Other Methods of Sterilization

Keep in mind that sterilization means any process by which microorganisms such as fungi, bacteria, viruses, and spores are killed. As such, there are many ways sterilization can be carried out depending on the material to be processed and for what reason it is to be sterilized. Sterilization processes include but are not limited to dry heat, pressurized moist heat, ethylene oxide (ETO), formaldehyde, gas plasma (hydrogen peroxide, H2O2), per-acetic acid, e-beam and gamma ray. Physical methods include moist heat, dry heat, and radiation whereas Chemical methods include gases other than water steam and various liquids.

Physical Methods

Physical methods include dry heat, moist heat, and radiation sterilization. We already discussed moist heat sterilization above so that won’t be included here. This section will cover dry heat and radiation only.

Dry Heat Sterilization

Dry heat sterilization is generally used for powders, oils and other substances that are sensitive to moist heat. Dry heat requires higher temperatures and a longer application to accomplish the same result. Dry heat methods include baking, flaming, and incineration.

Dry heat works by denaturing enzymes and nucleic acids by oxidation. By denaturing enzymes and nucleic acid you will have effectively killed the microorganism. Remember that this is a function of both temperature and time. One of the major problems of dry heat methods is that the sterilization process doesn’t provide for an even distribution of temperature. These sterilizers often have a fan to combat this problem. If you are sterilizing powders, it is possible that the powders may blow around. This method is best used for glass and metal materials. Temperatures and times are listed below.

    • 30 minutes or longer at 180 °C
    • 1 hour or longer at 170 °C
    • 2 hours or longer at 160 °C

Radiation Sterilization

Radiation methods include exposure to ultraviolet light or high-energy ionizing radiation for example gamma rays as well as high speed electrons. Therefore, radiation methods incorporate ionizing and non-ionizing forms of radiation.

X-rays and gamma rays are high in energy and have the ability to knock electrons off of molecules effectively ionizing. So, X-rays and gamma rays are ionizing forms of radiation sterilization. These high energy forms cause molecules to release free hydrogen radicals, hydroxyl radicals, and peroxides. These radicals are capable of doing serious intracellular damage.

Ultraviolet radiation on the other hand is lower in energy and can’t ionize molecules. Nevertheless, molecules absorb UV wavelengths and cause electrons to become excited and go to higher energy states which eventually will destroy cellular structure. UV radiation is often used in hospitals to kill microorganisms during and after surgical procedures to prevent the spread of disease. E-beam and gamma rays are often used to sterilize pharmaceuticals.

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Chemical Methods

Chemical methods make use of chemicals in gaseous or liquid form. This process exposes objects to gases such as ethylene oxide, formaldehyde, glutaraldehyde, propylene oxide, or per-acetic acid. We’ll concentrate on ethylene oxide and per-acetic acid.

Ethylene Oxide Sterilization

The most common gas method used ethylene oxide. This is a cyclic ether that acts by playing the role of an alkylating agent. Ethylene oxide is flammable and explosive so caution should be taken. It is often used with inert gases like carbon dioxide to prevent fire and explosion. This gas is often used to sterilize surgical instruments, glove, plastic syringes, disposable needles, tubing sets, and dialysis units. Sterilization times range anywhere from 2 to 6 hours and at a temperature of 55 °C.

Per-acetic Acid Sterilization

Per-acetic acid is a known biocidal oxidizer. It is good for removing contaminants on surfaces of surgical equipment such as protein. Per-acetic acid sterilizers are available that are microprocessor-controlled. The mode of action is not well understood and limited information is available. It is believed to denature proteins and effectively destroy cell walls.