Biomedical Engineering Reference
In-Depth Information
(a)
(b)
5 mm
(c)
(d)
Cover
Heat sink
SMA microactuator
ermal isolation
Spacer
membrane
Valve seat
Valve chamber
Housing
Inlet
Outlet
FIGURE 9.50
(a, b, d) Pictures of a TiNi pump; (c) schematic diagram of the pump's cross section. (Johnson, A.D., in Miyazaki
et al., eds.,
Thin Film Shape Memory Alloys: Fundamentals and Device Applications
, Cambridge University Press, UK,
2009, with permission to use from Cambridge University Press.)
current of 50-100 mA. The response time when operating in air is about 20 ms, and maxi-
mum flow is up to 2000 standard cm
3
/min at 1.3 atm (Johnson, 2009).
There are different designs for TiNi film-based micropumps or microvalves, and most
of them use TiNi membrane (or diaphragm, microbubble, etc.) for actuation with one
example shown in Figure 9.51 (Reynaerts, 1997; Benard et al., 1998). Both freestanding TiNi
films and constrained TiNi films are used. Although freestanding TiNi film has intrin-
sic two-way SME, to maximize this effect, extra process, such as 3-D hot-shaping of TiNi
film (membrane) and externally biased structure (such as a polyimide layer, or a bonded
Si cap or glass cap) are often applied. All of these extra processes could result in compli-
cated structure and difficulty in MEMS processing (Kohl et al., 1999; Makino et al., 2001).
Another concern is the effective thermal isolation between the heated TiNi films and the
fluid being pumped. TiNi/Si bimorph membrane-based micropumps and valves are more
commonly reported. The advantages of using TiNi/Si membrane as the driving diaphragm
include (Xu et al., 2001): (1) large actuation force; (2) simplicity in process and no special
bias structure needed because the silicon substrate can provide bias force; and (3) no iso-
lating structure is needed because silicon structure can separate the working liquid from
SMA film completely.
Two types of diaphragm-type microactuators were fabricated by Miyazaki et al. (2009),
that is, a multilayer diaphragm comprising a SMA film and a SiO
2
layer and a single layer
diaphragm comprising a SMA film as shown in Figure 9.52. The multilayer diaphragm
microactuators were operated by the martensitic transformation induced in the Ti-Ni,
