An autoclave is used in medical and laboratory settings to sterilize lab equipment and waste. Autoclave sterilization works by using heat to kill microorganisms such as bacteria and spores. The heat is delivered by pressurized steam. Pressurization allows the steam to reach the high temperatures that are required for sterilization.
Of all the methods available for sterilization, moist heat in the form of saturated steam under pressure is the most widely used and the most dependable. Steam sterilization is nontoxic, inexpensive , rapidly microbicidal, sporicidal, and rapidly heats and penetrates fabrics . Like all sterilization processes, steam sterilization has some deleterious effects on some materials, including corrosion and combustion of lubricants associated with dental handpieces; reduction in ability to transmit light associated with laryngoscopes; and increased hardening time with plaster-cast.
What is an Autoclave?
Autoclave is a device designed for sterilization of various tools – cosmetic, surgical, dental, equipment used in aesthetic medicine parlors, piercing salons and tattoo studios. Moreover, thanks to the development of technology, autoclaves are now much smaller than they used to be, while at the same time they continue to increase their capabilities – for example they are faster and easier to use. What is more, the price of such devices is decreasing, making them more and more available and affordable. They are an essential part of the equipment of any of the abovementioned parlors, as effective sterilization is the basis for the hygienic and safe functioning of such parlor. The autoclave is a kind of hermetically sealed, heated tank used for chemical processes. Thanks to the thick walls inside its chamber, high pressure can be maintained, and it’s crucial for applying a higher temperature, which ensures more effective sterilization. In addition, the condensation of water vapor leads to the release of energy, which completely eliminates harmful microorganisms.
The autoclave, thanks to its structure, allows the sterilization process to be carried out by using high pressure and steam, which kill microorganisms, including dangerous viruses and bacteria and all their spore forms. The use of steam-pressure autoclave is one of the most effective methods of sterilization. Sterilization by definition is the process of destroying all living forms of microorganisms, including spore forms. It can be done physically, chemically or mechanically. An instrument that is sterile – i.e. after a sterilization process – is completely safe for the human body. For sterilization to be effective, the material must be properly prepared, the process itself must be carried out correctly and the instruments have to be stored properly after sterilization. The equipment should first be pre-disinfected just after usage, washed under running water or using automatic washing machines, dried and then put into labelled packages to sterilize them. Correct sterilization should not damage or alter the equipment’s properties.
The history of the autoclave dates back to 1679. The autoclave with a valve (safety flap), which was also a prototype of a steam boiler, was invented by a physicist of French origin – Denis Papin. In those days it was called “Papin’s boiler”. It was also used as a pressure cooker. His invention was first demonstrated in 1682, during feast organized for members of the Royal Society in London. Thanks to Papin’s invention cooked dishes could be consumed during that celebration. Since then many changes in the production and use of autoclaves have occurred. Most of the processes have been automatized, the equipment is electronically controlled, supplied with distilled water or connected to the water supply system, and the processes are electronically archived. Some devices also have printer functions and displays showing the machine’s operating stages.
How Does an Autoclave Work?
Basic principle of steam sterilization
The basic principle of steam sterilization, as accomplished in an autoclave, is to expose each item to direct steam contact at the required temperature and pressure for the specified time. Thus, there are four parameters of steam sterilization: steam, pressure, temperature, and time. The ideal steam for sterilization is dry saturated steam and entrained water (dryness fraction ≥97%).Pressure serves as a means to obtain the high temperatures necessary to quickly kill microorganisms. Specific temperatures must be obtained to ensure the microbicidal activity. The two common steam-sterilizing temperatures are 121°C (250°F) and 132°C (270°F). These temperatures (and other high temperatures) must be maintained for a minimal time to kill microorganisms. Recognized minimum exposure periods for sterilization of wrapped healthcare supplies are 30 minutes at 121°C (250°F) in a gravity displacement sterilizer or 4 minutes at 132°C (270°F) in a prevacuum sterilizer. At constant temperatures, sterilization times vary depending on the type of item (e.g., metal versus rubber, plastic, items with lumens), whether the item is wrapped or unwrapped, and the sterilizer type.
