By Mike Schultz, Spectrum Plastics Group, Vice President of Innovation and Development
Minimally invasive surgeries (MIS) that utilize steerable catheter systems represent an expanding market in the healthcare space. 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 wide open for innovation.
MIS procedures, especially cardiovascular and neurovascular applications, must often navigate tortuous pathways to gain access to more distal vessels and specific targeted anatomy. The success of these procedures depends on advanced catheter systems that deliver therapies to precise locations through narrow access points, such as the femoral artery. As MIS procedures become more delicate and complex, physicians require enhanced control and steerability of catheters to precisely locate and deploy the treatment without damaging surrounding tissue.
The most important design considerations for steerable catheters are:
- Torque response— the ability of the catheter to transfer a twisting action from one end to the other (1:1 response is ideal). Torque response is dependent on several factors, including polymer durometer and the strength of the braid or reinforcing member
- Flexibility—the ability of a catheter to bend into a curve shape due to lateral force, for example in tortuous vessel. Flexibility is also impacted by durometer and braid density and therefore must be balanced with flexibility.
- Deflection—the ability to independently control the distal tip of the catheter, through a range of motions
- Steerability—the combined effect of torque transfer and deflection to maneuver the distal tip, with great accuracy, to the exact target location
- Kink resistance – the ability of the catheter to avoid collapse when bent or deflected
Building a Steerable/Deflectable Catheter
A typical steerable/deflectable catheter consists of four key parts:
- Ergonomic handle
- An adjustable wheel or slider that, when manipulated, controls deflection of the distal tip
- The shaft, which comprises a flexible, braid-reinforced polymer tubular body
- The distal tip, which deflects according to the users input and through which the therapy is deployed.
Generally, the handle is made from a rigid plastic, providing a comfortable grip and precise 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 becomes progressively more flexible from the handle to the distal tip, which is the most flexible part of the device. The respective lengths and stiffness of each section may vary according to the specific anatomy and procedure which the catheter is designed for.
Deflection of the distal tip is controlled by the user’s manipulation of the wheel or slider on the handle. The wheel is connected to one or more wires that run along the length of the catheter body to the distal tip; as the wheel is turned it creates tension in the wires, 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 navigated, and the range of motion/deflection required for treatment. For example, what are the bend angles and planes of deflection? How much deflection is required, and what is the optimum bend radius to fit the anatomy?
Flexibility is controlled by polymer type, durometer, and the density of the coiling or braiding reinforcement in respective sections of the catheter. A high-density braid at the distal tip maximizes flexibility; a lower-density braid makes the proximal shaft stiffer. Multiple durometers of materials can be used along the catheter body to create gradations in flexibility. Striking 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 key in allowing the designer to decide with some certainty how to optimize the performance such that the device will readily reach the target anatomy. Other reinforcing structures such as machined hypodermic tubing with a specialized flex pattern can also be added for enhanced deflection or more specific curve configurations.
Enclosed pull wires that run along the outside of the central catheter lumen control deflection of the distal tip. The number and position of the wires determines the degree of control that the user has over the distal tip deflection. Typical configurations are: uni-directional (one-way), bi-directional (two-way), and quad-directional (four-way). A quad-directional catheter has four pull wires that extend from the distal tip to the proximal handle to control curve actuation in two planes of motion. As tension is applied to the wires, the friction forces increase, so it is often necessary to select a low friction coating, such as PTFE, for the pull wires, the inner surface of the lumen, or both.
Dilator and Sheath Design
Access to internal targets in the body is gained via a sheath set, comprised of an access sheath and a dilator, which is inserted percutaneously into the selected blood vessel. The dilator is then removed 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. Main points of entry for most cardiovascular and neurovascular procedures are the femoral artery (groin) or radial artery (wrist). Emerging procedures that rely on steerable catheters include transeptal interventions in the heart, such as cardiac ablation, mitral valve repair, and pulmonary vein isolation.
The dilator/sheath consists of a hemostasis valve that controls blood flow, the central tubular sheath, which is typically made from flexible, low-friction materials such as Pebax , high density poly ethylene (HDPE), or fluorinated ethylene propylene (FEP) for easy insertion, and the sheath’s transition tip. It is imperative that the transition from the body of the sheath to the tip is seamless with no irregularities that would create friction. Hydrophilic coatings, especially for larger or longer devices, can also be applied, which become very slippery once wetted.
The dilator is often single durometer, as it may not need to advance through highly tortuous anatomy. Dilators that must penetrate further and/or must conform to tortuous anatomy often utilize a multi-durometer construction, with a stiff proximal end and a highly flexible distal tip. Sheaths are often made from FEP because of its flexibility and low coefficient of friction. In the case of larger diameter/thin-wall access sheaths a coiled reinforcement may be utilized to maximizes kink resistance while allowing flexibility. Flexible polymer marker bands can be loaded with microscopic tungsten particles, which allow the device to be visible under fluoroscopy. Distal holes can be laser-drilled to aid the perfusion or flushing of the dilator. Depending on the surgical application, the distal tip can be of various geometric design, which can be fabricated through grinding, a radio frequency dielectric heating process, or both.
Design and Development
Access to the targeted anatomy is a critical component to the success of any procedure, and for that reason, the sheath, dilator, and steerable catheter components must be optimized to ensure they provide a safe, easy, and reliable path for the therapeutic device. It is crucial that any cutting-edge devices employ the most advanced materials and design. Spectrum Plastics Group brings decades of experience and a deep knowledge of the materials, design, and processes that are employed 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 manufacturability.
Spectrum is deeply experienced in designing and manufacturing steerable/deflectable catheters and dilators/sheaths. Not only does Spectrum customize catheters for specific customer needs, we also provide off-the-shelf steerable catheters, catheter sub-assemblies, dilators/sheaths, and other catheter components online at webstore.spectrumplastics.com, which can be used to accelerate the design and development of your next project.
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