By Mike Schultz, Spectrum Plastics Group, Vice President of Innovation and Development

Minimally invasive surgeries (MIS) that use steerable catheter systems represent a growing market in the healthcare industry. As the variety of catheter-based procedures continues to grow, with physicians and therapies demanding tools and technologies that increase access and performance, the steerable catheter market is widely open to innovation.

MIS procedures, especially cardiovascular and neurovascular applications, often must follow tortuous pathways to access more distal vessels and specific targeted anatomy. The success of these procedures depends on advanced catheter systems that deliver therapy to precise locations through narrow access points, such as the femoral artery. As MIS procedures become more delicate and complex, physicians need improved control and steerability of catheters to accurately locate and deploy treatment without damaging surrounding tissue.

The most important design considerations for steerable catheters are:

• Torque response – the ability of the catheter to transfer twisting action from end to end (1: 1 response is ideal). The torque response depends on several factors, including the polymer durometer and the strength of the braid or reinforcing member
• Flexibility — the ability of a catheter to bend into a curved shape due to lateral force, for example in a tortuous vessel. Flexibility is also affected by the durometer and the density of the braid and therefore must be balanced with flexibility.
• Deviation — the ability to independently control the distal end of the catheter, through a range of motion
• Steerability: the combined effect of torque transfer and deflection to maneuver the distal end, with great precision, to the exact target location
• Kink resistance – the ability of the catheter to prevent sagging when bent or deflected

Construct a steerable / deflectable catheter

A typical steer / deflection catheter consists of four key components:

• Ergonomic handle
• An adjustable wheel or slider that, when manipulated, controls the deviation of the distal tip
• The rod, which comprises a flexible tubular body made of braid reinforced polymer
• The distal tip, which deflects based on user input and through which therapy is deployed.

Generally, the handle is made of rigid plastic, providing a comfortable grip and an accurate translation of the user’s movements. The proximal portion of the shaft closest to the handle is constructed from a relatively stiff polymer, which provides excellent torque response and minimal flexibility. The catheter body gradually becomes more flexible from the handle to the distal tip, which is the most flexible part of the device. The respective lengths and stiffnesses of each section may vary depending on the specific anatomy and procedure for which the catheter is designed.

The deviation of the distal tip is controlled by user manipulation of the thumbwheel or slider on the handle. The wheel is connected to one or more wires that extend along the length of the catheter body to the distal tip; when the wheel is rotated, it creates tension in the threads, which in turn deflects the flexible distal tip in a particular direction.

Designing a steerable catheter requires complete knowledge of the surgical procedure, the anatomy to be traversed, and the range of motion / deflection required for treatment. For example, what are the bend angles and deflection planes? What is the required deflection and what is the optimum radius of curvature to accommodate the anatomy?

Flexibility is controlled by the type of polymer, the durometer, and the density of the coil or braid reinforcement in the respective sections of the catheter. A high density braid at the distal end maximizes flexibility; a lower density braid makes the proximal rod more rigid. Several material durometers can be used along the length of the catheter body to create gradations of flexibility. Finding the balance between torque and flexibility requires careful modeling and testing. In general, the higher the flexibility, the lower the torque, so well-defined design inputs are essential to allow the designer to decide with some certainty how to optimize performance so that the device easily achieves the target anatomy. . Other reinforcing structures such as hypodermic tubes machined with a specialized flex pattern can also be added for improved deflection or more specific curve configurations.

Closed pull wires that run along the outside of the central lumen of the catheter control the deflection of the distal end. The number and position of the wires determine the degree of control the user has over the deviation of the distal tip. Typical configurations are: unidirectional (unidirectional), bidirectional (bidirectional) and four-way (four directions). A four-way catheter has four pull wires that extend from the distal tip to the proximal handle to control actuation of the curve in two planes of motion. As tension is applied to the threads, the frictional forces increase, so it is often necessary to select a low friction coating, such as PTFE, for the tensile threads, the inner surface of the lumen or the two.

Dilator and sheath design

Access to internal targets in the body is achieved through a set of sheaths, consisting of an access sheath and a dilator, which is percutaneously inserted into the selected blood vessel. The dilator is then withdrawn from the lumen of the sheath, leaving the sheath as the port into which the catheter or delivery system is inserted to start the procedure. The primary entry points for most cardiovascular and neurovascular procedures are the femoral artery (groin) or the radial artery (wrist). Emerging procedures that rely on steerable catheters include transseptal interventions in the heart, such as cardiac ablation, mitral valve repair, and pulmonary vein isolation.

The dilator / sheath consists of a hemostatic valve that controls blood flow, the central tubular sheath, which is typically made from low friction flexible materials such as Pebax, high density polyethylene (HDPE) or ethylene fluorinated propylene (FEP) for easy insertion, and the transition tip of the sheath. It is imperative that the transition of the body from the sheath to the tip be seamless and without irregularities that would create friction. Hydrophilic coatings, especially for larger or longer fixtures, can also be applied which become very slippery when wet.

The dilator is often a single durometer, as it may not need to advance through a very tortuous anatomy. Dilators that must penetrate further and / or must conform to a tortuous anatomy often use a multi-durometer construction, with a rigid proximal end and a highly flexible distal tip. Sleeves are often made of FEP due to its flexibility and low coefficient of friction. In the case of larger diameter / thin walled access ducts, a coiled brace can be used to maximize torsional strength while allowing flexibility. The flexible polymer marker bands can be loaded with microscopic tungsten particles, allowing the device to be visible under fluoroscopy. The distal holes can be laser drilled to facilitate perfusion or flushing of the dilator. Depending on the surgical application, the distal tip can be of a different geometric design, which can be fabricated by grinding, a radio frequency dielectric heating process, or both.

Design and development

Access to targeted anatomy is critical to the success of any procedure, and for this reason the components of the sheath, dilator, and steerable catheter must be optimized to ensure that they provide a safe, easy and safe path. reliable for the therapeutic device. It is crucial that all advanced devices use the most advanced materials and design. Spectrum Plastics Group brings decades of experience and in-depth knowledge of the materials, design and processes used in access and delivery systems. Our experienced development engineers and robust design control systems will ensure your device is optimized for maximum performance, quality and manufacture.

Spectrum has extensive experience in the design and manufacture of steerable / deflecting catheters and dilators / sheaths. Not only does Spectrum customize catheters for specific customer needs, but we also provide ready-to-use steerable catheters, catheter sub-assemblies, dilators / sheaths and other catheter components online at webstore.spectrumplastics .com, which can be used to speed up the design and development of your next project.

Content sponsored by Spectrum Plastics Group