Ž .Sensors and Actuators 77 1999 157–166www.elsevier.nlrlocatersna

Shape memory micro-actuation for a gastro-intestinal interventionsystem

Dominiek Reynaerts ), Jan Peirs, Hendrik Van BrusselKatholieke UniÕersiteit LeuÕen, DiÕision of Production Engineering, Machine Design and Automation, Celestijnenlaan 300 B, B-3001 HeÕerlee, Belgium

Received 2 July 1997; accepted 24 March 1999

Abstract

This paper describes the design of a prototype gastro-intestinal intervention system based on an inchworm-type of mobile robot. Thistype of device is a kind of vehicle for inspection through the colon, something that is currently impossible due to the large number ofturns in the intestinal system. Eventually, tools for intervention can be added in the future. The overall system is about 95 mm long andhas a diameter of 15 mm. The robot consists of three main modules: an extension and contraction module, a two degree-of-freedombending module and two locking modules. All these modules are to be actuated by shape memory elements. The main part of the paperdescribes a modular actuator for realising bending motion. The design can be compared to a single vertebra, where stacking severalelements can form a spinal column. It will be shown that this design greatly facilitates the control. The electromechanical interconnectionof the different parts was an integral part of the design as well as techniques for selectively addressing the different actuators. In order toincrease the performance of the proposed design, the last part of the paper discusses some production aspects of the shape memoryelements. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Gastro-intestinal; Shape memory alloy; Micro-actuator; Micro-electromechanical system; Microvalve; Minimal invasive surgery

1. Introduction

The idea of miniature mobile robots goes back to theGNAT robot developed at MIT, artificial intelligence labw x1 . The idea behind this GNAT is an insect-like, com-pletely autonomous, robot that can be mass-produced.Although this idea originally looked very futuristic, thegrowing interest in micro-electromechanical systemsŽ .MEMS caused the research community to consider thesesystems to be within the reach of current technology. Theterminology in this field is very poorly defined, but for thepurpose of this paper, a miniature robot is defined as

w xproposed by Dario et al. 2 : a robot with a size in theorder of a few cubic centimetres that operates in aworkspace and generates forces comparable to those appli-cable by human operators during fine manipulation.

The most promising future for this kind of robotsprobably lies in the field of inspection and intervention inmedical as well as in industrial applications. Within this

) Corresponding author. Tel.: q32-16-322-640; Fax: q32-16-322-987;E-mail: dominiek.reynaerts@mech.kuleuven.ac.be

field, several prototypes have been proposed: Aoshima andw xYabuta 3 proposed a miniature robot using piezo-ele-

ments; A giant magnetostrictive alloy actuated robot hasw xbeen proposed by Fukuda et al. 4 . The same author also

presented a micro optical robotic system with cordlessw x w xoptical power supply 5 ; Ikuta 6 presented the MEDI-

WORM, an active microrobot actuated by shape memoryalloys; a similar idea was also proposed by Hesselbach and

w xStork 7 .Some of these designs are based on the principle of the

inchworm actuator originally commercialised by BurleighInstrument. The inchworm principle was mostly used to

w xextend the stroke of piezoelectric linear drives 8,9 andconsists of two clamping modules and one expansionmodule. By intermittent clamping, expanding, andreclamping, these systems can creep over a rod or inside atube.

For all these systems, compact actuation is required.Ž .Shape Memory Alloy SMA actuators offer the advantage

of extremely high power-to-volume ratios that are compa-w xrable to those for hydraulic actuation 10 . Moreover, they

enable a very simple direct drive actuator design and anelectrical current using simple resistive heating can di-

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 99 00191-0

( )D. Reynaerts et al.rSensors and Actuators 77 1999 157–166158

rectly drive them. Shape memory alloys have been usedw x w xfor a wide range of applications 11 . Honma et al. 12 and

w xKuribayashi 13 presented shape memory based micro-ac-tuators. In the medical field, most applications are basedon the superelastic properties of SMAs or on a one-wayactuation like clamping. ‘Real’ actuators are mostly usedin minimal invasive surgery tools like endoscopes. Someexamples of this latter category are the active catheter tip

w xdeveloped by Dario and Montesi 14 , the active endoscopew xpresented by Ikuta et al. 15 , or the miniature active

