1. Stopping and Locomotion Mechanism for an
Endoscopic Microcapsule Robot
Abstract— This paper presents a stopping and a locomotion
mechanism to be used with an endoscopic microcapsule robot.
In the diagnosis of gastrointestinal diseases, microcapsules have
been developed recently as alternatives to conventional endoscopy.
However, they have less accuracy and functionality in diagnosis as
they lack the ability to control their position. We propose mecha-
nisms to be used with such microcapsules that would enable them
to anchor and crawl in any position inside the gastrointestinal
tract. The stopping mechanism, actuated by shape memory
alloys, makes use of dry and wet adhesive materials to attach
to the intestinal tract. Bio-inspired wet fibrillar adhesives that
create adhesion in a way similar to the attachment mechanisms
employed by beetles are proposed. The locomotion mechanism,
inspired by the locomotion principles of inchworms, is a modular
expansion of the stopping mechanism. Both the stopping and the
crawling locomotion mechanisms have been built and successfully
tested inside a flexible vinyl tube. Results showed stopping with
high repeatibily and 0.5 mm/sec locomotion speed.
I. INTRODUCTION
Traditional wired endoscopes are primarily used in the
investigation of the gastrointestinal tract for diagnosis of
deseases. Research is being conducted on the use of semi-
autonomous structures in order to avoid the inconveniences
introduced by traditional endoscopes [1]. In 2001, with the
invention of microcapsules, a much more convenient alterna-
tive was introduced to the market [2]. Microcapsules offer
virtually non-invasive diagnoses and require less complicated
operation procedures, resulting in overall higher convenience.
In addition, the use of microcapsules enables screening of the
small intestines, which is otherwise impossible with current
traditional endoscopy.
Despite their advantages over traditional endoscopes, mi-
crocapsules have a low accuracy and functionality in diag-
nosis mainly due to the lack of control over their position,
orientation and speed. The peristaltic motion provided by the
contraction of intestinal muscles pushes everything inside;
anything that does not deploy a mechanism of attachment
will be forcibly moved. The microcapsule developed by RF
Norika [3] has an external magnetically actuated system that
controls the orientation of the capsule, but it lacks the ability
to stop inside the intestines and further investigate a region
of interest. The need for such a mechanism is great, as it
will improve the accuracy of the diagnosis and provide a
level of control to the motion of the microcapsule, enabling
advanced applications such as biopsy, localized drug delivery
and surgery.
Previous studies [4]–[6] have been made on the development
of such stopping mechanisms. In this paper, we propose
a novel stopping mechanism with a biomimetic attachment
method that would enable an endoscopic microcapsule to
anchor itself in one position within the digestive tract. Futher-
more, we discuss how the usage of such a mechanism could be
expanded in order to be used as a crawling based locomotion
mechanism.
II. PROBLEM DEFINITION
The main challenges in the development of an attachment
and locomotion mechanism for a microcapsule robot are:
• Since the size of the capsule is very important (very
large pills are uncomfortable), the mechanism must be
sufficiently small as to not drastically increase the size
of the system.
• In situ, the microcapsule is subject to the peristaltic mo-
tion of the digestive tract at all times, so the atttachment
mechanism should be able to conform to the intestinal
environment.
• The attachment mechanism must be supplied with the
same power that is already onboard the microcapsule.
As these power systems are the limiting factor in the
endurance of most capsule systems, the attachment mech-
anism must be low power.
• When operating within the human body, safety is a major
concern; any attachment mechanism must be chosen with
safety and biocompatibility as a priority.
With these constraints in mind, many traditional attachment
mechanisms can be discarded. Mechanical interlocking mech-
anisms must not be used to avoid damaging the digestive tract.
Suction based systems require too much power to be useful.
Bio-inspired mechanisms are proposed as a solution.
III. STOPPING MECHANISM
Several research groups have proposed different miniature
mechanisms that make use of setae–like micro legs employed
by earthworms [4], [6]. The design idea of the stopping
mechanism presented in this paper relies on a biologically
inspired wet fibrillar adhesive material that would be pushed
against the intestinal walls, creating an adhesion [7].
