Effects of grain size on the rate-dependent thermomechanical responses of nanostructured superelastic NiTi

Concentric tube robots can enable new clinical interventions if they are able to pass through soft tissue, deploy along desired paths through open cavities, or travel along winding lumens. These behaviors require the robot to deploy in such a way that the curved shape of its shaft remains unchanged as the tip progresses forward (i.e., “follow-the-leader” deployment). Follow-the-leader deployment is challenging for concentric tube robots due to elastic (and particularly torsional) coupling between the tubes that form the robot. However, as we show in this paper, follow-the-leader deployment is possible, provided that tube precurvatures and deployment sequences are appropriately selected. We begin by defining follow-the-leader deployment and providing conditions that must be satisfied for a concentric tube robot to achieve it. We then examine several useful special cases of follow-the-leader deployment, showing that both circular and helical precurvatures can be employed, and provide an experimental illustration of the helical case. We also explore approximate follow-the-leader behavior and provide a metric for the similarity of a general deployment to a follow-the-leader deployment. Finally, we consider access to the hippocampus in the brain to treat epilepsy, as a motivating clinical example for follow-the-leader deployment.

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Concentric Tube Robots as Steerable Needles: Achieving Follow-the-Leader Deployment

Fatigue behavior and cyclic deformation of additive manufactured NiTi

The rotary bending fatigue properties of medical-grade Nitinol wires were investigated under conditions of 0.5-10% strain amplitudes to a maximum of 10(7) cycles. The results from this study provide insight into the behavior of Nitinol under fully reversed (emin/emax=-1) fatigue conditions for three compositions, two surface conditions and three test temperatures. For pseudoelastic conditions there are four distinct regions of the strain-cycle curves that are related to phases (austenite, stress-induced martensite, and R-Phase) and their respective strain accommodation mechanisms. In contrast, there are only two regions for the strain-cycle curves for thermal martensite. It was further observed that the strain amplitude to achieve 10(7)-cycles increases with both decreasing test temperature and increasing transformation temperature. Fatigue behavior was not, however, strongly influenced by wire surface condition. SEM of the fracture surfaces showed that the fatigue fracture area increased with decreasing strain amplitude. Finite element analysis was used to illustrate strain distributions across the wire as well as to calculate the tension-compression contributions to the rotary bending curves. The results from this investigation are discussed with respect to mechanisms of strain accommodation under cyclic tensile and compressive conditions.

https://www.nitinol.com/wp-content/uploads/2013/07/Pelton-et-al-2013-Rotary-Bending.pdf

Rotary-bending fatigue characteristics of medical-grade Nitinol wire.

Endovascular stenting has matured into a commonly used treatment for peripheral arterial disease (PAD) due to its minimally invasive nature and associated reductions in short-term morbidity and mortality. The mechanical properties of the superelastic Nitinol alloy have played a major role in the explosion of peripheral artery stenting, with modern stents demonstrating reasonable resilience and durability. Yet in the superficial femoral and popliteal arteries, even the newest generation Nitinol stents continue to demonstrate clinical outcomes that leave significant room for improvement. Restenosis and progression of native arterial disease often lead to recurrence of symptoms and reinterventions that increase morbidity and health care expenditures. One of the main factors thought to be associated with stent failure in the femoropopliteal artery (FPA) is the unique and highly dynamic mechanical environment of the lower limb. Clinical and experimental data demonstrate that the FPA undergoes significant deformations with limb flexion. It is hypothesized that the inability of many existing stent designs to conform to these deformations likely plays a role in reconstruction failure, as repetitive movements of the leg and thigh combine with mechanical mismatch between the artery and the stent and result in mechanical damage to both the artery and the stent. In this review we will identify challenges and provide a mechanical perspective of FPA stenting, and then discuss current research directions with promise to provide a better understanding of Nitinol, specific features of stent design, and improved characterization of the biomechanical environment of the FPA to facilitate development of better stents for patients with PAD.

Nitinol Stents in the Femoropopliteal Artery: A Mechanical Perspective on Material, Design, and Performance.

The successful medical application of Nitinol requires precise control of its transformational and mechanical properties. In this study the effects of heat treatments at 300-550°C for 2-180 minutes on Ti-50.8at%Ni wire with 30% and 50% initial cold work were investigated. Transformational and mechanical properties were characterized through the BFR technique and tensile testing. Thermally activated precipitation and annealing processes were observed. Annealing processes tended to increase the maximum slope of the BFR curves. The R-phase was observed with greater frequency and prominence in the 50% cold-worked wire after heat treatment. The general trends in Af are summarized in two TTT diagrams; both illustrate a maximum precipitation rate of Ni4Ti3 at 400-450°C. The trends in tensile properties are outlined for all heat treatment conditions. Recovery processes occurred at all temperatures. The onset of recrystallization occurred at approximately 450°C for both wires.

/pdf/the-effects-of-cold-work-and-heat-treatment-on-the-4wp5ee4vrp.pdf

Masao J. Drexel

Papers

The Effects of Cold Work and Heat Treatment on the Properties of Nitinol Wire