w xcatheter developed by Guo et al. 16 .The proposed research is part of a Brite-Euram funda-

mental research project studying shape memory alloy mi-cro-actuators for medical applications. A first developmentwithin this project consisted of an actively controlled

w ximplantable drug delivery device 17 . The second part ofthis project concerned the development of a gastro-intesti-nal intervention system based on an inchworm-type ofmobile robot. Semi-autonomous gastro-intestinal interven-tion systems have only been addressed in the recent pastw x6,18 . This paper first describes the general lay-out of thedesigned gastro-intestinal intervention system. Afterwards,a modular shape memory actuator for realising bendingmotion is discussed in more detail. A final part of thepaper discusses the production aspects of the shape mem-ory elements.

2. Design of a gastro-intestinal intervention system

When MEMS were proposed, operation in the bloodstream was probably the most popular application amongthe projected ideas. This kind of ‘free floating’ deviceshave to cope with a number of severe problems: thereaction of the immunosystem on strange objects, theclothing of the blood around these objects, the required

Ž w x.Fig. 1. Diagram of the human colon from Ref. 19 .

Fig. 2. Inchworm-type of robot for inspection of the colon.

ability to navigate in a viscous environment, if necessaryagainst the blood flow direction. All these problems werelargely underestimated. In view of the current state-of-the-art in microsystem technology, it was therefore decided tobuild a more realistic application consisting of a gastro-in-testinal inspection and intervention system. In fact, this canalso be considered as a mobile robot navigating through apipe, in this case, the colon, but the scale is quite different.

Fig. 1 shows a simplified representation of the humanw xcolon 19 . The colon has an average diameter of about 50

mm. The smallest radius is about 20 to 30 mm and islocated at the bending portion between the rectum and thesigmoid colon. The transverse colon, which is 400 to 500mm long, is the largest and most mobile part of the colon.Due to its horizontal position, the breathing process affects

w xits movements. According to Sturges and Laowattana 19 ,the use of a colonoscope is impeded by the peristalticaction of the gut, attempting to expel the device. The mainproblem for inspecting the colon with an endoscopic de-vice is the large number of turns, more specifically at thesigmoid, which have to be taken when entering the humanintestinal system. It is therefore also extremely difficult tomanoeuvre a classical endoscope around the bends of thecolon without damaging the gut. Therefore, the proposeddesign aims at a more advanced mobile system forcolonoscopy.

The system that is developed is basically an inchworm-type of mobile robot. Fig. 2 shows that, compared to theconventional inchworm with two clamping and an exten-sion actuator, the proposed design has two additionalbending degrees of freedom. This gastro-intestinal inter-vention system has an outer diameter of maximum 15 mm,which is acceptable for colonoscopes. Table 1 summarisesthe specifications for the design.

Table 1Specifications for inchworm design

Module Number Diameter Length StrokeŽ . Ž .mm mm

Translation 1 15 10 10 mmClamping 2 15 32.5 ø 15–50 mmBending 2 15 10 "458

( )D. Reynaerts et al.rSensors and Actuators 77 1999 157–166 159

As shown in this table, each module has a specificfunctionality that is to be realised within a very narrowvolume.

ØThe translation module has a length of 10 mm and hasto realise a stroke of 10 mm. This expansion can berealised with a spring-type SMA actuator. Also, the designhas to guarantee sufficient lateral stiffness.

ØThe clamping module has a length of 32.5 mm mustbe able to clamp in a human colon with a diameter rangingfrom 15 to 50 mm. The gut is a highly flexible andslippery environment and in the same time it is very fragileand sensitive to damage. Therefore, a clamping systembased on balloons seems most appropriate. This approach

w xwas already proposed by Slatkin et al. 18 .ØThe bending module has a length of 10 mm. It has to

realise a bending from y458 to 458.Both modules are stacked in such a way that a two

degree-of-freedom system is obtained. Several design teamswithin the project develop the different parts. In firstinstance, the interface between the different actuators is aflat disk of 15 mm diameter. After completion of the firstprototype, a further integration of the different actuatorswill be considered. For instance, empty space in onemodule can be used for the other actuators. For this firstdesign, the forces and torques to be developed should besufficient to support the weight of the device. A moredetailed specification will be made after experimental test-ing. No commercial endoscopic inchworms are yet exist-ing, so this design would be a completely new product.The proposed specs are on-the-edge, but appear to bereasonable for a first prototype.

