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The Importance of Transformation Temperature Testing of Nickel Titanium Wire used in Medical Devices, Part 2Start a Project
During wire processing, the transformation temperatures will be affected primarily by cold working and annealing temperatures. Cold work, specifically, has a great effect on the achievable transformation temperatures for the final device. In general, higher cold work percent leads to colder transformation temperatures and higher plateau stresses.
Cold work percentage has the opposite effect on fatigue life of the device – higher cold work percentage will lead to a decrease in fatigue resistance.
The annealing process during wire manufacturing is typically a partial anneal under tension. Final cold work percentage, a function of the reduction in cross-sectional area during wire drawing, is reported because of its influence on final device performance.
Transformation temperatures only increase during typical wire processing, the coldest state always being in the ingot form, which further highlights the importance of choosing the correct ingot temperatures for the application. A typical delta for medical grade Nitinol wire is a gain between 25°C and 35°C between the ingot Af and the active Af (active being used here to mean a material that has been heat-treated and is in its final, austenitic condition).
For the braided stent manufacturing processes, it is ideal to utilize Nitinol wire in its cold worked state, as it will be heat treated after it’s been braided. To maximize strength/plateau stresses of the finished device, minimizing the number of times the wire is heat treated is best.
Once the appropriate grade of material has been selected, and the wire has been processed into its final shape (braided into a stent, for example), heat treatment is necessary to bring the onset of phase transformation to Austenite and to lock in the new, desired shape.
For wire braiding applications, hard or cold worked material (wire that has not been heat-treated by the manufacturer and is in its martensitic phase) is normally selected due to its ease of conforming to a braided structure. Austenitic wire has been heat-treated to be straight off the spool, so its shape memory is to return to straightness making it difficult to maintain in a braided structure during processing and may have a greater tendency to fray at the cut ends of the braid.
During the process of forming and drawing nitinol wire a thin oxide layer typically forms on the surface of the material, early in the wire making process. This oxide layer provides a lubricious surface, which is generally desirable for most wire braiding applications. Nitinol wire continues to oxidize during heat treatment of braided constructs to achieve superelasticity.
For permanently implanted medical devices, the FDA requires removal of the oxide layer to eliminate the risk of fractured oxide particles being released inside the body. This typically is achieved via a post-process operation of electro-polishing or chemical etching.
Following removal of the oxide layer, the wire is passivated to prevent re-oxidation of titanium atoms and to reduce/eliminate nickel-ion release from the wire surface. However, electropolishing is inefficient at removing oxide residue at wire intersections on braided structures. Therefore, a substantial percentage of material is often removed from the construct to ensure the device is as free of oxide as possible.
Recently, some wire manufacturers have developed processes that minimize oxide formation on the surface of the wire, while maintaining the lubricious surface qualities of wire with an oxide layer. A minimal oxide surface finish can be a major benefit for braided constructs that use nitinol wire. Less oxide on wire at the braiding stage naturally leads to less oxide formed during heat treatment. Reduced surface oxide often improves the efficacy of certain laser-welding procedures performed to attach hubs or other artifacts to the braided constructs.
For some fine wire sizes, it can become especially critical to minimize the formation of surface oxides because uniform removal of oxide is difficult and may compromise the strength or function of the device. Vacuum heat treatment of Nitinol braided structures is an alternative method to achieve superelasticity without increasing oxide layer. However, vacuum heat treatment requires longer cycle times compared to other heating methods, and slow quenching is not optimal for certain nitinol properties.