Biomedical Engineering Reference
In-Depth Information
martensitic phase. Another unique property of SMA is the
superelasticity or pseudo-elasticity. In this case, the material is
deformed at a constant temperature in the austenitic phase leading to a
transformation into a martensitic phase. As soon as the load is
removed, the martensite reverses to austenite bringing the material
back to its original shape [BAR 00, DES 08]. In a superelastic
material, 8% strain may be recoverable [ONG 14].
Nitinol, a Ni-Ti alloy (55-56% Ni, 44-45% Ti - Table 3.1), is a
pseudo-elastic alloy used in medical applications since it exhibits the
shape-memory effect near room temperature. Nitinol is used in
orthodontic dental wires, medical staples and vascular stents [DUE 99,
MOR 04]. Nitinol exhibits a ductility comparable to most ordinary
alloys, which allows manufacturing in various forms, and a good
corrosion resistance due to the TiO
2
passive surface layer like in other
titanium implants. The alloy has a relatively low elastic modulus and
is more resilient than stainless steel or Co-Cr-based alloys [PAR 07].
Moreover, despite high Ni content, nitinol has a good
biocompatibility. As a matter of fact, the corrosion susceptibility of
nickel is mitigated by the TiO
2
protective layer which limits the
release of nickel ions and therefore the risk of cytotoxicity [BIE 12].
3.2.5.
Tantalum
Tantalum (Ta) is one of the most biocompatible metals because of
its low cytotoxicity and excellent corrosion resistance due to the stable
Ta
2
O
5
oxide formed on the material surface [BIE 12]. Ta can be used
as an alloying element in Ti-alloys as well as in its commercially pure
form (Table 3.1). Ta is a refractory metal with a high melting
temperature (3017°C). Its structure is a body-centered cubic α phase
which makes Ta very hard, and very ductile. However, due to its
mechanical properties and high density, Ta is restricted to a few
applications. It is successfully used in suture wires, clips and staples
[PAR 07]. Porous Ta has also been tested as bone graft substitutes and
cementless components in hip and knee arthroplasty considering its
resemblance to cancellous bone in terms of porosity (75-85%) and
stiffness. Its low stiffness (elastic modulus of 2.5-3.9 GPa) and high
friction coefficient (from 0.82 to 1.75) reduce stress shielding and
