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Surface-stimulationtechnologyforgraspingandwalkingneuroprostheses

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MilosRPopovic

UniversityofToronto

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ThierryKeller

Tecnalia

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IonPappas

GeneralElectric

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Surface-StimulationTechnology for Grasping andWalking NeuroprosthesesImproving Quality of Life in Stroke/Spinal Cord InjurySubjects with Rapid Prototyping and Portable FES Systems

An important aspect in the rehabilitationof stroke and spinal cord injured (SCI)

subjects is to help them improve or restorebody functions lost as a result of injury ordisease as well as help them become as in-dependent as possible [3, 43]. Absence ofbasic functions such as grasping, walking,breathing, and bladder voiding often ren-ders such subjects dependent on the assis-tance of others for daily living activities.One way to make these subjects inde-pendent is to provide them with assistivedevices such as a neuroprosthesis. Theneuroprosthesis can help the SCI andstroke subjects regain some lost bodyfunctions, such as walking and grasping[43]. A generic neuroprosthesis consistsof an electric stimulator, stimulation elec-trodes, and a variety of sensors, and, insome cases, is used in combination with amechanical brace [43]. A neuroprosthesisgenerates a train of short electrical pulsesthat when applied to a muscle causes it tocontract [3, 11] (see Fig. 1). By stimulat-ing properly selected muscles or musclegroups, the neuroprosthesis can enable anSCI or a stroke subject to move an other-wise paralyzed limb or body part.

In this article, portable grasping andwalking neuroprostheses, developed bythe Automatic Control Laboratory at theSwiss Federal Institute of Technology Zu-rich (ETHZ) and the Paraplegic Center atthe Universi ty Hospi ta l Balgr is t(ParaCare) are discussed [16, 33, 35].Both neuroprostheses employ surfacestimulation technology (discussed below)and are currently used by a number of sub-jects in daily living activities.

Functional Electrical StimulationFunctional electrical stimulation

(FES) is a methodology that uses short

bursts of electrical pulses to generatemuscle contraction [3, 11]. These pulsesgenerate action potentials in motor-neu-rons attached to a muscle, which causethat muscle to contract (Fig. 2). A neces-sary condition to use a neuroprosthesis isthat the motor-neurons of the muscles thatneed to be stimulated are intact; i.e., themuscles should not be “denervated.” Inorder to achieve a continuous muscle con-traction (tetanization), the FES system hasto induce at least 20 action potentials/s inthe motor-neurons [3, 11]. Otherwise, themuscle does not generate a steady outputforce, but only twitches.

In principle, motor-neurons can bestimulated by both monophasic andbiphasic current or voltage pulses [3]. Theinjected electric charge depolarizes themembrane of the motor-neuron, whichcauses the generation of an action poten-tial. It is generally believed that the in-jected charge should be removed from thebody and should not be allowed to accu-mulate over time. In the case of sur-face-stimulation FES systems, it is alsobelieved that it is often beneficial for boththe subject and the stimulation that the po-sitions of the anode and cathode alternateduring stimulation. Therefore, the major-ity of FES systems implement biphasiccurrent pulses, allowing control of theamount of charge delivered to and re-moved from the body.

The motor-neurons can be stimulatedusing either surface (transcutaneous),needle (percutaneous), or implanted elec-trodes [3]. Transcutaneous stimulation isperformed with self-adhesive ornonadhesive electrodes that are placed onthe subject’s skin, above the motor-neu-ron [3, 37]. Percutaneous stimulation useswire electrodes that are introduced into

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Milos R. Popovic1, Thierry Keller2,Ion P.I. Pappas1, Volker Dietz2,

and Manfred Morari11Automatic Control Laboratory,

Swiss Federal Institute of Technology Zurich - ETHZ2Paraplegic Center - ParaCare,

