(A) Moist heat sterilization:
The object to be sterilized is exposed to saturated steam under 1 Atmosphere pressure at a minimum temperature of 121°C for 15 minutes where microorganisms are destroyed by cellular protein coagulation. It is a device to sterilize equipment and supplies by subjecting them to high-pressure saturated steam at various temperatures and times (Table). Two types of autoclaves are used likely portable autoclave and stationary autoclave. A stationary autoclave is used as a large-scale sterilizer.
Table: Working condition of an Autoclave at various temperatures and times
|Temperature (degree C)||Pressure (lb/sq.inch)||Time (Minutes)|
Working of Autoclave:
All autoclaves operate on similar principles that they share with a common kitchen pressure cooker – the door is locked to form a sealed chamber, and all air within the chamber is replaced by steam. The steam is then pressurized to reach the desired sterilization temperature and time, before exhausting the steam and allowing the goods to be removed. The various phases of a sterilization cycle are given below (Fig.1).
1. Purge Phase: Steam flows through the sterilizer beginning the process of displacing the air; temperature and pressure ramp slightly to a continuous flow purge.
2. Exposure (Sterilization) Phase: During this phase, the autoclaves’ control system is programmed to close the exhaust valve causing the interior temperature and pressure to ramp up to the desired set point. The program then maintains the desired temperature (dwells) until the desired time is reached.
3. Exhaust Phase: The pressure is released from the chamber through an exhaust valve and the interior is restored to ambient pressure, although contents remain relatively hot.
(B) Dry heat sterilization:
Hot air oven:
It is an electrical device and uses dry heat to sterilize. It was originally developed by Pasteur. Generally, it can be operated from 50 to 300°C, using a thermostat to control the temperature. It requires direct heat at different times and temperatures (Table).
Table: Working condition of hot air oven
|Temperature (degree C)||Time (Minutes)|
The working of the hot air oven is based on the hot air inside the chamber of the oven by the forced circulation. When hot air reaches the top of the chamber, it is circulated back to the bottom by a fan installed inside the chamber and hence the optimum amount of heat is achieved gradually inside the hot air oven. After heating the content of the oven for two hours at 160°C, the articles are allowed to remain there, till the temperature comes down to 40°C. then the sterilized materials are then removed from the oven.
1. It is mainly used for the sterilization of glassware such as pestle and mortar, Petri dishes, flasks, pipettes, bottles, test tubes, etc.
2. It is used for the sterilization of powders such as sulfacetamide, sulphadiazine, kaolin, zinc oxide, starch, etc.
3. Injections where fixed oils are used as the vehicle are sterilized by the dry heat method like injections of progesterone, injection of testosterone propionate, and injections of oestradiols dipropionate.
4. It is also used for sterilization of scalpels, scissors, spatula, blades, and glass syringes.
5. The chemicals, glassware in laboratories, research institutions, industries, hospitals, and R&D centers used hot air ovens are suitable for temperatures up to 250°C.
6. Hot air ovens suits various applications like heating, drying, sterilizing, and baking.
Electromagnetic radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. Electromagnetic energy in the microwave region (225 MHz to 100 GHz, especially 2,450 MHz) is extensively used as an alternative energy source for sterilization. In a microwave oven heat is a byproduct and is controlled by temperature probes where sterilization is also achieved at 40°C.
Sterilization methods are carried out with the use of microwaves where a bactericidal solution is used for heating. The technique is being promoted for use with dental instruments and relies on heating a solution of a quaternary ammonium compound (benzalkonium chloride) to approximately 100°C. At present, these processes apply only to unpackaged instruments. Dry spores of Bacillus subtilis and B. niger are treated with heat simultaneously in a convection dry-heat oven and a microwave oven.
Microwaves are radio waves and the commonly used radio wave frequency is roughly 2,450 megahertz (2.4 gigahertz). These waves are absorbed by water and other nutrients and then converted directly into atomic motion and motion is converted into heat.
The microwaves are generated in a microwave by a device called a magnetron, which converts electricity from the power outlet into microwaves. These microwaves are absorbed by water molecules from the inside materials, which are rotated by the electric field from the microwaves. As these molecules rub against each other, they heat up, and then they heat molecules around them (Fig. 2).
(C) Mechanical sterilization
Air filter: (HEPA filter):
It is high-efficiency particulate air or originally called High-Efficiency Particulate Absorber (HEPA). It is used to describe filters that can trap 99.97 percent of particles that are 0.3 microns (Fig.3). Air particles are circulated through the HEPA filter in four directions viz.
(a) Direct Impaction: Large contaminants, such as certain types of dust, mold, and pollen, travel in a straight path, collide with fiber, and stick to it.
(b) Sieving: The air stream carries a particle between two fibers, but the particle is larger than the gap, so it becomes ensured.
(c) Interception: Airflow is nimble enough to reroute around fibers, but, thanks to inertia, particles continue on their path and stick to the sides of fibers.
(d) Diffusion: Small, ultrafine particles move more erratically than larger ones, so they are more likely to hit and stick to fibers (Fig.3).
- The method is suitable for the sterilization of thermolabile medicaments, such as blood products, insulin, and enzymes.
- All types of bacteria i.e., living as well as dead, are removed from the preparation.
- Both clarification and sterilization are done side by side.
- It is an excellent method for the rapid supply of a small volume of a parenteral solution in an emergency.
- The method is not a reliable one and therefore a sterility test is necessary.
- The suspension and oily preparations cannot be sterilized by this method.
- There are chances of absorption of medicaments from a solution by the filter.
- Defects in the media are not immediately detectable.
- An aseptic technique is necessary.
- Highly trained staff is required.
- The process is only suitable for medicaments that are in solution form.
This filtration method is useful for sterilization of parenteral solutions containing thermolabile medicaments without any decomposition e.g. insulin, blood serum, and other products containing protein matters, heat-sensitive injections, biological products, etc.
(D) Colour coded biomedical waste bags:
Various colored-coded bags are used for particular materials for disposal. They are discussed below (Table).
Table: Functions of various colored bags
|Yellow plastic bags||For human anatomical, animal, microbiological waste|
|Red disinfected container or plastic bag||Microbial waste|
|Blue or white plastic bag||Disposable tubing|
|Black plastic bags||Discarded medicines, incineration ash|
Make sure you also check our other amazing Article on : Chemical Methods of Sterilization