaceutical industry and is a popular alternative to ethylene oxide (ETO). Hydrogen peroxide can be used in two ways: a) vaporized hydrogen peroxide sterilization, and b) hydrogen peroxide plasma sterilization.

Vaporized Hydrogen Peroxide (VHP) Sterilization
How it works5, 6:
The VHP sterilizer is first filled with the objects to be sterilized. The basic VHP sterilization cycle consists of three stages (i.e., conditioning including vacuum generation, H2O2 injection, and aeration) and takes approximately 1 1/2 hours, including aeration time (removal of H2O2). After the aeration is complete, the sterilized objects are removed (batch processing).

The exact mechanism of action of HPV remains to be fully understood and probably varies with microorganisms. Nonetheless, H2O2 generates oxidative stress by producing reactive oxygen species, such as hydroxyl radicals, that attack multiple molecular targets, including nucleic acids, enzymes, cell wall proteins, and lipids.

Issues: VHP sterilization is suited for objects that cannot sustain the high temperature and moisture necessary for steam (autoclave) sterilization. Due to its low +25° to +50°C temperature operation, the VHP sterilization process is well suited for medical devices with embedded electronics. The vacuum may not, however, be acceptable for embedded batteries. VHP penetration capabilities are less than those of ETO, and the method has not been cleared by the U.S. FDA for sterilization of medical devices in healthcare facilities.

Hydrogen Peroxide Plasma Sterilization
How it works1, 7
:This method combines chemistry with physics. The hydrogen peroxide plasma sterilizer is first filled with the objects to be sterilized. The basic hydrogen peroxide plasma sterilization cycle consists of four stages (i.e., vacuum generation, H2O2 injection, diffusion, and plasma discharge) and takes approximately 1 to 3 hours. Aeration is not required.

After the cycle is complete, the sterilized objects are removed (batch processing). Hydrogen peroxide plasma sterilization inactivates microorganisms primarily by the combined use of hydrogen peroxide gas and the generation of free radicals (hydroxyl and hydroproxyl free radicals) during the plasma phase of the cycle.

Issues: Hydrogen peroxide plasma sterilization is suited for objects that cannot sustain the high temperature and moisture necessary for steam (autoclave) sterilization. The required vacuum is not as deep as with VHP sterilization. Although the low +40° to + 65°C process temperature is appealing, the 13.56 MHZ RF energy in the range of 200W to 400W during the plasma discharge phase is problematic for embedded electronics. Hydrogen peroxide plasma sterilization should not be used for objects containing semiconductors.

Radiation methods
Gamma Ray Sterilization7: Gamma radiation was discovered in 1900 when studying radiation emitted from radium. Later other sources were discovered, such as technetium-99m and cobalt 60. The industrial use of gamma radiation began in the 1950s with cobalt 60 as a radiation source. Cobalt 60 does not occur in nature; it is artificially produced in a reactor. The half-life time of cobalt 60 is 5.2714 years.

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