University Hospital Balgrist, Zurich

muscles close to the motor-neurons withan epidermal needle [13, 49]. Implantedand thumb electrodes are attached to themotor neurons or the muscles close to themotor neurons [5, 13, 37, 40, 46].Implanted electrodes and a better muscu-lar selectivity can be achieved, and therisk of infection is reduced compared topercutaneous electrodes [17, 21]. In addi-tion to the stimulation selectivity, oneother advantage of an implanted FES sys-tem is that once implanted, the stimulatorrequires less time to put on and take off,compared to a surface-stimulation sys-tem. On the other hand, a surface FES sys-tem does not require surgical intervention,which some subjects consider very impor-tant. In addition, surface FES systems canbe applied at a very early stage of the reha-bilitation, during the recovery and reorga-nization period of the central andperipheral nervous systems (plasticity),allowing early benefit for the patient. FEStraining during recovery may help a sub-ject restore a function to the point thathe/she no longer needs a neuroprosthesis.Subjects K.D. and Z.S. in Table 3; andsubjects F.M., M.B., and N.S. in Table 4,who participated in our study, all experi-enced function improvement to the pointthat they do not need a neuroprosthesisany longer. Similar findings have beenpreviously reported [1, 32]. We believethat the best approach is to start with FESas soon as possible after the injury, using asurface system. In this way, subjects whocould benefit from the central and periph-eral nervous systems plasticity would beable to recover some functions lost as a re-sult of disease or injury. Other subjectswould learn from the very early rehabili-tation program to accept the neuro-prostheses as devices they need to carryout daily living activities. We believe thatsubjects should not be advised to have animplanted FES system before he/she isable to successfully use a surface FESsystem on a daily basis.

State of the ArtGrasping Neuroprostheses

Grasping neuroprostheses are FES sys-tems designed to restore or improve func-tion in tetraplegic subjects. Some widelyknown grasping neuroprostheses are theFreeHand system [23, 40, 50], Handmaster[14, 44], Bionic Glove [36], FESmate [52],and the systems developed by Vodovnik,et al. [38] and Popovic, et al. [31]. Exceptfor the FreeHand system, all other devicesuse surface electrodes. The FreeHand sys-

tem has eight implanted stimulation elec-trodes and an implanted stimulator. Thestimulation electrodes are used to generateflexion and extension of the fingers. Handclosure and hand opening are commandedby using a position sensor placed on theshoulder of the patient’s opposite arm. Thesubject can choose a shoulder position tocommand hand opening and one for handclosing. Fast shoulder motions are used toindicate that the current stimulation shouldremain constant until the next fast shouldermotion (“locking”). The shoulder positionsensor and the controller are not implanted.The FreeHand system is the first graspingneuroprosthesis approved by the U.S. Foodand Drug Administration (FDA). Thus far,the FreeHand system has been provided tomore than 130 subjects.

One of the first grasping neuro-prostheses with surface FES technologywas developed by Vodovnik, et al. [38].This system has three stimulation chan-nels (two stimulation electrodes per chan-nel), which are used to generate thegrasping function by stimulating the fin-ger flexors, the finger extensors, and thethenar muscle. Although this device was

developed more than two decades ago, itis among the few FES systems that allowthe subject to control the stimulation trainvia different sensory interfaces: an EMGsensor, a sliding resistor, and a pressuresensor. As a result, the subject can choosethe most appropriate man-machine inter-face to control (communicate with) theneuroprosthesis. The research group thatdeveloped the FreeHand system [12] alsotried to use different man-machine inter-faces, but these interfaces did not becomea standard feature of their system. The optionto choose the neuroprosthesis control inter-face allows tailoring the neuroprosthesis tothe patient, rather than forcing the patient toadjust to the system.

The Handmaster [14], also an FDA-approved grasping neuroprosthesis, hasthree stimulation channels and is used togenerate the grasping function intetraplegic and stroke subjects. Ori-ginally, this system was envisioned as anexercise and rehabilitation tool, but it wasfound to be equally effective as a perma-nent prosthetic device. The Handmaster iscontrolled with a push button that triggersthe hand opening and closing functions.

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1. Schematic diagram of a neuroprosthesis with open hardware and software archi-tecture. Signals from a variety of sensors, such as EMG (eletromyography) elec-trodes, force-sensitive resistors, and goniometers (1), are inputs to theneuroprosthesis. These signals are processed (2) and provided to the stimulator con-troller (3). The controller, by way of the stimulator output stage (4), generates astimulation sequence that is provided to the subject via the surface stimulation elec-trodes (5). By stimulating properly selected muscles or muscle groups, theneuroprosthesis can enable the patient to move an otherwise paralyzed limb or abody part.

