Minimally invasive medical devices can strongly contribute to patient comfort compared to more traditional open surgery alternatives. However, designing such a device can be a challenging undertaking. A typical issue seen in catheter- or shaft-based minimally invasive device design is that improving the mechanical properties in one area may result in deterioration of mechanical properties in another area.

Common issue

For example, increasing the bending flexibility of the device shaft may be required to navigate through a particularly tortuous anatomy. It may be necessary to prevent plastic deformation from occurring when the device needs to get around an acutely angled segment. Doing so, however, can also result in a marked loss of torque transmission—angulation at the proximal end of the shaft will not be transmitted to the distal end, but rather the shaft will stall or even twist and kink. Adjustments in the shaft’s bending flexibility can also lead to an increase in the shaft’s longitudinal flexibility, negatively affecting elongation or compression resistance, potentially making control of the shaft or device delivery extremely difficult.

Key observations

- A closed pitch coil has high bending flexibility, but poor elongation resistance

- A stainless steel tube has high elongation resistance but poor bending flexibility.

Our solution

A particular shaft or catheter structure that can allow for a combination of adequate bending flexibility, torque transmission and elongation resistance is a ‘cable tube’—a structure very similar to a rope or strand as it is also formed by helically twisted wires over a core wire, but with the core wire removed to create a lumen inside said structure. As the cable tube’s constituent wires are helically twisted, angulation of the cable tube in its twist direction will result into its separate wires pressing against one another, consequently resulting in a high degree of torque transmission. The longitudinal orientation of the cable tube’s wires can be adjusted by changing the amount of wires it consists of. This makes it possible to find the best balance between bending stiffness and longitudinal elongation resistance.

Finding the balance

This is just one out of many examples—many more design trade-offs exist. Asahi Intecc Co., Ltd. is an ISO13485- and ISO9001-certified Japanese medical device components manufacturer that has specialized itself in solving these types of trade-offs.