Depyrogenation And Sterilization For Medical Devices
What is depyrogenation, and why is it needed for medical devices?
Depyrogenation is a process that removes pyrogens. The most prevalent and problematic pyrogens are the bacterial endotoxins found in the outer cell walls of gram-negative bacteria. Thus, depyrogenation is a process that will either destroy or remove bacterial endotoxins.
Medical devices and parenteral products must be pyrogen-free. Products can accumulate pyrogens from raw materials or other parts of the manufacturing process. The product determines the best pyrogen removal or destruction method (depyrogenation). Standard depyrogenation methods are dry heat, rinsing, and filtration.
What is sterilization, and why is it essential for sterile products?
Sterilization keeps patients safe from toxins and microbial illnesses when therapies or devices are consumed or used. Sterilization is any process that removes, kills, or deactivates all forms of life. Under the strictest definition of sterility, an item or product is sterile when there is the complete absence of viable microorganisms (bacteria, yeasts, viruses, and molds). Regulatory bodies define sterility by acceptance criteria based on calculated contamination probability for regulatory purposes. An acceptable level of contamination risk for most items is the probability of a single contaminated product out of a million manufactured products. However, sterility criteria may be more stringent or lax depending upon the intended use of the medical device or product.
Commonly, sterile products undergo sterilization processes that utilize chemicals, heat, radiation, or filters. Sterilization kills any microorganisms products collect during manufacturing. A less common version of sterilization is vapor phase sterilization.
What is steam sterilization (sterilization by moist heat)?
Steam sterilization (also known as moist heat sterilization) is performed in an autoclave. Moist heat sterilization destroys microorganisms on (or within) a product with steam under pressure. Steam kills the microorganisms by denaturing proteins within the cells. Steam sterilization is the most common method for medical device and product sterilization because it is non-corrosive, relatively fast, and inexpensive. Further, most healthcare facilities own one or more autoclaves on-site for reusable medical devices.
What products or medical devices can undergo steam sterilization?
Items traditionally sterilized with steam include mixing tanks, surgical medical devices, filling equipment, freeze-dryer chambers, and filled product containers that can withstand high-temperature exposure. Materials commonly autoclaved (steam sterilized) are rubber, metals, and durable plastic materials.
Why can’t the steam within autoclaves depyrogenate medical devices?
As mentioned earlier, the most prevalent and problematic pyrogens are the bacterial endotoxins found in the outer cell walls of gram-negative bacteria. Endotoxins require more extreme conditions to be destroyed or removed compared to bacteria and other microbes. There are three primary reasons why autoclaves can sterilize but not depyrogenate products.
#1: The use of moist heat
Both boiling and autoclaving cannot achieve depyrogenation because of the temperature limitations of steam (moist heat). Moisture prevents the destruction of pyrogens.
Steam sterilization equipment (autoclaves) can provide partial depyrogenation. However, if steam rather than dry heat is used to reach temperatures capable of destroying bacteria and other microorganisms, total depyrogenation cannot occur due to the moisture present during the heating process. As a result, dry heat vs. moist heat is used for the depyrogenation of heat-stable materials. While pyrogens aren’t completely irradiated with steam sterilization, autoclave steam sterilization processes provide better medical device sterilization than many other sterilization methods.
#2: Temperature limitations
Autoclaves often reach a temperature of 121 degrees Celsius for sterilization processes. However, endotoxins are resistant to heat. As a result, the destruction of endotoxin pyrogens requires a temperature of 250 degrees Celsius. Thus, while autoclaves will inactivate bacteria and prevent additional endotoxins from contaminating products, autoclaves will not destroy preexisting pyrogens that have contaminated products during manufacturing.
#3: Time limitations
Autoclave cycles vary depending on the medical device or materials that require sterilization. Cycles also differ depending upon the volume of items placed in the autoclave. Traditional heat-based depyrogenation requires holding products at 250 degrees Celsius for 30 minutes. Fragile items or items that cannot withstand high temperatures for extended periods will have sterilization cycle times that fall below the temperature and time depyrogenation requirements. As autoclaves do not reach the heat levels needed for depyrogenation, extending autoclave dwelling time will not be more effective at destroying or deactivating endotoxins.
How is steam sterilization performed?
Simply speaking, sterilization by moist heat is performed by steam under pressure. The most common devices used for sterilization by moist heat are autoclaves (pressurized vessels). Steam for moist heat sterilization must be pure and contain no air or other non-condensable gases. Autoclaves specialize in removing air from the chamber and replacing it with pure saturated steam. The removal of air is critical to steam sterilization. Effective air removal depends on the availability of moisture (steam) to displace air, the air removal system used (e.g., vacuum), the configuration of the load being sterilized, and the absence of air leaks in the autoclave.
