Selecting Plastic Animal Cages
By Dave Demorotski
Buying on price is one way. But if you can answer a few what’s and how’s about your facility caging needs and procedures, you can purchase cages that will last longer plus add value.
For many, selecting the right plastic caging for housing rodents can be a “poly nightmare.” There’s “poly-this type cage” and “poly-that type cage.” And the “poly-that type cage” must be better because it costs 20% more than the “poly-this type cage.” Yet the “poly-that type cage” didn’t last 20% longer because it became cloudy and hazy after only a few times through washing and sterilizing cycles.
So how should you determine what is the right caging to buy for your vivarium? The best answer probably lies in understanding how your cages are used and how they move through your facility… from the animal housing room to washing/sterilizing, to storage and back to holding the animals. Below are some questions to consider that can help you determine which plastic cages may best serve your facility.
Questions to Consider Before You Buy
• First, of course, is how many new plastic cages does your facility need? Do you have an absolute minimum (number of cages) to purchase or is there a budgeted amount to spend?
• What is the budget level or limitation?
• How long does the caging need to last?
• For what type of research are the cages being used?
• Does the study require only a one time or very limited use?
• Does it involve dangerous or toxic materials that need disposal? Or, do the cages just need to last as long as possible?
• How frequently do you changeout cages?
• How is the caging handled throughout your facility? What processes will they go through? For example, how is the soiled bedding removed, by hand or mechanically? How is sticky bedding removed? How are the cages transported and stored? In general, how much does your caging get knocked around in moving it from the animal lab through cleaning and sterilizing and back to the animal room?
• What kind of detergents are used in cleaning? How much alkali does the detergent have, <pH7.0?
• How hot is the water? Is it hard or soft water? Is the rinse cycle sufficient to make sure all residue is removed?
• What chemicals are used to disinfect your plastic cages?
• Will the cages be autoclaved? What are the usual autoclave settings? Are amine corrosion inhibitors added to the central boiler steam supply to protect the pipes? Will the cages be autoclaved with the bedding, feed, and water bottles in them? If so, what type of bedding is used?
• Finally, how will the sterilized cages be stored? How high will they be stacked?
Okay, so it’s more than a few questions. But the more you can find out about how the cages are handled and treated within your facility, the better you will be able to select the type of plastic that will serve your requirements, at the price you can afford.
|Suppliers and cage manufacturers conduct a variety of
tests on polymers. Most are to determine durability, chemical resistance, material clarity and steam resistance. This photo highlights an autoclave test of materials at 270° F for50 cycles under induced stress levels of 500 psi.
Selecting the Appropriate Type of Plastic Cage
Once you have completed your cage usage/care review and determined your caging requirements and budget, you can begin to quickly narrow the plastic options to the one or two polymers that will best fulfill your needs.
POLYSTYRENE OR POLYETHYLENE – These materials are low temperature and cannot be used in an autoclave. Polystyrene begins to distort at temperatures over 176°F (80°C) and polyethylene at temperatures over 200°F (93°C). The cages are rigid and are relatively low cost. For these reasons, cages of these materials are usually considered disposable and used in studies involving dangerous mate rials such as radiation. Polystyrene is a clear material so it is easy to view the animals, but its toughness and abuse resistance is fairly low. Polyethylene is opaque or translucent while offering a much higher level of impact resistance than polystyrene.
POLYCARBONATE (PC) – Polycarbonate is a popular material for animal cages and water bottles. It is also relatively inexpensive. PC can’t take the higher autoclave temperatures (>250°F/121°C) or the steam very well. It most likely will distort in the autoclave because of the temperature. Also, PC wants to absorb water (steam) which leads to its molecular breakdown evidenced by stress cracking after only a few sterilizer cycles. Stress cracking also occurs from chemical cleaning solutions such as alkalai and strong acid detergents as well as aromatic and chlorinated hydrocarbons.
HIGH HEAT POLYCARBONATE (PPC) – High heat PC is much the same as polycarbonate, just a little higher temperature tolerant (>270°F/132°C). The heat distortion is higher than PC, but the chemicals that attack it and the water absorption problem remain about the same.
POLYETHERIMIDE (PEI) – This polymer doesn’t have great impact strength to start and degrades further after autoclaving. It can withstand higher temperatures, up to 400°F (204°C), than the polycarbonates. It has good chemical resistance, especially against acidic detergents and organic solvents, but is susceptible to breakage because of its initial brittleness. PEI has very poor resistance to low pH solutions of alkaline chemicals. PEI should not survive more than 50 autoclave cycles unless it is given tender loving care. PEI is dark in color which doesn’t allow for much light transmission and makes it more difficult to view animals inside the cages.
