Today’s sophisticated catheters have become modern marvels of advanced design and polymer technology. These devices are used to perform an ever-increasing number of the minimally invasive surgeries that have revolutionized the treatment of many diseases. However, designers of next-generation diagnostic and interventional catheters face a major challenge: How can they balance the need for smaller and smaller catheters with the demand for catheters that possess strength and pushability, yet are extremely flexible and maneuverable?
The answer: Think extrusion and beyond. For optimal catheter performance, designers need to consider the biological, physical, and chemical characteristics of polymers as well as the growing roster of breakthrough manufacturing processes. It is also necessary to leverage design expertise and implement engineering know-how in order to produce tubing for mission-critical medical device applications.
High-performance interventional catheters are used for a range of procedures in neurovascular, neurologic, cardiovascular, endoscopic, urologic, peripheral vascular, and renal denervation applications. The dimensions of the catheter shaft depend on the particular application. In the neurovascular and neurologic areas, for example, ultrasmall catheters are used. Composed of three layers—the liner, the braiding or coiled reinforcement, and the outer sheath—such microcatheters can be as small as 1.1 Fr, or approximately 0.016 in. in diameter.
On the other end of the spectrum are large-diameter catheter shafts, which are often used in such applications as abdominal aortic aneurysm (AAA) and thoracic aortic aneurysm (TAA) cardiovascular procedures. These catheters can measure up to 35 Fr, or nearly 0.5 in. Such catheter tubing often incorporates longitudinal wire reinforcement, which helps to limit stretch while ensuring flexibility.
Interventional catheters are used in minimally invasive surgeries and therapies. Generally involving 10–26-Fr devices, this type of tubing measures between 1/8 and 3/8 in. in diameter and is typically 3–4 ft in length. Many procedures start in the femoral artery as the surgeon winds the catheter through the tortuous pathways of the human vasculature into the heart or brain, twisting and moving the proximal end to position the catheter tip where it is needed. Following the deployment of the device or therapy, the surgeon removes the catheter, attempting to cause as little harm to the vasculature pathway as possible.
Constructing the Layers
|Catheter shafts incorporate four layers: liner, braid reinforcement, marker band, and outer sheath.|
The first step in achieving optimal catheter performance is to choose the correct materials for the liner and the shaft. Fluoropolymers, such as PTFE and FEP, excel in medical device applications because they are lubricious and biocompatible. Of all the polymers on the market today, PTFE is the most lubricious, followed closely by FEP. Both polymers exhibit versatility for extruding catheter tubing in an extensive array of diameters and shapes with single or multiple lumens. The material readily accommodates secondary processing steps such as etching and cutting, and can undergo postextrusion expansion to make heat-shrink or spiral heat-shrink tubing.
Once the appropriate materials have been selected, production of the liner is the next step in the manufacturing process. Typically made from PTFE, the liner consists of ultra thin walls and is produced in small diameters.
The next layer could consist of braid or coil reinforcement commonly made from Type 304 and Type 316 stainless steel or from nonmetallic materials such as polyester and PEEK. In order to perform well, the catheter must exhibit sufficient strength and rigidity without bending back on itself or kinking. At the same time, the device must be flexible so that it can follow a winding, sometimes tortuous path. In order to achieve this balance, catheters are frequently constructed with braiding or coiling and have a relatively rigid proximal section and a more flexible distal section.
Coiling reinforces the catheter body against kinking and ovalization of the liner while it maximizes hoop strength. Alternatively, the coiling layer can feature continuous, variable pitching, which can provide different strength and flexibility characteristics along the 3- to 4-ft length of the catheter. A braided support layer provides excellent torque transmission from the proximal to distal tip of the catheters. In addition, braid reinforcement provides resistance to crushing, kinking, or radial expansion from internal pressure while adding substantial torsional stiffness.
The final layer of the catheter shaft is known as the outer sheath. This layer is typically composed of high-performance materials such as Pebax or FEP. Additional options include PTFE, ETFE, polyurethane, polyethylene, and nylon.
Unique Tubing Elements
By incorporating a variety of construction elements, manufacturers can create a catheter shaft with unique properties. One such element, a deflectable or steerable sheath, can be used in several different applications, including transcatheter heart-valve stent and ablation delivery.
Such high-performance catheter shafts can be produced with as many as eight steerable wires, enabling surgeons to control the catheter tip precisely in multiple directions. Surgeons must be able to position the catheter tip it in the right place in order to deploy the device properly.
Along with a steerable or deflectable shaft, it is often important to consider multidurometer segments along the shaft and tip—in other words, different extrusion segments with varying degrees of softness or hardness. These variations, in turn, enable the manufacturer to alter the device’s flexibility, bend radius, and deflection angles. Such variations are typically terminated with a soft radiopaque tip, which prevents tight vasculature trauma while allowing good visibility under radiographic imaging. An image of the tip informs surgeons exactly where the catheter ends so that they can position it correctly.
Metal marker bands are another construction element that provides surgeons with visibility under radiographic imaging. Composed of precious or nonprecious metals—typically silver, gold, or titanium—they are positioned along the shaft and used as a guide to distinguish between different catheter segments. Instead of incorporating marker bands, catheters can also be fitted with flexible radiopaque markers. Encapsulated with tungsten-filled Pebax, such markers provide visibility, like metal marker bands. However, while marker bands tend to be stiff, radiopaque markers are soft and pliable, enhancing the surgeon’s ability to navigate flexibly into tight vessels.
In addition to consisting of several layers and a variety of additional construction elements, interventional catheters can be enhanced with such finishing features as punched holes. For example, circular or even irregular-shaped apertures can be formed along the catheter shaft using suction. This process results in consistent and repeatable openings.
Other finishing operations include custom-shaped shafts and tips, mating hubs, and tip attachments. Shafts and tips can be specially configured to accommodate the target anatomy. Standard or custom mating hubs can be insert-molded based on the application, sterilization method, or choice of mating components. Accomplishing this task requires overmolding expertise and the ability to match the outer material of the catheter to the substrate. For catheters ranging in size from 3 to approximately 30 Fr, tip attachments can be bonded with components of different sizes, enabling the manufacturer to terminate the coil or braid precisely and ensure that the catheter does not contain exposed construction elements.
The Big Picture
In the medical device industry, extrusion is a nearly universal technology. However, while many suppliers offer extrusion services, they must also be able to make myriad other decisions in order to produce high-performance interventional catheter tubing. At each step of the process, these decisions can positively or negatively impact the overall function of the device.
Jessica Lenhardt is director of worldwide marketing for OEM at Teleflex Medical OEM and a member of the company’s executive team. An expert in precision extrusion technologies and applications, she has more than 20 years of marketing, management, and product development experience at Teleflex Medical OEM, Cardinal Health, Leica Microsystems, and Cole-Palmer. Lenhardt received a bachelor’s degree in molecular biology and chemistry from Knox College.
Bob Michaels is senior technical editor at UBM Canon.