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Transformational Process: Making A Rigid Steel Shaft Flexible

Our engineers are developing a medical device to guide surgical instruments through the beating heart. It features a straight and rigid stainless-steel shaft with a working channel and lumens for fluid management and electronics. One of the many challenges was to maintain shaft strength while also achieving the required bending of the distal end for navigation. 

The Challenge
Our key criteria were to design a structure to that provides bi-directional 90-degree bend in one plane, while maintaining durability and strength, with working channel and other lumens remaining operational during bending.

The Solution
We have developed a specific cut pattern of the stainless-steel shaft. With cuts and bridges placed strategically, the shaft could bend easily, with each bridge flexing slightly, while maintaining the required level of shaft stiffness. The design had to optimize bridge width, bridge distance and curvature, and minimize sharp edges and overlaps. The elimination of any sharp point or line-like contacts and opting for smooth surface connections while designing cut-out geometry enabled smooth and secure bending. Calculating the removal of material was also vital for the desired result, achieving the desired flexibility without risking fracture. Covering and sealing the cut pattern is essential to ensure safety. FEP shrink tubes were chosen for their even shrinking, effective sealing, and smoothness. An anticoagulant coating is also required on the external surface to prevent blood clotting. Finally, the flexible shaft had to be made maneuverable. Two pull wires were added, which needed to be fixed securely along the length of the shaft to prevent tangling, slipping, or breakage. These attached to a dial on the handle. 

An Iterative Design Process
Our team focused on rapidly planning, designing, prototyping, testing and revising consecutive iterations to meet the customer’s requirements. Prototype modifications were made until the desired outcome was achieved, satisfying the customer.

Exploring EDM Technology and Laser Cutting
For this particular design, we used electrical discharge machining (EDM) to achieve the desired cut pattern. This metal fabrication process removes material from a workpiece through recurring current discharges between a wire electrode and the workpiece. An alternative technology to consider would be laser cutting. The choice between the two technologies comes down to specifics of the tubing and the cut pattern. The next iteration will involve exploring new cut patterns through laser cutting for enhancing axial movement.