There are many factors to consider when choosing the right tubing for your research. From fit and material to kink resiliency and air permeability, it’s important to understand the pros and cons of materials, sizes, and the intended application.
Choose the best tubing for your application based on seven key factors:
- Fit - How well do the tubing dimensions match the rest of your system?
- Material - Do the physical properties of the material make sense for your use?
- Kink Resistance - Will the tubing need to make tight turns?
- Compound Compatibility - How compatible is the tube material with the fluid that will be flowing through it?
- Air Permeability - How important is it that air not permeate through the tube?
- Suitability for Implantation - If the tube will be implanted, how suitable is it for use inside the body (animal research only)?
- Cost - Will it fit your budget?
1. Fit
Tubing’s purpose is really quite simple: transport fluid (or gas) from point A to point B. How the tubing connects to point A and point B is important. In the human medical world, tubing connections are standardized with luers fittings and other connectors that are bonded onto the tube. These fittings make reliable connections, but they also add dead volume and cost which can make them impractical in research.
For many years Intramedic™ polyethylene (PE) tubing from the Clay Adams division of Becton Dickinson was the standard for small diameter laboratory tubing. It came in a range of sizes with a somewhat random set of inner and outer diameters. In fact, Instech’s first fluid swivel, model 375/22, had 22ga connections to fit PE-50, the most popular of the Intramedic sizes. (Actually 23ga fits PE-50 better, but PE tubing has a problem that it takes a set over time and so the founder of Instech chose 22ga for a tighter fit.) And for that reason, today 22ga is the standard for Instech’s rat system connections. 25ga is the standard for mouse systems.
While it varies with material and tubing size, in general, for an ideal friction fit the flexible tubing should have an inner diameter (ID) of 0.002-.004in (0.05-0.10mm) smaller than the outer diameter (OD) of the metal coupler onto which it will be placed. The outer diameter of stainless hypodermic tubing is almost always right on spec, but extruded tubing will vary by +/- 0.002in (0.05mm) or more due to the nature of the manufacturing process. So if you have a tube with a nominal ID very close to the OD of the connector, you can be fairly sure at some point it will disconnect or leak. Download this guide which contains a printable Guide to Tubing Fit.
2. Tubing Material
Tubing material is just as important as the dimensions for a good connection. Here is how they stack up:
Silicone is soft and stretchy, so you can have a small inner diameter stretch over a relatively large connector which is sometimes useful if you are trying to minimize dead volume. Silicone tubing does not take a set; i.e., it will come back to its original size when pulled off the connector. However, silicone is slippery and will easily pull off connectors, so it is a poor material for friction-fit connections.
Dow Corning Silastic® brand medical-grade silicone is commonly used for research tubing; beware of industrial-grade silicone which can have impurities.
Polyethylene is stiff and will not stretch far over connectors, but at first it will make a good connection. Over time, however, PE will take a set (permanently stretch out), weakening the connection. PE is not good for friction fit connections.
Polyurethane stretches well and grabs. If you try to pull a PU tube that has a good fit over a connector it will stretch down like a Chinese finger trap and grab even more; so much so that the tube might break or the connector might pull out of the device before it will disconnect. PU is a good material for friction fit connections, and the only material we recommend for subcutaneous connections.
In some cases, it is wise or necessary to take additional steps to prevent disconnection. Options include:
Bonding. Special light-cure glues can bond some tubing to the stainless steel connectors. Some glues are medical grade and can be safely implanted. PU can be bonded, but silicone and PE cannot. Instech has co-extruded PE/PVC tubing that can be bonded because the glue binds to the PVC layer. Downsides: you can’t disconnect and replace the tubing at the end of the experiment; if you are bonding a subcutaneous connection, such as a catheter to a Vascular Access Button™, your surgical procedure may need to be modified.
Sleeve. Placing a segment of thick-walled stretchy tubing, usually silicone, over the joint is a relatively simple solution that can reinforce the connection, though it is not as foolproof as bonding. A sleeve can also help prevent kinking at the connector. It can be used for external or subcutaneous connections.
Suture. If you are connecting a catheter to a connector during surgery and are already suturing the vessel, it can be a simple step to throw a loop around the connection. The only downside is the time to do it.
3. Kink Resistance
Just like your garden hose, research tubing can kink if it makes too sharp a bend. How likely it is to kink depends on the tubing wall thickness and material. Temperature is also a factor – tubing can suddenly kink as it warms up and the material relaxes, for example when body-temperature blood flows through it.
