Presented by Phillip Leopold at MD&M West 2013
Good afternoon, everybody. Thanks for coming by and participating in this presentation. I’m going to talk about Complex Catheters and try to give you some tips on how to go about making special products such as this.
I’m going to give you a real brief introduction of what Medical Murray does, and then try to define Complex Catheter for you with three different examples of products that we’re making. I will go into some construction tips related to braiding, and molding hubs onto multi-lumen catheters, silicone rubber balloon, bonding thermoplastics, and then some words about integrating sensors into the catheter and how to go about it, followed by a summary of what I’ve presented.
Our company is in the Chicago area, and we started in April of 1996 between myself and my son, Andy. We spent about the first 10 years on developing products, medical devices – primarily disposables, and then expanded into manufacturing in 2008 with a separate manufacturing facility that is located about four miles from our development facility. We just recently expanded our development operation into Charlotte, North Carolina, late last year.
Our focus has been on designing and developing devices for other companies. We don’t make any of our own products – we just make things for others. We specialize in things that take unique processes – things that nobody else knows how or wants to make – we take on as a challenge. Devices that involve special processes or materials, or designs that meet those requirements. This includes complex catheters that I’m talking about today, as well as components for them and implant materials.
So what is a complex catheter? For us, it’s a catheter that integrates a number of different features. This is a table that I prepared that shows three different catheters, and it shows the features in them. We consider when something has six or seven different features in it, it becomes complex and difficult to make.
Typically, they’re multi-lumen so that you have one lumen that’s used for a guidewire, another one for balloon inflation or a sensor. Often, you need a low friction liner and one of those lumens to be able to easily move a guide wire through it.
Balloons often are needed to make the function of the catheter work. Variable stiffness – sometimes the distal ends you need very flexible to navigate the anatomy, but you want the proximal end to be stiff, so that you have to combine different materials to achieve that. Also, they often are reinforced with either braid wire or axial reinforcement to get special stiffness in the catheter for, again, positioning accuracy. These days, there’s many that wants sensors such as temperature sensors or pressure sensors integrated into the catheter.
We have a few cases where you want to deliver a sensor or a device such as a stent, and you need a release mechanism built into the catheter so it lets go of that. I’ll show you an example of a release mechanism we’ve worked with on several cases.
They also sometimes need to be steerable – you put multiple pull wires into the wall so that you can tip the end of the catheter one way or the other and steer it into the anatomy. Soft Tips. Radiopaque markers so you can see it under fluoro and know where the catheter is. Side ports for delivering drugs or other materials into the anatomy. Usually, if you have multi-lumens, you need to separate them out on the proximal end so that you get access to them individually.
Shaped tips. Printing on the outside of the catheter to know the depth and where the catheters position. Hydrophilic coatings to make it slippery so that moves easily. Finally, to make all these things happen, you have multiple materials, different plastic materials, maybe nitinol, wire, Teflon, and so on.
So to show some examples, this is a example of a catheter, that we wanted to have a needle come out the side of the catheter to puncture at a different direction than what the main body of the catheter was coming through with the guide wire. To make this happen, we needed to be able to know which way this was coming out the side under fluoro. It had a Platinum Iridium tube that was bound in it, and it becomes like a pointer under fluoro. You can see the end of it under fluoro and tell which way it was pointed. There was a marker band embedded into the side of the catheter, so you could see which direction it was pointed. If you want to be able to have this control, you need a very torsionally stiff catheter. We did a double braid over the outside and we incorporated a polyamide tube around the top surface here – that is where the guidewire comes through. It’s a combination of these features that we considered making a pretty complex.Another application is a catheter that the customer wants to deliver steam. This means that you have to have heat resistance in the catheter. It had a balloon on the end of it, so that it needs to be a dual lumen to inflate that balloon.
You need a high pressure connector on the proximal and so the steam can be controlled at pressure. So in this case, we have a silicone rubber balloon bonded on to the distal end of the catheter. And it’s braided to give it good strength and resistance to the internal pressure, the steam and a soft tip on the end of it.
The last example, I want to give it as a drug delivery catheter. On the bottom here, you can see that we have two balloons on the catheter. These are inflated and a drug is delivered between the two balloons.
It’s a three lumen catheter – one for the guidewire going down it, one for the inflation of the balloons, and a third lumen to infuse the drug out between between these two balloons in this area. Then in the end, we had to separate the three lumens in the hub area and the three separate lines to keep them all isolated.
