Online proceedings - EDA Publishing Association

The FEM model allowed calculating the structural
stiffness, as well as the accuracy of kinematics. An
optimization work was done by changing the configuration,
length, and width of the different parts of the grippers. The
results of all analysis can be seen in Table 5. To select the
material, bio-compatibility of the gripper was taken into
account. After performing a comprehensive survey in the
literature, the SU-8 polymeric material was selected because
of its properties, which are particularly suitable for cell
manipulation devices. For the FEM simulations, the
following properties of SU-8 were considered: coefficient of
thermal expansion α = 52x10 -6 K -1 , Young’s modulus E =
4.02 GPa, Poisson’s ratio ν = 0.22 and ultimate tensile stress
of 34 MPa.
Since in all microgrippers previously proposed in the
literature, due to angular motion of the jaws, the gripping
planes at the tip of the tweezers do not remain parallel
during cell manipulation (see Table 2), in this study an
attempt was performed to change this strategy, by leading to
surround a cell and keep it within the room defined by the
gripper tips, in closed configuration. Furthermore, by
increasing the contact area of the cell and gripper tip, this
approach causes less stress on the cells membrane during
gripping. Above all, a single mask layer is enough to
fabricate this layout and there is no need to dope any kind of
metal on substrates and use several mask layers in
fabrication process that is usual in all thermal actuators.
The first configuration in Table 1 is our first idea to
achieve the above mentioned goals. One upper and one
lower arm which are connected together with some inclined
connecting parts compose this simple configuration. The
upper arm is fixed and the lower arm is attached to an
external actuator. In this configuration the most important
problem was the y-direction movement of the upper arm of
the gripper due to its bending. Figure 1 shows the deformed
and undeformed shape and also stress distribution of this
layout. The results of all analyses can be seen in Table 5. By
applying 20 µm displacement to the lower arm in FEM
simulation, more than 100 µm displacement in y direction
occurred. To solve this problem a wider upper arm and
longer connecting parts were used in second proposed
configuration. Moreover, the inclined connectors were
changed to vertical position in order to increase the
movement in direction of x in comparison to y and to
decrease the amount of force required by the lower arm. To
keep the gripper tip dimensions proper for the considered
cell size (35 µm) the shape of the tip was changed
accordingly (Fig. 2).
As can be seen in Table 5 these modifications did not
solve the problem. Still were found about 20 µm of
displacement along y direction when displacement was
equal along direction x. Even though the second
configuration decreased the movement along y direction in
comparison to the first layout, long connecting parts
increased the dimension of whole gripper structure. The
next idea to solve these difficulties is shown in Fig. 3. An
additional arm was joined to the structure. Displacement in
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May 2011, Aix-en-Provence, France
FEM simulation was induced on the upper arm while the
lower two arms were fixed. This improvement allowed
finding good results in terms of stress and opening range of
the gripper tip. As it can be seen in Fig. 3 length of the
upper arm can be a problem for the microfabrication,
because of its compliance. This effect can be seen in Fig. 3,
since a significant deflection of the upper arm occurred after
applying displacement. To solve this final problem the
upper arm was supported by another structure to prevent its
deflection. Figure 4 shows the final configuration. In this
layout a large opening at the gripper tips occurs if only 10
µm of displacement are applied (half of the first two
layouts). As can be seen in Table 5, the most opening range
with minimum stress can be achieved by this layout.
Furthermore, the least stiffness is related to this
configuration while the width of the gripper does not exceed
140 µm.
Fig. 1. First configuration, left: deformed and undeformed shape,
right: stress analysis
Fig. 2. Second configuration, left: deformed and undeformed shape,
right: stress analysis
Fig. 3. Third configuration, left: deformed and undeformed shape,
right: stress analysis
Fig. 4. Fourth configuration, left: deformed and undeformed shape,
right: stress analysis
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