A. Adhesives
Synthetic dry adhesives that seek to duplicate the per-
formance of the natural fibrillar adhesive mechanism most
commonly associated with geckos have been the focus of
much recent research [8]–[10]. The natural system is capable
of robust attacment to most materials, regardless of the surface
roughness. In addition, as a passive mechanism no extra energy
is required to maintain the hold once adhesion is achieved.
Another attractive feature of dry adhesives is that by utilizing
a peeling motion, very little energy is required for detachment.

2. As another alternative, beetles and other insects use similar
fibrillar structures but enhance adhesion through capillary
forces that arise from a mostly hydrophobic oily secretion [11],
[12]. These systems require less complex fibers, but the
composition of the liquid as well as the secretion mechanism
itself are challenging tasks.
The past year has seen great improvement in the quality of
fabricated synthetic biomimetic fibrillar adhesives. High aspect
ratio, densely packed fibers with diameters of two, three, and
five microns have been successfully fabricated (see Figure 1)
with both molding and photolithography techniques. Thus, this
project aims to use either the polymer polydimethylsiloxane
(PDMS) or the photoresist SU-8 for the fiber material. A
candidate for a liquid secretion to enhance adhesion is a
silicone oil with high viscosity and long evaporation time that
will be stored in a reservior and secreted via integrated pores
by the pressure created by the contact of the foot with the
substrate. Until these fabricated fibers are further refined into
viable adhesive systems, a simple flat pad of PDMS provides
enough adhesion for testing purposes.
B. Design Principle
The design of the stopping mechanism was geared towards
the use of synthetic biomimetic fibrillar adhesives. This re-
quires a mechanism that will push the adhesive material onto
the intestinal walls, the simplest of which is a set of adhesive-
tipped legs that can open to stabilize the capsule.
Many factors must be considered in the selection of the
actuator for this purpose. The main concern is that it must
be biocompatible. Also, high output force and low power
consumption are required. Several actuators, including piezo-
electric materials, polymer actuators and shape memory alloys
were considered in the design stage. While the piezoelectric
materials have high force output and low power consumption,
they typically need very high driving voltages. This, in addtion
to the limited strain capabilities of the actuator, make it
an inappropriate choice for our purpose. Polymer actuators
have much better strain output, and they are biocompatible.
However, they are slow, they require high power and they are
incapable of high output force.
Shape Memory Alloy (SMA) wires are selected here like
other several research groups that proposed these actuators for
microcapsule applications [4], [6]. Shape memory alloy helical
wires have high output forces and large strain up to 50 percent.
Fig. 1. 5 µm diameter high aspect ratio, densely packed SU-8 microfibers
fabricated using photolithography.
The actuation is through heating; a voltage applied across the
wire causes current flow, and the power is dissipated as heat.
This also means that they have a very high power consumption
and low efficiency that limits their untethered usage. Despite
this fact, these actuators are chosen for their high output force
and small size. The power consumption issue will be addressed
in the future. After a search for commercially available SMA
actuators, we have decided to use coil type SMA actuator
manufactured by Toki Corporation, due to its large strain.
Figure 2 shows the design concept of the stopping mecha-
nism. The mechanism is built on a hollow cylindrical casing
(turqouise). Three legs (blue) are each attached on a cylindrical
pulley (yellow) that is free to rotate. A rubber spring (pink)
is attached to the pulley on one end, and to the casing on the
other end. An adhesive pad (pink) is attached to the footpad,
which is attached to the leg with a polymer hinge (not shown).
The SMA wire (dark blue) connects the upper surface of the
leg to the casing.
The initial state of the leg is closed (SMA wire not ac-
tuated). When the SMA wire is heated by passing current
through it, it pulls the leg, creating a torque. This torque
causes the pulley to turn, thereby rotating the leg. The rubber
spring is stretched as the pulley turns, creating a torque in
the other direction. The leg stops opening when it reaches an
equilibrium point where the torque created by the SMA wire
equals the torque created by the rubber spring. Once the SMA
wire is shut off, the leg is pulled back by the rubber spring
since the SMA wire no longer creates a strong resisting torque.
The foot carries an adhesive pad that will stick and generate
high adhesion forces when pushed on the intestinal walls. As
the adhesive pad reaches the walls and is preloaded, the leg
does not open futher, but it applies a preload as the SMA is
continuously actuated. Three legs are placed symmetrically on
the casing, so that a strong grip from three points is formed.