3. Design of a modular bending actuator

3.1. Bending actuator general concept

Fig. 3 shows that the actuator can be compared to asingle vertebra, where stacking several elements can forma spinal column. Each vertebra is controlled in a binary

Fig. 3. Vertebral design of a bending actuator.

Fig. 4. Design concept of a single vertebra actuator.

mode. This means that a single vertebra has only twoŽ .powered positions left bending or right bending and the

central position in the unpowered condition. This alsomeans that the complete ‘spinal column’ can only reach adiscrete number of positions. However, for the envisagedendoscopic application this is more than sufficient.

Fig. 4 shows a design concept for a single vertebraactuator. The actuator consists of a central rotating partactuated by two antagonistic SMA actuators. The centralelement also contains two return springs with constant andequal torque. This means that the unpowered positioncorresponds to the neutral position.

Fig. 5 shows a photograph of a preliminary prototype.For this specific example, the actuator has a stroke of"158 and a diameter of 17 mm. The major parts of thisprototype are made of brass, but could be easily made in

Ž .another material e.g., plastics .As each actuator is controlled in a binary mode, only a

mechanism for selection of the different actuators has to beprovided. The electrical control of the different actuators

Fig. 5. Prototype of a single vertebra actuator.

( )D. Reynaerts et al.rSensors and Actuators 77 1999 157–166160

Fig. 6. Electromechanical connection of the actuators.

can be realised with a bus system. Each single vertebracontains an integrated electronic circuit to enable actuatorselection, or eventually selection of several actuators withinone single bending module. With plastic modules, thisintegration can be realised by integrating electrical connec-tions on the parts of the module with the SIL 1 technologyw x17 . Fig. 6 shows the principle of electromechanical con-nection for the different modules. Adjacent modules can berotated 908 to each other so that a two-dimensional bend-ing actuator is obtained. In this case, five signals areprovided: a supply voltage for the electronics V , a supplycc

voltage for the SMA elements V , a common ground, as

select channel, and a clock channel.Fig. 7 shows two principles for driving the different

actuators based on the above signals. The first principleŽ .frequency modulation based communication realises ac-tuator selection with a filter or a phase locked loop. Thisallows simultaneous actuation using only one signal wirecarrying a frequency modulated selection signal. No clocksignals are required. This design has the drawback that allmodules differ by their filter, which is a disadvantage formass production and results in a decreased degree of

Ž .modularity. The second principle serial communicationuses digital addressing of the different modules. Thissystem also enables simultaneous actuation but requires anadditional clock channel. In this case, all modules differ bytheir address. As it is much easier to change a digitaladdress, this solution is preferable for real mass produc-tion.

3.2. Detailed actuator design

This section describes a general study of different alter-natives for the vertebra actuator design. The vertebra is

1 ŽSpritzgiesteile mit Integrierten Leiterzugen Injection moulded parts¨.with integrated conducting pads . SIL starts from a plastic part that is

galvanically covered with copper and tin. The tin layer is patterned bylaser evaporation. Afterwards, this layer is used as a mask to etch theunderlying copper. Finally, tin is removed and the result is a structuredcopper layer.

considered as a flat disk with diameter 15 mm. The desiredstroke of an actuator is "158. This means that threevertebras are required for realising the bending movementspecified in Table 1. The external torque corresponds tothe weight of the links to rotate. It is estimated to be 6Nmm. Other requirements are a sufficiently high electricalresistance for control by an electronic circuit embedded inthe vertebra and the possibility for miniaturisation. Theassumed SMA material characteristics can be found in theupper left corner of Table 2. It is assumed that the returnforce to deform a cold shape memory element is one-thirdof the force it generates when heated. Table 2 comparesnine different actuation principles for the vertebra actuator.

In each row of Table 2, a comparison is made betweenpure antagonistic SMAs, an SMA working against anelastic joint, and a combination of both.

