What is liquid phase sterilization?
What is sterilization, and why is it essential for sterile products?
Sterilization is any process that removes, kills, or deactivates all forms of life. Sterilization keeps patients safe from toxins and microbial illnesses when therapies or devices are consumed or used. Many companies offer product sterilization services to support medical device and healthcare companies. 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 liquid sterilization (also known as liquid phase sterilization), which uses a liquid sterilization agent (from liquid sterilant selection) for product 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 for liquid sterilant selection are aldehydes, acids, bases, and strong oxidants. These liquid sterilization agents are able to destroy resistant bacteria (spores) as well as fungi.
What do liquid sterilants include:
- Aldehydes—glutaraldehyde, formaldehyde
- Acids—peracetic acid, nitric acid, sulfuric acid
- Bases—sodium hydroxide, potassium hydroxide
- Oxygenating compounds—hydrogen peroxide, ozone, chlorine dioxide
- Halides—sodium hypochlorite, chlorine
What products or medical devices can undergo liquid phase sterilization?
Liquid phase sterilization is useful for the outer surface sterilization of items. Products that require the sterilization of inner surfaces or are sensitive to corrosion by liquid sterilization agents should not be sterilized with this method.
How is liquid phase sterilization performed?
Fluid-based sterilizations are simple once liquid sterilant selection occurs. 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. There aren’t any widely accepted biological indicators for liquid phase sterilization. Thus, resistant bacterial spores like Bacillus atrophaeus or B. subtilis are used to verify sterility.
How do you select a liquid sterilant for Product Sterilization Services?
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 validate liquid sterilization process For Product Sterilization Services?
Liquid sterilization processes can be validated with two approaches, a half-cycle approach or a bracketing validation method.
Half-Cycle Approach Product Sterilization Services
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 Product Sterilization Services
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.
Sterilization validation activities needed to validate liquid sterilization processes:
- Equipment qualification— The equipment qualification for liquid sterilization confirms equipment installation and operation. Equipment performing temperature control and agitation or recirculation rates are the qualification essentials for sterilization by liquid chemicals.
- Component and load definition—Each item to be sterilized must be completely submerged into the liquid sterilant to kill microbes effectively. Thus, recirculation or mixing of the chemical sterilant is needed to keep items uniformly coated with liquid during the sterilization process. Items to be sterilized may have closely fitted parts, lumens, or porous materials. The sterilizing liquid will need to penetrate all components of the item to perform appropriate sterilization. Thus, it is important to define the components of the items to be loaded into the chemical sterilant solution. A maximum load per vessel or container should be the worst-case product load scenario and provide the greatest surface area to be sterilized with the validated process.
- Biological indicators— atrophaeus and B. subtilis are spores used as biological indicators for liquid phase sterilization. In validations for liquid phase sterilization, spores with a bioburden that correlates to manufacturing-accumulated microbe levels are directly added to items under sterilization for a microbial challenge.
- Process and microbiological challenge confirmation— This process confirms that the sterilization process can repeatedly kill biological indicators and microbial challenges. A linear death curve for the challenge microorganism will be created, and the death of all challenge organisms will be confirmed. If needed, the liquid sterilization process can be modified to increase lethality through adjusting chemical sterilant concentration, chemical exposure time, liquid agitation, and other factors. In some cases, liquid sterilant selection may be repeated.
- Liquid sterilization agent neutralization or removal— Like with gaseous sterilants, the sterilizing liquid must be removed from the items or neutralized before items can be handled or processed further. Liquid neutralization or removal can be tricky as this process must still maintain the sterility of the sterilized products. Thus, liquid neutralization or removal is often handled under aseptic conditions. Liquid sterilant selection plays a big part in the available neutralization or removal methods for the sterilization process.
Essential practices required to maintain validated status for liquid 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
Summary
Overall, medical devices, products, and therapies must be sterile. Sterilization is any process that removes, kills, or deactivates microbes. 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. 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. Liquid sterilization processes can be validated with two approaches, a half-cycle approach or a bracketing validation method. 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>).
