Sterilization By Moist Heat
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 moist heat?
Sterilization by moist heat is also known as steam sterilization. Moist heat sterilization destroys microorganisms in a product with steam under pressure. Sterilization by moist heat is the most common method for medical device and medical product sterilization.
What items can be sterilized by moist heat?
Items traditionally sterilized by moist heat include rubber, durable plastic materials, mixing tanks, surgical equipment, filling equipment, freeze-dryer chambers, and filled product containers that can withstand high-temperature exposure.
How is sterilization by moist heat 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.
In autoclaves thermocouples monitor temperature. Bowie–Dick or Dart indicators verify that the temperature measured is steam heat vs. dry heat. Dark brown stripes appear across the Bowie–Dick tape when enough steam penetration has occurred. Other indicators for sterilization validation of moist heat sterilization processes are temperature, pressure, and biological indicators.
Lethality of microorganisms depends on:
- Degree of heat exposure
- Duration of heat exposure
- Moisture level
Heat-based sterilization methods kill microorganisms by denaturing proteins within the cells. For moist heat sterilization, saturated steam that hits a cooler surface than itself will increase the temperature of the surface and release heat of condensation during the phase change of water from gas to liquid. The heat of condensation releases hundreds of calories of energy, thus killing any microorganisms in the area the steam penetrates. If moisture cannot reach an item, such as oil, sterilization by moist heat will not effectively sterilize the item.
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
In order to create steam, water’s boiling point is raised from 100◦C to 121◦C by applying 15 pounds per square inch of pressure above atmospheric pressure. 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 the point 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.
What type of autoclave should you use for your medical device or product?
There are several different designs of autoclaves that are used. Each differs in how the post-sterilization stage is accomplished. Post-sterilization is a depressurization stage where steam is replaced by air. Examples of these various autoclave designs are described below.
Autoclave with vacuum and time-controlled vacuum maintenance
The product load after sterilization cycle completion is dried and cooled with vacuum purges. Autoclaves using time-controlled vacuum maintenance are used for solid materials (porous and nonporous).
Autoclave with circulating cold water in the jacket
For this autoclave type, steam is removed as compressed sterile air is introduced. The compressed air is added at pressures equal to the sterilization pressure, which prevents solutions from boiling. This autoclave also has cold water circulating within the jacket of the autoclave to support cooling. Culture media and other liquids are sterilized using this type of autoclave.
Autoclave with nebulized spray water
After the sterilization cycle, these autoclaves spray nebulized cool water onto the sterilized load to rapidly condense steam and reduce pressure. Glass-sealed ampoules and plastic containers are often sterilized in this type of autoclave.
Autoclave with superheated water spray (water cascade)
This autoclave is used to sterilize flexible containers that can’t tolerate sudden changes in temperature and pressure together. At the beginning of the sterilization process for water cascade autoclaves, no air in the chamber is removed. Instead, water is circulated in a heat exchanger and sprayed onto the load. This process provides excellent temperature uniformity, which decreases sterilization time. Cold tap water flows into the heat exchanger’s plates to replace the steam and cool the load. Sterile air counter-pressure within the autoclave chamber is always maintained so that no thermal or pressure shock occurs. This type of autoclave cannot dry the containers during the cycle.
Autoclave With Air Over Steam Counter Pressure
Counter pressure autoclaves are similar to water cascade autoclaves. The benefits of counter-pressure autoclaves are that you can dry containers during the cycle. The disadvantage is that the cooling phase takes much longer than for water cascade autoclaves. Like water cascade systems, no air in the chamber is removed before the cycle. Pressures inside the chamber of this autoclave are much higher than conventional saturated steam autoclaves. Partial air pressure of air and steam is adjusted during the entire autoclave process with fans and flow deflectors in the chamber, assuring a homogeneous steam and air mixture. The cooling phase occurs by feeding air into the chamber to condense the steam while maintaining the sterilization phase pressure.
Sterilization-In-Place (SIP)
SIP is used to sterilize large equipment items, mixing tanks, vessel-filter-filler systems, and isolator units.
For successful SIP, equipment must:
- Withstand pressures required for steam sterilization
- Have adequate air venting using microbial retentive filters
- Trap condensate
- Have no leaks
- Have no inner surfaces that cannot be exposed to steam
What are the problems with sterilization by moist heat?
The greatest problem with sterilization by moist heat is that not all items can be exposed to pressurized steam and maintain their integrity. Thus, sterilization by moist heat will not work for all products, especially products containing electronics or flexible plastics. Also, oils or enclosed dry systems cannot effectively be terminally sterilized by moist heat as steam cannot reach these items.
Summary
Overall, sterilization by moist heat is the cheapest and most common sterilization method. Sterilization by moist heat kills microbes through exposure to pressurized steam. Sterilization validations for sterilization by moist heat often use the overkill method. 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>).