POLYSULFONE (PSU) – Polysulfone can withstand higher temperature than PC or high heat PC, about 300°F (149°C) and should withstand about 100 autoclaves with minimal affect. PSU has good chemical resistance to all commercial cage cleaners, only being attacked by high levels of ionic surfactants. While polysulfone is completely unaffected by pure steam, amine corrosion inhibitors used in some central steam supply systems can cause crazing and cracking in areas of cages under high stress. It can also be attacked by ketones, aromatic and chlorinated hydrocarbons.
POLYPHENYLSULFONE (PPSU) – This is the highest level of the transparent polymer materials. Polyphenylsulfone offers great chemical resistance, great steam sterilizing (up to 380°F/193°C) capability, and high impact strength. Heat deflection begins to appear at the high range above 400°F (204°C). Tests have shown PPSU to withstand 2000 autoclave cycles with very little affect. PPSU also shows good to excellent resistance to both inorganic and organic cleaning chemicals. The hindrance to PPSU is a somewhat higher cost than other polymers used for animal cages.
A Little TLC Can Go a Long Way
As described above, the main areas that can determine the longevity of your plastic cages are its chemical resistance, steam resistance and toughness. If you follow some of the practices listed here, you may increase the useful life of your plastics.
Be as careful as possible in handling and transporting cages. In particular, avoid hitting or banging the plastics against hard surfaces. One common practice to avoid is hitting the cage against a surface to remove soiled and stuck bedding. This can significantly shorten the life of your cages. Even the higher end polymers with greater impact strength lose some of their resistance after repeated chemical cleanings and sterilization. To remove stubborn bedding, it is best to use a soft polymer spatula or scraper. Do not use a metal scraper because it can scratch the plastic surface.
Also, do not overstack the caging, especially cages with bedding. The stress on the lower cages can cause cracking. Again, this is especially true for older cages that have been through several cleaning cycles. As a general rule, mouse cages should not be stacked more than 15 high. Rat cages should not be stacked more than ten high.
Plastic cages and bottles should be washed in hot, soft water with a manufacturer recommended detergent solution. Washing plastics in hard water could cause a milky-gray discoloration on the surface after frequent washing. Optimum water temperature is in the 140°-150°F (60°-66°C) range. A short rinse at approximately 180°F (82°C) is helpful for disinfection.
If an alkaline detergent is used, a short acidic rinse cycle followed by a final fresh water rinse is recommended. In general, selecting a detergent with a pH between five and seven will work well for most polymers.
Be sure all residue is removed from cages and bottles by the final rinse. This is especially important if autoclaving is to follow. The extreme heat of autoclaving will most likely cause the residue to be baked onto the plastic, resulting in loss of clarity and gradual deterioration.
To help prolong the life of your plastic materials, you should consult your cage wash supplier to ensure that the cycle times and temperatures are correct for the plastics you are using.
There are many disinfectants that can harm plastics. Check with your disinfectant supplier regarding the use of their products for your particular plastic materials. You should NEVER heat cages that contain a disinfectant residue.
It is important that plastic cages and bottles are washed, thoroughly dried, and free from any residue before autoclaving. Effective autoclaving depends upon proper temperature controls and appropriate steam supply. Autoclaves should be regularly checked to ensure effectiveness.
While polycarbonate (PC) is considered autoclavable, it will deteriorate after repeated autoclaving. It is recommended that PC cages should be autoclaved only on an as needed basis and at a temperature no greater that 250°F (121°C) for a short cycle, about 20 minutes.
While cages with bedding, feed, and water bottles can be autoclaved together, it is important to note that heating these combined materials may possibly release damaging substances which can attack the plastic. This could cause clouding and/or cracking of the polymer material.
For steam sterilization systems supplied by a boiler feed, check for the use of corrosion inhibitors with amine. These could also dull or damage the plastic material.
Answering a few questions regarding the chemical, disinfecting/sterilizing procedures and how your plastic cages are handled within your facility should be the first part of your selection process. Finding answers to the chemical and steam resistance as well as impact strength of the various plastic resins you are considering for your cages is the second step. Finally, determine your budget and longevity goals. With these answered, you will be able to select the plastic animals cages that will best serve the needs of your facility.