Jan_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 | J A N U A R Y 2 0 1 6 3 0 NEWPROCESS JOINSNITINOL TOSTAINLESS STEEL A new solid-state joining process for medical guidewire applications increases joint strength, provides superior bending properties, and does not require tertiary metals or ferrules. Pankaj Gupta,* Arne Rimmereide, and Roger Dickenson, Lake Region Medical, Chaska, Minn. A guidewire isamedical deviceused in various minimally invasive vas- cular applications. Its foundation leading to unpredictable brittle joints. To avoid this, current joining methods use either a transition section made of a ter- tiary metal or a ferrule joining process. JOINING OPTIONS Metallurgically, joining Nitinol

developed by researchers at Lake Re- gion Medical (LRM). The resulting joint strength is approximately 80%of the ten- sile strength of the raw Nitinol wire with excellent bending properties. Complete 0.014-in. outer diameter guidewires were built using solid-state weld technology, tested, and compared to a competitor’s product with a hypotube joint design. The solid-state weld joint’s metallurgical characteristics aswell as data fromguide- wire functional tests are presented here. EXPERIMENTAL PROCEDURES Solid-state weld joints were creat- ed using pre-straightened superelastic binaryNitinol (54.5%-57.0%Ni) and 304v spring-tempered stainless steel wires with subsequent evaluation of joint strength, durability, and microstructure. Parts went through preconditioning by cycling the joint 10 times through a U-bend fixture with a 0.10-in. radius, pri- or to obtaining tensile strength data by pulling the joint to failure using an MTS testing system. Joint microstructures were examined using standard metallo- graphic methods of polishing and etch- ing the longitudinal joint sections. In addition, optical microscopy and scan- ning electron microscopy (SEM) con- firmed overall joint quality. Energy dis- persive spectroscopy (EDS) analysis on the cross-section determined the weld zone length with intermixed Nitinol and stainless steel. A grinding study was conduct- ed on the solid-state welded bimetal joints using 0.018-in. stainless steel to 0.020-in. Nitinol wires. This allowed

is a metal core wire, typically constructed of stainless steel or Nitinol. A metal coil, polymer jacket, or combination of the two covers the core wire on the distal end in order to make the tip atraumatic, kink resistant, and flexible. Designing a guidewire is an in- tricate exercise in balancing strength and flexibility. For example, a guide- wire with a spring-tempered stainless steel core has good pushability and torque transmission due to its high yield strength and Young’s modulus. These properties are important in order to nav- igate to the desired treatment sites and deliver the desired clinical therapy. How- ever, exceeding the yield strength of the material in a bending mode results in permanent bends and kinks, which se- verely reduces guidewire performance. Nitinol is a superelastic material pro- viding great kink resistance, but it lacks pushability due to an inherently lower Young’s modulus, which results in less support in delivering therapies or devic- es. Ideally, a guidewire core combines the excellent mechanical properties of stainless steel in the main body, with the kink resistance of Nitinol at the distal tip. A bimetal medical guidewire with a stainless steel proximal section and Nitinol distal section enhances perfor- mance compared to guidewires made of either alloy alone. However, standard fusion welding of Nitinol (NiTi) to stain- less steel (SS) is challenging because it causes brittle intermetallic Fe-Ti to form, *Member of ASM International; now at St. Jude Medical

to stainless steel via fusion welding is problematic due to the formation of brittle Fe-Ti intermetallics [1,2] , as pre- viously mentioned. One method of avoiding brittle intermetallics is to use a tertiary metal, such as Nickel, when joining the stainless steel to Nitinol [3] , but this adds cost and complexity to the design and can degrade performance. Solid-state processes such as fric- tion welding [4] , explosive welding [5] , and ultrasonic welding [6] can also be used to join dissimilar metals while avoiding the formation of brittle intermetallics in the joint. Another method used in guidewire applications is to insert the ends of the stainless steel and Nitinol into a ferrule (a section of hypotube) and then secure both ends using ad- hesive or solder. This method requires preprocessing to reduce the diameter at the ends of each core in order to fit the parts together, which adds cost and complexity. Further, this decrease in core diameter, along with the stiffer section of hypotube, and the addition of joint material, creates a kink point and reduces clinical performance. An alternative proprietary solid- state butt joining process for Nitinol and stainless steel wires ranging in diame- ter from 0.013 to 0.020 in. that does not require tertiary metals or ferrules was