Sterilizing Fittings: The Critical Factors
Consider several important factors when choosing a fitting for an application: the size and connection style, chemical compatibility, and certification/approval requirements. These factors alone can be challenging. However, for many applications, an additional factor may be equally important—sterilization compatibility.
Sterilization is the required process of eliminating microorganisms (including spore-forming and non-spore-forming bacteria, viruses, fungi, and protozoa) that contaminate the surface of materials and equipment before use. Potential health hazards, contaminated research, and altered experiments can result when these microorganisms have not been eliminated. Sterilization is most commonly performed for pharmaceutical, research, and food applications. To prevent deleterious effects, most facilities in these industries have established sterilization guidelines on how to treat various materials for any given process.
Sterilization Techniques Overview
The most common forms of sterilization include autoclaving (steam sterilization), dry heat, ionizing radiation (gamma and electron-beam irradiation), and gas (ethylene oxide or formaldehyde). To ensure that a process has been successful requires close monitoring and validation of the steps of each type of process. Use a test organism as a control to verify and ensure the effectiveness of the sterilization process.
Autoclaving is the process of exposing materials to a combination of high temperature and pressure over a fixed period of time. This process is the preferred for most applications because it requires lower heat than the dry-heat sterilization technique. At higher pressures, the boiling point of water increases which allows the water to carry more energy to “cook” any microorganisms in the autoclave chamber. At 2 atmospheres, the water temperature can get close to 120ºC, which is enough energy to kill most of the microorganisms over 15 minutes. The World Health Organization (WHO) recommends the sterilization in an autoclave for 15 minutes at 121 to 124°C at 200 kPa. Additional ranges are listed below at different temperatures.
|Temperature (°C)||Approximate corresponding pressure (kPa)||Minimum sterilization time (min)|
|126 to 129||250 (~2.5 atm)||10|
|134 to 138||300 (~3.0 atm)||5|
Minimum sterilization time should be measured from the moment when all the materials to be sterilized have reached the required temperature throughout. Monitoring the physical conditions within the autoclave during sterilization is essential. To provide the required information, temperature-monitoring probes should be inserted into representative containers, with additional probes placed in the load at the potentially coolest parts of the loaded chamber (as established in the course of the validation program). The conditions should be within ±2°C and ±10 kPa (±0.1 atm) of the required values. Each cycle should be recorded on a time-temperature chart or by other suitable means.
WHO recommends the following test organism for autoclaving: Bacillus stearothermophilus.
Dry Heat Sterilization is the process of heating equipment to a temperature high enough to “cook” or kill the majority of microorganisms that might be found on equipment. Because there is no addition of pressure (as with autoclaving), the process must happen at a higher temperature and for a longer time period. Below are the WHO guidelines for dry heat sterilization.
|Temperature (°C)||Minimum sterilization time (min)|
WHO recommends the following test organism for dry-heat sterilization: Bacillus subtilis.
Gamma Irradiation involves exposing a material to a specific dose of ionizing radiation. The radiation causes mutations in the DNA of the microorganisms which results in their death. One of the advantages of gamma irradiation is that the materials can be placed in their final container and penetrated by the radiation. This is an advantage for items that need to remain sterile for a longer time period before being used. The WHO notes that the usual level of absorbed radiation for sterilization is 25 kGy or 2.5 Mrad, although other levels may be employed.
Depending on the dose of radiation, the WHO recommends the following test organisms for gamma irradiation: Bacillus pumilus, Bacillus cereus, or Bacillus sphaericus.
Gas Sterilization is the technique of exposing materials to a highly volatile and therefore toxic gas for a controlled amount of time. This process is used when the ability to elevate a material to a high temperature is not available or practical. Ethylene oxide is the most common gas used. It is mixed with other inert gases to a more usable and less toxic level. An air-tight container or room is filled with the materials to be sterilized. Then the concentration of the gas, the humidity, the temperature, and the time of exposure are monitored to ensure effective disinfection.
WHO recommends the following test organisms for gas sterilization: Bacillus subtilis or Bacillus stearothermophilus.