ØThe pure antagonistic system has two stable poweredpositions, but is not stable when not powered. The forcerequired to deform a SMA element is 1r3 of the force itgenerates. Consequently, the activated SMA has to gener-ate force for external use and for deformation of his

Ž .antagonist as expressed in Eq. 1 . By simplifying thisequation, this means that the SMA has to generate a torque1.5 times higher than the external torque.

1 3M GM q M ´M G M 1Ž .SMA ext SMA SMA ext3 2

Ž .Eq. 1 is the Torque balance for antagonistic system.ØThe SMA in combination with an elastic joint has one

powered position and also a stable neutral position. Theelastic joint serves as both spring and joint. The activatedSMA has to produce the external torque but also to deform

Ž .the spring as expressed by Eq. 2 . In the unpoweredsituation, the spring has to be able to deform the SMA andto generate the external torque. The torque generated bythe bias spring is assumed to be constant. This is correct if

Ž .super-elastic joints are used see Section 4 . Combination

Fig. 7. Two principles for selective addressing of the actuators.

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Table 2Comparisons of actuation principles for the vertebra actuator

of both formulas leads to the conclusion that the SMA hasto generate 3 times the external torque.

M GM qM ¶SMA ext spring

•1 ´M G3M 2Ž .SMA extM GM q M ßspring ext SMA3

Ž .Eq. 2 is the Torque balance for SMA with bias springŽ .super-elastic joint .

ØThe combination of an antagonistic system and anelastic joint creates three stable positions, two powered andone unpowered. Here the torque of the SMA is 6 timeshigher than the external torque.

1 ¶M GM qM q MSMA ext spring SMA3 •´M G6MSMA ext1

M GM q M ßspring ext SMA3

3Ž .Ž .Eq. 3 is the Torque balance for combined system.

The columns of Table 2 show different mountings inwhich the SMA material can be used: a straight wire, apulley system, or a spring.

ØFor the simple wire system, the maximum wire lengthŽ .of 15 mm i.e., the diameter of the module is an additional

constraint. The lever follows from the wire length, themaximum strain and the specified stroke. Lever and speci-fied torque define the force and consequently, also the wirethickness and resistance.

ØThe pulley system and the spring system assume thatthe maximum possible lever is used in order to reduce theforces and the section of the wire. For the pulley system,the force can be calculated from the specified torque andthe known lever. Wire length is calculated from lever andstroke.

ØThe SMA springs are calculated with the normalspring formulas where the E-modulus is replaced by theratio t rg . This is correct under the followingSMA SMA

assumptions.Ž .1 The maximum strain is proportional to the stress.

( )D. Reynaerts et al.rSensors and Actuators 77 1999 157–166162

Ž .2 The formula is only used to calculate full strokedeformation: assumption 1 is only valid for the maximumstrain at a specific stress.

Ž .3 The strain is proportional to the distance to thecentre of the wire: zero in the centre, maximum at thesurface. Together with assumption 1, this means that stressand strain are proportional to the distance to the centre asfor normal springs. Therefore, the E-modulus can be re-placed by t rg .SMA SMA

The conclusions of this table can be summarised asfollows.

ØThe torque to be generated by the SMAs is higherthan the required external torque: the pure antagonisticapproach asks only 1.5 times the external torque, thesystem with the elastic joint and one SMA element re-quires already 3 times the external torque and the combina-tion of antagonistic SMAs with elastic joints requires theSMAs to generate a 6 times higher torque. In the last case,the SMAs have to generate 36 Nmm instead of 6 Nmm.The combined system asks more, but offers more function-ality: three stable positions instead of two and the neutralposition is unpowered leading to lower power consumptionas the inchworm will mainly move straightforward suchthat the bending actuator will be used only during a smallpart of the intervention.

ØNone of the nine combinations satisfies all constraintsŽ .for this specific design: 1 For straight SMA wires, the

forces in the SMA, the mechanical connections and thejoint are very high. As a result, thick wires are neededsuch that the electrical resistance is extremely low. Fur-thermore, these wires are not flexible such that bending

Ž .forces of these wires become important. 2 The pulleysystem has a high electrical resistance but requires a verylong wire passed over a number of pulleys. Depending onthe case, at least five or 10 of these pulleys per SMA wireare needed. These cause friction and are not suited for

Ž .miniaturisation. 3 SMA springs offer the easiest way for

construction of the actuator but their problem is again theextremely low electrical resistance.

