Present and Future Applications of Shape Memory and Superelastic Materials
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Figure 1: Superelasticity in a Ti-50.8 at. % Ni alloy pulled in tension. Shown are the design parameters: loading plateau-a 1 , unloading plateau-au, permanent set-sp. characterized by plateaus during both loading and unloading (Figure 1). In cases, the plateau may be somewhat ill-defined, but should in all cases exhibit at least an inflection point during unloading. As shown on Figure 1, the effect is parameterized by its loading stress, unloading stress, hysteresis (by implication), and permanent set. Strains of 10% can be nearly fully recovered, with the stress induced martensite responsible for approximately 8% and conventional elasticity for the balance. Superelastic properties can be observed at temperatures above Af and below Md. In fact full superelastic effects are found over an even narrower range-typically only 20-40*C in width. Further information concerning the origin and characteristics of the effect has been presented elsewhere 6,7. At first it may not be clear why superelastic properties are an indication of a "smart" material, and in fact the term may be poorly applied-after all, any material dumbly obeys the laws of nature as programmed by its hopefully smart human designers. Still, two important features should be considered. First, the material reversibly alters its crystal structure and shape in order to relieve applied stresses. Secondly, and often neglected, is that the stress applied by a constrained superelastic device will rise and fall with temperature in a linear fashion, thus acting as a temperature sensor and actuator. A superelastic spring does not follow Hook's Law, but in fact delivers a constant stress when deformed between 1.5% and 7%. This can be very important in the field of medicine since one can engineer a device to deliver a certain, physiologically ideal stress and rely on the fact that the stress will be held constant. Prototypical of this type of application are orthodontic archwires, which also happen to be one of the first medical applications8' 9 . Here the archwire is constrained while being installed into brackets mounted on the mal-aligned teeth. During treatment, the arch struggles to restore the teeth to their proper location, but always applies forces according to the unloading plateau of the arch's stress-strain curve. This maintains a therapeutically ideal force while eliminating adjustments, causing the patient less discomfort and accelerating treatment. Superelastic archwires were researched in the mid-1970's, and have been in widespread use since the early 1980's. They are now an appliance used by virtually all orthodontists. A second key property of Ni-Ti alloys is their biocompatibility. In 1975, Simon proposed
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Figure 2: The Simon filter is inserted through a 2.3mm catheter (Frame 1), then thermally expands to form a filter used to prevent recurrent pulmonary embolism. Frame 6 (far right) shows the fully deployed filter from an end view. using Ni-Ti for a vena cava filter (Figure 2), to break-up blood clots (thrombus) in the vena cava10 wh
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