Designed for life

Mak Joshi, Teleflex Medical OEM, discusses designing a high performance interventional catheter for stent delivery

Demand for minimally invasive procedures has increased the need for specialised catheters to deliver stents, stent grafts, and other medical devices in the body. These catheters are technological wonders that are complex, multifunctional, and have small profiles. Engineering next-generation stent delivery systems requires a deep understanding of materials and advanced manufacturing processes to develop devices with even smaller profiles and, seemingly conflicting, performance characteristics of strength, pushability, flexibility and maneuverability.

Stents are typically used in the treatment of: coronary heart disease; carotid artery disease; peripheral artery disease; aortic tears; blockage or obstruction in the bile ducts and intracranial aneurysms.

Stent delivery procedures start in arteries in the thigh, arm, or neck as the clinician winds catheter through tortuous pathways of the human vasculature, twisting and moving the proximal end to position the catheter tip at the targeted organ. Following deployment of device or therapy, the clinician removes the catheter while minimising harm to the vascular pathway.

Design considerations

Vascular health is a key catheter design consideration. Tortuosity and rigidity of the vasculature, minimal tissue trauma, anatomical location, and size of vessels are major factors impacting design, material selection, and choice of manufacturing processes.

To function at its optimal level, a catheter must possess several desirable features and attributes:

• High lubricity of the inner liner for smooth stent delivery
• Reinforcement for strength, rigidity, and torque control along the length of the catheter while balancing the need for flexibility and kink resistance to navigate tortuous vascular pathways
• High lubricity of the outer sheath for low insertion force and minimal vascular trauma
•  Soft tip and multi-durometer segments along the length of the catheter for excellent pushability and maneuverability
• Radiographic contrast of tip and key segments for visibility of insertion and anatomical placement on radiographic imaging apparatus
• Steerability and deflection for navigation of vasculature

Dimensions of the catheter shaft depend on the anatomy of the vasculature and the dimensions of the stent, for example:

• CHD, CAD, and PAD applications generally require diameters between 5F and 8F (0.066 in. and 0.105 in.), and are typically 40 to 50 inches in length.
• Catheters used for deployment of biliary stents are between 6F and 9F (0.079 in. and 0.118 in.), and are typically 30 to 60 inches in length.

Catheters used for stent delivery in neurovascular conditions such as intracranial aneurysms generally require extremely small diameters (1.1F or 0.014in.) but may go as high as 6F (0.079 in.) and are typically 70 to 90 inches in length.

Choosing the right materials and manufacturing processes

An important first step in achieving optimal catheter performance is to choose the right materials for the inner liner and the outer sheath. Fluoropolymers, such as PTFE and FEP, excel in medical device applications because of their desirable properties of lubricity, chemical inertness, and biocompatibility. PTFE is the most lubricious polymer available today, followed closely by FEP. Both polymers can be extruded in an extensive array of diameters and shapes with single- and multi-lumens, and readily accept secondary processing such as etching, cutting, and printing, as well as post-extrusion expansion to make heat-shrink tubing. 

Once materials have been selected, production of the liner is the next step. The liner is constructed from PTFE and typically consists of ultra-thin walls and is produced in small diameters.

The next layer typically consists of some type of reinforcement. Interventional catheters are frequently constructed with braiding or coiling to have a relatively rigid proximal section and a more flexible distal section. Materials commonly used for reinforcement are metals such as stainless steel and nitinol, or nonmetallic materials such as polyester and PEEK.

Reinforced PTFE-lined catheters are made with flat or round wire braid, and/or coiled configurations. Flat wire is generally used for thin walled applications where "anti-kink" performance is critical. Coiled shafts are more flexible and kink resistant, but they do not have the same torque capabilities as a braided shaft. In addition, coiling reinforces the catheter against crushing, kinking, and radial expansion from internal pressure, while adding negligible bending stiffness to the body. A braided support layer also provides resistance to crushing, kinking, or radial expansion from internal pressure, while providing exceptional torsional stiffness. 

