Liquid vs. Vapor Phase 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.
What is liquid phase sterilization?
Liquid phase sterilization kills microbes through immersing products, packages, or other items in a chemical solution. Liquid sterilant lethality depends upon the chemical agent’s concentration and temperature. Examples of chemical agents (by chemical families) used as liquid sterilants are aldehydes, acids, bases, and strong oxidants. These chemicals are able to destroy resistant bacteria (spores) as well as fungi.
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 (H2O2), peracetic acid (CH3CO3CH), formaldehyde (CH2O), and glutaraldehyde [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.
What products or medical devices can undergo liquid vs. vapor sterilization?
Vapor sterilization works well for heat-sensitive materials and for the sterilization of surfaces. In contrast, liquid phase sterilization is useful only for sterilizing the outer surface of products and other items. Products that require the sterilization of inner surfaces or are sensitive to corrosion by liquid chemical agents should not be sterilized with this method.
How is liquid phase sterilization performed?
Liquid phase sterilizations are simple. Objects to be sterilized are fully immersed in a sterilant solution under a designative temperature and for a set time. Then aseptic processes are used to inactivate or remove the liquid sterilant and extract sterilized products. Recontamination can occur during the liquid sterilant inactivation/removal process, unlike other sterilization methods. Thus, including steps to avoid recontamination and remove the liquid sterilant is a part of the sterilization process for liquid phase sterilization. There aren’t any widely accepted biological indicators for liquid sterilization. Thus, resistant bacterial spores like Bacillus atrophaeus or B. subtilis are used to verify sterility.
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 exposing them to 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.
How do you select a liquid sterilant for liquid phase sterilization?
The antimicrobial activity of the sterilant is impacted by pH, concentration, processing temperature, contact time, the extent of liquid agitation (mixing) during sterilization, and the presence of particulate or cellular debris on items undergoing sterilization. Moreover, sterilants vary in their stability and interaction with product materials undergoing sterilization. When selecting a sterilant, the impact of the chemical agent on equipment, product, and packaging materials is the most critical consideration. Another consideration is personnel safety during sterilant use as liquid sterilization agents are often highly toxic and require additional safety precautions to be taken.
How do you select between hydrogen peroxide or peracetic acid for vapor 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 do you validate a liquid sterilization process?
Liquid sterilization processes can be validated with two approaches, a half-cycle approach or a bracketing validation 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.
How do you validate a 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, there are no standardized biological indicators or D-values that 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.
Essential practices required to maintain validated status for liquid and vapor phase sterilization include:
- Regular calibration
- Regular chemical and physical measurements
- Biological indicator usage and regulation
- Ongoing process control
- Ongoing change control
- Preventive maintenance
- Periodic reassessments
- Training of personnel
What are the advantages and disadvantages to vapor phase and liquid phase sterilization?
Liquid phase sterilization is a fast sterilization process. However, liquid sterilants are harsh and can only sterilize a subset of materials without causing chemical degradation or corrosion. Further, liquid phase sterilization is a surface sterilization technique. If inner surfaces need to be sterilized, this method is not a preferred sterilization technique. In contrast, vapor phase sterilization is a slower sterilization method since sterilant gas components kill microbes at a slower rate than liquid phase components of the vapor. Advantageously, vapor phase sterilization can do more than surface-deep sterilization. However, vapor phase sterilization can only be performed on products that can withstand exposure to vapor sterilants without deterioration.
Summary
Overall, medical devices, products, and therapies must be sterile. Sterilization is any process that removes, kills, or deactivates microbes. Vapor phase sterilization inactivates microbes through exposure to sporicidal agents suspended in the air. Liquid phase sterilization kills microbes through immersing products, packages, or other items in a chemical solution. Vapor sterilization works well for heat-sensitive materials and the sterilization of surfaces, whereas liquid phase sterilization is useful for sterilizing the outer surfaces of items. Products that require the sterilization of inner surfaces or are sensitive to corrosion by liquid chemical agents should not be sterilized with this method. Typical vapor phase sterilization agents are hydrogen peroxide, peracetic acid, formaldehyde, and glutaraldehyde. Examples of chemical agents (by chemical families) used as liquid sterilants are aldehydes, acids, bases, and strong oxidants. 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. All in all, 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
Michael J. Akers. Sterile Drug Products Formulation, Packaging, Manufacture, and Quality. Drugs and the Pharmaceutical Sciences. Informa Healthcare. 2010.
United States Pharmacopeial Convention. <1229.6> Liquid-Phase Sterilization. Rockville, MD, USA. 2021. (USPC <1229.6>).
United States Pharmacopeial Convention. <1229.11> Vapor Phase Sterilization. Rockville, MD, USA. 2021. (USPC <1229.11>).