One critical mechanism is the polymer hinge through which
the adhesive pad is connected to the legs. The purpose of
the compliance created by the polymer hinge is to aid the
adhesive pad in sticking to and detaching from the walls of the
intestine. When the pad starts to touch the intestinal wall, the
polymer hinge bends, applying both shear and normal preload.
Consequently, when the leg closes, this structure creates a
peeling motion, greatly reducing the force necessary to detach.
This compliant structure is formed using a stiff polymer film
which connects the two separate parts of the leg.
(a) (b)
Fig. 2. (a) Solidworks drawing of the conceptual design, (b) zoomed.
A: Adhesive Pad, B: Leg, C: SMA wire, D: Casing, E: PDMS Spring, F:
Pulley

3. Fig. 3. Diagram showing the dimensions used in the mathematical model.
The mechanism is drawn in the unactuated position.
C. Modeling
A mathematical model of a single leg mechanism was
created to predict the forces between the foot and substrate
when current is applied to the SMA actuator. Figure 3 shows
the relevant dimensions of the mechanism used in this model.
The leg and foot were treated as rigid beams. The force
required to bend the ankle joint (polymer hinge) was calculated
using large beam deflection theory. The PDMS spring was
treated as a linear spring whose spring constant could be
calculated through simple axial beam loading equations. These
assumptions were experimentally validated with simple force
measurements using a load cell. The current-force and strain-
force relations for the SMA wire were also found experimen-
tally and incorporated into the model. For the the 100 µm
coil-type SMA, the emprical output force FSMA in mN was
found to be reasonably approximated by
FSMA = 0.0341c2
− 0.0966c + (772.72 − 17.053c) ε (1)
where c is the applied current in mA and ε is the strain. This
equation is only valid above a threshold current of 60 mA and
below a burnout current of 180 mA.
If enough current is applied to the SMA, the leg will rotate
open and push the footpad against the substrate. There are two
modes of contact as displayed in Figure 4: the “flat mode”
where the footpad is pressed flat against the substrate and the
“angled mode” where only the distal end of the footpad is in
contact with the substrate.
The shear force Fshear is the friction force on the footpad,
calculated with Fshear = µFpreload, assuming high preloads.
By balancing moments about the pulley, the leg angle θ,
preload force, shear force, and contact mode can be calculated
(a) (b)
Fig. 4. A schematic of the two contact modes and the forces that act
on the leg: a) In the flat mode, the footpad completely contacts the
substrate. b) In the angled mode, only the distal end of the footpad
is in contact with the substrate.
for any given input current to the SMA wire. Assuming a
uniform pressure on the footpad in the flat mode, the preload
force for the flat and angled mode were found to be:
Fpreload,flat =
(r + t ) FSMA − rFspring
Dflat
(2)
Dflat = (L + µr) cos θ + (µL − r) sin θ
+ (L /2 + µtf )
Fpreload,angled =
(r + t ) FSMA − rFspring
Dangled
(3)
Dangled = (L + µr) cos θ + (µL − r) sin θ
+ (L + µtf ) cos (θ − α) + (µL − tf ) sin (θ − α)
where r, L , Lf , t and tf are mechanism dimensions defined
in Figure 3, θ is the opening angle of the leg (see Figure 4),
and α is the bend angle of the ankle joint.
The factor that determines which mode of contact the
mechanism will achieve is the ankle joint. In order to reach and
maintain the flat mode, sufficient force must be applied to the
ankle to maintain the bend. Given a contact mode assumption,
the resulting preload force can be calculated. This force can
then be used to determine the force on the ankle joint, which
will then validate or refute the assumption.
A graph of the calculated preload and shear forces for var-
ious distances (d) from the substrate can be seen in Figure 5.
There are two clear transition points on the graph that indicate
a switch in contact modes. On the left side of the graph,
where the preload force decreases as the distance increases,
the mechanism maintains a flat mode of contact. At d = 6.7
mm, the contact changes to angled mode; the preload force
stays relatively constant (even increasing slightly). Finally, at
d = 9.2 mm, the foot no longer reaches the wall, and the
forces drop to zero.