With a sliding resistor, the subject canregulate the way in which the thumbflexes, allowing adjustment to the size andthe shape of the object being grasped. Inaddition, the subject can increase or de-crease the grasping force by using twopush buttons. One of the advantages of theHandmaster is that it is very easy to put on(donning) and to take of (doffing).

The Belgrade Grasping System(BGS), proposed by Popovic et al. [31], isa neuroprosthesis that, in addition to thegrasping function, also provides a reach-ing function. The BGS has four stimula-tion channels, three of which are used togenerate the grasping function, and thefourth channel is used to stimulate the tri-ceps brachii muscle to allow the subject toextend the elbow in order to reach objectsotherwise unreachable. The graspingfunction is controlled via a push buttonthat triggers the hand opening and closing.The reaching function is performed bymeasuring the subject’s shoulder velocitywith a goniometer and then stimulatingthe triceps brachii muscle generating asynergistic elbow motion that resemblesnormal shoulder-elbow coordination.Similarly, the Cleveland group also com-bined grasping and reaching functions us-ing their FreeHand system, but they didnot replicate an able-bodied subject’sshoulder-elbow synergy [10]. Instead,their neuroprosthesis measured the posi-tion of the arm in space and, for certainarm positions, automatically triggeredstimulation of the triceps brachii muscle.Simultaneously with the triceps brachiimuscle stimulation, the subject had to vol-untarily contract the biceps muscle to reg-ulate the arm’s position.

Finally, we would like to describe theBionic Glove [36]. This neuroprosthesiswas designed to enhance the tenodesisgrasp in subjects who have active controlof wrist flexion and extension. The systemuses a position transducer mounted on the

subject’s wrist to detect flexion or exten-sion. When the subject flexes the wrist,the finger extensors are stimulated, gener-ating the hand-opening function. Whenthe subject extends the wrist, the fingerflexors and the thenar muscles are stimu-lated, causing hand closure (tenodesisgrasp). The Bionic Glove was success-fully tested by more than 40 SCI subjectsworldwide. Saxena, et al. [39], proposed agrasping system similar to the BionicGlove, which, instead of a position trans-ducer, uses the EMG signal of the wristextensor muscle to trigger the onset of thestimulation sequence.

Walking NeuroprosthesesThe first walking neuroprosthesis was

proposed in 1961 by Liberson and his col-leagues [20]. This system was developedto compensate for the “drop foot” prob-lem in hemiplegic subjects (a subject with“drop foot” problem often has control ofone leg, but poor or no control over theother leg. The subject cannot flex or ex-tend the ankle of the disabled leg and alsohas reduced control over the hip and theknee joints of the disabled leg). By stimu-lating the peroneal nerve, the prosthesiselicits a flexion reflex that generates si-multaneous hip, knee, and ankle flexion,allowing the subject to take a step with thedisabled leg. Since 1961, a number ofwalking neuroprostheses have been de-signed and tested with various subjects.The widely known walking neuro-prostheses are Fepa [47]; MikroFES [22];Parastep [8, 9]; LARSI [51]; FESmate[52]; HAS [30]; WalkAid [48]; RGO [41,42]; Praxis24 [7]; Odstock 2 [45]; the sys-tem proposed by Kralj, et al. [19]; and theimplanted FES system proposed byKobetic, et al. [17, 18]. These devices canbe divided into the systems that are de-signed mainly to compensate for the“drop foot” problem, such as Fepa,MikroFES, WalkAid and Odstock 2, andthe systems that facilitate walking in sub-jects who have both legs paralyzed, suchas the Parastep, RGO, HAS, and Praxis24.

