Biomedical Engineering Reference
In-Depth Information
In general, on the rougher surfaces there are lower cell numbers, decreased rate of cellular prolif-
eration, and increased matrix production compared to smooth surface. Bone formation appears to
be strongly related to the presence of transforming growth factor β
1
in the bone matrix (Kieswetter
et al., 1996).
1.4.2 TiNi Alloys
he
titanium-nickel
alloys show unusual properties, that is, after deformation the material can snap
back to its previous shape following heating of the material. This phenomenon is called
shape memory
effect
(SME). The SME of TiNi alloy was first observed by Buehler and Wiley at the U.S. Naval Ordnance
Laboratory (Buehler et al., 1963). The equiatomic TiNi or NiTi alloy (Nitinol) exhibits an exceptional
SME near room temperature: if it is plastically deformed below the transformation temperature, it
reverts back to its original shape as the temperature is raised. The SME can be generally related to a dif-
fusionless martensitic phase transformation which is also thermoelastic in nature, the thermoelasticity
being attributed to the ordering in the parent and martensitic phases (Wayman and Shimizu, 1972).
Another unusual property is the
superelasticity
, which is shown schematically in Figure 1.9. As can be
seen, the stress does not increase with increased strain after the initial elastic stress region and upon
release of the stress or strain the metal springs back to its original shape in contrast to other metals such
as stainless steel. The superelastic property is utilized in orthodontic archwires since the conventional
stainless-steel wires are too stiff and harsh for the tooth. In addition, the SME can also be utilized.
Some possible applications of shape memory alloys are orthodontic dental archwire, intracranial
aneurysm clip,
vena cava
filter, contractile artificial muscles for an artificial heart, vascular stent, cath-
eter guide wire, and orthopedic staple (Duerig et al., 1990).
In order to develop such devices, it is necessary to understand fully the mechanical and thermal
behavior associated with the martensitic phase transformation. A widely known NiTi alloy is 55-Nitinol
(55 wt% or 50 at% Ni), which has a single phase and the mechanical memory plus other properties, for
example, high acoustic damping, direct conversion of heat energy into mechanical energy, good fatigue
properties, and low-temperature ductility. Deviation from the 55-Nitinol (near stoichiometric NiTi) in
the Ni-rich direction yields a second group of alloys which are also completely nonmagnetic but dif-
fer from 55-Nitinol in their ability to be thermally hardened to higher hardness levels. Shape recovery
Stainless steel wire spring
Stress
TiNi SMA wire spring
Permanent set
Strain
FIGURE 1.9
Schematic illustration of the stainless steel wire and TiNi SMA wire springs for orthodontic arch-
wire behavior. (Modified from Wayman, C.M. and Duerig, T.W. 1990.
Engineering Aspects of Shape Memory Alloys
.
London: Butterworth-Heinemann, pp. 3-20.)
