Modern Mechanical Engineering, 2011, 1, 31-37
doi:10.4236/mme.2011.12005 Published Online November 2011 (http://www.SciRP.org/journal/mme)
Micro Transportation Systems: A Review
Phuc Hong Pham1, Dzung Viet Dao2
1School of Mechanical Engineering, Hanoi University of Science and Technology (HUST), Hanoi, Vietnam
2Research Institute for Nanomachine System Technology, Ritsumeikan University, Kusatsu , Shiga , Japan
Received August 31, 2011; revised October 12, 2011; accepted October 22, 2011
Abstract
This paper summarized and reviewed recent studies of micro transportation systems (MTS) in the MEMS
(Micro Electro-Mechanical System) field. Micro transportation systems can be identified and classified into
three categories based on the contact types between the objects and the actuators (i.e. liquid-based, solid-
based and air-bearing type). Their advantages and disadvantages were also analyzed and compared. The au-
thors have proposed and developed three types of solid-based MTS utilizing electrostatic comb-drive actua-
tors and ratchet mechanisms to drive the micro container in straight and curved paths. These MTSs have
been fabricated with silicon-on-insulator (SOI)-MEMS technology and tested successfully. In the near future,
MTSs can be applied in different fields such as medicine (to classify and test blood cells), in bioengineering
(to capture, sort and combine bio-cells, DNA), or in micro robot systems.
Keywords: Micro Transportation System (MTS), Electrostatic Comb-Drive Actuator (ECA)
1. Introduction
By definition, a transportation system or a conveyance
system is commonly used to transport or convey human,
goods, devices etc. from a place to another.
Transportation systems are very important in daily life,
as most people have to move everyday or even every hour
for work, study, or shopping etc. Goods need to be con-
veyed to stores or to supermarkets after production. Some
ty pical examples of transportation systems are vehicles net-
work in cities with the size of kilometers, or automatic
conveyance belts in factories with the size of meters. We
can imagine how terrible our life would be if the trans-
portation systems were not available.
Now, let us con sider the ca se when the tran sporte d ob-
jects are in micrometer or nanometer scale, such as bio
cells, DNA, biomedicine samples, micro/nano beads, car-
bon nano tube etc. Obviously, in order to transport or con-
vey those small samples effectively, it is necessary to de-
velop transportation systems having size proportionally
to the size of the transported objects. Those transporta-
tion systems are refered to as Micro Transportation Sys-
tems (MTS).
In other words, micro transportation systems are trans-
portation systems having the feature size in the microme-
ter scale, to move micro/nano objects from one place to
another. The MTS should include fundamental features
as in a macro transportation system, i.e. it should be able
to move objects in straight and curved paths, to load and
unload, or change the velocity of the objects in different
applications.
2. Classification of MTSs
Micro/Nano Transportation Systems are important for con-
veying and sorting of small samples in assembly of mi-
cro systems [1], bioengineering [2],
 -TAS (micro total
analysis systems), robotics and automation, and so on. Up
to date, there has been substantial number of reports on
micro/nano transportation systems that can be grouped into
three main categories corresponding to different contact
types between the objects and the actuators, i.e., liquid-
based, solid-based and air-bearing transportati on syst ems.
2.1. Liquid-Based Transportation Systems
Liquid-based transportation systems are applied in nano
bio -technology [2,3],
 -TAS or micro fluidic systems [4-6]
and can manipulate micro objects such as biological cells
by pushing or pulling them. Electrostatic, electro-wettin g,
electrophoresis, dielectrophoresis, magnetic and chemical
effects are usually used as the driving principle. However,
it is difficult to obtain high accuracy of the movement
with this type of transportation system.

P. H. PHAM ET AL.
32
2.2. Solid-Based Transportation Systems
In solid-based transportation systems, objects directly con-
tact with the solid actuators to receive the motion. Much
work on these systems has been reported. In 2-D trans-
portation system based on ciliary motion, objects were
elevated and moved by the ciliary type thermal actuator
arrays [7-14] or by permalloy magnetic actuator arrays
[15] underneath. In array-driven ultrasonic micro-actua-
tors [16], objects were conveyed by ultrasonic swing of
the pillars array beneath. XY-plane transportation systems
based on surface acoustic waves in piezoelectric substrate
[17,18], 2-D micro conveyor based on electrostatic ac-
tuator [19,20], inchworm motors with bidirectional XY
electrostatic actuators [21,22] or transportation system
based on vibrations of a slider combined with wedge me-
chanism [23], etc.
Almost of above-mentioned systems where the actua-
tors are directly contacted with the moving objects have
been realized by arrays of movable legs erected from the
silicon surface [8-13], two clutch-drive actuators [21], or
in-plane twisting actuators [22].
The legs and actuators are actuated by using different
principles such as thermal, electrostatic, magnetic or pie-
zoelectric actuation. In these cases, a complex driving te-
chnique requires at least two groups of actuators which
are turned on and off at different times, alternately hold-
in g and driving the objects. But higher speed and smoothe r
motion can be obt ai ned with this dri ving.
2.3. Air-Bearing Transportation Systems
Air-bearing-based transportation systems were also seen
in MEMS. The objects were levitated and moved by pneu -
matic air flow beneath [24-28]. In the micro transportation s
by using pneumatic or electromagnetic forces [29,30] to
create a cushion on which the mover levitates, there is
not direct contact between objects and actuators (contact
is free). Magnetic levitation can be achieved by using
permanent magnets, electromagnets or diamagnetic (i.e.
a superconductor) bodies. The main advantages of these
types are low friction, the scratch and damage of the ob-
jects can be avoid ed, but the drawback is high sen sitivity
to the cushion thickness (i.e. load dependent) while the
cushion thick ness can be quite difficult to control.
In order to have a comprehensive view about each type
of MTS developed so far, Table 1 shows some typical
characteristics of the MTS in chronologic order based on
three categories mentioned above.
3. Solid-Based Micro Transportation System:
Three Cases of Study
The au thors have been published three types of so lid-based
MTS using electrostatic comb-drive actuators to drive mi-
cro containers in the defined path [31-34]. The first MTS
is the tangential dr iving MTS, in which the driven micro
containers are moved by tangential force [31]. The sec-
ond is the perpendicular driving MTS, which drives mi-
cro containers by the perpendicular driving force, i.e. the
moving direction of the container is perpendicular to the
actuation direction [32,33]. The perpendicular MTS with
novel design and faster movement of the container has
been improved and tested in [34].
Main configuration and characteristics of three types
of MTS will be described as below:
3.1. Tangential Micro Transportation System
Figure 1 shows the working principle of the tangential
MTS. When applying a periodic voltage (V12) between
fixed electrodes (pad ) and movable comb fingers (pad
; note that pad  is connected to movable comb fin-
gers through the beams), the movable fingers will move
back and forth due to electrostatic force and elastic force
of the beams. The left and right ratchet racks are con-
nected to the m ova bl e c om bs of act uators , s o t hey c a n mo ve
together. These ratchet racks are engaged to two sides of
the micro container at the same time. The driving voltage
is applied to the left and right actuator in the way so that
their motions are in opposite directions. When the right
ratchet actuator moves up, it drives the micro container
to move up, while down movement of the left ratchet
actuator leads to slide between the teeth of the micro
container and of the ratchet actuator. The micro container
can not move back due to the ratchet teeth arrangement.
Figure 2 shows the tangential MTS after fabrication.
In this work [31], the movement of the micro container
has been tested with voltage Vpp = 100 V and driving fr e-
quency ranges from 5 Hz to 40 Hz. The velocity of the
container was proportional to the driving frequency, and
the maximum obtaine d value was about 0.7 08 mm/sec.
3.2. Perpendicular Micro Transportation System
A structure of the ratchet-actuator and the container are
shown in Figure 3. The fabrication perpendicular MTS
and micro container are shown in Figure 4.
When applying a periodic voltage (V12) between fixed
electrodes (pad ) and movable comb fingers (pad ),
the movable fingers will move forward and backward in
lateral direction (i.e. horizontal direction) due to electro-
static force and the elastic force of the beams. The driv-
ing voltage is applied to the left and right comb actuator
in the way so that the motions of the left and righ t ratchet
racks are in opposite directions. When the right ratchet
rack moves to the left, the left ratchet moves to the right
at the same time with the same displacement, the wings
of the container will rotate inward around the elastic

P. H. PHAM ET AL.
33
Table 1. Overview of some recent micro transportation systems.
Authors Principle
Maximum
velocity Displacement (m) per cycle Dimensions and power consumption
Pister et al.,
1990 [24] Pneumatic air-bearing (low friction levitation). Driving by electrostatic force. 0.1-1 mm/s 100 – 500 (at f = 1 – 2 Hz) Not specified
Kim et al.,
1990 [29] Magnetic levitation. Driving by magnetic Lorentz force. 7.1 mm/s Not specified Not specified
Ataka et al.,
1993 [8] Array of thermobimorph polyimide legs. Electrical heating (asynchronous) 0.5 mm/s 80 (f < fc= 10 Hz)* 8  2  16 legs at 500 m.
Total size: 5  5 mm2. 33mW
Konishi et al.,
1994 [25] Array of pneumatic valves. Electrostatic actuation. Not specifiedNot specified 9  7 valves at 100  200 m2. Total size: 2  3 mm2
Liu et al.,
1995 [15] Array for magnetic in-plane flap actuators. External magnet for actuation (synchronous)2.6 mm/s 500 (fc = 40 Hz) 4  7  8 flaps at 1400 m.
Total size: 10  10 mm2
Suh et al,
1997 [12]
Array of thermobimorph polyimide legs.
Thermal and electrostatic actuation (asyn-
chronous) 0.2 mm/s 10 (f = 1 Hz). fc 30 H z f o r
thermal actuation 8  8  4 legs at 430 m. Total size: 10  10 mm2.20 mW
Hirata et al,
1998 [28] Pneumatic (air jets) 50 mm/s (flat object) Not specified 2x10 splits at 50 m. Size of slider: 3  3 mm2. Total size: 20  30 mm2
Kladitis et al.,
1999 [13] Array of erected Si legs. Thermal actuation (asynchronous) 0.0075 mm/s3.75 (f < fc = 3 Hz) 96 legs at 270 m. Total size: 10  10 mm2. 175 mW
Nakazawa et al.,
1999 [30] Array of planar electromagnets 30 mm/s Not specified Mover size: 5  5 mm2. Total size: 40  40 mm2
Ruffieux et al.,
1999 [14] Array of non-erected Si legs. Piezoelectric or thermal actuation Not specified20 125 triangular cells (legs) at 400m
(300 m). Chip  18 mm2
Ebefors et al.,
2000 [10,11] Array of erected Si legs. Thermal actuation of polyimide joints (asynchronous) 12 mm/s 48 (f = 250 Hz) 2x6 legs at 500 m. Size of Si object: 14  7  0.5 mm3. 216 mW/leg
Yeh et al.,
2002 [21] Bidirectional electrostatic actuation 4 mm/s 2 (fc is high-kilohertz range) Total size of one motor: 3  1 mm2. Power density: 190 W/m3
Kim et al.,
2005 [22] Electrostatic in-plane twisting actuation 3 m/s 0.12 (fc = 1.68 kH z ) Total size: 77 mm2
Wu et al.,
2006 [9] Thermal bimorph actuation of polyimide layer 0.0015 mm/ s0.2 - 0.3 (at fc = 5 Hz) Total size: 900  100  4.5 m3
Chung et al.,
2007 [4] Liquid-based using micro bubble fluidics. Electrowetting-on-dielectric actuation Not specifiedNot specified Size of each electrode: 500  500 m2
Pham et al.,
2006-2010 [31-33] Solid-based using electrostatic actuation and ratchet mechanism 0.708 mm/s 10 - 20 (fc = 20 Hz) Size of each module: 6  6 mm2
Dao et al.,
2010 [34] Solid-based using electrostatic actuation and ratchet mechanism 1 mm/s 20 (fc = 50 Hz) Size of each module: 6  6 mm2