The Fepa, MikroFES, WalkAid, andOdstock 2 systems use surface stimula-tion (one generation of the Fepa systemhad an implanted electrode, but this ideawas abandoned in favor of surface stimu-lation [47]) with one or at most two stimu-lat ion channels . The st imulat ionsequences are triggered with a push but-ton, foot switch, or a pendulum resistor.These FES devices are most frequently

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2. The FES system with surface-stimulation electrodes causes a muscle contractionby electrically stimulating the motor-neurons that are attached to the muscle. Theelectrical stimulation generates action potentials in the motor-neurons, which prop-agate along the motor-neurons toward the muscle. When the action potentials reachthe muscle, they cause the muscle to contract.

One of the main

features of both

stimulators is that

they can reliably

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activity and use this

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used for short-term therapeutic treatmentin the clinical environment, althoughsome subjects use them as permanentorthotic devices [2]. All of these systemsare small, fairly reliable, and simple touse. Some of them, such as the Odstock 2and the MikroFES, have been fitted tomore than 500 subjects. Thus far, only theWalkAid has been FDA approved.

The walking FES systems such as theParastep, HAS, RGO, Praxis24, and thesystems proposed by Kobetic, et al.[17,18] and Kralj, et al. [19], were de-signed for subjects who have both legsparalyzed. Parastep and the system pro-posed by Kralj, et al., are walking FESsystems with six stimulation channels.Two channels are used to stimulate theperoneal nerves bilaterally, two channelsto the quadriceps muscles bilaterally, andtwo channels to stimulate the paraspinalsor the gluteus maximus/minimus mus-cles bilaterally [9]. The last two channelsare used with subjects who cannot volun-tarily extend the lower back. Thequadriceps muscles are stimulated duringstanding up and during the stance to pro-vide body support. The peroneal nervestimulation is used to generate simulta-neous hip, knee, and ankle flexion, allow-ing the subject to take a step. Thestimulation sequences are triggered with apush button, which is attached to thewalker or crutches. The Parastep systemwas successfully applied to more than 400subjects and is the first FDA-approved

FES system. The system proposed byKralj, et al., was also successfully usedwith more than 50 subjects.

The FESmate and the system proposedby Kobetic, et al., are implanted FES sys-tems with 24 and 32 electrodes, respec-tively. They are used to restore walking inparaplegic subjects. Besides walking, thePraxis24 system also provides blad-der-voiding function, which is not dis-cussed in this article. Both of thesesystems were designed to perform thewalking and standing functions, similar tothe functions generated by surface-stimu-lation systems mentioned above. The onlydifference is that these two systems areimplanted and therefore should providebetter stimulation selectivity and a morenatural walking pattern; both are still inthe development phase.

The HAS and the RGO walkingneuroprostheses are devices that, in addi-tion to using surface FES, also apply activeand passive braces, respectively. Thebraces were introduced to reduce the highmetabolic rate observed in subjects duringFES walking [9] and to provide additionalstability during standing and walking [29].Thus far, the RGO system has been suc-cessfully applied to more than 40 subjects.

ETHZ-ParaCareFunctional Electrical StimulatorsPrior to developing a neuroprosthesis,

one has to select a stimulator to be used asthe hardware platform. In 1995, when our

project was initiated, a portable andprogrammable surface stimulator that sat-isfies the requirements for EMG con-trolled neuroprostheses was not availableon the market. Therefore, our team, whichintended to develop an EMG-controlledgrasping neuroprosthesis, had to designits own stimulator. As a result, in 1998,two different stimulators were developedby our group: a system for rapidprototyping of the neuroprosthesis[15,35], and a portable FES system thatsubjects can use at home in daily living ac-tivities [16, 33, 35]. The rapid prototypingsystem (RPS) consists of a data acquisi-tion system (LabPC+ board from NationalInstruments), a constant current electricalstimulator, a custom made software writ-ten using LabView 5.1 from National In-

January/February 2001 IEEE ENGINEERING IN MEDICINE AND BIOLOGY 85

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3. Neuroprosthesis rapid prototypingsystem (RPS).