How Autoclaves Work
Once you close the autoclave sterilizer chamber, a vacuum pump removes all the air from inside the device or it is forced out by pumping in steam. If done the first way, the sterilizer is pumped with high pressured steam to quickly raise the internal temperature. On every autoclave there is a thermometer that is waiting for the thermal sweet point, 268-273 degrees Fahrenheit, and then it starts its timer. During the sterilizing process, steam is continuously entering the autoclave to thoroughly kill all dangerous microorganisms. Once the required time of autoclave sterilization has the elapsed, the chamber will be exhausted of pressure and steam allowing the door to open for cooling and drying of the contents.
The various phases of a sterilization cycle
For a more detailed explanation of the various phases of a sterilization cycle, please refer to the list and image shown below:
1. Purge Phase: Steam flows through the sterilizer and starts to displace the air; temperature and pressure ramp slightly to a continuous flow purge.
2. Exposure (Sterilization) Phase: During this phase, the autoclave’s control system is programmed to close the exhaust valve, thereby causing the interior temperature and pressure to increase to the desired setpoint. The program then maintains the desired temperature (dwells) until the desired time is reached.
3. Exhaust Phase: Pressure is released from the chamber through an exhaust valve and the interior is restored to an ambient pressure (though contents remain relatively hot).
Critical Components of an Autoclave
The typical laboratory autoclave is comprised of the following components:
The vessel is the main body of the autoclave and consists of an inner chamber and an outer jacket. Laboratory and hospital autoclaves are constructed with “jacketed” chambers (see Figure 4), where the jacket is filled with steam, reducing the time that it takes to complete a sterilization cycle and reducing condensation within the chamber. A vessel designed and manufactured with a full jacket is superior to that of a partial jacket or blanketed jacket for the following reasons: a full jacket improves temperature uniformity within the chamber, it reduces the likelihood of wet packs, and helps minimize wet steam, which is not good for sterilization.
In the United States, every autoclave vessel is inspected and tagged with an American Society of Mechanical Engineers (ASME) nameplate that includes a National Board number. Manufacturers are required to hydrostatically test each vessel and apply an ASME nameplate before putting an autoclave into use. This inspection and the ASME nameplate are key indicators of a properly functioning autoclave.
Laboratory and hospital autoclave vessels can vary in size, from 100L to 3,000L, and are typically constructed from 316L stainless steel. Inner chambers are either 316L or nickel-clad, and outer jackets are made of 316L, 304L stainless steel, or carbon steel.
2. Control System
All modern autoclaves are equipped with a controller interface, not unlike what you would find on a microwave or oven. That said, autoclave control systems tend to be a bit more sophisticated than those of household appliances. A sterilization cycle follows a preprogrammed software formula that opens and closes valves and other components in a specific sequence. Therefore, all autoclaves require some form of control system, whether it’s as simple as a “push button” system with a microprocessor or as complex as a programmable logic controller with color touch screen.
3. Thermostatic Trap
All autoclaves feature some form of thermostatic trap or steam trap, a device designed to allow air and water (condensate) to escape from the chamber. Although a steam delivery system/steam autoclave might use a variety of traps, they all perform the same basic function: removing condensate while preventing the passage of dry steam. Most often, steam traps are temperature-sensitive valves that close when heated part a certain setpoint. Thermostatic traps are a critical component of any well-designed autoclave.
4. Safety Valve
All autoclaves operate under elevated pressure (14–45 pound-force per square inch gauge) and must therefore be manufactured with an incredibly robust construction and fitted with a number of safety features and devices to ensure they present no danger to users. One of these safety devices is the safety valve, which is the final fail-safe device for the pressure vessel should all electronic controls fail. It is imperative that the safety valve be inspected, tested, and verified to be in proper working condition based on the recommendations of the sterilizer and/or valve manufacturer, as well as local inspection and insurance agencies.
5. Waste-Water Cooling Mechanism
Many autoclaves are equipped with a system to cool effluent (air, steam, and condensate) before it enters the drain piping. Many municipalities and buildings do not allow effluent above 140˚F to enter the floor drain. In order to avoid damage to the facility’s drain piping, the steam must be cooled before it can be sent down the drawn. The simplest method for cooling this steam is to mix it with additional cold tap water, but the amount of water required can cause an autoclave to be a major contributor to a building’s water usage. Some autoclaves come equipped with systems designed to reduce, or even eliminate, water consumption.
6. Vacuum System (if applicable)
In order to ensure proper sterilization, it’s vital that all air inside the autoclave chamber be replaced with steam. Certain commonly sterilized goods — especially porous materials such as animal bedding or cloth or containers with small opening such as flasks or goods in bags — tend to retain air pockets. If an air pocket is present during the cycle, any microorganisms within that pocket will survive, and the goods will not be sterile.
