Biomedical Engineering Reference
In-Depth Information
the precise location of pathology. Without tissue diagnosis, it is often difficult to differenti-
ate inflammatory lesions from tumor infiltration. The former may require only medical
treatment, whereas the latter may need surgical solution. Therefore, there are two poten-
tial microactuator applications in capsule endoscopy: (1) microgripper for biopsy or tissue
sampling (Sugawara et al., 2006); (2) microclipper or pin tagging device, to firmly attach to
the tissue. SMA thin film-based microactuators are promising for these applications.
Grasping and manipulating small or micro-objects with high accuracy is required for a
wide range of important applications, such as the assembly in microsystems, endoscopes
for microsurgery, and drug injection micromanipulators for cells. There are some basic
requirements for microgrippers, for example, large gripping force, sufficient opening dis-
tance for assembling works, and so on. TiNi films are promising in these applications. So
far, two types of TiNi film-based microgripper designs are available.
The popular design is out-of-plane bending mode, mostly with two integrated TiNi/
Si cantilever (or other substrate, such as SU-8 or polyimide, etc.) with opposite actuation
directions (see Figure 9.59) (Seidemann et al., 2002; Lee et al., 1996). This SMA cantilever
structure was heated to different temperatures (from room temperature to 423 K), results
show that there is actually the SME and the tip displacement is quite large (up to 60
μ
m).
This type of cantilever structure can be further fabricated as a microgripper, as shown in
Figure 9.60, which can be actuated at relatively low temperatures with internal integrated
heaters forming by polysilicon patterns. These grippers can be used as the end-manipulator
for microassembly for industry, minimally invasive surgery for medical application, and
handling of small particles in hazardous environment for military application.
Takeuchi and Shimoyama (2000) reported a novel microelectrode with TiNi clipping
structure, which can be used for minimally invasive microelectrodes to clip a nerve cord
or other living organisms. The TiNi film is actuated when a current is applied to the elec-
trode. The clipping force of the electrode to the nerve is enhanced by a hook structure and
two C-shaped probes as shown in Figure 9.61a and b. Another gripper design is in-plane
mode, in which the deformation of two arms (using freestanding TiNi films or TiNi/Si
beams) is within a plane realized by compliant structure design (Fu and Du, 2003). Wang
et al. (2002) reported a microtweezer structure, in which residual stress in TiNi film is
used as a bias force load. This can eliminate the need for providing bias force for device
operation. However, the force from the deformation of the freestanding films is not large
enough to grasp large objects. The other problem in this type of design is how to prevent
SE
×70 500 µm
FIGURE 9.59
Cantilever structure with a thickness of 15
μ
m fabricated by conventional MEMS process. (From Fu et al.,
Surf.
Coat. Technol.
, 145, 107-112, 2001, with permission from Elsevier.)
