Tech Talk – Anatomy Models and Medical Device Testing

One important consideration to take into account while designing and testing medical devices is how they’re going to be used. In the case of vascular devices, they will be used in arteries and veins and I’ll describe some of the test considerations. While performing your verification testing, you want to ensure that you have a reasonable clinical model to perform testing under.

For example you might have a device that is intended to be used in the Right Coronary Artery (RCA) as shown below (image from Wikipedia):

How do you ensure your device works correctly without using it on a person? You find an appropriate model.

Models of various anatomies are available from Elastrat and Shelley Medical Imaging Technologies. ASTM F2394 also contains a schematic of a two dimensional (2D) model recommended by the FDA for some applications. Although it is best to ensure you pick a model based on the vessel characteristics that your device will likely see (such as vessel diameter and tortuosity). Using the ASTM or another off the shelf model can get you into trouble if for example you’re testing a guide catheter and pushing it into the smallest vessels when it is only intended to sit in the aortic arch.

A 2D model is easy to construct and use, but not necessarily the most accurate, it generally consists of just two pieces of machined plastic (bottom with a clear top) with a piece of tubing inserted. The easiest model consists of just a radius as shown below (by the way, I did the diagram myself in MS PowerPoint!).

Ask your clinical source what the tightest radius your device is likely to see in-vivo and simply machine a curve of that diameter. Insert a piece of silicone tubing into that radius to simulate a vessel wall more accurately than hard plastic and you’re ready to get started (you can use pig vessels to line your tortuous pathway, but this is no fun to set up). It is easy to create 2D models of various tortuous pathways. Creganna (Medical Device Technology, May 2006) shows an example tortuous pathway in some of their coronary block model simulated use testing:

You can see the simulated aortic arch as the large looping arch (start from the top most pathway) and then into tighter arteries as the model continues.

A 2D model obviously lacks some realism as it is possible that your device will need to turn multiple planes during use. Although it may be argued that the worst case scenario is actually the 2D model as it stresses only those two dimensions, adding the third dimensions allows additional flexibility in most devices as they are generally concentric. In the case of a non-concentric model, you can perform your testing to the worst case in the 2D model, setting your weakest side up to take the most abuse. The 2D model is also arguably tougher on your device than clinical use as the give is limited to the wall thickness of the tubing you insert.

For 3D models, you’re probably better off buying one from the companies listed above, although they can get expensive, and can be damaged so you need to be careful. An example heart model from Elastrat is shown below.

Once you have your anatomy model, you generally want to further simulate clinical conditions, either by flowing 37C saline through the model, or a mixture of glycerin and water to simulate blood flow.

Now you want to run whatever tests you can using the model. For example, a guide wire or catheter turns to failure test is simple enough, just hold the distal end and turn the proximal end until the device breaks. However, to really simulate use, you should put the device through the model, and while the device is still in the model, hold the distal end then turn the device until it breaks. The addition of the model adds a bit of complexity to the test, but is a better test system.

I have seen the FDA question models, so document your clinical justification why the model you use is appropriate in the protocol. If you’re using a model that you bought, you should ask them to provide you with the justification.

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.