Sterilization Resistance

Heat Sterilization (Steam, Dry Heat)

Water saturated steam at high temperature (121-134°C) and pressure (1.1-2.1 bar) is the most common sterilization technique used. Steam is non-toxic, inexpensive, and has a high sporicidal effect with short application time (4-15 min). Steam is highly recommended for sterilizing heat-resistant materials, however, inapplicable to heat sensitive materials. Plastics that have higher softening temperature than the sterilization temperature must be used when considering steam sterilization.

Hence, steam sterilization plays a major role in hospital sterilization procedure for reusable and procedure trays.

Dry heat sterilization (150-170°C for 60-150 min) is obtained in an oven (Pasteur stove) by hot air that destroys pathogens by oxidation, volatilization of light components, and deep dehydration of microorganisms. Different from saturated steam, it does not induce superficial corrosion and deposits and allows the effective sterilization of complex designs.

Ethylene Oxide

Ethylene oxide (EtO) sterilization is a commonly used sterilization technique to sterilize items that are heat or moisture sensitive. Used in the gaseous state, EO gas must have direct contact with microorganisms on or in items to be sterilized.

• The sterilization process includes a vacuum phase in autoclave and a subsequent injection of EtO gas in concentrations ranging from 600 to 1200 mg/L.
• The chamber is kept between 30 and 50°C with a relative humidity of 40% - 50%.
• The cycle lasts for 2–8 hours according to the specific sterilization protocol and sterilizer load.

EO gas sterilization is dependent upon four parameters:

1. EO gas concentration
2. Temperature
3. Humidity
4. Exposure Time

The main advantage of EtO sterilization is related to the low-temperature process and the wide range of material compatibility. The sterilization cycle should include a post process aeration for detoxification of gas residual.

Plasma Techniques

Cold plasma techniques are currently the most effective alternative to EO for low-temperature sterilization of thermosensitive materials. Cold plasma is a partly ionized gas including ions, electrons, ultraviolet photons, and neutral reactive species such as radicals and excited molecules. They can be generated through the action of either a strong electric or magnetic field, somewhat like a neon light.

Materials and devices that cannot tolerate high temperatures and humidity are often sterilized by hydrogen peroxide gas plasma. This method is compatible with most materials and applications.

Radiation

Sterilization by exposure to ionizing radiation is widely used as a room temperature treatment and end-point process of chain production. Ionizing radiation produces ions by knocking electrons out of atoms.

• Gamma radiation sterilization is the most popular form of radiation sterilization. Co-60 and, to a lesser extent, Cs-137 serve as radiation sources and undergo decomposition to release high energy gamma rays.
• Sterilization can alternatively be accomplished using electron beam irradiation. High energy electrons capable of inducing biological damage are generated by electron beam accelerators.

Gamma rays can deeply penetrate low-density materials, such as polymers, whereas high-energy electrons usually interact with the superficial layer of materials.

Chemical Sterilization

Chemical sterilants such as formaldehyde, glutaraldehyde, and chlorine dioxide are excellent at sterilizing many heat sensitive polymers and materials, but they will leave residuals or by-products, and will not penetrate all designs, configurations, plastics, packaging etc.

Check out this overview of common sterilization methods used for polymeric materials:

In researching and selecting the most suitable sterilization technique for a specific device, it is important to perform in-depth polymer compatibility analyses with the process used.

A variety of factors must be carefully considered in selecting a sterilization process without affecting the properties of plastics (physio-chemical, biocompatibility, stability etc.).

• Steam or dry heat sterilization will melt and degrade some plastics
• EO has toxic residuals, and it has limited penetration, but it can sterilize almost every plastic
• Radiation may discolor or degrade some plastics, but it has excellent penetration and no residuals. However, it can damage many plastics on repeated sterilization. It has a larger impact on materials than EO, particularly on PTFE, polyacetal, and unstabilized PP.

Overall, sterilization compatibility, material type, bio-compatibility, qualification time, turnaround time, and cost are the key drivers in the selection of sterilization process.

There are several requirements and guidance that exist to validate the level of sterility and maintain the routine controls to deliver such a process on a repeated basis.

• ISO 14937 – Developed for generic guidance for any sterilization method
• ISO 11135 – Describes requirements for an ethylene oxide sterilization process intended to sterilize medical devices
• Moist Sterilization – ISO 1134, 13683, 554