Table 1. Data Sheets for the Rapid Prototyping and Portable FES Systems

Parameters Rapid Prototyping System Portable System

Stimulation Channels 4 channels:• current regulated• pulse frequency 20-50 Hz• pulse amplitude 0-100 mA• pulse width 0-500 µs

4 channels:• current regulated• pulse frequency 20-50Hz• pulse amplitude 0-100 mA• pulse width 0-500 µs

Analog Input Channels 8 channels:• for EMG, FSR, goniometer, inclinometerand other sensors• 8 channels multiplexed with maximumsampling frequency of 64 kHz

6 channels:• for EMG, FSR, goniometer, inclinometerand other sensors

Controller LabView programmable:• for custom made applications• control frequency 25 Hz

Assembler programmable:• for custom made applications• control frequency 200 Hz

Work Station 200 MHz Pentium PC

Power Source 110/220 VAC and 4 Ah/8.4 VDC NC battery(12 h of stimulation)

2.8 Ah/7.2 VDC Li-ion battery (8 h of stimu-lation)

Dimensions Standard PC 200 mm × 120 mm × 56 mm

struments, and a personal computer (PC),which combines these elements into an in-tegrated system (Fig. 3 and Table 1). Theelectrical stimulator consists of four mul-tiplexed stimulation channels (separatedby an Hewlett Packard 2630 optocoupler)that are controlled with a Motorola HC11microcontroller. The stimulator commu-nicates with the PC via galvanically sepa-rated digital ports and is battery poweredfor safety. The RPS was developed as atool to rapidly design custom-madeneuroprostheses. With this system, in amatter of minutes, one can program theneuroprosthesis stimulation sequencesand the stimulation parameters such aspulse amplitude, pulse width, and pulsefrequency. In addition, one can choose asensor the subject can use to interact(communicate) with the neuroprosthesis,such as EMG sensor, push button, acceler-ometer, etc. The RPS is used to assess whichfunctions can be improved or restored in thepatient, to identify which muscles and mus-

cle groups can be stimulated to restore thedesired function, to test different stimula-tion electrode configurations, to identify thebest stimulation sequence and control strat-egy for the proposed neuroprosthesis, and totest the prosthesis.

Once the neuroprosthesis is developedusing RPS, the software from this systemis transferred to the portable stimulator toprovide a neuroprosthesis the subject cantake home. The portable system, shown inFig. 4 and discussed in Table 1, has identi-cal stimulation characteristics to the RPS.The only difference is that once the pros-thesis program is downloaded into theportable stimulator, it cannot be changedas readily as is possible with the RPS. Onthe other hand, the portable system is sig-nificantly smaller and can be used in dailyliving activities, unlike the RPS (Table 1).

As mentioned above, one of the mainfeatures of both stimulators is that theycan reliably measure muscle EMG activ-ity and use this signal to trigger and con-trol the stimulation sequences. Theconcept of using the EMG signal for trig-gering purposes is not new, and some de-vices that perform this task were patented20 years ago (Hodgson - UK patentGB2098489). Today, advanced arm pros-theses for amputees, such as the BostonElbow, Utah Arm, and the Otto Bock arm,apply either a proportional EMG controlstrategy or use the EMG activity of a mus-cle to generate ON and OFF commandsfor the prosthesis. In the case of the pro-portional EMG control, the prosthesisjoint moves only when the muscle, whichis instrumented with the EMG sensor, iscontracted. The second control strategyuses the onset of the EMG activity to trig-

ger the joint motion, and the second onsetof EMG activity of the same muscle to ter-minate the motion.

There is a fundamental difference inmeasuring EMG activity for a prosthesisfor amputees and for a neuroprosthesis.Due to the electrical stimulation, the sub-ject with the neuroprosthesis has an in-duced voltage noise in his/her body, withmagnitude often significantly larger thanthat of the EMG signal. The EMG signalof a healthy muscle is usually in the rangeof 10 to 1000 µV (in SCI subjects, theEMG amplitude is often lower) while thestimulation artifacts can easily reach 5 to10 V. (Our most difficult case thus far, butalso successful, was stimulation of thewrist extensor muscle, with 80 V, distancebetween the stimulation electrodes of 12cm, the EMG electrodes placed on thesame wrist extensor muscle between thestimulation electrodes, and the distancebetween EMG electrodes at 2 cm.)

