The proliferation of minimally invasive procedures has prompted engineers and extruders alike to push the boundaries of medical tubing development. As a result, catheter diameters have shrunk dramatically to enable better navigation of the device through the narrow and tortuous anatomy of the body. But while decreasing the diameter at the distal end of a catheter is beneficial, it can be problematic at the proximal end of the tube, which must often accommodate luers, extension tubes, or other medical devices. To reconcile these varying size requirements at the proximal and distal ends of the catheter, engineers are increasingly turning to bump tubing as a design solution for peripherally inserted central catheters (PICCs) and other devices.

Benefits of the Bump
"Prior to 2000, bump tubing was out there but only being done on a limited basis," comments Tim Steele, founder and CEO of Microspec Corp. (Peterborough, NH). "It has become more popular in the last 10 years. Practicality and design benefits for the user and the patient have fueled its growth."

Also known as a taper tube, bump tubing is characterized by a changing diameter along its length that typically transitions from a smaller-diameter distal end to a larger-diameter proximal end. This configuration allows catheter designers to minimize the diameter of the distal end to improve patient comfort while maximizing functionality of the proximal end and providing easier device access, according to Aubrey Sanders, medical market specialist for Zeus Inc. (Orangeburg, SC). It can also help to limit friction during the procedure, she adds, because there is less material at the distal tip.

While bump tubing enables a catheter to feature different diameters at either end, it also offers the added bonus of doing so in a single, continuous structure. "There's a huge advantage to that; the tube is functionally stronger and not going to break apart easily," Steele says. Non-bump methods of producing such a tube would require the use of multiple parts bonded together in some fashion.

And because it is extruded as a continuous piece, bump tubing can benefit medical device manufacturers as well in terms of time and cost savings. These designs, Steele notes, eliminate the need for added strain relief as well as associated welding and assembly costs. "Generally, if you're doing postprocessing, you would also have to form the tube over some sort of mandrel," Sanders adds. "That can be very labor intensive and costly, and can generate a lot of scrap. If you do a bump tube with a supplier, it can be a cost reduction."

Extrusion of Bump Tubing
In contrast to catheter flaring, which is usually performed as a secondary operation, bump tubing is achieved in-line during the extrusion process. "Because you're drawing the tube away from the extruder, it's a moving target that you have to hit," explains Steele. "And if your process is highly controlled, you can extrude a bump tube with high precision."

The exact process and equipment employed in extruding bump tubing, however, tends to differ slightly among service providers. Microspec, for example, designs all of its extrusion tools in-house and has a unique crosshead. While the company does employ off-the-shelf equipment, it redesigns and integrates systems in such a way that is proprietary. It modifies the extruders, haul-off equipment, error regulators, and various factors that can affect the extrusion process, according to Steele.

Despite differences in the details, bump extruders all focus on several variables that affect the finished product. "The first and probably most important variable is the rate at which the tube is drawn from the extrusion die, which we call line speed," Steele says. "The second variable would be the internal air pressure; the third variable would be the output of the extruder." These variables, he adds, can be manipulated to achieve the desired diameter and specifications of the finished bump tube.

But line speed, air pressure, and extruder output aren't the only factors to consider when developing taper tubes, according to bump extrusion specialists. When designing these unique tubes, engineers need to keep in mind that the ratio, ID, and OD on one end should be somewhat proportional to the other end, Sanders says.

Apur Lathiya, vice president and general manager of Vesta Thermoplastics (Corona, CA) echoes that advice. "Depending on the plastic they're choosing and how much they want the diameter to change from one end to the other, that ratio can be an issue," he says. "Sometimes the plastic is just not capable of going from very big to very small [diameters]; some plastics don't like to be drawn down that much." Lathiya cites high-density polyethylenes as one such example, noting that attempting drastically different ratios with these materials may result in bad yields or wavy surfaces.

Lathiya also stresses the importance of allocating a minimum transition length between the proximal and distal ends of the catheter. "There's always a transition length, and depending on how much you're varying the diameter, that transition length will vary." He notes that Vesta has extruded bump tubing with transition lengths as small as 0.5 in., but 2-in. minimum transition lengths tend to be common in the industry. Likewise, Sanders says that Zeus recommends a transition length of at least 2 in. for bump tubing.

Next-Generation Bump Tubing
In response to the success of bump tubing in minimally invasive applications, extrusion companies have expanded their related capabilities to better serve the market. Microspec, Vesta, and Zeus, for instance, can all extrude both single- and multilumen bump tubing. In fact, Zeus recently worked on a bump tube project that required the extrusion of six lumens. "That's not a constraint, though," Sanders says, "that's just one project I'm aware of for gastrointestinal endoscopy. It increases the number of working channels you have for an endoscope."

Microspec, Steele states, has extruded multilumen bump tubing with as many as eight lumens for drug-delivery applications. However, he points out that the company does not often extrude more than four lumens in a given bump tube. It can provide coextruded bump tubing and microbump tubing with ODs measuring less than 2 French as well.

Building on these various techniques, the next phase of bump tubing could entail braiding on top of bump tubes, Lathiya suggests. "That will allow especially for [progress in] neurovascular applications, where the anatomy is very tortuous and very small," he says. "For people to go in and navigate that area without the catheter kinking, they braid it; that gives it flexibility without kinking. It also gives it column strength so you can push it." Additional advantages for braided bump tubing, according to Lathiya, could include smaller ODs, thinner walls, and a large working area.

"I'm sure it's being done right now," he adds. "I think that's where we're going to see more and more opportunities for bump tubing in the future."