The basic steam sterilization cycle has three steps:
- Preconditioning of the chamber and load within the chamber to remove air and replace it with saturated steam
- The chosen sterilization cycle
- Removal of steam and release of pressure
Water’s boiling point is raised from 100◦C to 121◦C by applying 15 pounds per square inch of pressure above atmospheric pressure to create steam. The steam sterilization cycle is dependent on the steam’s capacity to penetrate the materials being sterilized thoroughly. The container walls must be heated to raise the solution’s temperature to where microbial proteins are denatured for solution sterilization. Any sealed or covered container must have some degree of moisture inside the sealed or covered system. Otherwise, steam cannot penetrate the container, and the container’s interior will not be appropriately sterilized. For steam-sterilized solutions, glass containers are used, as plastic containers or syringes may burst under pressure.
What are the advantages and disadvantages of steam sterilization?
Steam sterilization is inexpensive, ubiquitous, and easily accessible. Further, steam sterilization is non-toxic and sterilizes materials at lower temperatures than dry heat. Moreover, steam is penetrative and can sterilize both inner and outer surfaces. However, steam sterilization cannot be performed on materials that cannot withstand humidity, certain temperature levels, and vacuum pressure levels necessary to remove air from the steam sterilization chamber. Additionally, steam can prevent the growth of bacteria (and endotoxin production) through sterilizing products. However, steam sterilization cannot completely destroy pyrogens on products with endotoxin contamination.
When is depyrogenation for medical devices necessary?
Depyrogenation is only necessary if the pyrogens existing on sterile products exceed regulatory endotoxin limits. Bacterial Endotoxin Testing can be used to determine whether medical device pyrogen levels are high enough to warrant depyrogenation. Pyrogens will not be found on items that had no significant pyrogen burden upon initial sterilization and are sterilized regularly within an autoclave, as the bacteria that create endotoxins will be consistently irradicated. Indeed, consistent bacteria destruction eliminates pyrogens for existing in the first place. However, unexpected gram-negative bacterial contamination could result in pyrogen contamination that steam sterilization will be unable to destroy.
Summary
Medical devices, products, and therapies must be sterile and pyrogen-free. Depyrogenation is a process that removes pyrogens, whereas sterilization is any process that removes, kills, or deactivates microbes. The most prevalent and problematic pyrogens are the bacterial endotoxins found in the outer cell walls of gram-negative bacteria. Steam sterilization destroys microorganisms on (or within) a product with pressurized steam. Items traditionally sterilized by moist heat include mixing tanks, surgical medical devices, filling equipment, freeze-dryer chambers, and filled product containers that can withstand high-temperature exposure. Materials commonly sterilized with steam are rubber, metals, and durable plastic materials.
Steam sterilization is inexpensive, ubiquitous, and easily accessible. Further, steam sterilization is non-toxic and sterilizes materials at lower temperatures than dry heat. Moreover, steam is penetrative and can sterilize both inner and outer surfaces Steam can prevent the growth of bacteria (and endotoxin production) through sterilizing products. However, steam sterilization is unable to destroy pyrogens on products with endotoxin contamination due to the use of moisture and the temperatures reached during the steam sterilization process. All in all, ensure you choose a contract testing organization that can provide appropriate sterility testing and bacterial endotoxin testing (pyrogenicity testing) for your product needs.
Ethide Labs is a contract testing organization specializing in Sterilization Validations and Sterility Testing. Ethide Labs also offers Microbiology Testing, Bioburden Testing, EO Residual Testing, Bacterial Endotoxin Testing, Cytotoxicity Testing, Environmental Monitoring & Package Integrity Testing services for medical device companies and allied industries. Ethide is an ISO 13485 certified facility.
References
International Organization for Standardization. Sterilization of health care products- Moist heat- Part 1: Requirements for the development, validation, and routine control of a sterilization process for medical devices. Geneva (Switzerland): ISO; 2006. (ISO 17665-1:2006/(R)2016).
MES Medical Equipment Services Admin. Why Doesn’t Depyrogenation Occur in Autoclaves? 2019.
Michael J. Akers. Sterile Drug Products Formulation, Packaging, Manufacture, and Quality. Drugs and the Pharmaceutical Sciences. Informa Healthcare. 2010.
United States Pharmacopeial Convention. <1211> Sterility Assurance. Rockville, MD, USA. 2021. (USPC <1211>).
United States Pharmacopeial Convention. <1228.1> Dry Heat Depyrogenation. Rockville, MD, USA. 2021. (USPC <1228.1>).
United States Pharmacopeial Convention. <1229> Sterilization of Compendial Articles. Rockville, MD, USA. 2021. (USPC <1229>).