Selecting a Sterilization Technique
Many materials will respond differently or even become damaged after exposure to some sterilization techniques. Even with a variety of accepted techniques, the recommended technique is usually chosen for a specific application because it will produce the lowest chance of any surviving microorganisms. This usually means it is better to find materials that are compatible with the favored sterilization technique than to try to identify the best technique for the materials in a process. Fortunately, many fittings are available in a variety of raw materials to make finding a suitable fitting easier.
Use the table below as a general guideline when selecting a fitting material. This table is for reference purposes only (see disclaimer below), as many of the materials listed are available in multiple resin formulations and may respond differently to any of the sterilization techniques listed.
The use of a fitting designed for a specific application may affect its sterilization compatibility. For example, a fitting that might be rated good for autoclaving at a low pressure might become slightly more brittle, making it acceptable for a higher pressure reading. Sterilization often will reduce the overall life of the fitting or material as well. One last consideration: the fittings, or materials, reaction to other chemicals may also alter the sterilization compatibility. Each part should be verified for compatibility and tested before use.
Sterilization Stability of Materials (see the disclaimer below)
|Material||Gamma Irradiation||Ethylene Oxide (EtO)||Autoclave|
|ABS||Compatible up to 5 Mrads||Good||Not suitable due to low heat deflection temperatures|
|Acetyl||Varies on resin; compatible from up to 1 to 5 Mrads||Excellent||Very good|
|Acrylic||Very good up to commonly used doses (6 Mrads)||Excellent||Not recommended|
|Co-polyester||Very good up to commonly used doses (6 Mrads)||Excellent||Not recommended|
|Nylon and Glass-Reinforced Nylon||Very good; may discolor to brownish hue||Very good. Some susceptibility to oxidizing agents||Excellent. Components may swell slightly due to water absorption|
|Clear Polycarbonate (PC)||Compatible up to 10 Mrads with minor loss of physical properties; will discolor to yellow-green hue||Highly compatible||Poor. May craze or stress crack due to molding stresses|
|Tinted Polycarbonate (PC)||Excellent up to 10 Mrads with minor loss of physical properties; light violet hue turns clear upon sterilization||Highly compatible||Not recommended|
|Polypropylene (PP)||Varies on specific formulation; excellent to commonly used sterilization doses (6 Mrads)||Varies on specific formulation. Highly compatible but may react poorly to EtO/CFC mix||Poor. Components may distort due to low heat deflection temperatures|
|Polysulfone (PSF)||Highly compatible; will discolor to brownish hue||Excellent||Excellent|
|PVDF||Highly compatible; will discolor to brownish hue||Excellent||Excellent|
|Brass||Data not available||Data not available||Excellent|
|PTFE, PFA||Data not available||Excellent||Excellent|
*Stainless Steel is also rated as excellent for the following methods of disinfecting and sterilization: formalin, isopropyl alcohol, ethyl alcohol, E-Beam, and dry heat.
Disclaimer: The data presented in this chart is for reference only. It was compiled primarily from outside sources provided by feedstock materials suppliers and resin manufacturers and is offered as a means of comparing the characteristics of resins and materials. The particular conditions of your use and application of our products are beyond our control. Thus, it is imperative that you test our products in your specific application to determine their ultimate suitability. All information is provided without implied or expressed warranty or guarantee by Masterflex, or the resin and feedstock manufacturers. Masterflex also assumes no liability with respect to the accuracy or completeness of the information contained herein and none of the information provided constitutes a recommendation or endorsement of any kind by Masterflex.
This article covers only a small sample of the available sterilization techniques. Different regulations favor various methods for certain applications. For example, some fluids can be sterilized though a process of filtration. For more information on various sterilization techniques or further information on sterilization, please visit the following organizations websites:
- US Sterilization Guidelines c/o the Center for Disease Control
- International Pharmacopoeia c/o World Health Organization
Disclaimer: Masterflex products are not approved or intended for, and should not be used for medical, clinical, surgical or other patient-oriented applications.
Last Updated: 03/13/19