Vapor Phase vs. Steam Sterilization
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). For regulatory purposes, sterility is defined by acceptance criteria based on calculated contamination probability. 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. The basics of steam sterilization, hydrogen peroxide vapor sterilization, and steam sterilization of medical devices will be covered in this article.
What is vapor phase sterilization?
Vapor is an agent or molecule that is suspended in the air. Vapor phase sterilization sterilizes products through exposure to sporicidal agents suspended in the air. Traditional vapor phase sterilization agents are hydrogen peroxide vapor sterilization (H2O2), peracetic acid sterilization (CH3CO3CH), formaldehyde sterilization (CH2O), and glutaraldehyde sterilization [CH2(CH2CHO)2]. Sterilizing gases and liquids differ from vapor phase agents, as vapor phase agents expose products to multiple phases (liquid, gas, etc.) during sterilization. Hydrogen peroxide vapor sterilization is the most popular vapor sterilization method.
What is steam sterilization?
The basics of steam sterilization are simple. Steam sterilization of medical devices (also known as moist heat sterilization) is performed in a device called an autoclave. The steam sterilization of medical devices destroys microorganisms on (or within) a product with steam under pressure. Steam kills the microorganisms by denaturing proteins within the cells. The basics of steam sterilization differ from dry heat sterilization in that hot water vs. hot air is used for sterilization. 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 vapor vs. steam sterilization?
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. The steam sterilization of medical devices made from materials such as rubber, metals, and durable plastic materials is common. Steam sterilization is not used for materials such as biodegradable plastics. In contrast, vapor sterilization works well for non-corrosive, heat-sensitive materials and for sterilizing surfaces.
How is vapor phase sterilization performed?
At room temperature, vapor phase agents (liquids or solids) vaporize and can be utilized for sterilization within a sealed chamber or vessel. Vapor sterilization must have a correct sterilant concentration, chamber temperature, and relative humidity for the items undergoing sterilization. Typically, the sterilant concentration (amount) will be determined from its injection quantities into the sterilization chamber. Vapor phase agents are most often introduced as an aqueous solution. For example, a standard vapor sterilization process involves adding items into the sterilization chamber and then adding heat and humidity. Next, the sterilant agent is introduced (sometimes through an atomizer), and the products are marinated in the vaporized sterilant for a set time. And finally, allowing the vapor to be removed from the system or evaporate before opening the chamber and removing the newly sterile items. If introduced as an aqueous solution, water moisture is introduced along with the sterilant. This added moisture is factored into humidity and condensation considerations for the sterilization process. Out of all vapor phase agents, hydrogen peroxide and peracetic acid are widely used and the most established for sterilization.
Hydrogen Peroxide Vapor Phase Sterilization
Hydrogen peroxide has a long history of being a liquid sterilant in healthcare and other industries. Hydrogen peroxide can be added into a sterilization chamber through multiple approaches. These approaches are continuous administration, intermittent administration, or injecting the entire dose of hydrogen peroxide all at once. Some vapor phase sterilization protocols have a drying step before adding the hydrogen peroxide. This drying step allows the hydrogen peroxide concentration within the sterilization chamber to increase without additional condensation. Hydrogen peroxide can also be introduced to a chamber as a liquid and exposed to targeted heating to create the vapor phase. Like gaseous sterilization methods, the sterilization chamber is aerated, and the sterilant gas is allowed to dissipate before the sterilized medical devices, products, and other items are removed.
Peracetic Acid Vapor Phase Sterilization
Peracetic acid may be used alone or mixed with hydrogen peroxide to sterilize medical products and devices. Peracetic acid is a liquid sterilant. An atomizer is used to distribute peracetic acid for vapor phase sterilization. The atomizer allows both liquid and vapor forms of peracetic acid to be present during sterilization. After peracetic acid exposure, evaporation is used to remove all peracetic acid from the system.
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 100ºC to 121ºC by applying 15 psi 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 a heat 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 do you validate vapor sterilization process?
Vapor phase sterilization can be challenging to validate as relative humidity, sterilant concentration, and condensation rate varies throughout the sterilization process. These variations cause localized differences in sterile conditions within a sterilization chamber. Thus, some products or product areas may not experience the same microbial lethality as other products or product parts. Further, there is no standardized biological indicator for vapor systems as it is a liquid and a gas combined sterilization system. D-values (which determine the lethality of a sterilization process) can be tricky to calculate for vapor sterilization systems because gas-phase conditions, surface conditions, and microbial lethality do not have known correlations. D-values can only be calculated under well-defined, system-to-system specific conditions.
