Autoclave
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- For the article about the early 1990s Washington, D.C.-based indie rock band, see Autoclave (band).
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An autoclave is a pressurized device designed to heat aqueous solutions above their boiling point. It was invented by Charles Chamberland in 1879.[1]
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[edit] Introduction
Under ordinary circumstances, heating water above the boiling point in an open vessel is impossible. Whenever the temperature reaches the boiling point, water ceases warming. This is due to the extensive evaporation that occurs during boiling. As the water reaches its boiling point it begins to evaporate, a process that requires 2.2*106 Joules of energy per kilogram of water. Any additional heating beyond this point only evaporates the water and does not heat it further due to the principle of latent heat. If water is boiled long enough, it turns to vapor.
When water, however, is heated in a sealed vessel (such as an autoclave), it is possible to increase its boiling point. When the temperature reaches approximately 90° C, extensive evaporation takes place. Water vapor, being a gas by nature, creates excessive pressure within the vessel. As vapor pressure in the vessel reaches the value that corresponds to the temperature, evaporation stops. Since evaporation stops, not all the water turns into vapor. The higher the temperature, the higher the pressure of the vapor. The heat generated under pressure is called latent heat and has more penetrative power to squeeze through bacteria and even their dormant, heat-resistant form — the endospores. This works well on solid objects; when autoclaving hollow objects, however, (hypodermic needles, tools, etc.), it is important to ensure that all of the trapped air inside the hollow compartments is vacuumed out. Otherwise, it will act as thermal insulation for the bacteria inside.
Simple autoclaves use a single pulse pre-vacuum, while a modern day autoclave has fractioned pre-vacuum that pulls the air out in several stages to achieve 100% steam penetration in the sterilization process.
Autoclaves are widely used in medicine, veterinary science, dentistry and metallurgy.
[edit] Autoclaves in medicine
A medical autoclave is a device that uses steam to sterilise equipment and other objects. This means that all bacteria, viruses, fungi, and spores are inactivated. However, in 2003 scientists discovered a single-celled organism, Strain 121, that survives traditional autoclave temperatures. Prions also may not be destroyed by autoclaving.
Autoclaves work by allowing steam to enter, then maintaining pressure at 103 kPa (15 psi). This causes the steam to reach 121 °C, and this is maintained for at least 15 minutes. However, this technique is dated; modern autoclaves operate at higher temperature and pressures.
Autoclaves are found in many medical settings and other places that need to ensure sterility of an object. They were once more common, but many procedures today use single-use items rather than sterilized, reusable items. This first happened with hypodermic needles, but today many surgical instruments (forceps, needle holders, scalpel handles, etc.) are commonly single-use items rather than reusable.
Because damp heat is used, heat-labile products (such as some plastics) cannot be sterilised this way or they will melt. Some paper or other products that may be damaged by the steam must also be sterilised another way. In stovetop autoclaves, items should always be separated to allow the steam to penetrate the load evenly.
[edit] Chemiclave
Unlike the humid environment produced by conventional steam, the unsaturated chemical vapor method is a low-humidity process. No time-consuming drying phase is needed, because nothing gets wet. The heat-up time is shorter than for most steam sterilizers, and the heaters stay on between cycles to minimize warm-up time and increase the instrument turnover.
[edit] Autoclave quality assurance
There are physical, chemical and biological indicators that can be used to ensure an autoclave reached 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 indicates that the object inside the package, or under the tape, has been autoclaved sufficiently.
Biological indicators include Attest devices. These contain spores of a heat resistant bacterium Bacillus stearothermophillus. If the autoclave did not reach the right temperature, the spores will germinate, and their metabolism will change the colour of a pH-sensitive chemical.
Physical indicators often consist of an alloy designed to melt only after being subjected to 121 °C for 15 minutes. If the metal has melted, the change will be visible.
As well as these separate indicators, autoclaves have temperature and pressure gauges visible from the outside.
There are certain plastics that can withstand repeated temperature cycling greater than the 121 degrees Celsius required for this process. PFA is an example.
Computer-controlled autoclaves use an F0 (F-naught) value to control the sterilization cycle. F0 values are set as the number of minutes of equivalent sterilization at 121 °C (e.g: F0 = 15 min.). Since exact temperature control is difficult, the temperature is monitored, and the sterilization time adjusted accordingly.
[edit] Loading an autoclave
(These directions are for a typical front loading autoclave, but apply to most autoclaves)
- Clean and inspect the autoclave regularly. Check the autoclave door gasket, drum, and steam return filter for corroded pipes.
- Use distilled De-ionized water, or other clean neutral pH water.
- Autoclave dissimilar metals (aluminum, brass, steel) in different loads as electroplating may occur which will permanently stain the tool.
- Use towels to separate instruments and absorb moisture if needed.
- Autoclave hinged instruments in the non locked open position, if locked the tool may become damaged (usually at the hinge).
- No sharp instruments should be allowed to touch other instruments.
- Do not over pack the autoclave, steam should be able to get to all sides of all pouches.