ØThe problem with the straight wire system can besolved by replacing the thick SMA wire by a number ofthin SMA wires mechanically in parallel and electrically in

Ž .series without using pulleys of course . This way, theelectrical resistance is higher and the thin wires are muchmore flexible. Problem is the equal load distribution overthe different wires. Cutting the pattern out of a ribbonsolves this problem. This technique is explained more indetail in Section 5.

ØFinal conclusion: the selected design is the straightwire system with antagonistic SMAs, super-elastic joints,and a patterned ribbon.

4. Construction of an optimised actuator prototype

Fig. 8 shows top, side and isometric view of the actua-tor design. The body of the actuator consists of two parts,an upper and a lower part, that have an identical form butone of them is rotated 1808. They are made of aluminium

Ž .by electro-discharge machining EDM , final versionscould be made of plastic by use of injection moulding.Both parts are connected to each other via two super-elas-tic joints. When the upper SMA strip is heated, it shortensand forces the upper part to rotate to the right. When thelower strip is heated, the upper part rotates to the left. Thestroke in both directions is 158 and is limited mechanically.Heating both strips at the same time has to be avoided, asthis will overload both strips and the joint. When no stripis heated, the elastic joints hold the actuator in its middleposition. A mechanical stop guarantees a fixed distancebetween the SMA strip and the axis of rotation.

A 0.15-mm thin glass plate assures thermal and electri-cal isolation between the SMA strip and the aluminiumbody. Assembly of all parts is done by gluing. To connect

Fig. 8. Bending actuator design.

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Fig. 9. Stacking of three actuators to obtain specified stroke.

the glass plate to the body and the SMA strip to the glassplate, epoxy glue is used. To obtain the specified stroke of"458, three actuators have to be used as shown in Fig. 9.Activation of one, two, or three actuators causes, respec-tively 158, 308 or 458 rotation. Seven useful combinationsare possible leading to following positions: y458, y308,y158, 08, 158, 308, 458.

The actuator contains two super-elastic joints with di-mensions shown in Fig. 10. Bending is limited to thenarrowed zone of 0.12 mm thickness. Two joints, one oneach side of the actuator, are used to enhance stiffness andstrength in the other directions. They are made of aCuAlNi alloy by EDM. Rolled strips are an alternative butwere not available in this material.

A super-elastic joint is an elastic joint made of super-elastic material. As all elastic joints, they have a number ofadvantages over normal rotary joints.

Ø The joint acts also as bias spring such that no extraspring has to be included in the design. This leads to areduction in the number of parts and a simplification of thedesign, both improving the possibilities for miniaturisation.

Ø An elastic joint exhibits no friction or backlash.Elastic joints suffer from a limited stroke and a return

force that can be substantial. However, these disadvantagesdo not apply for this development. Super-elastic materialshave a non-linear characteristic that permits to build actua-tors that are stiff in their neutral position while the forcesto bend them to their extreme position are kept low. A

Ž .high stiffness or high Young’s modulus in the neutralposition is important to avoid small disturbing forces frombending the actuator. With a normal elastic material thishigh modulus results in high elastic forces at the end of the

Ž .Fig. 10. Dimensions of super-elastic joint in mm .

Fig. 11. Characteristic of super-elastic joint.

stroke of the joint. This effect can be minimised by use ofsuper-elastic materials. The material behaves linear until astress of 450 MPa is reached. Then the material deformswithout any significant increase in stress. During recoveryof the material, the stress is about 50 MPa lower. Super-elastic behaviour is closely related to the shape memoryeffect. The difference is that super-elastic materials havetransformation temperatures far below ambient tempera-ture. These transformation temperatures increase propor-

Žtional with stress, such that at a certain stress level here.450 MPa , the martensitic transformation temperature ex-

ceeds the ambient temperature. At this point, the materialstarts to deform without any significant increase in stress.As long as the stress remains below this stress level, thematerial behaves linear. The super-elastic deformation is100% reversible. In fact, the material used is monocrys-talline and has better characteristics than polycrystallinematerial such as larger strains, less hysteresis and a flatterplateau.

Fig. 11 compares the bending characteristics of thesuper-elastic joints used in the prototype with normal

Fig. 12. Prototype of bending actuator.

