Designing the optimum shaft for a device requires the skillful and meticulous balancing of a number of performance characteristics. These attributes consist of: Pushability, Trackability, Torqueability, Kink Performance, and Transition.
Achieving the best performance in one category can often directly impact that of other. For example, decreasing the wall thickness of a hypotube to improve its trackability may reduce the overall kink performance of the hypotube. Therefore, the challenge faced by the product designer is to find the best possible combination of characteristics for their particular application.
There are five key performance defining characteristics to consider when designing a hypotube shaft.
1. Push: the ability of a device to transmit a longitudinal force from the proximal end of the shaft to the distal end. When push is optimized in combination with other hypotube design features it will be easier for the physician to maneuver the device to the exact treatment site. Pushability can be improved by: (1) Maximizing the cross-sectional area of the device (2) Maximizing the modulus of elasticity by using a stiffer material (3) Maximizing tensile strength.
2. Torque: the ability of the device shaft to transmit a rotational displacement along the length of the device. Rotational movements by the physician translate effectively to the device tip within the anatomy when torque performance is high. Torque can be improved by: (1) Maximizing the polar moment of inertia of the tube (2) Maximizing the shear modulus using a stiffer material.
3. Kink Performance: also known as Kink Resistance, it is the ability of a device shaft to maintain it’s cross sectional profile during compressive deformation. When kink performance (resistance) is high, the physician can rely on the device shaft to negotiate difficult routes without fracturing or breaking. Kink Resistance can be improved by: (1) Maximizing wall thickness (2) Maximizing the ductility of the material.
4. Trackability: describes the ability of a device to travel through complex anatomies and is influenced by a number of factors including the shaft flexibility, strength and its friction within the anatomical environment. Trackability describes the “feel” of the device to the physician when manipulating and positioning the treatment device. Trackability can be improved by: (1) Selecting a low friction outer layer or coating (2) Reducing the modulus of elasticity (3) Reducing the wall thickness of the device (4) Reducing the outer diameter of the shaft.
5. Transition: refers to the change in stiffness along the device shaft. Physicians prefer a device shaft that is flexible at the distal end and stiff at the proximal end as it maximizes the push and torque performance of the device. A well-designed transition along the device shaft will also decrease the likelihood of kinking. Transition can be improved by: (1) Increasing the flexibility of the distal end of the shaft (2) Removing some of the material from the distal end of the shaft (3) Adding a component of intermediate stiffness to the transition area.
The design of the catheter shaft is a significant factor in determining the formation of curves, angles of deflection and levels of steerability. The choice of material determines the level of pushability, torque and flexibility and it can be manipulated along the length of the catheter through a variety of means to achieve the desired results.
Below are the most common steerable and deflectable catheter options:
Steerable Fixed Curve Catheters - These catheters have a predefined distal curve shape. They have high torque transmission that allows you to turn the tip of the catheter with almost like-for-like rotational movement of the proximal handle. These types of catheters are widely used to access challenging anatomy, during diagnosis, delivery and biopsy etc.
Deflectable Catheters (Uni-Directional Catheters) - Deflectable catheters feature a tip that can be pulled into a defined curve. This is achieved by using a wire connected to a pull or anchor ring near the tip. The tip returns to its original shape through natural spring-back. Almost all deflectable catheters will have a degree of steerability. These devices are useful when making highly angulated turns in distal anatomy or to control exact positioning of the catheter tip. Examples include guiding catheters, implant delivery systems or EP mapping and ablation catheters.
Bi-Directional Catheters - Feature a tip that can be pulled in two opposing directions. This is achieved by using two pull wires connected to a distal pull ring. These devices are particularly useful for controlled movement and placement of the distal tip as they can be steered forwards and backwards. Typical examples include EP Mapping and ablation catheters and implant delivery systems.
4-Way Deflectable Catheters - These devices can be pulled in 4 directions using a device handle. They require four wires connected to a distal pull ring. The most common 4-way deflectable devices are ultrasound imaging systems, such as ICE catheters. These can be manipulated to access multiple chambers of the heart and also view images from different angles.
Omnidirectional catheters - These are 4-way deflectable catheters remotely controlled via a robotic device to allow tip orientation in any direction. Steering is achieved by manipulating one or more pull wires simultaneously. Robotic catheters can be used for a variety of applications and provide the physician with greater control and less exposure to radiation.
PTFE - PTFE is the most ubiquitous fluoropolymer for vascular catheters. It is most commonly used as a lubricious inner catheter liner because it has the lowest coefficient of friction of all catheter materials. Catheters using PTFE are usually hand-assembled because PTFE cannot be melt-processed by conventional extrusion methods.
PU- PU is one of the key polymers in the vascular catheter market. The diversity of urethanes used in catheters is significant. Polyurethanes varieties include polyester, polyether, and polycarbonate-based varieties, as well as aromatic and aliphatic grades.
Polyamide (Nylon) - Polyamide such as Nylon 11 and Nylon 12 are materials of choice in applications such as percutaneous transluminal coronary angiography (PTCA) applications, including balloon and stent delivery devices, where greater stiffness is required and softening at body temperature is not desired.
Polyamide (Pebax) - Pebax are modified nylons with soft segments that provide more elasticity than polyamide 11 or 12. They have become materials of choice in many interventional catheters because they combine the flexibility and softness of polyurethanes with the strength of nylons.
Polycarbonate-based PU - Polycarbonated-based PU exhibit excellent long-term biostability and are commonly used in applications that are in the body for long periods. Aliphatic and aromatic polyether-based polyurethanes soften at body temperature, which promotes patient comfort and reduces the risk of vascular trauma. These are commonly used for in-dwelling catheters, such as a peripherally inserted central catheters (PICC) and central venous (CV) catheters.
HDPE - HDPE is commonly used as a liner for the inner lumen of vascular catheter designs and as a complete shaft because it can be melt-processed by extrusion. It does not have as low a coefficient of friction as PTFE but is superior to polyamides and polyurethanes. It is also harder than PTFE, which can result in a lower effective coefficient of friction compared with PTFE in some applications. Where devices or components are advancing down a working channel, PTFE has a tendency to plow up in front of the component, increasing the deployment force. Because HDPE is harder, it does not plow up, reducing the deployment force even though the coefficient of friction is higher.
PEEK - PEEK is considered one of the most rigid polymer with high tensile modulus and ultimate tensile strength. It is used in catheters where high strength or heat resistance is required, such as ablation devices. PEEK has similar mechanical properties to polyimide, but it is a true thermoplastic and can be extruded in a continuous process.