text: Fatigue failures of nitinol devices are a constant subject of discussion.  Because it is the material of choice for applications requiring enormous flexibility and motion (e.g., peripheral stents, heart valves, smart thermomechanical actuators and electromechanical microactuators), it is necessarily exposed to much greater fatigue strains compared to other metals. While the strain-controlled fatigue performance of nitinol is superior to all other known metals, fatigue failures have been observed in the most demanding applications. There is a great deal of effort underway trying to better understand and define the durability limits of nitinol.
  Nitinol is half nickel, and thus there has been a great deal of concern in the medical industry regarding the release of nickel, a known allergen and possible carcinogen. (Nickel is also present in substantial amounts in stainless steel and cobalt-chrome alloys.)  When properly treated (via electropolishing or passivation), nitinol forms a very stable protective TiO layer that acts as a very effective and self-healing barrier against ion exchange. It has been repeatedly shown that nitinol releases nickel at a slower pace than stainless steel, for example.  With that said, very early medical devices were made without electropolishing, and corrosion was observed.  Today's nitinol vascular self-expandable metallic stents, for example, show no evidence of corrosion or nickel release, and the outcomes in patients with and without nickel allergies are indistinguishable.
  There are constant and long-running discussions regarding inclusions in nitinol, both TiC and TiNiO.  As in all other metals and alloys, inclusions can be found in nitinol. The size, distribution and type of inclusions can be controlled to some extent. Theoretically, smaller, rounder and few inclusions should lead to increased fatigue durability.  In literature, some early works report to have failed to show measurable differences, while novel studies demonstrate a dependence of fatigue resistance on the typical inclusion size in an alloy.
  Nitinol is difficult to weld, both to itself and other materials. Laser welding nitinol to itself is a relatively routine process. Strong joints between NiTi wires and stainless steel wires have been made using nickel filler. Laser and tungsten inert gas (TIG) welds have been made between NiTi tubes and stainless steel tubes. More research is ongoing into other processes and other metals to which nitinol can be welded.
  Actuation frequency of nitinol is dependent on heat management, especially during the cooling phase. Numerous methods are used to increase the cooling performance, such as forced air, flowing liquids, thermoelectric modules (i.e. Peltier or semiconductor heat pumps), heat sinks, conductive materials and higher surface-to-volume ratio (improvements up to 3.3 Hz with very thin wires and up to 100 Hz with thin films of nitinol). The fastest nitinol actuation recorded was carried by a high voltage capacitor discharge which heated an SMA wire in a manner of microseconds, and resulted in a complete phase transformation (and high velocities) in a few milliseconds.
  Recent advances have shown that processing of nitinol can expand thermomechanical capabilities, allowing for multiple shape memories to be embedded within a monolithic structure. Research on multi-memory technology is on-going and may deliver enhanced shape memory devices in the near future,
   and the application of new materials and material structures, such hybrid shape memory materials (SMMs) and shape memory composites (SMCs).