Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
A Novel SU-8 Microgripper with External Actuator
for Biological Cells Manipulation
M. Mehdi S. Mousavi 1, 2 , Giorgio De Pasquale 1 , Aurelio Somà 1 , Eugenio Brusa 1
1 Department of Mechanics, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129, Torino, Italy
2 Italian Institute of Technology, Center for Space Human Robotics, Corso Trento 21, 10129, Torino, Italy
mehdi.mousavi@polito.it, giorgio.depasquale@polito.it, aurelio.soma@polito.it, eugenio.brusa@polito.it
Abstract- Specification and target impose several limitations
and difficulties in micro manipulators design. These obstacles
are even more important when the target of microgripping is
biological cells. Even though a variety of designs and solutions
has been proposed in the literature, the problems of
temperature at microgripper tip and applied voltage in
gripping jaws are still unsolved. In this paper a new approach
to eliminate this kind of problems in biological cells
manipulation is introduced. The proposed kinematics is
externally actuated and an optimization procedure based on
FEM simulations has been performed to improve the micro
gripper design. Some considerations on fabrication process
show that the new approach can sensitively decrease the cost
and time of fabrication processes, as well as the complexity of
the technologies involved.
I. INTRODUCTION
Common micro grippers cannot be used to manipulate
biological samples, such as living cells, because of their
actuation methods. The actuation mechanism shall be
suitable for operating in electrolytic aqueous media because
of ionic environment of cells [1, 2]. This prerequisite limits
the application of high voltage to actuator that is necessary
in piezo-actuated grippers since bubble formation, caused
by electrolysis, occurs at 1.5–2 volts in water [3]. Moreover,
any exposure to magnetic or electrical fields may have some
negative effects on biological cells. This also limits the
application of electrostatic or electromagnetic actuated
micro grippers. In addition, shape memory alloy (SMA)
actuators are not a good candidate for micro grippers due to
lack of reliability for a reasonable number of operations.
Furthermore, the maximum allowed temperature for
manipulation of human cells in many applications such as
Intracytoplasmic Injection or Pro-nuclei DNA injection is
around 37°C that is quite lower than the required high
temperature (more than 100° C for bare extended arms of
gripper) in an electro-thermal gripper. Therefore, even
though electro-thermal actuators are of great interests
among researchers for cell manipulation, they show many
difficulties when are used [4-6]. Another point is biocompatibility
of gripper materials that places some
restrictions in choosing of actuation method and fabrication
process.
From above explanation, it is clear that whatever the
actuation method is, there are many points that must be
taken into consideration for microgripper design. This work
proposes a survey of the literature about the kinematic and
actuation solutions adopted for microgrippers; then a new
design approach is proposed and the FEM simulation of few
candidate devices for cells manipulation are reported.
II. DESIGN ISSUES IN MICROGRIPPING
A survey of the literature reveals the following key
features in design of microgrippers for biological cells:
- actuation principle
- kinematics
- fingertips shape
- force feedback
- releasing strategy
The actuation strategy is usually determined by selecting
internal or external actuators. About the first category, it is
possible to build some specific parts of the gripper with
piezoelectric (PZT) material to generate a localized force
when an electric voltage is provided [7]. The electrostatic
force can be used as an actuation by applying a voltage
difference on a capacitor with movable armature [8]. The
thermal actuation, widely used for both biological and nonbiological
manipulation, is based on the thermal expansion
of the gripper arms due to the Joule effect in presence of
electric currents [9]. A faster response of the arms can be
achieved with shape memory alloys (SMA) [10]: they are
able to restore almost immediately the memorized shape
when a threshold temperature is passed. The
electromagnetic actuation is based on micro-coils and is
able to generate weak confined magnetic fields [11].
Hydraulic and pneumatic actuation can be used to
manipulate bio-cells with micro-pipes integrated in small
circuits including micro-pumps and valves [12]. There are
strong limitations in using internal actuators for the
manipulation of biological particles. PZT actuators have
strong nonlinear output, high supply voltage, small motion
range and other problems such as creep, mechanical fatigue,
hysteresis and biocompatibility; as a consequence, they
require an embedded force feedback control. The
electrostatic actuators are generally disadvantaged by the
small dimensions of the capacitors. To increase to the force,
very complicated shapes of the gripper are necessary by
introducing many comb drives; then, the motion range is
strongly reduced by the small gaps between the armatures
and the applied voltage easily causes electrolysis of watered
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