( )D. Reynaerts et al.rSensors and Actuators 77 1999 157–166164

Fig. 13. The bending actuator in the left, middle, and right position.

elastic ones. An elastic joint with the same stiffness in theneutral position needs a 5 times higher torque to bend it toits extreme position. An elastic joint with the same maxi-mum torque has a 5 times lower stiffness in the neutralposition.

Fig. 12 shows a photograph of a first prototype. Itrequires a current of at least 0.8 A. Faster motion can be

Žachieved by increasing the current to 1.2 A each time for.a full stroke of 158 left or right . The resistance of the strip

is 0.3 W for the part on the actuator itself and 1 Wbetween the two clamps. This results in a power consump-tion of 0.2 to 0.4 W. With the depicted system in alaboratory environment without any extra cooling, a band-width of 0.25 Hz can be attained.

If, as explained in Section 5, patterned strips are usedthe electrical characteristics will improve. These powervalues do not pose any problem for application in agastro-intestinal inspection system. The inchworm systemas depicted on Fig. 2 has an umbilical system supplying airand water for flushing the colon. Flushing is absolutelyrequired to move forward in the colon. Both media could

therefore also be used for cooling purposes. The strips aremelt spun ribbons, 1.5 mm wide and 40 mm thick that cangenerate a force of 6 N. As the lever is 0.5 mm, the torquegenerated by the SMA is 3 Nmm, such that the external

Ž . Žtorque 6 times smaller is 0.5 Nmm 12 times lower than.specified . This can be increased by using wider and

thicker strips, but these were not available. Thicker stripsshould be carefully used because their flexibility is lower.The final prototype measures 15 mm in diameter and is 4mm high which is according to the specs in Table 1. Fig.13 shows the prototype in its three stable positions: bent tothe left, middle position and bent to the right.

5. Production of the actuator elements

An important problem with SMA actuators is the lowelectrical resistance, in particular when the electric currenthas to be supplied by integrated electronic circuitry. In-creasing the resistance is possible by replacing the ribbonby a number of wires mechanically in parallel and electri-

Fig. 14. Different steps in the production of a thin film actuator.

( )D. Reynaerts et al.rSensors and Actuators 77 1999 157–166 165

Ž . Ž .Fig. 15. Ribbons cut with EDM top , CO laser middle and excimer2Ž .laser bottom .

cally in series. Suppose a ribbon is cut into n mechanicallyparallel wires that are put electrically in series. This causesa multiplication of the resistance by n2. When the totalconsumed power remains the same, the required current isn times lower and the voltage is n times higher. It isimportant that the different wires mechanically are loadedequally. Therefore, the wires have to be kept together untilthey are fixed to the actuated device.

The thermal properties of the parallel wire system aresuperior to those of the flat ribbon, as a larger contactsurface is available for the cooling the same cross-section.Cooling is the limiting factor for the bandwidth of a shape

w xmemory actuator 10 .Fig. 14 shows the different steps in the production of

such a thin film actuator. For realising a patterned ribbon,several technologies are possible: EDM, laser cutting, andetching. All three of these technologies are tested. Wire-EDM has the disadvantage that a starting hole is neededfor the wire unless the wire starts from the side of theworkpiece. The upper photograph of Fig. 15 shows aribbon cut with EDM. The wires are 0.3 mm wide and 5.5

Ž .mm long. A Differential Scanning Calorimetry DSC testwas performed on the material after cutting. The transfor-mation temperatures shifted 108C upwards, but this may becaused by the mechanical test performed on it.

Laser cutting is faster and allows more complex pat-terns. Laser cutting was performed in an argon atmospherebecause of the high reactivity of titanium with oxygen. Toavoid movement of the tiny wires, the ribbon was gluedwith strain gauge glue on a substrate. An aluminiumoxideplate was used as substrate to avoid welding of the ribbonon the substrate. The middle photograph of Fig. 15 showsthe CO laser cut ribbon with wires of 0.2 mm wide and 52

mm long. The cut surfaces are very rough, which is verydisadvantageous for fatigue. As shown on the lower photo-graph of Fig. 15, superior results were obtained with anexcimer laser.

