Liquid 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. This article covers medical liquid sterilization, contract steam sterilization, liquid phase sterilization standards, and steam sterilization standards.
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 liquid chemicals are able to destroy resistant bacteria (spores) as well as fungi and meet sterilization standards for some medical products.
What is steam sterilization (sterilization by moist heat)?
Steam sterilization (also known as moist heat sterilization) and contract steam sterilization are performed in an autoclave. Moist heat sterilization meets steam sterilization standards by destroying microorganisms on (or within) a product with steam under pressure. Steam kills the microorganisms by denaturing proteins within the cells. Steam sterilization (including contract 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. Thus, healthcare facilities are able to meet steam sterilization standards inhouse and without the need for contract steam sterilization.
What products or medical devices can undergo liquid vs. steam sterilization?
Items that can meet steam sterilization standards without damage include mixing tanks, surgical medical devices, filling equipment, freeze-dryer chambers, and filled product containers that can withstand high-temperature exposure. Materials commonly autoclaved that meet steam sterilizations standards without damage are rubber, metals, and durable plastic materials. In contrast, medical products undergoing liquid sterilization will only be sterilized at the outer surface of the product or package. 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?
Medical products with liquid sterilizations undergo a simple process. 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 or 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 for medical products with liquid sterilizations.
How do you select a liquid sterilant for liquid phase sterilization?
The antimicrobial activity of the sterilant for medical products with liquid sterilizations 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 is steam sterilization performed?
Simply speaking, contract steam sterilization is performed by steam under pressure. The most common devices used to meet steam sterilization standards 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 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). Contract steam sterilization 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 steam 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 undergoing inhouse or contract steam sterilization.
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 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.
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.
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.
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.
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 moist heat 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.
What are the advantages and disadvantages of steam vs. 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. Steam sterilization is less corrosive than liquid sterilants. Further, steam is penetrative and can sterilize both inner and outer surfaces. However, steam sterilization is slower than liquid phase sterilization. Also, steam sterilization requires much higher temperature exposure than liquid phase sterilization.
Summary
Overall, medical devices, products, and therapies must be sterile. Sterilization is any process that removes, kills, or deactivates microbes. Liquid phase sterilization kills microbes through immersing products, packages, or other items in a chemical solution, whereas steam sterilization destroys microorganisms on (or within) a product 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, 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. Liquid sterilization processes utilize a half-cycle approach or bracketing validation methods. In contrast, steam sterilization uses an overkill validation method. 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
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. <1229.6> Liquid-Phase Sterilization. Rockville, MD, USA. 2021. (USPC <1229.6>).
