autoclave

Autoclave

A modern front-loading autoclave
Uses	Sterilization
Inventor	Charles Chamberland
Related items	Waste autoclave

An autoclave is a device to sterilize equipment and supplies by subjecting them to high pressure steam at 121° C or more. It was invented by Charles Chamberland in 1879,^[1] although a precursor known as the steam digester was created by Denis Papin in 1679.

Uses

Widely used in microbiology, medicine, body piercing, veterinary science, dentistry and podiatry.

Typical loads include glassware, waste, utensils, redbags, packs, animal cage bedding, and LB broth.^[2]

A notable growing application of autoclaves is in the treatment and sterilization of waste, such as pathogenic hospital waste. Machines in this category largely operate under the same principles as the original autoclave in that they are able to neutralize potentially infectious agents by utilizing pressurized steam and superheated water. A new generation of waste converters is capable of achieving the same effect without any pressure vessels.

Air removal

It is very important to ensure that all of the trapped air is removed as hot air is very poor at achieving sterility. Steam at 134° C can achieve in 3 minutes the same sterility that hot air at 160° C takes two hours to achieve.^[3] Methods of achieving air removal include:

Downward displacement (or gravity type) - As steam enters the chamber, it fills the upper areas as it is less dense than air. This compresses the air to the bottom, forcing it out through a drain. Often a temperature sensing device is placed in the drain. Only when air evacuation is complete should the discharge stop. Flow is usually controlled through the use of a steam trap or a solenoid valve, but bleed holes are sometimes used, often in conjunction with a solenoid valve. As the steam and air mix it is also possible to force out the mixture from locations in the chamber other than the bottom.

Steam pulsing - Air dilution by using a series of steam pulses, in which the chamber is alternately pressurized and then depressurized to near atmospheric pressure.

Vacuum pumps - Vacuum pumps to suck air or air/steam mixtures from the chamber.

Superatmospheric - This type of cycle uses a vacuum pump. It starts with a vacuum followed by a steam pulse and then a vacuum followed by a steam pulse. The number of pulses depends on the particular autoclave and cycle chosen.

Subatmospheric - Similar to superatmospheric cycles, but chamber pressure never exceeds atmospheric until they pressurize up to the sterilizing temperature.

Autoclaves in medicine

Stovetop autoclaves - the simplest of autoclaves

A medical autoclave is a device that uses steam to sterilize equipment and other objects. This means that all bacteria, viruses, fungi, and spores are inactivated. However, prions, like those associated with Creutzfeldt-Jakob disease, may not be destroyed by autoclaving at the typical 134° C for 3 minutes or less common 121° C for 15 minutes. Also, some recently-discovered organisms, such as Strain 121, can survive at temperatures above 121° C.

Autoclaves are found in many medical settings and other places that need to ensure sterility of an object. Many procedures today use single-use items rather than sterilized, reusable items. This first happened with hypodermic needles, but today many surgical instruments (such as forceps, needle holders, and scalpel handles) are commonly single-use items rather than reusable. See waste autoclave.

Because damp heat is used, heat-labile products (such as some plastics) cannot be sterilized this way or they will melt. Some paper or other products that may be damaged by the steam must also be sterilized another way. In all autoclaves, items should always be separated to allow the steam to penetrate the load evenly.

Autoclaving is often used to sterilize medical waste prior to disposal in the standard municipal solid waste stream. This application has grown as an alternative to incineration due to environmental and health concerns raised by combustion byproducts from incinerators, especially from the small units which were commonly operated at individual hospitals. Incineration or a similar thermal oxidation process is still generally mandated for pathological waste and other very toxic and/or infectious medical wastes.

Autoclave quality assurance

Two large autoclaves used for processing substantial quantities of laboratory equipment prior to reuse, and infectious material prior to disposal. (The machine in the middle is a washing machine)

Sterilization bags often have a "sterilization indicator mark" that typically darkens when the bag has been processed. Comparing the mark on an unprocessed bag (L) to a bag that has been properly cycled (R) will show an obvious visual difference.

There are physical, chemical, and biological indicators that can be used to ensure an autoclave reaches the correct temperature for the correct amount of time.

Chemical indicators can be found on medical packaging and autoclave tape, and these change color once the correct conditions have been met. This color change indicates that the object inside the package, or under the tape, has been processed. Biological indicators contain spores of a heat-resistant bacterium, Geobacillus stearothermophilus. If the autoclave does not reach the right temperature, when incubated the spores will germinate, and their metabolism will change the color of a pH-sensitive chemical. Some physical indicators consist of an alloy designed to melt only after being subjected to the relevant holding time. If the alloy melts, the change will be visible.

Some computer-controlled autoclaves use an F₀ (F-nought) value to control the sterilization cycle. F₀ values are set as the number of minutes of equivalent sterilization at 121° C or 249° F (e.g: F₀ = 15 min.). Since exact temperature control is difficult, the temperature is monitored, and the sterilization time adjusted accordingly.

Superheating accidents

It is critical that autoclaves be opened only after checking both the pressure and temperature gauges. If the water inside the autoclave has managed to become superheated, the pressure gauge may not indicate the presence of steam even though the temperature may be significantly higher than the local boiling point for water. If the autoclave is opened in this state and the superheated water is disturbed, a steam explosion becomes possible. This phenomenon can easily produce fatal burns to people in the vicinity of the explosion.

See also

• Aerated autoclaved concrete
• Pressure cooker
• Sterilization (microbiology)
• Waste autoclave

References