Biomedical Engineering Reference
In-Depth Information
Pulsation Sensor ................................................................................................................ 446
TiNi-Based Microcage ....................................................................................................... 449
Freestanding TiNi-Based Microcage............................................................................... 450
TiNi/DLC Microcage ........................................................................................................ 452
Intravascular Medical Stent and Tube Devices.................................................................. 456
Covered Stent ..................................................................................................................... 456
Heart Valve ......................................................................................................................... 457
Fabrication of Superelastic Thin Film Tubes ................................................................. 458
Summary and Future.................................................................................................................. 460
References..................................................................................................................................... 461
Introduction
Shape memory alloy (SMA) is a metal that can “remember” its geometry, that is, after
a piece of SMA is deformed from its original shape, it regains its original geometry by
itself upon heating (shape memory effect, SME) or, simply upon unloading (superelastic-
ity, SE). These extraordinary properties are due to temperature-dependent reverse mar-
tensitic transformation from a low-symmetry phase (martensite) to a highly symmetric
crystallographic structure (austenite) upon heating and a martensitic transformation in its
opposite direction upon cooling. SME has been found in many materials, such as metals,
ceramics, and polymers. Among all these materials, TiNi-based alloys, often called nitinol,
have been extensively studied and have found many commercial applications (Miyazaki
and Otsuka, 1989; Humbeeck, 1999; Hane, 2000; Otsuka and Ren, 2005).
Biomedical Application of TiNi Alloys
Nitinol was first discovered by Buehler et al. in 1963. It has inspired a long-standing inter-
est in both material research and industrial development. Although many applications of
nitinol have been developed for different industries, their real success only came after the
introduction of microsurgery. Nitinol provides a perfect solution for problems presented
by microsurgery because of its unique mechanical properties—“shape memory effect,”
“superelasticity,” and the excellent biocompatibility. The SME is characterized by a revers-
ible phase transition between martensite (low temperature phase, soft) and austenite (high
temperature phase, hard). At low temperature, nitinol can be plastically deformed, and
the deformation can be recovered by heating through a phase transition. If the nitinol
is deformed at a temperature above the phase transition temperature, the stress-induced
martensitic transition can be recovered spontaneously once the stress is removed. This is
known as SE. These remarkable shape memory properties are not present in any other
conventional materials. Both superelastic and thermal recovery SMAs can provide large
deformation and force for robust surgical devices for biological applications. More impor-
tantly, the nitinol is biocompatible with low toxicity and high corrosion resistance, thus, it
can be used for implants, for example, stent, staple, and sutures without any adverse long-
term effects on patients. Nitinol is also nonferromagnetic and can thus be used as medical
devices for magnetron resonance imaging (MRI) guided surgery and minimal invasive
interventions (Melzer et al., 2006). A brief history of SMAs (bulk materials and SMA thin
films) for medicine is summarized in Table 9.1.
