This is an effective but expensive method that consumes large sums and time, and can consider relatively few testing conditions. A common design method is the so-called “testing for survival”, where several devices are experimentally verified under in-vivo like boundary conditions. To fulfill certification requirements, the stents are designed to survive a minimum required number of loading cycles, which could be up to 10 8 for cardiovascular applications, while it is limited to 10 7 for peripheral stents ( ASTM, 2013 Kinkel et al., 2009). Though companies manufacturing peripheral stents commonly use their own design rules, combining different arrangements of rings, v-struts and connecting links, a common feature can be recognized in the bending-dominated deformation mode of the v-struts, in which high gradients arise. Such an effect is rather distinctive of Ni-Ti, which, conversely to conventional metal materials like stainless steel or chromium-cobalt alloys, exhibits an increase in the fatigue limit with increasing mean strain ( Pelton et al., 2008). The latter more efficiently verifies the effect of the mean strain (that could be related to the radial oversizing of the stent after the in-vivo deployment into the artery) on the fatigue behavior. The results of these tests are commonly reported in a “strain-life diagram”, where strain amplitude is plotted against the number of cycles to failure (under fixed mean value), or in a “constant-life diagram”, which examines strain amplitude versus mean strain (at fixed number of cycles to failure). Ni-Ti fatigue characterization is usually performed through uniaxial tensile tests on ad-hoc specimens, investigating different levels of strain mean value and amplitude. Few studies have analyzed fracture modes in these small parts as it is difficult to properly identify nucleation and propagation phases. ![]() ( Robertson et al., 2012) studied Ni-Ti fatigue behavior, highlighting how fatigue cracks in Ni-Ti specimens, such as stents with v-struts about 0.2 mm thick, quickly lead to the failure of the component. A final surface finishing, such as electro-polishing, reduces defect propagation to fatigue crack formation. Peripheral stents are usually laser-cut from a source tube (about 2 mm in diameter) – subsequent shape-setting (multiple heatings and quenchings) produces the definitive expanded configuration and desired mechanical properties. The material behavior is strongly affected by the thermomechanical treatment during manufacturing. The Ni-Ti stress-strain curve exhibits a flag-shaped morphology, with a wide stress plateau, allowing a strain-based focus for durability assessment. When implanted in peripheral vessels interaction with by muscle and ligaments with low extremity motion produces a complex cyclic set of non-proportional axial compression, bending and torsion that is transmitted through the peripheral vessels into the stents ( Ansari et al., 2013). This overzing induces mean strain in the v-struts elements of the stent. Typically, the implantation occurs with 1 mm oversizing: it means that, for example, a 6 mm nominal outer diameter is implanted into a 5 mm vessel to ensure just the right amount of radial force to keep the vessel open without subsequent injury. Stents come in a large variety of length/diameter combinations. Peripheral stents are crimped onto a delivery catheter which allows in-situ deployment into an atherosclerosed vessel their diameter is reduced up to about 1 mm and maintained in such configuration until release which triggers a pseudo-elastic response that restores the original dimension. Though so strong that the material was used in the nose cones of missiles, fatigue failure in Ni-Ti peripheral stents remains a multi-factorial open issue due to: i) the complexity of the applied loads, ii) the non-linear material characteristic curve and iii) the design procedure. stents for the treatments of peripheral occlusive diseases ( Duerig et al., 1999 Petrini and Migliavacca, 2011). Its temperature-dependent pseudo-elasticity allows design of self-expandable devices for mini-invasive deployments e.g. ![]() Nickel-Titanium (Ni-Ti) is a shape memory alloy widely used in biomedical fields.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |