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Catheter shafts are a core component of medical devices designed for minimally invasive procedures. The main objective when designing a catheter shaft is to get the right balance of performance between the various functional requirements of the device.
For example, getting the right balance of torque control, pushability, kink resistance, flexibility, compression resistance, etc.
During the development process, engineers work through various design considerations to achieve the right performance balance and ensure the device is suitable for the application it is being designed for.
Polymers are a crucial part of a catheter’s shaft, but most minimally invasive medical devices have performance requirements that are beyond the capabilities of any available polymer. In particular, the need to have ultra-thin wall thicknesses to minimise the overall diameter of the shaft.
Therefore, reinforcement technologies are used – specifically braid and coil reinforcement technologies. Depending on the application, they can be used individually, or a shaft can be designed with a combination of the two. Other variations are also possible, such as varying the pattern and/or pitch of the braid along the length of the shaft.
One of the main benefits of braid reinforcement is improved torque control, i.e., the ability of the clinician during the procedure to twist the proximal end of the catheter to manipulate the distal tip. Not enough torque control, and the clinician will not have sufficient manoeuvrability to complete the procedure, while too much torque control creates a whipping effect that can cause trauma inside the vessel.
Coil reinforcement techniques also offer benefits, including improved kink resistance and pushability. Coil reinforcement can also enhance the hoop strength (pressure resistance) of the catheter shaft.
Durometer is a method of measuring the hardness of materials, including plastics. In catheter shaft design, the durometer of the material used for the shaft’s exterior jacket is an essential design consideration. It can be important for the shaft’s liner as well.
It is also possible to vary durometers along the length of the shaft to achieve the required performance characteristics, particularly in relation to stiffness versus flexibility. Different durometers can be achieved by varying the durometer of a single material and/or using different materials along the shaft’s length.
Stiffness improves the tensile strength of the shaft while flexibility makes it possible for the catheter to navigate tortuous vasculatures. In many cases, flexibility is most important at the distal tip.
It’s also important to note that the durometer of the catheter shaft isn’t the only design element that influences flexibility and tensile strength. With braid reinforcement, for example, the durometer can be lowered to increase flexibility, with the reinforcement layer providing the required torsional rigidity.
It isn’t just about adding braid reinforcement, either, as the braiding pattern and pitch will also have an impact on the performance of the shaft.
For many applications, ultra-thin walls and excellent lubricity are crucial considerations when designing the liner of a catheter. This particularly applies in applications where the catheter is delivering an implant or other therapeutic or diagnostic device.
Material selection is also crucial. PTFE is the most commonly used material in the design of catheter shaft liners. It has a very low coefficient of friction, so offers excellent lubricity. Durometers can also be varied along the length of the shaft with a PTFE liner, and the liner can be designed with ultra-thin walls.
Other materials that can be considered for the liners of catheter shafts include FEP, HDPE, and polyamides. Each of the alternatives has advantages and disadvantages over PTFE. FEP is more flexible, for example, but it has a higher coefficient of friction. Polyamides, on the other hand, offer benefits in high-pressure applications because they have a high material strength, although there are downsides in terms of other performance characteristics.
Chemically resistant fluoropolymers can also be used, including for some drug delivery systems.
Advanced catheter technologies increasingly require sensors or electrodes to be embedded in shaft walls or liners. For example, electrodes for EP testing or ablation catheter applications. There could also be sensors for continuous cardiac output monitoring as well as temperature and pressure sensors, tactile sensors, or sensors to assist with positioning the catheter.
Any requirement for integrated technologies in the catheter shaft must be considered as those technologies can impact other areas of performance. The capabilities and performance characteristics of the integrated technologies themselves are also an essential design consideration.
Catheter shafts are complex components of minimally invasive medical devices, with the level of complexity increasing every day as new innovations are developed. This is having a positive effect on patient outcomes, but it also brings challenges to the catheter design and development process.
As a result, it is important to work with a medical device design partner that has experience in advanced catheter technologies and catheter shaft design. To speak to one of our engineers about your idea for a new medical device, complete the form below and we’ll get back to you.