In order to measure reliably voluntaryEMG activity of a muscle that is stimu-lated, we had to develop a real-time signalprocessing routine that discriminates vol-untary EMG activity from stimulation ar-tifacts. We have developed a softwareroutine that eliminates the stimulation ar-tifacts from the measured EMG signals byblanking the EMG signals for 2 ms fol-lowing the stimulation pulse. The EMGsignal is measured using a “BiofeedbackSensor” (Model 2M4456, Compex SA,Ecublens, Switzerland) (gain 1400;high-pass cut-off frequency 300 Hz;low-pass cut-off frequency 4 kHz) [4].The measured EMG signal is further recti-fied and filtered using a 1.5 Hz lowpassfilter. Our grasping neuroprosthesis can

86 IEEE ENGINEERING IN MEDICINE AND BIOLOGY January/February 2001

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Electrodes EMG Electrodes

4. Portable FES system.

Table 2. Examples of the SCI Disability Classification

C5 Complete SCI Subject

Injury: intact sensory and motor neurons coming out of the spine above cervical vertebra No.5; sensory and motor neu-rons coming out of the spine below cervical vertebra No.5 are affected by the injury

Symptoms: subject is unable to perform finger flexion and extension, wrist flexion and extension, shoulder extension,supination, pronation and elbow extension; as a result the subject cannot grasp objects using a single hand andcan grasp only few objects using both hands

C7 Incomplete SCI Subject

Injury: intact sensory and motor neurons coming out of the spine above cervical vertebra No.7; sensory and motor neu-rons coming out of the spine below cervical vertebra No.7 are partially affected by the injury

Symptoms: the worst case scenario: the subject would have a disability similar to C7 complete SCI subject: unable to per-form finger abduction and adduction, thumb flexion and adduction. The subject could have difficulties performingfinger extension, wrist extension and pronation; as a result the subject would have rudimentary grasp but wouldnot be able to perform fine object manipulationthe best case scenario: the subject would have few disabilities and would be able to perform almost all functionsas the able-bodied subject

thus be controlled using EMG controlstrategies, as discussed below.

Since January 1999, our team has beencollaborating with the company CompexSA, located in Ecublens, Switzerland,which is one of the leading manufacturersof electrical stimulators for medical andsports applications worldwide. The objec-tive of this collaboration is to develop anew portable electrical stimulator withsurface stimulation electrodes. Thestimulator will be used for variousneuroprosthetic, rehabilitation, and scien-tific applications, and will be pro-grammed with a Windows-basedgraphical user interface. The dimensionsof the new stimulator are 148 × 85 × 30mm, and weight 420 g.

Grasping NeuroprosthesisThe grasping neuroprosthesis, devel-

oped using the above rapid prototyping andportable FES systems (see above), was de-signed to improve or restore the graspingfunction in complete high lesioned SCIsubjects [16, 33, 35]. In this article, theterm “high lesioned SCI subjects” is usedto describe C5 SCI subjects [25] (see Table2). If the part of the spinal cord below thelesion is completely disconnected from theupper part, this lesion is called complete.All other lesions are called incomplete. Al-though the grasping neuroprosthesis wasdesigned for C5 complete SCI subjects, itcan also be used by subjects with lowerlevel lesions, such as C6 and C7 completeSCI subjects; or C5 and lower lesion in-complete SCI subjects.

The ETHZ-ParaCare graspingneuroprosthesis is designed to generateeither a palmar or a lateral grasp, but notboth. The palmar grasp is used to graspbigger and heavier objects such as cans,bottles, and an electrical razor (Fig. 5).The lateral grasp is used to grasp smallerand thinner objects such as keys, papersheets, and floppy disks. A pinch grasp,which is used to hold a pen, is obtained us-ing the lateral grasp strategy (Fig. 6). Thelateral grasp is generated by first flexingthe fingers, followed by the thumbflexion. The palmar grasp is generated bysimultaneous flexion of both the thumband the fingers. Finger flexion is per-formed by stimulating the flexordigitorum superficialis and the flexordigitorum profundus. Thumb flexion isperformed by stimulating the thenar mus-cle of the thumb or the median nerve. Fin-ger extension is performed by stimulatingthe extensor digitorum.

