An autoclave is a machine, that is used for the sterilization of the object. In other words, it helps to kill bacteria, viruses, fungi, and spores present on an object. You can measure the effectiveness of the sterilization with the help of measuring tools, a biological indicator or the chemical indicator.
(How does an autoclave work? )
An autoclave is based on the principle of moist heat sterilization.
When the water present inside a closed container is heated, the water begins to vaporize. Since the container is sealed from all sides, the vapor will not be able to escape. Due to this reason, the vapor will create pressure within the chamber.
The vapor circulates throughout the chamber. Thus, it will be able to exert pressure from all sides. So, the high pressured vapor will be able to penetrate the external layer of the micro-organisms. Then the vapor will cause the precipitation of the protein, which is a major component of the micro-organisms. So, the micro-organisms will not be able to utilize this protein causing its death.
Also, there is a direct relationship between the boiling point and the vapor pressure. Since, the pressure within the autoclave increases, the boiling point of the water will also increase. Thus, the rise in the boiling point of water will allow heating the water even more, without changing its form. So, the micro-organisms will not be able to handle this excess heat, leading to its death.
Autoclave Cycle Stages
An autoclave machine undergoes the following stages during the process of moist heat sterilization.
- Chamber pre-heating:- During this process, the water inside the chamber begins to heat up. This causes the rise in the formation of vapor within the chamber.
- Vacuum Performance:- This is only available in the vacuum displacement autoclaves. During this process, the pump draws out the cold air from the chamber to the atmosphere.
- Sterilization:- During this process, the temperature and the pressure reach the set value. The sterilization should last for 15 minutes or more.
- Vacuum Drying:- This is only available in the advanced form of vacuum displacement autoclaves. During this process, the pump draws out the steam from the chamber to the atmosphere. The atmospheric cold air enters the machine.
In other types of the machine, we simply open the steam release valve. Due to lower atmospheric pressure, the steam from the chamber escapes out. However, this method will not be able to completely remove the steam.
- Door Open
Autoclave Temperature and Pressure
The pressure exerted by the vapor on the walls of the container is known as vapor pressure. As the vapor pressure increases, the boiling point of the water will also increase. In other words, there is a direct relationship between vapor pressure and the boiling point of the water.
When the boiling point of the water increases, the temperature of the water will also increase. But, for effective sterilization, the autoclaves have been designed to operate at 121°C (15 Psi pressure).
At a temperature below 121°C, the process of sterilization will not be effective. Similarly, on increasing the temperature above 121°C, there will not be any significant impact on the quality of sterilization. Thus, for normal sterilization, you need to maintain 121°C of temperature or 15 Psi of vapor pressure for at least 15 minutes.
How to check efficiency of autoclave?
The effectiveness of the autoclave should be checked from time to time. So, for this, one of the main ways is to check the effectiveness of moist heat sterilization. For checking the effectiveness of the sterilization there are mainly three ways. They are as follows.
- Use of devices that measures the temperature and the vapor pressure within the chamber.
- Use of the chemical indicators which change their color when the specified temperature has been reached. Autoclave tape is the most common type of chemical indicator available in the market.
(Note:- Autoclave tape is a type of tape that you stick on the physical object, that you want to autoclave. Initially, it is yellow in color. But, after the process of sterilization is over, the color of the tape changes from yellow to black. This marks, that the process of sterilization is successful. )
- Use of the biological indicators such as spores of Bacillus stearothermophilus which gets passive when it is exposed to 121°C for 20 minutes.
Types of Autoclave
1. Based on the method of air replacement
The presence of air within the chamber can be a major cause of hindrance for the sterilization process. So, you need to replace the cold air present with the hot steam. So, based on the method of replacement of air, the autoclave is of two types.
Gravity Displacement Autoclave
This is the most common type of autoclave. In this method, the hot steam rises up in the chamber. During this process, the steam pushes the cold air that is already present in the chamber. So, the cold air escapes from the exhaust valve of the machine.
Since the machine is based upon simple mechanisms they are simple in design. They are easier to operate. Also, the cost price is relatively low.
Gravity displacement autoclave is mainly suitable for those materials, that do not need deep penetration. For example, plasticware, glassware, liquid media, etc.
Vacuum Displacement Autoclave
This method creates a vacuum within the chamber with a help of a pumping system. Thus the steam can easily cover all the space within the chamber.
Since the machine is based upon a much more complex mechanism, they are quite complex in design and operation. They are also expensive to some extend.
Vacuum displacement autoclave is mainly suitable for those materials which are porous or are hard to reach. For example, fabrics wrapped surgical kits, cotton, etc.
2. Based on Classes
The autoclaves have further been classified into three classes. They are as follows.
Class N Autoclave
N type of autoclave is equivalent to that of gravity displacement autoclave.
Class S Autoclave
S type autoclave is an intermediate type of N and B class. It is a type of vacuum displacement autoclave. However, it only allows for a single pre-vacuum system. Due to this reason, the effectiveness of the class S autoclave is less than that of class B.
Class B Autoclave
The most advanced form of the autoclave that has the function of the fractioned pre-vacuum system is the class B autoclave. It has also the property of vacuum drying. The result of class B autoclave is superior to that of the class N and S. It is mostly used in dentistry, tattoo studio, etc.
3. Based on Design
Based on the design, they are further classified into the following types.
- Pressure Cooker Type Autoclave
- Horizontal Autoclave
- Vertical Autoclave
- Benchtop Autoclave
(Materials that can be autoclaved)
An autoclave machine can be applied in the sterilization of equipment in a number of industries. Some of the applications are as follows.
- Useful in scientific research, laboratory research, etc.
- Used in the sterilization of autoclavable glassware and plasticware, used in the laboratory, microbiology, pharmaceuticals, etc.
- Used for sterilization of biohazardous waste materials before disposal.
- Medical autoclaves are also used in the sterilization of surgical equipment, surgical blades. etc. They are also used for the sterilization of the aprons, gowns, etc, of the health workers.
- For the sterilization of culture media.
- Industrial autoclaves are used in many industrial sectors such as automobiles, agriculture, etc.
Even though you can sterilize multiple objects, there are certain limitations to it. The first limitation is that there is a boundary for temperature. Some materials may not be able to sustain high temperatures. In such a case, the materials will get damaged.
The second limitation is that it follows the principle of moist heat sterilization. So, after the sterilization, the object will become wet.
So based on these two limitations, a few of the objects which are not autoclavable are stated below.
- Should not be used to sterilize waterproof materials such as oil, grease, etc, or dry materials such as gloves powder.
- Should not be used to sterilize materials that are susceptible to corrosion.
- Low-quality glass and plastic materials will not be able to handle the high temperature.
- The proper sterilization of the over-loaded materials within the autoclave will not be possible.
- The sterilization of liquid enclosed within a closed vessel is not possible. This is because the liquid when vaporizes for a long period of time can break the vessel enclosing it.
- Wrapping the objects with aluminum foil, plastic cover, etc, reduces the effectiveness of the sterilization.