Stopping and Locomotion Mechanism for anEndoscopic Microcapsule Robot

Abstract— This paper presents a stopping and a locomotionmechanism to be used with an endoscopic microcapsule robot.In the diagnosis of gastrointestinal diseases, microcapsules havebeen developed recently as alternatives to conventional endoscopy.However, they have less accuracy and functionality in diagnosis asthey lack the ability to control their position. We propose mecha-nisms to be used with such microcapsules that would enable themto anchor and crawl in any position inside the gastrointestinaltract. The stopping mechanism, actuated by shape memoryalloys, makes use of dry and wet adhesive materials to attachto the intestinal tract. Bio-inspired wet fibrillar adhesives thatcreate adhesion in a way similar to the attachment mechanismsemployed by beetles are proposed. The locomotion mechanism,inspired by the locomotion principles of inchworms, is a modularexpansion of the stopping mechanism. Both the stopping and thecrawling locomotion mechanisms have been built and successfullytested inside a flexible vinyl tube. Results showed stopping withhigh repeatibily and 0.5 mm/sec locomotion speed.

I. INTRODUCTION

Traditional wired endoscopes are primarily used in theinvestigation of the gastrointestinal tract for diagnosis ofdeseases. Research is being conducted on the use of semi-autonomous structures in order to avoid the inconveniencesintroduced by traditional endoscopes [1]. In 2001, with theinvention of microcapsules, a much more convenient alterna-tive was introduced to the market [2]. Microcapsules offervirtually non-invasive diagnoses and require less complicatedoperation procedures, resulting in overall higher convenience.In addition, the use of microcapsules enables screening of thesmall intestines, which is otherwise impossible with currenttraditional 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 thecontraction of intestinal muscles pushes everything inside;anything that does not deploy a mechanism of attachmentwill be forcibly moved. The microcapsule developed by RFNorika [3] has an external magnetically actuated system thatcontrols the orientation of the capsule, but it lacks the abilityto stop inside the intestines and further investigate a regionof interest. The need for such a mechanism is great, as itwill improve the accuracy of the diagnosis and provide alevel of control to the motion of the microcapsule, enablingadvanced applications such as biopsy, localized drug deliveryand surgery.

Previous studies [4]–[6] have been made on the developmentof such stopping mechanisms. In this paper, we proposea novel stopping mechanism with a biomimetic attachmentmethod 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 beexpanded in order to be used as a crawling based locomotionmechanism.

II. PROBLEM DEFINITION

The main challenges in the development of an attachmentand locomotion mechanism for a microcapsule robot are:

• Since the size of the capsule is very important (verylarge pills are uncomfortable), the mechanism must besufficiently small as to not drastically increase the sizeof the system.

• In situ, the microcapsule is subject to the peristaltic mo-tion of the digestive tract at all times, so the atttachmentmechanism should be able to conform to the intestinalenvironment.

• The attachment mechanism must be supplied with thesame power that is already onboard the microcapsule.As these power systems are the limiting factor in theendurance of most capsule systems, the attachment mech-anism must be low power.

• When operating within the human body, safety is a majorconcern; any attachment mechanism must be chosen withsafety and biocompatibility as a priority.

With these constraints in mind, many traditional attachmentmechanisms 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 miniaturemechanisms that make use of setae–like micro legs employedby earthworms [4], [6]. The design idea of the stoppingmechanism presented in this paper relies on a biologicallyinspired wet fibrillar adhesive material that would be pushedagainst 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 mostcommonly associated with geckos have been the focus ofmuch recent research [8]–[10]. The natural system is capableof robust attacment to most materials, regardless of the surfaceroughness. In addition, as a passive mechanism no extra energyis required to maintain the hold once adhesion is achieved.Another attractive feature of dry adhesives is that by utilizinga peeling motion, very little energy is required for detachment.

As another alternative, beetles and other insects use similarfibrillar structures but enhance adhesion through capillaryforces that arise from a mostly hydrophobic oily secretion [11],[12]. These systems require less complex fibers, but thecomposition of the liquid as well as the secretion mechanismitself are challenging tasks.

