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Principles and methods of sterilization Sterilization mainly classified into two types 1.Physical method 2.Chemical method
1.Physical method • Heat • Filtration • Radiation Method
Heat • Dry heat - Hot air oven • Incineration • Moist heat- Autoclave, Pressure cooker • Tyndallization(Fractional sterilization)
A. Dry heat - Hot air oven In dry heat sterilization, dry heat is used for sterilizing different materials. Heated air or fire is used in this process. The temperature is usually higher than 356° F or 180 °C. Example: Hot air oven Principle of HOT AIR OVEN : Sterilizing by dry heat is accomplished by conduction. The heat is absorbed by the outside surface of the item, then passes towards the Centre of the item, layer by layer. The entire item will eventually reach the temperature required for sterilization to take place.
Dry heat does most of the damage by oxidizing molecules. The essential cell constituents are destroyed and the organism dies. • The temperature is maintained for almost an hour to kill the most difficult of the resistant spores. • The most common time-temperature relationships for sterilization with hot air sterilizers are 170°C (340°F) for 30 minutes, 160°C (320°F) for 60 minutes, 150°C (300°F) for 150 minutes or longer depending up the volume.
Advantages • Hot air ovens use extremely high temperatures over several hours to destroy microorganisms and bacterial spores. • The ovens use conduction to sterilize items by heating the outside surfaces of the item, which then absorbs the heat andmovesit towards the center of the item. Disadvantages • Time consuming method because of slow rate of heat penetration and microbial killing. • High temperatures are not suitable for most materials.
B. Incineration • Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials. Incineration and other high-temperature waste treatment systems are described as "thermal treatment". Incineration of waste materials converts the waste into ash, flue gas and heat.
C. Moist heat- Autoclave • Water at high pressure level is used in moist heat sterilization. Autoclave is the instrument in which this process is carried out. Example: Autoclave, Pressure cooker Autoclaves principle : • High-pressure steam sterilization (autoclaving) to kill all microorganisms, including spores , sterilization is the best, the most widely used methods of sterilization. • At 103.4kPa (1.05kg/cm2) vapor pressure , the temperature reached 121.3 ℃, for 15 ~ 20 minutes .
Uses of Autoclave: • Autoclaving is used to sterilize culture media, instruments, dressings, intravenous equipment, applicators, solutions, syringes, transfusion equipment, and numerous other items that can withstand high temperatures and pressures. • The laboratory technician uses it to sterilize bacteriological media and destroy pathogenic cultures.
D. Tyndallization (Fractional sterilization) • Tyndallization is a process dating from the nineteenth century for sterilizing substances, usually food, named after its inventor, scientist John Tyndall, that can be used to kill heat-resistant endospores. Although considered old-fashioned, it is still occasionally used. • Tyndallizationessentially consists of heating the substance to boiling point (or just a little below boiling point) and holding it there for 15 minutes, three days in succession.
Filtration Types of filters • Seitz filter • Membrane filters
A. Seitz filter Seitz filter A filter disc (originally of asbestos) with pores so fine that they will not permit passage of bacteria; solutions emerge sterile.
B. Membrane filters • Membrane filters have a known uniform porosity of predetermined size (generally 0.45 µm ) sufficiently small to trap microorganisms. • Using the membrane filter technique, sample is passed through the membrane using a filter funnel and vacuum system. • Any organisms in the sample are concentrated on the surface of the membrane. The membrane, with its trapped bacteria, is then placed in a special plate containing a pad saturated with the appropriate medium.
The passage of nutrients through the filter during incubation facilitates the growth of organisms in the form of colonies, on the upper surface of the membrane. Discrete colonies thus formed can be easily transferred to confirmation media. • Membrane filter technique is an effective, accepted technique for testing fluid samples for microbiological contamination.
Radiation Methods • UV Radiation • X-rays
A. UV Radiation • UV light sterilization is an environmentally friendly method of killing bacteria, mold, fungi, and viruses without the use of harmful chemicals and does not produce corrosive materials or disinfection by-products (DBPs). or • short-wavelength ultraviolet (UV-C) light to kill or inactivate microorganisms by destroying nucleic acids and disrupting their DNA, leaving them unable to perform vital cellular functions.
Advantages: • It does penetrate air, effectively reducing the number of airborne microorganism and killing them on surfaces on operating rooms and rooms that will contain caged animals. • To help sanitize the air without irradiation humans, these lights can be turned on when there rooms are not in use. Disadvantage: • A major disadvantage of UV light as a disinfectant is that the radiation is not very penetrating, so the organism to be killed must be directly exposed to the rays. • Another potential problem is that UV light can damage human eyes, and prolonged exposure can cause burns and skin cancer in humans. And it may cause damage in human skin cells and permanent damage the eyes.
B. X-rays • X-rays make up X-radiation, a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers. • It have good penetration. • A wide variety of healthcare products from implants to pharmaceuticals employ X-ray technology’s sterilizing capability, while industrial applications from semiconductors to micro-etching can benefit from its use.
2.Chemical method or Disinfections • Ethylene oxide • Alcohols • Phenols • Aldehydes
A. Ethylene oxide(EO or EtO) • Ethylene Oxide (also known as EO or EtO) sterilization is a simple chemical compound that is commonly used for gaseous sterilization of disposable medical devices and healthcare products. • EtOgas infiltrates packages as well as products themselves to kill micro organisms that are left during production or packaging processes. • This gas, mixed with air at a ratio of at least 3% EtO gas, forms an explosive mixture. Pure EtO gas boiling point is 10.73 ºC at atmospheric pressure.
Most of the time, it is mixed with Nitrogen or CO2. This explosive condition requires Intrinsic Safe material (ATEX) zoning, for security of people as well as security of the process itself. • Safety of personnel is an important issue due to the harmful effect of EtO on humans. • When this toxic gas is removed from the room it needs to be treated using thermal burners, scrubbers or oxidation for environmental protection or be transported to an alternate facility for treatment.
B. Alcohols • The types of alcohol used in disinfection are ethanol (80%), propanol(60%), and isopropanol (70%). • Alcohols are quite effective against bacteria and fungi, less so against viruses. • They do not kill bacterial spores. Due to their rapid action and good skin penetration, the main areas of application of alcohols are surgical and hygienic disinfection of the skin and hands. • One dis-advantage is that their effect is not long-lasting (no depot effect). Alcohols denature proteins.
C. Phenols • Lister was the first to use phenol (carbolic acid) in medical applications. • Phenol derivatives substituted with organic groups and/or halo-gens (alkylated, arylated, and halogenated phenols), are widely used. Onecommon feature of phenolic substances is their weak performance againstspores and viruses. • Phenols denature proteins. • They bind to organic materials to a moderate degree only, making them suitable for disinfection of excreted materials.
D. Aldehydes • It can be used in a special apparatus for gas sterilization. • Formaldehyde is a water-soluble gas. • Formaldehyde is a broad-spectrum ger-micidefor bacteria, fungi, and viruses. At higher concentrations, spores are killed as well. • This substance is used to disinfect surfaces and objects in 0.5–5% solutions. • It was commonly used in gaseous form to disinfect the air inside rooms (5 g/m3).
