Tech Talk – Catheter Bonding

Tech Talk – Medical Device Catheter Bonding

For my second tech talk I thought I’d briefly cover catheter bonding. The typical vascular catheter is made of several types of polymer; it is of obvious importance to connect them in a precise way. A typical bond will consist of two types of tubing; a popular choice is Arkema’s Pebax. Pebax is a USP class VI material that stands up well to sterilization and takes colorants well; it is widely used in current catheters. Bonding will consist of something like 72 durometer on the proximal end of a catheter to 63 durometer on the next step, then moving down the catheter until you reach 35 durometer at the tip. Ideally, the two pieces of tubing are the same size, but they can be of slightly different inner and outer diameters. Starting with your two pieces of Pebax tubing:

Since Pebax isn’t as smooth as most physicians desire, a PTFE or HDPE liner is usually used on the inside of the catheter. The liner allows for smooth delivery of other devices such as guide wires or other catheters down the catheter. A liner will not be required for something like an inflation lumen. Liners may also be multiple layers to better bond with the Pebax. A mandrel is placed inside the liner or lumen. The mandrel is usually coated with PTFE (by companies like Applied Plastics) or parylene for easy removal later on in the process.

Fluorinated ethylene propylene (FEP) tubing is then placed over the Tubing joint. FEP is available off the shelf in multiple diameters and wall thicknesses, it can also be custom made if required by companies such as Zues and Fluortek. Using FEP forms a more consistent and even joint.

Apply heat to the FEP tubing to shrink it and melt the Pebax. Pebax melts at around 174°C so usually 190°C is enough heat. To heat, Beahm Designs makes off the shelf heaters that apply a constant stream of air at a set temperature. Alternatively heat guns can be used, although long term you should look for a more controllable solution.

After heat is applied and the part is allowed to cool, cut off the FEP tubing with a razor blade (slice more parallel to the tubing, not straight into the middle of the FEP) and remove the mandrel and you have a very nice piece of tubing that is very difficult to tell where the transition is.

For a catheter you may actually have five or six transitions such as this depending on the stiffness you want and the number of lumens you want at each point. It is also possible, but not as desirable to join two types of tubing using adhesive, this may be required if the polymers are too different to heat bond. In this case, one piece of tubing fits into the other.

The adhesive used for medical devices is generally made by Dymax or Loctite and can be UV or heat/time cured. UV cure is superior for manufacturability as you can quickly pass it to the next operation, but it may not be possible to use UV adhesives in all cases.  The geometry of this catheter is more difficult and the liner may have to be tapered.

Advanced Polymers also sells a polyester heat shrink tubing that can be left on the device and used in catheter bonding if these two methods don’t quite work for you.

After bonding you will want to validate your design and process using methods such as tensile test to ISO 10555 and burst pressure to ensure it is high quality. Catheter bonding is something that has been fairly extensively developed and you can get lots of support from suppliers, but it always turns out you need to know one extra trick that you have to develop yourself.

References for this post are The Medical Device R&D Handbook by Theodore R. Kucklick, Beahm Designs, Arkema, and personal experience.