The past year has seen great improvement in the quality offabricated synthetic biomimetic fibrillar adhesives. High aspectratio, densely packed fibers with diameters of two, three, andfive microns have been successfully fabricated (see Figure 1)with both molding and photolithography techniques. Thus, thisproject aims to use either the polymer polydimethylsiloxane(PDMS) or the photoresist SU-8 for the fiber material. Acandidate for a liquid secretion to enhance adhesion is asilicone oil with high viscosity and long evaporation time thatwill be stored in a reservior and secreted via integrated poresby the pressure created by the contact of the foot with thesubstrate. Until these fabricated fibers are further refined intoviable adhesive systems, a simple flat pad of PDMS providesenough adhesion for testing purposes.

B. Design Principle

The design of the stopping mechanism was geared towardsthe use of synthetic biomimetic fibrillar adhesives. This re-quires a mechanism that will push the adhesive material ontothe 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 theactuator for this purpose. The main concern is that it mustbe biocompatible. Also, high output force and low powerconsumption are required. Several actuators, including piezo-electric materials, polymer actuators and shape memory alloyswere considered in the design stage. While the piezoelectricmaterials have high force output and low power consumption,they typically need very high driving voltages. This, in addtionto the limited strain capabilities of the actuator, make itan inappropriate choice for our purpose. Polymer actuatorshave much better strain output, and they are biocompatible.However, they are slow, they require high power and they areincapable of high output force.

Shape Memory Alloy (SMA) wires are selected here likeother several research groups that proposed these actuators formicrocapsule applications [4], [6]. Shape memory alloy helicalwires have high output forces and large strain up to 50 percent.

Fig. 1. 5 µm diameter high aspect ratio, densely packed SU-8 microfibersfabricated using photolithography.

The actuation is through heating; a voltage applied across thewire causes current flow, and the power is dissipated as heat.This also means that they have a very high power consumptionand low efficiency that limits their untethered usage. Despitethis fact, these actuators are chosen for their high output forceand small size. The power consumption issue will be addressedin the future. After a search for commercially available SMAactuators, we have decided to use coil type SMA actuatormanufactured 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 cylindricalpulley (yellow) that is free to rotate. A rubber spring (pink)is attached to the pulley on one end, and to the casing on theother 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 theleg to the casing.

The initial state of the leg is closed (SMA wire not ac-tuated). When the SMA wire is heated by passing currentthrough it, it pulls the leg, creating a torque. This torquecauses the pulley to turn, thereby rotating the leg. The rubberspring is stretched as the pulley turns, creating a torque inthe other direction. The leg stops opening when it reaches anequilibrium point where the torque created by the SMA wireequals the torque created by the rubber spring. Once the SMAwire is shut off, the leg is pulled back by the rubber springsince the SMA wire no longer creates a strong resisting torque.

The foot carries an adhesive pad that will stick and generatehigh adhesion forces when pushed on the intestinal walls. Asthe adhesive pad reaches the walls and is preloaded, the legdoes not open futher, but it applies a preload as the SMA iscontinuously actuated. Three legs are placed symmetrically onthe casing, so that a strong grip from three points is formed.One critical mechanism is the polymer hinge through whichthe adhesive pad is connected to the legs. The purpose ofthe compliance created by the polymer hinge is to aid theadhesive pad in sticking to and detaching from the walls of theintestine. When the pad starts to touch the intestinal wall, thepolymer hinge bends, applying both shear and normal preload.Consequently, when the leg closes, this structure creates apeeling motion, greatly reducing the force necessary to detach.This compliant structure is formed using a stiff polymer filmwhich 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

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 wascreated to predict the forces between the foot and substratewhen current is applied to the SMA actuator. Figure 3 showsthe relevant dimensions of the mechanism used in this model.

The leg and foot were treated as rigid beams. The forcerequired to bend the ankle joint (polymer hinge) was calculatedusing large beam deflection theory. The PDMS spring wastreated as a linear spring whose spring constant could becalculated through simple axial beam loading equations. Theseassumptions were experimentally validated with simple forcemeasurements 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 µmcoil-type SMA, the emprical output force FSMA in mN wasfound 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. Thisequation is only valid above a threshold current of 60 mA andbelow a burnout current of 180 mA.

If enough current is applied to the SMA, the leg will rotateopen and push the footpad against the substrate. There are twomodes 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 incontact 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 acton the leg: a) In the flat mode, the footpad completely contacts thesubstrate. b) In the angled mode, only the distal end of the footpadis in contact with the substrate.

for any given input current to the SMA wire. Assuming auniform pressure on the footpad in the flat mode, the preloadforce 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 definedin 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 themechanism will achieve is the ankle joint. In order to reach andmaintain the flat mode, sufficient force must be applied to theankle to maintain the bend. Given a contact mode assumption,the resulting preload force can be calculated. This force canthen be used to determine the force on the ankle joint, whichwill 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 indicatea 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 = 6.7mm, the contact changes to angled mode; the preload forcestays relatively constant (even increasing slightly). Finally, atd = 9.2 mm, the foot no longer reaches the wall, and theforces drop to zero.

D. Fabrication

Two prototype stopping mechanisms were built and suc-cesfully tested. The casings, cylindrical pulleys and legs were

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Forc

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PreloadShear

Fig. 5. Predicted preload and shear forces using the developed mathematicalmodel. 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.

machined from Delrin® which is a hard but easy to machinepolystyrene. The casing is a hollow cylinder which is 9 mmlong and 8 mm in diameter, having 0.8 mm thick walls. Theactuators are 100 µm diameter coil type SMA manufacturedby Toki Corporation. The adhesive pad is a 3 mm by 1 mmPDMS pad.

After having all three legs and the casing machined, theparts were assembled using Loctite 495 Super Glue. Theelectrical connections for the SMA wires and the external con-nectors were done using clamping. The SMAs were connectedin series for simplicity, requiring only two connectors per threeactuated legs.

E. Experiments

A series of tests were conducted to gain a measure of theviability of the fabricated stopping prototype. Measurementswere made of the preload and shear forces produced by asingle leg mechanism. In addition, in vitro tests of the completemechanism were performed inside a vinyl tube.

1) Single Leg Test: A single leg mechanism was actuatedwith a current of 150 mA while the distance between theunactuated footpad position and the substrate was varied. Theapplied 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 thecapsule, two deformable substrates were also tested. Thesesubstrates were in the form of a membrane; mounted only onthe edges, the leg could push well past the undisturbed sub-strate surface as is expected inside the human gastrointestinaltract. The first membrane was 0.75 mm thick PDMS rubber,and the second was 0.1 mm thick nitrile rubber. The resultsof these characterization tests are shown in Figure 6.

In addition to measuring just the preload force, a systemwas developed to measure the shear forces with the load cell.This setup was used to determine the coefficient of friction µfor various combinations of substrate and adhesive materials.The results are shown in Table I.

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Fig. 6. Single leg experimental preload characterization results. The prototypesingle leg mechanism was actuated with 150 mA and 2.30 V for all data points.

When compared to the model prediction (see Figure 5) thecharacterization results show the same three distinct contactmodes. Indeed, visual observations confirmed the type ofcontact at each point corresponded to theory. In addition,Figure 5 was generated with parameters that matched thefabricated prototype so it can be seen that the rigid glasssubstrate results compare favorably to the model. The contactmodes transition at approximately the same distances andthe initial slopes of the curves match. The characterizationresults appear to be simply shifted to a lower force fromthe model predictions, a phenomenon that might be explainedby fabrication imperfections and friction. These results showthat the developed mathematical model provides a reasonableestimate of the performance of the leg mechanism. In thefuture, this may be used to further refine the design via theoptimization of various parameters.

2) Whole Mechanism Test: The successful tests of thesingle leg led to whole mechanism tests. Figure 7 shows theopening and closing of the 3 legs assembled on the casing.

