Biomedical Engineering Reference
In-Depth Information
All the aging-treated thin films exhibited two-stage transformation behavior both upon
cooling and heating, that is, the R-phase transformation in a higher temperature region and
the martensitic transformation in a lower temperature region. The major part of the two-
way SME was associated with the R-phase transformation. The two-way shape memory
behavior is sensitive to aging conditions. For example, the film aged at 573 K for 3.6 ks and
the film aged at 723 K for 3.6 ks bend in the same direction as the constrained direction in
boiling water. However, in iced water, they show different behavior; the film aged at 573 K
bends forward, but the film aged at 723 K bends backward and shows an opposite curva-
ture. As described earlier, Ti
3
Ni
4
plate precipitates are formed in the TiNi matrix during
age-treatment. These precipitates form in a disk shape on {111} planes of the TiNi matrix
and reduce the volume by 2.3% along
→
111
→
directions. When a film is aged under con-
straint, the precipitates form on one of the {111} planes selectively, relaxing the constraint
stress by the volume change. If the film is aged at a low temperature for a short time,
the precipitation of Ti
3
Ni
4
is not sufficient for relaxing the constraint stress. In this case,
when the constraint is removed, the film tends to go back to the original shape before the
constraint-aging. However, even after the removal of the constraint, internal stress seems
to remain locally between relaxed and unrelaxed regions in this film. This internal stress
may determine a specific R-phase variant upon the transformation, so that the film shape
approaches the constrained shape. However, if the constraint stress is fully relaxed after
aging at a high temperature for a long time, such internal stress becomes small. Instead,
the coherent strain around the precipitates becomes large. This stress field determines the
specific R-phase variant so that the film shape returns to the original shape. This effect
has been also found in bulk specimens by Nishida et al. (1984). They called it “all round
shape memory effect” because this effect is so prominent that the film curvature reverses.
However, “two-way shape memory effect” is more appropriate to reflect the nature of the
phenomenon.
TiNi Diaphragms, Micropump, and Microvalves
MEMS-based micropumps and microvalves are attractive for many applications such as
implantable drug delivery, chemical analysis, and analytical instruments, and so on. TiNi
thin films are suitable for building microvalves and pumps (Shih et al., 2006). Control of
fluid flow is essential to operation of all pneumatic and hydraulic systems from implantable
insulin pumps to heating, ventilating, and air conditioning systems. The trend to miniatur-
ization is driven by the needs for portability and improved performance. Miniaturization
of fluidics systems requires miniaturization of all the components including microvalves
and pumps. Microvalves are potentially useful in microfluidics, pumps, thermal switches,
and a wide range of other applications. The main purpose of microvalves is to open and
allow fluid to flow or close and prevent fluid to flow. Both shape memory microvalves (i.e.,
active valves) and superelastic microvalves (i.e., passive check valves) have been fabricated.
The microvalve manufactured by the TiNi Alloy Company (Figure 9.50; Johnson, 2009)
was the first miniature SMA-actuated pneumatic control device to be offered commer-
cially. It consists of an actuator die with a poppet controlled by the 8 TiNi thin film strips,
3.5
μ
m thick and 250
μ
m wide, a silicon orifice die, a spacer, and a bias spring. All elements
are assembled in a plastic package. The bias spring forces the poppet toward the orifice.
Resistive heating of the SMA thin films supporting the poppet causes it to transform from
the martensite phase to the parent phase. By this transformation, TiNi strips recover the
original length, lifting the poppet against the bias force and opening the valve. This device
has a poppet displacement of ~100
μ
m and bias force of 0.5 N. It is operated with an electric