I want to talk now about making multi-lumen braided catheters. Here, the problem is that you’re putting braid wire over that multi-lumen. Typically, you put a Pebax or soft urethane over the braid, and then heat shrink it down with a Teflon heat shrink tube. But, if you don’t reinforce those lumens, they all collapse when you do that process. You then can put small Teflon tubes or the wire inside of them, so that after you’ve shrunk them, you can pull the wire out and then pull the Teflon tube out.
In other cases, we put polyamid tubes into the inside of that lumen and it has enough stiffness and resistance and doesn’t melt because it’s high temperature Polymer. You can form a lumen just from that polyamid tubing. And finally, you can use shaped extrusions of PTFE for irregular shaped lumens, but usually the issue with that is how you get them out. It’s really hard to pull them out after it’s been shrunk down on them.
Another tip that can make a high axial stiffness and strengthen in a catheter is to do linear reinforcements along the length and this picture here, which is part of a braider, you can see these stainless tubes sticking out. What happens is you feed the axial fiber through those and then the braider braids it into the weave as its braiding. It becomes very straight and very consistent along the length and gives you both axial reinforcement and torsional.
You know, an example of this is down here in the bottom where we put in four polyamide tubes that we braided with thin polyester fiber. It held them in place, and then we heat shrank the pebax over the outside and left these four lumens in that were used to inflate four balloons at the tip end that catheter. In some cases, they use kevlar fibers for these and it gives good axial stiffness and strength during a certain procedure.
I wanted to talk about how do you separate these from molding. The biggest problem with these is that you have a very thin wall between each of the lumens. When you injection mold over it, it’s difficult to have all three separated and not get any leakage across that very thin wall. You also have to avoid any voids inside the molding because that creates crosstalk between the lumens. Extension tubes with wires to create preload for the injection mold. Over molded hub with uniform wall thickness. Start with bump tubing to increase lumen size and wall thickness .Add a lower durometer strain relief tubing for mold shut-off.
One example to improve this is to start with bump tubing – where the extruder makes it so that the proximal end is much larger in diameter, with thicker walls between each of the lumens to make it easier to mold. We then place a softer strain relief material over the catheter, so that when we put it in the mold, we can shut off on that soft strain relief and get a consistent shut off. Next, you put wires with the individual lumens that I showed earlier into that and this becomes a preform, then it’s put into the injection mold. Finally, you injection mold a polymer that will bond with a catheter.
You try to make it so that all these wall thicknesses are very uniform and it avoids shrinkage voids during the molding. It’s about the design of the molded hub, as well as how you put the wires in and prevent flow and prevent the voids coming out.
This is an example of a hub that we’re molding now that have six lower fittings going into it and one fitting for a pressure sensor that goes down the side of it. It separates out six different lumens in the individual connections on the hub. Obviously, it’s a lot easier to mold just a single lumen with a lower hub on the back for high pressure applications. We also use molding to mold tips for catheters. These aren’t really complex, but it’s a way to create integrated products.
Next, I want to mention how to bond silicone rubber on to a thermoplastic elastomer catheter. The most critical thing with this is to get a circular bond line around it – if there’s any variation or it’s tipped or irregular, then the silicone rubber balloon won’t open concentrically – it’ll all open on one side of the catheter.
One of the ways we avoid that problem is to put a little block that compresses the silicone rubber and prevents adhesive getting under it. You put the adhesive in both ends. It makes a very uniform bond, you cure it and then you take the fixture off and it leaves the balloon unbonded in the right area. Adhesive selection wise – we use primers to make a good bond between them and typically a silicone rubber type adhesive.T The alternate to this is a urethane balloon that you can heat bond, but you can’t get as big a diameter out of it. With silicone you can go up to maybe 8 to 10 times its starting diameter when it’s inflated.
The next thing I wanted to mention was integrating sensors into catheters. Typically, you need to bring wires back from the sensor to connect to them. The problem is if you embed that copper wire into the catheter wall, it breaks the wire when you bend it into the body. One way to avoid this is to put the wires inside of a polyamid tube, so that they can stretch and move inside. The other is to do a spiral wrap around the outside and then that prevents that axial lengthening of them during the bending of the catheter.
Finally, I want to show you an example of a release device mechanism we’ve used in several products. It uses a release wire that is threaded through a hole in the catheter that you want to deliver with, through the device – this Christmas tree shaped part. This wire holds everything together when you’re moving it, but once you extract this wire out the back of it, it pulls out of those holes and releases the device completely. It’s a very simple way to hold something and then release it reliably.
So in summary, Complex Catheters means that you have to use lots of different materials and many different processes. I’ve shown you some ideas about braiding and hub design, how to bond silicone rubber balloons, release mechanisms, and sensor integration. We’d be happy to help you out on any challenges you might have.
Presented Live at MD&M West 2013 by Phillip Leopold, President of Medical Murray.