D. Fabrication
Two prototype stopping mechanisms were built and suc-
cesfully tested. The casings, cylindrical pulleys and legs were
1 2 3 4 5 6 7 8 9 10
0
10
20
30
40
50
60
70
Distance (mm)
Force(mN)
Preload
Shear
Fig. 5. Predicted preload and shear forces using the developed mathematical
model. For these calculations, L = 9.5 mm, t = 0.9 mm, Lf = 4.1 mm,
tf = 1.7 mm, r = 1.35 mm, c = 150 mA, and µ = 0.75.

4. machined from Delrin®
which is a hard but easy to machine
polystyrene. The casing is a hollow cylinder which is 9 mm
long and 8 mm in diameter, having 0.8 mm thick walls. The
actuators are 100 µm diameter coil type SMA manufactured
by Toki Corporation. The adhesive pad is a 3 mm by 1 mm
PDMS pad.
After having all three legs and the casing machined, the
parts were assembled using Loctite 495 Super Glue. The
electrical connections for the SMA wires and the external con-
nectors were done using clamping. The SMAs were connected
in series for simplicity, requiring only two connectors per three
actuated legs.
E. Experiments
A series of tests were conducted to gain a measure of the
viability of the fabricated stopping prototype. Measurements
were made of the preload and shear forces produced by a
single leg mechanism. In addition, in vitro tests of the complete
mechanism were performed inside a vinyl tube.
1) Single Leg Test: A single leg mechanism was actuated
with a current of 150 mA while the distance between the
unactuated footpad position and the substrate was varied. The
applied preload force was measured using a load cell (GSO-
10, Transducer Techniques).
The initial preload test was done with a rigid glass substrate.
To more accurately represent the working environment of the
capsule, two deformable substrates were also tested. These
substrates were in the form of a membrane; mounted only on
the edges, the leg could push well past the undisturbed sub-
strate surface as is expected inside the human gastrointestinal
tract. The first membrane was 0.75 mm thick PDMS rubber,
and the second was 0.1 mm thick nitrile rubber. The results
of these characterization tests are shown in Figure 6.
In addition to measuring just the preload force, a system
was developed to measure the shear forces with the load cell.
This setup was used to determine the coefficient of friction µ
for various combinations of substrate and adhesive materials.
The results are shown in Table I.
0 1 2 3 4 5 6 7 8 9 10
0
5
10
15
20
25
30
35
40
45
50
Distance (mm)
Force(mN)
Glass
PDMS membrane
Nitrile membrane
Fig. 6. Single leg experimental preload characterization results. The prototype
single leg mechanism was actuated with 150 mA and 2.30 V for all data points.
When compared to the model prediction (see Figure 5) the
characterization results show the same three distinct contact
modes. Indeed, visual observations confirmed the type of
contact at each point corresponded to theory. In addition,
Figure 5 was generated with parameters that matched the
fabricated prototype so it can be seen that the rigid glass
substrate results compare favorably to the model. The contact
modes transition at approximately the same distances and
the initial slopes of the curves match. The characterization
results appear to be simply shifted to a lower force from
the model predictions, a phenomenon that might be explained
by fabrication imperfections and friction. These results show
that the developed mathematical model provides a reasonable
estimate of the performance of the leg mechanism. In the
future, this may be used to further refine the design via the
optimization of various parameters.
2) Whole Mechanism Test: The successful tests of the
single leg led to whole mechanism tests. Figure 7 shows the
opening and closing of the 3 legs assembled on the casing.
The stopping mechanism was then tested in vitro. By the
use of fishing line attached to the casing, the mechanism was
lowered into a 19 mm inner diameter vinyl tube that was set
up vertically on the test bench. As current was applied to the
actuators, the legs opened, stopping the capsule within the
tube (see Figure 8). When the current was removed, the legs
retracted to their original position, peeling the adhesive pads
off of the wall per the design and detaching the capsule from
the walls of the tube. To ensure that it was not the tether that
was supporting the capsule but the adhesive pads themselves,
the tether was wiggled, released and even pulled; the prototype
always held its position within the tube. In addition, the
capsule prototype was tested within a larger vinyl tube with
an inner diameter of 25 mm. The capsule performance was
comparable to that in the smaller tube. These tests showed
Substrate Adhesive µ
Glass Flat PDMS 0.79 ± 0.04
PDMS fibers (4 µm ) 0.62 ± 0.08
Glass w/ Oil (WD-40) Flat PDMS 0.17 ± 0.08
PDMS fibers (4 µm ) 0.31 ± 0.13
Glass w/ Water Flat PDMS 1.62 ± 0.09
PDMS fibers (4 µm ) 0.40 ± 0.09
TABLE I
EXPERIMENTALLY MEASURED FRICTION COEFFICIENTS FOR VARIOUS
SUBSTRATES AND ADHESIVES
(a) (b)
Fig. 7. The stopping mechanism: a) Legs closed (initial state), b) Legs
opened.

