Sterilization By Dry Heat 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.
What is sterilization by dry heat?
Heat-based sterilization methods kill microorganisms by denaturing proteins within the cells.
Lethality of microorganisms depends on:
- Degree of heat exposure
- Duration of heat exposure
- Moisture level
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.
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. Dry heat is both simple and one of the most effective ways to destroy endotoxins. Temperatures for depyrogenation are a minimum of 250◦C. Thus, depyrogenation via dry heat requires higher temperatures (and often longer exposure times) than dry heat sterilization alone. An additional advantage of dry heat sterilization is that the sterilized materials are dry at the end of the sterilization cycle, so there is no risk of metals corroding after sterilization. Further, unlike in gas sterilization, off-gassing time and residuals testing are not needed.
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 because of the long exposure times required to kill spores. The long exposure time needed to kill spores is the other reason dry heat sterilization cycles are lengthy. The high temperatures necessary for dry heat sterilization often cause material degradation, which is why dry heat is used as a sterilization method for only a few applications.
Temperatures experienced by items under dry heat sterilization will vary. For example, heat penetration through steel is faster than penetration through glass. Additionally, shiny surface reflectance and variances in air density have significant effects on the rate of dry heat sterilization. Fans or blowers aid heat circulation by minimizing air density issues and keeping air from stratifying. 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 door 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 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 ± 15◦C.
Summary
Overall, dry heat sterilization is a common method for glass, metal, and some dry powders. Sterilization by dry heat kills microbes through exposure to extremely high temperatures, 170◦C or more, for long periods of time. The benefits of dry heat sterilization are that a lengthy holding time at high temperatures (250 ◦C) also kills off pyrogens (endotoxins). 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>).
