autoclave

Square chamber laboratory autoclave with mechanically hoisted power door

An autoclave is an instrument used to sterilize equipment and supplies by subjecting them to high pressure saturated steam at 121 °C for around 15–20 minutes depending on the size of the load and the contents.^[1] It was invented by Charles Chamberland in 1879,^[2] although a precursor known as the steam digester was created by Denis Papin in 1679.^[3] The name comes from Greek auto-, ultimately meaning self, and Latin clavis meaning key — a self-locking device.^[4]

Uses

Autoclaves are widely used in microbiology, medicine, tattooing, body piercing, veterinary science, mycology, dentistry, chiropody and prosthetics fabrication. They vary in size and function depending on the media to be sterilized.

Typical loads include laboratory glassware, surgical instruments, medical waste, patient pair utensils, animal cage bedding, and lysogeny broth.^[5]

A notable growing application of autoclaves is the pre-disposal treatment and sterilization of waste material, such as pathogenic hospital waste. Machines in this category largely operate under the same principles as conventional autoclaves 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 a pressure vessel to sterilize culture media, rubber material, gowns, dressing, gloves, etc. It is particularly useful for materials which cannot withstand the higher temperature of a hot air oven. For all-glass syringes, sterilizing in a hot air oven is a better method.^{[citation needed]}

Autoclaves are also widely used to cure composites and in the vulcanization of rubber. The high heat and pressure that autoclaves allow help to ensure that the best possible physical properties are repeatably attainable. The aerospace industry and sparmakers (for sailboats in particular) have autoclaves well over 50 feet long, some over 10 feet wide.

Air removal

It is very important to ensure that all of the trapped air is removed from the autoclave before activation, as hot air is a very poor medium for achieving sterility. Steam at 134 °C can achieve in three minutes the same sterility that hot air at 160 °C takes two hours to achieve.^[6] Methods of achieving air removal include:

Downward displacement (or gravity type) - As steam enters the chamber, it fills the upper areas first as it is less dense than air. This compresses the air to the bottom, forcing it out through a drain which often contains a temperature-sensing device. Only when air evacuation is complete does the discharge stop. Flow is usually controlled by 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 - a vacuum pump sucks air or air/steam mixtures from the chamber.

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

Subatmospheric cycles - similar to the superatmospheric cycles, but chamber pressure never exceeds atmospheric pressure 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 three minutes or 121 °C for 15 minutes. Also, some recently-discovered organisms, such as Strain 121 microbes, can survive at temperatures above 121 °C.

Autoclaves are found in many medical settings, laboratories, and other places that need to ensure the sterility of an object. Many procedures today employ single-use items rather than sterilizable, 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 rather than reusable items (see waste autoclave).

Because damp heat is used, heat-labile products (such as some plastics) cannot be sterilized this way or they will melt. Paper and other products that may be damaged by 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 become more common as an alternative to incineration due to environmental and health concerns raised because of the combustion by-products emitted by 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 waste.

Autoclave quality assurance

The machine on the right is an autoclave used for processing substantial quantities of laboratory equipment prior to reuse, and infectious material prior to disposal. (The machines on the left and in the middle are washing machines.)

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

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

Chemical indicators on medical packaging and autoclave tape change color once the correct conditions have been met, indicating that the object inside the package, or under the tape, has been appropriately processed. Biological indicators contain spores of a heat-resistant bacterium, Geobacillus stearothermophilus. If the autoclave does not reach the right temperature, the spores will germinate when incubated 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 a given temperature for 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 for the number of minutes of sterilization equivalent to 121 °C (250 °F) at 15 psi (100 kPa) above atmospheric pressure for 15 minutes . Since exact temperature control is difficult, the temperature is monitored, and the sterilization time adjusted accordingly.

References