A kink can be disastrous. In a drug infusion study a kink will stop flow completely; the catheter can clot in the meantime and you have the danger of a large bolus of fluid when the kink is released. If your pump does not have an occlusion alarm, high pressures can build up and cause a leak at the weakest point in the line.
PE-50 (polyethylene) is the most commonly used tubing in research, but unfortunately its stiffness and the relatively thin wall size makes it the most likely to kink. Once it has kinked, it is much more likely to kink again in the same place. Using PE tubing with a co-extruded layer of PVC around it practically eliminates the chance that it will kink.
Silicone is relatively resistant to kinking. Its issue is that it is so soft that if it is bent over a connector it can puncture and will leak.
Polyurethane is relatively resistant to kinking except when the tubing walls are thin. We recommend thick-walled PU tubing for external segments where tight friction-fit connections are critical. We use it inside the springs of our rodent tethers which experience a lot of pulls and movement.
A kink in an implanted catheter may be even more dangerous because it can’t be seen. It will act just like a blockage. It is a surgeon’s responsibility to know the nature of his or her catheter tubing and make sure the bends are not too tight.
4. Compound Compatibility
Chemical compatibility. Certain chemical compounds or vehicles can react with the tubing material. Other compounds can stick to tubing and release later. If you are unsure, test your compound through an isolated tubing segment before starting your experiment.
- Silicone is non-reactive but is porous and some compounds, such as oils, can permeate into it.
- Polyethylene is generally considered the most inert tubing material.
- Polyurethane is occasionally incompatible with certain compounds or chemicals; because PU makes reliable connections and is good for implantation, substitute materials often involve sacrifices.
Light sensitivity. Most tubing is translucent, which is a problem when compounds are sensitive to light. You can cover syringes with aluminum foil and surround tubing with black shrink tubing, or use PE tubing that is coextruded with a surrounding layer of black PVC for this purpose.
5. Air Permeability
If fluid is static in a tube that is permeable to air, as is often the case with the exteriorized portion of a catheter, the fluid will evaporate through the tube. In the case of an exteriorized catheter this evaporation pulls blood into the tip, which can clot and lead to a blockage.
Silicone is the most permeable tubing material.
Polyethlyene and PE/PVC tubing have very low air permeability, but are not ideal for implantation.
Polyurethane is moderately permeable to air, and this can be an issue when the tubing walls are thin as is often the case with implanted catheters. Instech’s Vascular Access Buttons™ solve this issue because no part of the catheter is exteriorized. When PU is needed for external connections, use tubing with relatively thick walls, not catheter tubing.
See the results from this study we did on air permeability in tubing
6. Suitability for Implantation
If you are choosing tubing for an implanted small animal catheter several additional considerations come into play.
Size. First, a catheter must be the appropriate size for the vessel into which it will be placed. If it is too large it will be difficult to insert, which can lead to extra time and trauma during surgery; too small and flow can be restricted. The outer diameter of a flexible catheter is measured using the French scale, where 3Fr=1mm. Most rat catheters are 3Fr; mouse jugular vein catheters are often 2Fr, whereas catheters for mouse carotid arteries and femoral veins are usually 1Fr. Intrathecal catheters are typically even smaller – 0.8Fr.
Material. Next, the material must be biocompatible and stiff enough to advance to the appropriate location in the vessel but not so stiff that it causes trauma. Polyurethane is generally considered the best material for catheters due to its biocompatibility and moderate stiffness. PE is still often used for short-term catheterizations because of its ubiquity in the lab, but it is too stiff for long-term implantation.
Tip. Sharp edges of tubing can damage the vessel wall and lead to an occlusion. Beveled tips may be easy to insert but can puncture straight through a vessel, particularly if the tube is made of a stiff material like PE. Rounded tips are generally considered the best for long-term patency, but they add cost to the catheter as they are time consuming to make and require special catheter-tipping machines.
Sterility. Finally, per animal welfare regulations, any tubing implanted into a laboratory animal in a survival surgery must be sterile. Silicone is the only tubing material that can be autoclaved; the others should be sterilized with ethylene oxide gas. If your facility does not have a gas sterilizer, purchase either pre-sterilized polyurethane tubing segments or finished sterile catheters.
7. Cost
Tubing cost can vary from less than $3 / meter for standard PE tubing to more than $20 / m for the very small diameter sizes. For the standard sizes, however, the cost differences are not so great that price should outweigh any of the other factors discussed here.
Price per meter (Bulk tubing offered by Instech, 2017 price list)
As you can see, choosing the best tubing for a job involves many factors. If you still have questions after reading this guide please contact us.
