I– Nitinol

Nitinol is an alloy of roughly 50% Nickel and 50% Titanium. Discovered onlyrecently (1960s) as part of a Naval research project [1], Nitinol derives its name from its chemical components and its founders: Ni (Nickel) + Ti (Titanium) + NOL (NavalOrdinance Lab). It has good biocompatibility (response is similar to stainless steel) [2-5]and good magnetic resonance imaging opacity (i.e. can readily be seen by X-ray or MRI)[6, 7], making it ideally suited for design of biomedical implantable devices. Moreimportantly are two characteristics that make Nitinol extraordinarily unique from other bioengineering metallic alloys, namely its superelasticity and shape-memory, which aredescribed in Section 1.3. These two unique properties are attributed to a first-order phasetransformation from the parent austenite phase to the daughter martensite phase (seeSection 1.2) which can occur via the addition of an energy source either in the form ofheat or stress [8].Since its commercial introduction in the 1970s, Nitinol has been used for a variety ofapplications: pipe couplings, bra underwires, earthquake dampeners, eyeglass frames,orthodontic wires, mobile phone antennas, micro-actuators, and a variety of biomedical devices . Recently, however, the Nitinol community’s trend has been to turn itsefforts to the biomedical device field .

II– Current research

For our project, Nitinol is used as an actuator to bend the needle. This is accomplished by Nitinol’s capability to remember its own shape ( hence Shape Memory Alloy). This means that when Nitinol is stretched at a lower temperature ( either in martensite phase or mixed martensite-austenite phase), when the load removed, the stretched length ( strain deformation) remains the same). However, when you heat Nitinol wire up to complete austenite phase, the wires’ original length is recovered. The wires are heated via electrical current ( Joules’ heating).

Figure 1: Phase transformation diagram of Nitinol [2]

Composite Lab is currently conducting experiments to better under stand Nitinol’s thermo-mechanical behavior. Due to its complexity, Finite Element Analysis (FEA) will be used to predict its behavior.

II– Reference

[1] Robertson, Scott W.(2006). On the Mechanical Properties and Microstructure of Nitinol for Biomedical Stent Applications. Lawrence Berkeley National Laboratory: Lawrence Berkeley National Laboratory. LBNL Paper LBNL-62454. Retrieved from: http://escholarship.org/uc/item/1d5926nx

[2]L. C. Brinson, M. S. Huang, C. Boller and W. Brand, 1997, “Analysis of Controlled Beam Deflections Using SMA Wires,” Journal of Intelligent Material Systems and Structures 8; 12