Steam Sterilization vs. Dry Heat Sterilization For Medical Devices & Products
What is sterilization?
Sterilization is any process that removes, kills, or deactivates all forms of life. Sterilization is related to the term sterile, which means a complete absence of viable microorganisms or microbes that have the potential to reproduce. Thus, sterile products that undergo sterilization are often chemically or heat sterilized after being placed in their final packaging. The chemical or heat sterilization kills any microorganisms inside the products (obtained during manufacturing and packaging). This chemical or heat sterilization process after final product packaging is known as terminal sterilization.
How do thermal sterilization methods work?
Heat-based sterilization methods kill microorganisms by denaturing proteins within the cells.
Thermal sterilization lethality depends on:
- Degree of heat exposure
- Duration of heat exposure
- Moisture level
What is sterilization by dry heat?
Sterilization by dry heat uses exceptionally high temperatures, 170◦C at minimum, to inactivate microorganisms. Dry heat kills microorganisms by oxidation (cell bursting) because of the high temperatures experienced during sterilization.
What items can be sterilized by dry heat?
Items typically sterilized by dry heat are glassware, metal parts, oils, and some dry powders.
What is sterilization by moist heat?
Sterilization by moist heat is also known as steam sterilization. Moist heat sterilization destroys microorganisms in a product with steam under pressure. Sterilization by moist heat is the most common method for medical device and medical product sterilization.
What items can be sterilized by moist heat?
Items traditionally sterilized by moist heat include rubber, durable plastic materials, mixing tanks, surgical equipment, filling equipment, freeze-dryer chambers, and filled product containers that can withstand high-temperature exposure.
What are the problems with sterilization by moist heat?
The greatest problem with sterilization by moist heat is that not all items can be exposed to pressurized steam and maintain their integrity. Thus, sterilization by moist heat will not work for all products, especially products containing electronics or flexible plastics. Also, oils or enclosed dry systems cannot effectively be terminally sterilized by moist heat as steam cannot reach these items.
What are the problems with sterilization by dry heat?
The greatest problem with sterilization by dry heat is that the sterilization process is slow. Thus, dry heat sterilization can only sterilize items capable of holding their integrity at high heat for long periods. Additionally, the dry heat sterilization process is difficult to control within precise temperature limits, which is why the USP states that an acceptable operating temperature range for the empty chamber is ± 15ºC.
How is sterilization by moist heat 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
In order to create steam, water’s boiling point is raised from 100ºC to 121ºC by applying 15 pounds per square inch of pressure above atmospheric pressure. 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.
How is sterilization by dry heat performed?
Sterilization by dry heat is a simple process where loaded items are placed into a heating cabinet or conveyor tunnel. Items to be sterilized are then exposed to high temperatures for an extended time, and filtered air blower fans enable the heat to be uniformly distributed in the sterilizer. Fans or blowers also aid heat circulation by minimizing air density issues and keeping air from stratifying. Temperatures experienced by items under dry heat sterilization often vary from 170ºC (the minimum temperature for microbe sterilization) to 250ºC (the minimum temperature for depyrogenation). Depyrogenation via dry heat requires higher temperatures (and often longer exposure times) than dry heat sterilization alone. During sterilization by dry heat, materials expand during heating and contract during cooling. Thus, all openings must be securely covered to protect microorganisms from being drawn into materials during contraction.
As mentioned above, sterilization by dry heat is performed in cabinet ovens or conveyor tunnels. In these systems, temperature, time, and blower speed are controlled during sterilization. In cabinet ovens, HEPA filtered air flows across the load, moved by a blower. Though HEPA filters remove most particulates, there is always a risk that particulate matter generated from the heat source could collect on the sterilized load. In order to prevent particulates from entering the cabinet and consistent temperature during sterilization, the cabinet dryer door must be appropriately sealed before sterilization. Note that the size of the cabinet oven chamber is limited. Limited chamber size, along with manual loading and unloading, reduces the processing rate for dry heat sterilization. Dry heat sterilization processing rates are much higher for tunnel sterilizers. Tunnel sterilizers are dry heat conveyor systems. In the conveyor system, items are sterilized and depyrogenated as they move from heating zones through cooling zones. The heat source for a dry heat sterilization tunnel is either convection or radiant heat. Cooling zones contain vertical laminar airflow units under HEPA filtration. Tunnel sterilizers contain a stainless-steel conveyor belt. The conveyor belt often moves nonsterile containers through the dry heat sterilization cycle and onto a collection table for immediate sterile product filling. Tunnel dry heat sterilizers are primarily used to sterilize glass containers and are part of a sterile fill system. Tunnel sterilizers, like cabinet ovens, may generate particles from the heating source. Where tunnel sterilizers provide advantages in dry heat sterilization loading and unloading speeds, tunnel sterilizers are more challenging to validate than cabinet ovens, as it is tricky to control uniform heating throughout the entire conveyor system.
What are the primary differences between dry heat and steam sterilization methods?
Dry heat penetrates to interior surfaces of items via conduction. Air is a poor heat conductor, which is one reason why dry heat sterilization is so slow. Heat transfer through the air is much slower than heat penetration through steam heat.
Thus, dry heat requires longer heat exposure times to kill spores than steam sterilization. Indeed, for moist heat sterilization, saturated steam that hits a cooler surface than itself will increase the temperature of the surface and release heat of condensation during the phase change of water from gas to liquid. The heat of condensation releases hundreds of calories of energy, thus killing any microorganisms in the area the steam penetrates. In contrast to the hundreds of calories of energy released via the heat of condensation, dry heat at the same temperature will only release about one calorie per gram.
Unlike steam sterilization, dry heat sterilization can depyrogenated materials. However, the high temperatures necessary for dry heat sterilization can cause material degradation, which is why dry heat is used as a sterilization method for only a few applications. With time and temperature control, steam sterilization may also be used to sterilize items made from rubber and polypropylene. While steam sterilization offers far more flexibility than dry heat sterilization, materials that at sensitive to heat should be sterilized via alternative means such as radiation or gas.
Summary
Overall, steam sterilization and dry heat sterilization are the cheapest and most common sterilization methods for medical devices and products. Steam sterilization heat kills microbes through exposure to pressurized steam. Dry heat sterilization uses exceptionally high temperatures, 170ºC at minimum, to inactivate microorganisms. Steam sterilization is often preferred over dry heat sterilization as steam sterilization can kill microbes at lower temperatures, thus minimizing material degradation following sterilization. However, heat-sensitive materials should be sterilized using neither steam sterilization nor dry heat sterilization techniques. All in all, ensure you choose a contract testing organization that can provide appropriate sterilization validations for your product needs.
Ethide Labs is a contract testing organization specializing in Sterilization Validations & Sterility Testing. Ethide Labs also offers EO Residual Testing, Microbiology Testing, Cytotoxicity Testing, Bacterial Endotoxin Testing, Bioburden Testing, Package Integrity Testing & Environmental Monitoring 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).
Michael J. Akers. Sterile Drug Products Formulation, Packaging, Manufacture, and Quality. Drugs and the Pharmaceutical Sciences. Informa Healthcare. 2010.
United States Pharmacopeial Convention. <1115> Bioburden Control of Non-Sterile Drug Substances and Products. Rockville, MD, USA. 2021. (USPC <1115>).
United States Pharmacopeial Convention. <1116> Microbiological Control & Monitoring of Aseptic Processing Environments. Rockville, MD, USA. 2021. (USPC <1116>).
United States Pharmacopeial Convention. <1211> Sterility Assurance. Rockville, MD, USA. 2021. (USPC <1211>).