5. that this prototype is capable of basic stopping performance
in accordance with the design parameters.
The prototype leg mechanism constructed works robustly
and promises to cover the basic requirement; it appears to
provide enough preload on the adhesive pads.
IV. LOCOMOTION MECHANISM
The stopping mechanism is designed to be hollow, allowing
the space inside to be used for a cameraand other modules to
extend the capabilities of the capsule. The stopping capsule is
an advantageous mechanism for applications like monitoring
the gastrointestinal tract; a locomotion mechanism would be
even more advantageous, as it would allow forward and
backward position control. The current design enables us to
extend the usage of the stopping mechanism and use it as a
locomotion mechanism. Two stopping modules can be inte-
grated to perform locomotion using an inchworm locomotion
principle. The CAD drawing of the conceptual design is shown
in Figure 9.
The two stopping mechanisms are connected with a com-
pression spring (grey) and a hollow cylinder, forming a piston.
There is also a coil type SMA wire (red) connected to the two
casings on the ends. The SMA wire and the spring work in
an antagonist fashion. When the SMA wire is not actuated,
the spring pushes the casings apart so the capsule is in its
expanded state. When the SMA wire is actuated, it works
against the compression spring, pulling the casings together
(see Figure 10).
Locomotion is performed by sequentially opening and clos-
ing the legs and actuating the piston. Figure 11 gives a
graphical representation of the inchworm movement. At any
time, three of the legs are always open, anchoring the capsule
to the intestine walls, while the other module either pushed
or pulled to achieve the desired movement. The mechanism
works as follows: first, the front legs are closed, and the piston
is extended; then the front legs are opened again and this time
the rear legs are closed; after that, the rear legs are pulled
by shrinking the piston; once pulled, they are again opened,
returning to the initial state.
The locomotion prototype has been built. Figure 12 shows
the stills of a movie in which the prototype is operated as
desired. The capsule prototype was also tested in a 19 mm
diameter vinyl tube. The mechanism was lowered into the
(a) (b) (c)
Fig. 8. The stopping mechanism inside the tube: a) Free to move (legs
closed), b) anchored (legs open),c) Detached, free to move (legs closed).
Fig. 9. The CAD drawing of the proposed inchworm locomotion mechanism
(exploded view).
tube and the lower legs were opened. The control of the three
parts (lower legs, upper legs and the piston) was performed
manually by switches. Figure 13 shows four states of the
crawling locomotion taking a step.
Tests have shown that the capsule could achieve robust
motion inside the tube with an average velocity of 0.5 mm/sec.
V. CONCLUSION
This paper demonstrates progress toward the development
of a complete stopping and locomotion mechanism for en-
doscopic microcapsules. Characterization and optimization of
important design parameters through the modeling of the
mechanisms and measurements have been achieved. The de-
signs for stopping and locomotion mechanisms for an endo-
scopic microcapsule robot perform robustly.
The mechanisms demonstrated in this paper are promising.
The designs are simple yet effective, achieving both stopping
and locomotion action in in vitro environments. The modu-
larity of the stopping mechanism enables it to be used as an
expansion module for microcapsules that are currenty being
used. It allows other components such as a miniature camera
to be integrated.
The design continues to be optimized using the developed
model in conjunction with in vitro tests. Possible further
works include the research of actuator power minimization,
(a) (b)
Fig. 10. CAD Drawings of the locomotion mechanism: a) shrunk state, b)
expanded state.

6. Fig. 11. The proposed inchworm based crawling locomotion mechanism
motion steps.
(a) (b)
Fig. 12. Locomotion mechanism: a) initial state, b) legs open and body
shrunk
additional modules, improved fabrication techniques and an
improved testing environment.
ACKNOWLEDGMENT
This section suppressed for anonymous submission.
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