Also etching was investigated. A major problem here isthe high resistance of NiTi to etching products. An effec-

Ž . Ž .tive solution is HF 5% and H 0 95% . But all hydro-2 2

gen containing etching products must be avoided becauseof hydrogen brittleness of NiTi. Furthermore, most pho-toresists do not withstand these strong acids. The solutionis to use electrochemical etching with a weak acid thatdoes not contain hydrogen. FeCl was used and showed3

good results.

6. Conclusion

This paper described the design of a prototype gastro-intestinal intervention system based on an inchworm-typeof mobile robot. The proposed device can be a kind ofvehicle for inspection through the colon, something that iscurrently impossible due to the large number of turns inthe intestinal system. The prototype has three main mod-ules: an extension and contraction module, a locking mod-ule, and a two degree-of-freedom bending module. Allmodules are actuated by SMA elements. This paper merelyconcentrated on a modular actuator for realising bendingmotion. The design can be compared to a single vertebra,where stacking several elements can form a spinal column.The concept takes into consideration the electromechanicalinterconnection of the different parts and also techniquesfor selectively addressing the different actuators are pro-posed. A detailed comparison between several design alter-natives shows that for this application, a straight wiresystem with antagonistic actuators, is the best, althoughnot ideal, solution. In order to enhance the performance ofthe proposed system, several technologies for producingstraight SMA elements with high electrical resistance werediscussed. Excimer laser cutting gave the best results.

Acknowledgements

This research was sponsored by the Brite-Euram pro-gramme of the European Union, project number BE-7596-93 and contract number BRE2-CT93-0579 and by the

Ž .Belgian programme on Interuniversity Poles IUAP4-24of attraction initiated by the Belgian State, Prime Minister’sOffice, Science Policy Programming. The authors assumethe scientific responsibility of this paper. D. Reynaerts is apostdoctoral Fellow of the Fund for Scientific Research

Ž .Flanders—Belgium F.W.O.

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Dominiek Reynaerts received his Mechanical Engineering degree fromthe Katholieke Universiteit Leuven, Belgium, in 1986. He has beenworking at the Mechanical Engineering Department of that same univer-

Ž .sity, as a research assistant 1986–1991 , and since 1996 as seniorŽ .research assistant of the F.W.O. Fund for Scientific Research, Flanders .

He obtained his PhD degree in mechanical engineering in 1995 with thethesis ‘Control methods and actuation technology for whole-hand dexter-ous manipulation’, and became Assistant Professor at the KatholiekeUniversiteit Leuven in 1997. His research activities are in design andcontrol of multi-fingered grippers, shape memory alloy actuators, preci-sion mechanics, and micromechanical systems. He is a member of IEEE.

Ž .Jan Peirs graduated as Mechanical Engineer K.U. Leuven, 1993 and, asa research assistant at the Division of Production Engineering, MachineDesign and Automation of K.U. Leuven, he is currently working towardsa PhD Degree in Mechanical Engineering. His research interests includethe design of shape memory alloy micro-actuators for medical applica-tions and micromechanical systems in general.

ŽHendrik Van Brussel is mechanical engineer HTI-Oostende, Belgium,. Ž .1965 and electronics engineer K.U. Leuven, Belgium, 1968 . He re-

ceived a PhD Degree in Mechanical Engineering in 1971 from K.U.Leuven. From 1971 until 1973, he was ABOS-expert at the MetalIndustries Development Centre in Bandung, Indonesia, where he set upan engineering research centre, and Associate Professor at ITB, Bandung.In 1973, he became lecturer at K.U. Leuven and is now full professor inautomation and Head of the Department of Mechanical Engineering. Hewas a pioneer in robotics research in Europe and an active promoter ofthe mechatronics idea as a new paradigm for concurrent machine design.He has published more than 200 papers on different aspects of robotics,mechatronics and flexible automation. His present research interests areshifting towards holonic manufacturing systems and precision engineer-ing, including microrobotics. He is Fellow of SME and IEEE and in1994, he received a honorary doctor degree from the ‘Politehnica’University in Bucharest, Romania and from RWTH, Aachen, Germany.He is also a corresponding member of the Royal Academy of Sciences,Literature and Fine Arts of Belgium and an active member of CIRPŽ .International Institution for Production Engineering Research .