For this reason, many sterilizers will include a vacuum system. This not only enables the user to forcibly remove air by using a vacuum on the chamber before a cycle (known as pre-vacuum), is also enables them to use a vacuum after the cycle (known as post-vacuum) to remove any steam that remains in the chamber and to dry off the goods inside the autoclave.
7. Steam Generator (if applicable)
A central “house” boiler is the most common steam source for an autoclave. However, if house steam is unavailable or insufficient for the autoclave, one must resort to using an electric steam generator, also known as a boiler. These boilers typically sit underneath the autoclave chamber and utilize electric heating elements to heat water and generate steam.
Types of Autoclaves
The two basic types of steam sterilizers (autoclaves) are the gravity displacement autoclave and the high-speed prevacuum sterilizer. In the former, steam is admitted at the top or the sides of the sterilizing chamber and, because the steam is lighter than air, forces air out the bottom of the chamber through the drain vent. The gravity displacement autoclaves are primarily used to process laboratory media, water, pharmaceutical products, regulated medical waste, and nonporous articles whose surfaces have direct steam contact.
For gravity displacement sterilizers the penetration time into porous items is prolonged because of incomplete air elimination. This point is illustrated with the decontamination of 10 lbs of microbiological waste, which requires at least 45 minutes at 121°C because the entrapped air remaining in a load of waste greatly retards steam permeation and heating efficiency.The high-speed prevacuum sterilizers are similar to the gravity displacement sterilizers except they are fitted with a vacuum pump (or ejector) to ensure air removal from the sterilizing chamber and load before the steam is admitted.
The advantage of using a vacuum pump is that there is nearly instantaneous steam penetration even into porous loads. The Bowie-Dick test is used to detect air leaks and inadequate air removal and consists of folded 100% cotton surgical towels that are clean and preconditioned. A commercially available Bowie-Dick-type test sheet should be placed in the center of the pack. The test pack should be placed horizontally in the front, bottom section of the sterilizer rack, near the door and over the drain, in an otherwise empty chamber and run at 134°C for 3.5 minutes.
The test is used each day the vacuum-type steam sterilizer is used, before the first processed load. Air that is not removed from the chamber will interfere with steam contact. Smaller disposable test packs (or process challenge devices) have been devised to replace the stack of folded surgical towels for testing the efficacy of the vacuum system in a prevacuum sterilizer.These devices are “designed to simulate product to be sterilized and to constitute a defined challenge to the sterilization process.”They should be representative of the load and simulate the greatest challenge to the load.Sterilizer vacuum performance is acceptable if the sheet inside the test pack shows a uniform color change. Entrapped air will cause a spot to appear on the test sheet, due to the inability of the steam to reach the chemical indicator. If the sterilizer fails the Bowie-Dick test, do not use the sterilizer until it is inspected by the sterilizer maintenance personnel and passes the Bowie-Dick test.
Another design in steam sterilization is a steam flush-pressure pulsing process, which removes air rapidly by repeatedly alternating a steam flush and a pressure pulse above atmospheric pressure. Air is rapidly removed from the load as with the prevacuum sterilizer, but air leaks do not affect this process because the steam in the sterilizing chamber is always above atmospheric pressure. Typical sterilization temperatures and times are 132°C to 135°C with 3 to 4 minutes exposure time for porous loads and instruments.
Like other sterilization systems, the steam cycle is monitored by mechanical, chemical, and biological monitors. Steam sterilizers usually are monitored using a printout (or graphically) by measuring temperature, the time at the temperature, and pressure. Typically, chemical indicators are affixed to the outside and incorporated into the pack to monitor the temperature or time and temperature. The effectiveness of steam sterilization is monitored with a biological indicator containing spores of Geobacillus stearothermophilus (formerly Bacillus stearothermophilus). Positive spore test results are a relatively rare event and can be attributed to operator error, inadequate steam delivery,or equipment malfunction.
Portable (table-top) steam sterilizers are used in outpatient, dental, and rural clinics. These sterilizers are designed for small instruments, such as hypodermic syringes and needles and dental instruments. The ability of the sterilizer to reach physical parameters necessary to achieve sterilization should be monitored by mechanical, chemical, and biological indicators.