A Guide to Autoclaving Plastics and GlassRyan White
What is Autoclaving?
Autoclaving is used to sterilise plastic and glass containers, plastic caps, medical instruments, and other items used in a laboratory setting. Some items must be sterilised beyond what can be achieved with hot water and detergent, so autoclaving is used to achieve microbial limit testing on these products. Using heat, steam, pressure – in a similar way to a pressure cooker – autoclaving achieves a high level of sterilisation. It is the preferred method of sterilisation for materials that can withstand the heat and pressure of the autoclave.
What Materials Are Safe?
Glass and plastics can both be autoclaved, although only some plastics are considered to be safe for autoclaving. Polypropylene and polypropylene copolymer containers are safe to be autoclaved repeatedly, as can fluoropolymer products. On the other hand, polycarbonate containers are less resilient and may only withstand between 30 and 50 autoclave cycles, as sterilising this plastic reduces the mechanical strength of the material. Resins such as HDPE, LDPE, PET, and PETG cannot be autoclaved and should instead be sterilised using gas.
Glass is usually considered to be safe for autoclaving, although precautions should be taken to ensure it is done safely. If glass bottle caps are not loosened, for example, the bottles may explode from the pressure in the autoclave. It is also worth noting that it is usually recommended that only borosilicate glassware is used, as it withstands the high stress of autoclaving better than other glassware. The only closure liners which can safely be autoclaved are rubber and Teflon.
Guide to Use
Like a lot of equipment you’ll find in your lab, if used incorrectly autoclaves can be dangerous. Following your lab’s procedure when using the autoclave is important to ensure both proper sterilisation and your own safety. The following gives a general outline of the autoclaving procedure, but you should always refer to your lab’s own guidelines.
Prepare – The first step is to prepare your research equipment for sterilisation. Pyrex bottles should have the cap placed on loosely, to prevent explosions caused by heat and pressure expansion. Bottles that are not made form safety glass may need to be covered with aluminium foil if they are sterilised by autoclave. Check that any plastics are suitable for autoclaving. Items will often require secondary containment, for which polypropylene or stainless steel tubs should be used. Space should be left between the tubs to allow steam to circulate.
Secure – It is important to ensure that the door to the autoclave is sealed before selecting the cycle. Although autoclaves are self-sealing due to the high pressure (the name autoclave means automatic locking) it is important to also use the manual seal on the outside for safety.
Cycle – If you are autoclaving liquids you will usually use the liquid or slow exhaust cycle. Your lab should have guidelines on which cycle is recommended for sterilising dry goods or equipment. If you’re unsure, always ask your lab manager. Once you have chosen the cycle, you should set the time for sterilisation and drying. This will usually be around 30 minutes for sterilisation and 20 minutes for drying if you are autoclaving dry goods.
Start – Once the cycle, sterilisation, and drying time are set, press the start button to initiate the cycle.
Log – Each machine will have an autoclave log where records of the machine’s use are kept. Usually the date and operator will be recorded, along with other details.
Safety – Once the autoclave cycle has finished it is important to follow the safety precautions to protect yourself and the equipment. Ensure that you are wearing a lab coat, eye protection, closed-toe shoes, and heat-resistant gloves. Glassware will be especially hot and can cause serious injury if you do not protect your hands.
Always wait for the pressure gauge to drop to zero with zero time remaining before you open the door. You should stand behind the door as you open it and allow all the steam to fully escape before reaching into the autoclave. Let glassware and plastics fully cool before touching them without protective gloves.
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