Since vapor sterilization has multiple phases (liquid and gas) that vary over time, no standardized biological indicators or D-values can be used across the board for vapor phase sterilization validations. An empirical approach is taken with vapor sterilization processes since D-values are inconsistent. D-value inconsistencies occur because the lethality of a sterilant is different in the gas vs. the liquid phase. Generally, liquid phase kill rates are greater than gaseous kill rates. Thus, sterilization process parameters must be modified until a complete kill of all microbes is achieved, no matter the location of the items within the sterilization chamber. The vapor sterilization parameters for a total kill are the minimum conditions needed to kill a particular amount of bioburden. In some cases, vapor sterilization may be validated using a half-cycle or bracketing approach, like liquid chemical sterilization. A bracketing approach is better for defining maximum and minimum operating ranges for critical sterilization parameters than the half-cycle method.
Half-Cycle Approach
The half-cycle approach was initially created for gaseous ethylene oxide sterilization. This approach establishes the minimum conditions to completely kill a certain amount of a resistant microorganism (e.g., a type of bacterial spore). Processes utilizing a half-cycle approach will double the minimum sterilant exposure time to sterilize products. In process validations, the product’s exposure time under optimal sterilization conditions is known as “dwell time.” Doubling the minimum dwell time statistically supports a probability of only one nonsterile unit in a million. In other words, doubling the dwell time of the validated half-cycle approach meets the sterilization criteria for medical devices, parenteral products, and other sterile items.
Bracketing Approach
The bracketing approach defines sterilization conditions (e.g., sterilant concentration, processing temperature, relative humidity) that cover a product’s minimum (under treatment) and maximum (overtreatment) microbial elimination. This method gets its name because identifying a minimum and maximum range for the sterilization process “brackets” the sterilization process conditions. Bracketing occurs through finding the minimum lethality conditions and incrementally increasing sterilization lethality until an ideal maximum lethality metric is reached. A quick neutralization method for sterilants is needed for bracketing method success with liquid phase processes. Otherwise, accurate microbial counts after exposure to different liquid phase process parameters cannot be obtained. Many liquid sterilants have rapid kill rates, so product exposure periods often need to be brief to determine maximum and minimum process lethality parameters. The bracketing approach provides better data on the operating ranges for critical sterilization parameters than the half-cycle method since it defines maximum and minimum values vs. minimum values alone for hydrogen peroxide vapor sterilization and other vapor sterilization protocols.
How do you perform a sterilization validation for steam sterilization?
Steam sterilized products utilize an overkill method to prove an autoclave’s sterilization cycle and parameters can destroy a certain quantity of bioburden. The overkill method requires successfully killing reference microorganisms (bacterial spores) to establish a certain level of sterility. Bacterial spores are a worst-case scenario for bioburden. Thus, the lethality for sterilization cycles that pass an overkill method test will far exceed any unexpected rises in microbial contamination for manufactured products. Understanding the basics of steam sterilization validations helps with the selection of temperature, pressure, humidity, and time parameters that optimize product sterilization.
Steam sterilization validations require multiple formally documented stages. The first sterilization validation stage is the process development stage. In the process development stage, operating parameters and controls used for the sterilization process are investigated and selected. The next stage is the installation qualification stage, which ensures that equipment controls and instrumentation are installed and calibrated appropriately. As part of the installation qualification, systems to regulate steam, water, and air should be verified and documented. The third sterilization validation stage is the operational qualification stage. Operational qualification ensures that installed equipment functions within the set sterilization process parameters. After the operation of the equipment is verified, the performance qualification stage begins. Performance qualifications assess the sterilization of materials, items, and biological indicators that pass through the sterilization process under validation. Performance qualifications measure sterilization cycle controls and the effectiveness of the sterilization cycle in overcoming worst-case biological challenges. The fifth and final stage of sterilization validation is the routine process control stage. This final stage ensures that sterilization processes are continuously monitored and controlled to maintain the efficacy of product sterilization. Understanding the basics of steam sterilization validations helps in creating sterilization process monitoring that ensures medical device and product safety overtime.