P. H. PHAM ET AL.
34
Left ratche t syst e mRight ratchet systemContainer


V12

Fixed partsMovabl e parts




t (sec)
V 12
O
V
pp
t/ 2
Left ratche t syst e mRight ratchet systemContainer


V12

Fixed partsMovabl e parts




t (sec)
V 12
O
V
pp
t/ 2
t (sec)
V 12
O
V
pp
t/ 2
Figure 1. An electrical diagram for creating a straight mo-
vement of the micro container in the tangential MTS.
Figure 2. SEM image of the tangential MTS.
points due to electrostatic force. As a result, the container
is pushed to move in vertical directi on. Therefore, the anti-
reverse hairs also move in vertical direction. The free end
of anti-reverse hairs slides on the teeth of ratchet rack in
forward direction on ly. Solid arro ws in Figure 3 indicate
the movement directions of ratchet racks and of the mi-
cro container. Next, when the driving voltage is reduced
to zero, the two ratchet racks move outward and return to
the initial positions due to elastic force of the beams. The
mic ro con tainer w ill no t mov e b ack due to four an ti- r ev erse
ha irs of the container engag ed in the ratchet racks. In order
to guarantee forw ard movement of the conta iner, after each
cycle, the container or, in other words, the antireve rs e h a i r
should move at least one pitch of the ratchet teeth.
In our experiments, various driving frequencies rang-
ing from 1Hz to 20 Hz were used to accelerate the micro
container up to 0.2 mm/sec. A power consumption at a
frequency f = 10 Hz, bias voltage V0 = 50 V (Vpp = 100
V) can be calculated to be about 55.7 mW [32,33]. In the
both types, each module of MTS has a size of 6 × 6 mm2.
3.3. The Improved Perpendicular Micro
Transportation System
Different from the work mentioned in paragraph 3.2, in
this improved type of MTS, the contai ner (Figures 5 and 6)
has been newly designed. In the container, an elastic joint
of the driving wings in previous design is replaced by
spring system, and therefore, the driving wings can rotate
easier with larger angle without the risk of being broken
due to overloading or fatigue failures problems. Thanks
to this improvement, the improved container could stand
longer than one hour operational test at velocity of 1mm/-
sec without any problems observed. In this experiment,
driving voltage and frequency were 140 V and 50 Hz,
respectively [34].
In our work, the container with a length, width and
thickness of 450 μm, 250 μm and 30 μm, respectively,
moves unidirectional in straight and curved paths. The
V 12






V 12






Figure 3. A working principle of the micro container in the
perpendi cular MTS.
Figure 4. SEM image of the perpendicular MTS.

35 P. H. PHAM ET AL.
~
1
2
2
1
2
2
Figure 5. A working principle of the improved perpendicu-
lar MTS with new container’s structure.
Figure 6. SEM image of new container in the improved per-
pendicular MTS.
velocity of the container can be changed by varying the
frequency and/or amplitude of the driving voltage. The
MTS including several of basic modules with the same
size has been fabricated from SOI (silicon-on-insulator)
wafer by utilizing silicon mi crom achini ng t echnol ogy wi th
only one mask.
The advantages of proposed MTS using electrostatic
comb-drive actuators are simple configuration and con-
trol, small size, robust and can be batch fabricated by the
bulk micromachining process.
4. Discussion and Conclusions
While the macro transportation systems have been deve-
loping well over hundreds years and are operating effect-
tively in daily life to transport people and goods, the sys-
tem to convey micro samples are still an attractive research
to pic in MEMS f ield. In order to transport or convey micro
samples efficiently, it is obviously necessary to bu ild the
MTS with dimensions or scale similar to the transported
objects so that the system can be manipulated, checked
and characterized in small working areas of t he micro world,
such as under optical microscope, inside SEM (scanning
electron microscope), inside AFM (atomic force micro-
scope), or under other measurement instruments of mi-
cro/nano research fields. Here, similarity in scale also
includes the other features of the systems, such as move-
ment velocity, accuracy, movement step, etc.
To achieve high performance and low cost micro trans-
portation systems, it is important to create small size de-
vice with simple configuration, simple control and using
standard fabrication technology. The design of the MTS
should be flexible so that the system can be assembled or
extended to different configurations conveniently. Dur-
ing the design process of a device, the trade-off between
the range of stepping motion and power consumption,
accuracy and cost, etc. must be taken into consideration.
Therefore, design and trial fabrication of novel structures
are the challenges and motivation in MEMS research.
In the last few years, the authors have reported three
types of solid-based silicon MTS utilizing the electro-
static actuation and ratchet mechanism. In order to facili-
tate the flexible development of sophisticated MTS, we
introduce the functional modules included starting or
loading module, straight module, turning module, T-junc-
tion or separation module, ending module, etc. Each mo-
dule was designed to achieve a specific moving task, i.e.
the straight and turning modules were used to drive the
containers in straight and turning pa ths, respectively. The
T- junction module allows the container to turn left or right.
The loading and ending modules were for convenient
loading of the containers to the MTS and storing them
after it finished the task respectively. All modules had
the same dimensions and design rule so that they could
be assembled together to form different MTS configure-
tions for different applications.
As the suggestion for future work, one of the potential
applications of our MTS is in biochemical analysis. For
example, the micro/nano samples are loaded and reacted
on the containers, and moved continuously to the check-
points, where these samples are evaluated optically or
electrically before going to the T-jun ction modules to se-
parate or classify. Then, good samples will be carried to
the ending module or store-room for the next experiments
and the others will be rejected.
5. Acknowledgements
The authors gratefully acknowledge to the Vietnam Na-
tional Foundation for Science and Technology Devel-
opment (NAFOSTED) for funding (Code: 107.01.26.09).

P. H. PHAM ET AL.
36
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