October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 4 8 CoNiCr-NITINOL COMPOSITEWIRE FOR GUIDEWIRE-TYPE APPLICATIONS Dissimilar metals, each with their own special properties, can be integrated into a single composite wire to achieve a functionally driven design. David Snider,* Eric Dietsch, Adam Griebel,* David Plumley,* Mark Michael, and Jeremy Schaffer* Fort Wayne Metals, Fort Wayne, Indiana C omposite wires, such as drawn filled tube (DFT), com- bine the physical properties of multiple metals to meet specific performance needs in a variety of applications. The demand for these composite wires in medical technology has increased steadily over the past two decades. An example of simultaneously strong, stiff, and superelastic guidewires that provide proximal stiffness and distal flexibility is dis- cussed in this article. The properties of the guidewires could improve the crossing of chronic total occlusions (CTOs) by of- fering improved physician control and tip performance with a jointless core-wire design [1-3] . MATERIALS OF CONSTRUCTION A composite wire was constructed from a 35Co-35Ni-20Cr- 10Mo (35N LT alloy) outer shell and a Ni 50.8 Ti 49.2 nitinol core. The 0.34 mm (0.0135 in.) diameter wire was produced using the DFT production method similar to that described in an earli- er publication [4] , which involves filling seamless 35N LT tubing with a nitinol rod core, and co-processing using conventional wire drawing to achieve the target finish diameter with pro- portionate reduction of shell and core (Fig.1). Intermediate annealing temperatures were used to soften both species for consistent draw reduction, grain size control, and property *Member of ASM International tuning. Final cold-reduction ratios of 20 to 60% reduction of area and heat treatment were selected to optimize final wire straightness, 35N LT strength levels, and nitinol core super- elastic properties including plateau stresses and total recov- erable strain potential. Terminology for the final DFT-wire configuration is 35N LT-DFT-30%NiTi, indicating a nitinol core within a 35N LT sheath comprising about 30% of the wire’s overall cross-sectional area. Performance testing of the composite wire included tensile testing of the bulk wire, cyclic tensile testing of the nitinol core after centerless grinding, torque control testing comparing proximal to distal rotation response, and bendmo- ment testing. PERFORMANCE RESULTS OF COMPOSITE WIRE Figure 2 gives an overviewof geometric and performance results of the 35N LT-DFT-30%NiTi wire construction. Data in- cludeanas-polishedcross sectionof thewiregeometry,mono- tonic uniaxial tensile testing (curves a, b, and c), and bend load performance (curves d and e) over a constant moment-arm test for comparison against superelastic binary nitinol alone. No voids were observed at the as-polished interface between the CoNiCr and nitinol species and centerless grinding to Fig. 1 — Example DFT composite wire produced for use in a tapered guidewire component to provide relatively stiff, high-strength proximal material (the handle, left side) with a relatively flexible, kink-resistant superelastic tip without forming a joint during guidewire assembly. The superelastic nitinol tip is exposed simply by grinding or through laser machining or chemical removal. 1 0 FEATURE