The overkill method is utilized as a part of the performance qualification for steam sterilization validations. The overkill methods are used to validate sterilization and to sterilize reusable products. Overkill supports a sterilization process designed to exceed the treatment required to achieve a certain level of sterility, thus accounting for variances in microorganism burden that may occur during pre-sterilization cleaning procedures. Two types of overkill methods can be performed. One involves a full-cycle approach, and the other involves a reduced level of treatment known as a partial cycle approach. An example of a partial cycle approach is a half cycle approach.
In order to perform an overkill sterilization cycle, appropriate biological indicators (or live microorganisms) are placed in product areas that are most difficult to sterilize and are likely to pick up a high level of bioburden (such as device lumens). Next, products are packaged routinely and loaded for sterilization in the location most challenging to achieve sterilizing conditions. Understanding the basics of steam sterilization validations aids in determining difficult to sterilize areas ahead of time and designing medical devices such that areas where bacteria can grow and thrive are minimized.
Overkill Method Partial Cycle Approach
Finding the reduced treatment point needed to inactivate one million microorganisms on an ISO 11138-3 compliant biological indicator (BI) is the goal for a partial cycle approach. Once found, this sterilization treatment level is performed three times to prove reproducibility. The autoclave cycle’s confirmed microorganism inactivation rate can then be used to predict the probability of microorganism survival. Probability is determined using the inactivation kinetics of the sterilizing agent and the number and resistance of the microorganisms on the BI. Understanding the basics of steam sterilization validations will support you in knowing whether a partial or full cycle overkill approach should be used for your sterilization process.
Overkill Method Full Cycle Approach
For a full-cycle approach, the sterilization load should be exposed to the sterilizing agent under normal conditions designed to deliver a particular level of sterilization. The population on the biological indicators used should account for microbial variations and changes in microbial resistance caused by unplanned contact with contaminated material. Microorganisms with high resistance to steam that are suitable for use include G. sterarothermophilus, B. coagulans, C. sporogenes, and B. atrophaeus. Sterilization load is then exposed to a sterilizing agent for the normal cycle to confirm no survivors. Once a successful sterilization cycle is established, the overkill method is to be performed two other times to ensure the repeatability of the process. Understanding the basics of steam sterilization validations supports in correct selection of test microorganisms for biological indicators.
What are the advantages and disadvantages of vapor vs. steam sterilization?
Advantageously, vapor phase sterilization can be used on items that are heat-sensitive. Further, vapor sterilization processes are typically faster than steam sterilization. Vapor sterilization is faster as the liquid phase components of the vapor kill microbes at a faster rate than steam alone.
However, vapor phase sterilization can only be performed on products that can withstand exposure to vapor sterilants without deterioration. In contrast, steam sterilization is less corrosive than vapor sterilants. Further, steam is more penetrative compared to vapor and can sterilize deeper into products. As a limitation, steam sterilization requires much higher temperature exposure than vapor phase sterilization. Thus, understanding the basics of steam sterilization and steam sterilization temperatures is critical for sterilizing medical devices with steam.
Summary
Overall, medical devices, products, and therapies must be sterile. Sterilization is any process that removes, kills, or deactivates microbes. Vapor phase sterilization kills microbes through exposure to sporicidal agents suspended in the air, whereas steam sterilization of medical devices destroys microorganisms 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. In contrast, vapor sterilization works well for non-corrosive heat-sensitive materials and sterilizing surfaces. Hydrogen peroxide vapor sterilization is the most common vapor sterilization method. However, there is no set sterilization validation process for vapor sterilization. However, liquid sterilization processes (half-cycle approach or bracketing validation methods) can be used for vapor phase validations. The bracketing approach provides better data on the operating ranges for critical sterilization parameters than the half-cycle method since it defines maximum and minimum values vs. minimum values alone. Steam sterilization uses an overkill validation method. All in all, you should ensure you choose a contract testing organization that can provide appropriate sterility 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).
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. <1229> Sterilization of Compendial Articles. Rockville, MD, USA. 2021. (USPC <1229>).
United States Pharmacopeial Convention. <1229.11> Vapor Phase Sterilization. Rockville, MD, USA. 2021. (USPC <1229.11>).
