Types of Autoclaves
Autoclaves function primarily through either gravity or vacuum-induced or pre-vacuum (pre vac) sterilization methods, though some types of autoclaves combine both methods to sterilize. Though both types of autoclaves sterilize through high-temperature steam and use pressure as a means to allow steam to displace ambient air in the chamber to penetrate sterilization media, how these mechanisms occur differ and thus, are more conducive to certain types of media over others. This article will outline the basic function of these autoclaves and list the types of sterilization media most associated with each type of autoclave.
Gravity autoclaving, also known as gravity displacement autoclaving is the most basic form and is suitable for sterilizing the most common laboratory media, including steel utensils, glassware, and bio-hazardous waste. Gravity-induced autoclaving involves pumping steam into the autoclave chamber, which displaces the ambient air and forces it out of exhaust valves so that the remaining steam can sterilize the contents. This mechanism is advantageous in the simplicity of its design and lack of dependency on peripheral mechanisms to displace ambient air with steam, making these types of autoclaves more affordable and dependable. The majority of autoclave media or items to be sterilized are simple in design and do not contain spaces or obstacles for steam to penetrate within, thus the steam displacement function is ample for proper sterilization. It is for these reasons that gravity autoclaves are the most common types of autoclaves in the market and are usually the recommended type of autoclave for most uses.
On the other hand, vacuum autoclaving, also known as pre-vacuum autoclaving or sterilizing is more suited in cases where air cannot be easily removed from sterilization media. This may include large or porous items such as animal cages and bedding sterilization as well as wrapped surgical kits. The vacuum function in these autoclaves allows deeper sterilization of the contents, as it completely evacuates the ambient air within, allowing high-temperature steam to penetrate and sterilize areas that would normally be occupied by ambient air, and can be more efficient at sterilizing certain items with hard-to-reach areas within.
Autoclave Media & the Appropriate Autoclave Type
As mentioned above, your choice of an autoclave system largely depends on your autoclave media, ie. the items to be sterilized. Overall, steam sterilization is highly dependable, effective, fast, and non-toxic, and offers an inexpensive way to rapidly heat and penetrate the chamber’s contents, including appropriately contained liquids. However, this method cannot sterilize powders or oils, and can only work with heat and moisture-resistant goods. Keeping this in mind, below is a summary of the primary types of media used by gravity and vacuum-autoclave types.
Gravity autoclaves are appropriate for sterilizing non-porous items (i.e. those with a hard surface):
- Most metals, particularly stainless steel surgical instruments and lab utensils
- Polypropylene Pyrex® or Type I borosilicate glassware
- Biohazard waste
- Unwrapped goods
Vacuum (pre and post) are appropriate for sterilizing large or porous items:
- Media solutions in appropriate containers, such as tissue culture flasks with loose caps for a steam autoclave cycle
- Pipette tips and other high-density polyethylene products, such as syringes
- Wrapped dry items that can trap air
- Animal cages and bedding
Choosing an Autoclave
Choosing the right autoclave for your purposes requires you to not only assess the type of media to autoclave, but also throughput capacity, available lab space, and access to utilities including electricity (with proper power output), water and house steam.
Due to the simplicity of the gravity-displacement mechanism, which requires an autoclaving chamber, a heating mechanism, intake and exhaust valves, there is great flexibility in design for gravity autoclaves, including front and top-loading types.
The top-loading type autoclave is particularly advantageous, as it allows for maximum loading space, minimum floor space requirement and does not require any building steam connection, as the vertical chamber design allows for water to rest at the bottom, which in turn is turned into steam through a heating element located at the bottom. Note that many autoclaves on the market, especially compact top-loading autoclaves often require no more than access to the proper electrical outlet type and enough space to place your autoclave.
Gravity autoclaves are also particularly advantageous when used in geographical areas of high humidity or higher altitudes as they consistently retain the relationship between pressure and heat within the autoclave chamber and overcome differences in boiling points at higher altitudes by opening and closing the exhaust valve. Known as high altitude autoclaves, their functions allow for proper sterilization to occur in laboratories and hospitals located in markets with high altitude terrains.
Regarding the size of your autoclave, to limit energy use and costs especially for smaller labs, it is important to buy the proper size autoclave that will accommodate the equipment that needs sterilizing, while not going overcapacity. For this, several autoclaves range between 50 L and 70 L, which tend to strike a good balance between relatively high capacity and low energy use.
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