Several braid patterns can be utilised based on desired torque control features:

• Regular braid pattern common pattern that uses 16 wires in a ‘one under two, over two’ pattern.
• Diamond braid pattern is a pattern that uses 16 wires, but in a ‘two wire under two, over two” pattern. This pattern tends to provide better torque and more kink resistance than the regular braid pattern, but at a slightly higher cost.
• Diamond braid pattern, half load utilises half the number of wires. The diamond pattern can be produced in a ‘one wire under one, over one’ pattern which provides more torque than the regular diamond pattern but incurs a much higher cost.

Designers are not limited to these patterns. Several original equipment manufacturers can create custom-engineered braid and coiling variations. Now it is even possible to utilise variable pitch, continuous reinforcement that can vary performance characteristics along the three to four ft. length of the shaft. Also, there are novel technologies for connecting dissimilar sections of the shaft without sacrificing shaft flexibility or performance. Designers can create precise catheter characteristics by combining any number of diameters, reinforcements, and hardnesses.

The outer sheath is the final layer. High-performance materials such as PTFE, FEP, ETFE, PE, Polyurethane, Pebax, and nylon are typically preferred.

A coating can enhance performance

Hydrophilic coating is used to enhance lubricity of the inner liner and outer sheath. The coating, usually negatively charged, exhibits ‘water loving’ properties, meaning it performs hydrogen bonding with surrounding water. This lowers the coefficient of friction of the material and wets the surface more evenly, reducing the insertion force, allowing the catheter to move through the vasculature more easily.

Unique catheter features

By incorporating a variety of construction elements, manufacturers can create catheter shafts with unique features.

One such feature, a deflectable or steerable sheath, can be used in several different applications, including AAA stent graft delivery, transcatheter heart-valve delivery, and renal denervation. A steerable catheter shaft can be produced with as many as eight steerable wires, enabling clinicians to maneuver the tip precisely in multiple directions. Clinicians must be able to position the tip in the right place so that they can deploy the device properly.

Another important feature is multi-durometer segments along the shaft and tip—in other words, different extrusion segments with varying degrees of softness or hardness which enable the manufacturer to alter the catheter’s flexibility, bend radius, and deflection angles. Multi-durometer shafts are typically terminated with a soft radiopaque tip, which prevents vascular trauma while allowing good contrast under radiographic imaging.

A third feature is use of metal marker bands to provide clinicians with visibility under radiographic imaging. Bands of high density precious metals—typically tantalum, gold, or platinum —are positioned along the shaft and used as a guide to distinguish key areas along the length of the catheter.

Flexible radiopaque markers are sometimes used in lieu of metal marker bands. Encapsulated with tungsten-filled Pebax, these markers provide similar radiographic visibility, while being soft and pliable.

Finishing operations

Interventional catheters can be further enhanced with such finishing features as punched holes, shaping, tipping, and printing. For example, circular or irregular-shaped apertures can be formed along the catheter shaft using a suctioning process that results in consistent and repeatable openings.

Finishing operations also include custom-shaped shafts and tips, mating hubs, and tip attachments. Shafts and tips can be shaped to accommodate the target anatomy. Standard or custom mating hubs can be insert-molded to the catheter. In all finishing operations, it is important to terminate the coil or braid precisely and ensure that the catheter does not contain exposed construction elements that can cause functional defects or vascular trauma during procedures.

Finally, the manufacturer can print useful information on the shaft to provide gradations or instructions for the clinician. Printing also allows the manufacturer to brand the device.

The long haul

A plethora of decisions go into producing high-performance catheters for stent delivery. At each step of the process, these decisions can positively or negatively impact the overall function of the device. In addition, taking a concept from a functional prototype to a marketable catheter requires in-house expertise, and superior design and manufacturing capabilities. Finding the right partner that can deliver along those attributes can go a long way toward a successful product launch.