The stopping mechanism was then tested in vitro. By theuse of fishing line attached to the casing, the mechanism waslowered into a 19 mm inner diameter vinyl tube that was setup vertically on the test bench. As current was applied to theactuators, the legs opened, stopping the capsule within thetube (see Figure 8). When the current was removed, the legsretracted to their original position, peeling the adhesive padsoff of the wall per the design and detaching the capsule fromthe walls of the tube. To ensure that it was not the tether thatwas supporting the capsule but the adhesive pads themselves,the tether was wiggled, released and even pulled; the prototypealways held its position within the tube. In addition, thecapsule prototype was tested within a larger vinyl tube withan inner diameter of 25 mm. The capsule performance wascomparable to that in the smaller tube. These tests showed

Substrate Adhesive µ

Glass Flat PDMS 0.79 ± 0.04PDMS fibers (4 µm �) 0.62 ± 0.08

Glass w/ Oil (WD-40) Flat PDMS 0.17 ± 0.08PDMS fibers (4 µm �) 0.31 ± 0.13

Glass w/ Water Flat PDMS 1.62 ± 0.09PDMS fibers (4 µm �) 0.40 ± 0.09

TABLE IEXPERIMENTALLY MEASURED FRICTION COEFFICIENTS FOR VARIOUS

SUBSTRATES AND ADHESIVES

(a) (b)

Fig. 7. The stopping mechanism: a) Legs closed (initial state), b) Legsopened.

that this prototype is capable of basic stopping performancein accordance with the design parameters.

The prototype leg mechanism constructed works robustlyand promises to cover the basic requirement; it appears toprovide enough preload on the adhesive pads.

IV. LOCOMOTION MECHANISM

The stopping mechanism is designed to be hollow, allowingthe space inside to be used for a cameraand other modules toextend the capabilities of the capsule. The stopping capsule isan advantageous mechanism for applications like monitoringthe gastrointestinal tract; a locomotion mechanism would beeven more advantageous, as it would allow forward andbackward position control. The current design enables us toextend the usage of the stopping mechanism and use it as alocomotion mechanism. Two stopping modules can be inte-grated to perform locomotion using an inchworm locomotionprinciple. The CAD drawing of the conceptual design is shownin 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 twocasings on the ends. The SMA wire and the spring work inan antagonist fashion. When the SMA wire is not actuated,the spring pushes the casings apart so the capsule is in itsexpanded state. When the SMA wire is actuated, it worksagainst 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 agraphical representation of the inchworm movement. At anytime, three of the legs are always open, anchoring the capsuleto the intestine walls, while the other module either pushedor pulled to achieve the desired movement. The mechanismworks as follows: first, the front legs are closed, and the pistonis extended; then the front legs are opened again and this timethe rear legs are closed; after that, the rear legs are pulledby shrinking the piston; once pulled, they are again opened,returning to the initial state.

The locomotion prototype has been built. Figure 12 showsthe stills of a movie in which the prototype is operated asdesired. The capsule prototype was also tested in a 19 mmdiameter vinyl tube. The mechanism was lowered into the

(a) (b) (c)

Fig. 8. The stopping mechanism inside the tube: a) Free to move (legsclosed), 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 threeparts (lower legs, upper legs and the piston) was performedmanually by switches. Figure 13 shows four states of thecrawling locomotion taking a step.

Tests have shown that the capsule could achieve robustmotion inside the tube with an average velocity of 0.5 mm/sec.

V. CONCLUSION

This paper demonstrates progress toward the developmentof a complete stopping and locomotion mechanism for en-doscopic microcapsules. Characterization and optimization ofimportant design parameters through the modeling of themechanisms 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 stoppingand locomotion action in in vitro environments. The modu-larity of the stopping mechanism enables it to be used as anexpansion module for microcapsules that are currenty beingused. It allows other components such as a miniature camerato be integrated.

The design continues to be optimized using the developedmodel in conjunction with in vitro tests. Possible furtherworks include the research of actuator power minimization,

(a) (b)

Fig. 10. CAD Drawings of the locomotion mechanism: a) shrunk state, b)expanded state.

Fig. 11. The proposed inchworm based crawling locomotion mechanismmotion steps.

(a) (b)

Fig. 12. Locomotion mechanism: a) initial state, b) legs open and bodyshrunk

additional modules, improved fabrication techniques and animproved testing environment.

ACKNOWLEDGMENT

This section suppressed for anonymous submission.

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(a) (b)

(c) (d)

Fig. 13. Photos from a movie showing the capsule crawling: a) initially onlylower legs are open, the capsule is anchored, b) body is shrunk, c) upper legsare opened, d) lower legs are closed and body extended.

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