The proposed grasping neuro-prosthesis consists of the portablestimulator (see above), three pairs of theCompex self-adhesive surface stimula-tion electrodes (5052MID [6]), a wrist re-tainer/splint, and the Compex EMGsensor “Biofeedback Sensor” (2M4456[4]). One pair of surface stimulation elec-trodes is placed on the subject’s skinabove the flexor digitorum superficialisand the flexor digitorum profundus mus-cles to generate finger flexion. The secondpair of electrodes is placed on the sub-ject’s skin, above the median nerve, togenerate thumb flexion. The third pair of

electrodes is placed on the subject’s skin,above the extensor digitorum muscle, togenerate finger extension. The currentpulse amplitudes and widths generated bythe grasping neuroprosthesis are in therange of 16 to 40 mA and 0 to 250 (s, re-spectively. The wrist splint is used to sta-bilize the wrist and to provide supportduring grasping. The EMG electrodes areused to monitor the subject’s voluntarymuscle activity.

The proposed grasping neuro-prosthesis offers four different controlstrategies:

January/February 2001 IEEE ENGINEERING IN MEDICINE AND BIOLOGY 87

Push Button

Neuroprosthesis

5. Subject S.O. performs a palmar grasp with the graspingneuroprosthesis.

Neuroprosthesis

SlidingResistor

6. Subject D.K. performs a pinch grasp with the graspingneuroprosthesis.

The ETHZ-ParaCare

grasping

neuroprosthesis is

designed to generate

either a palmar or a

lateral grasp, but

not both.

� Proportional EMG control can beused by SCI subjects who can volun-tarily, selectively, and gradually con-tract the anterior and posteriorbranches of their deltoid muscle.Two EMG sensors are placed on theanterior and posterior branches of thedeltoid muscle on the arm opposite tothe one instrumented with theneuroprosthesis. By contracting theanterior side of the deltoid muscle,the SCI subject commands finger ex-tension (hand opening), and by con-tracting the posterior side of thedeltoid muscle, the subject com-mands fingers and thumb flexion(hand closing and grasping). Thesubject has to keep the anteriorbranch of the deltoid muscle continu-ously contracted during the timehe/she wants the hand to be open.Similarly, the subject has to keep theposterior branch of the deltoid mus-cle continuously contracted duringthe time he/she wants the hand to beclosed. The amplitude of the differ-ence between the anterior and theposterior EMG signals is used to con-trol the force exerted by the subject’shand during grasp.

� Discrete EMG control can be usedby subjects who can voluntarily con-tract a muscle but cannot contract itgradually or cannot maintain themuscle contraction for prolongedperiods of time. Such subjects gen-erate on and off commands similarto Morse code by contracting and re-laxing a muscle. These commandsare interpreted by the prosthesis,which in turn generates an appropri-ate stimulation sequence. For exam-ple, our C6 complete SCI subjectK.D. (see Table 3), who has volun-tary control of the wrist extensormuscle on the stimulated arm, usedtwo rapid contractions of this wrist

extensor muscle to trigger handclosing. The neuroprosthesis re-sponded to this command by open-ing the hand, keeping it open for twoseconds, and closing it automati-cally. The hand remained closed un-til the subject contracted the wristextension muscle once again andkept it contracted for at least 2 swithout a break. This is the handopening command to which theprosthesis reacted by opening thehand, maintaining it open for twoseconds, and then ceasing stimula-tion, which caused the hand to relax.When the subject needed his hand tobe open longer than 2 s prior tograsping, he had to issue thehand-closing command every sec-ond for as long as he needed the handto be open. Once the subject ceasedto generate the hand-closing com-mand, the hand closed automaticallyafter 2 s and remained closed untilthe hand-opening command was is-sued again.

� Push button control can be used byan SCI subject who cannot or doesnot want to use the EMG controlstrategies. By pressing a push button,the subject initiates the hand-closingfunction, discussed above under thediscrete EMG control strategy. Thehand remains closed until the subjectpresses the push button once again.By pressing the push button for thesecond time, the hand opening func-tion described under the discreteEMG control strategy (Fig. 5) is ini-tiated. In order to keep the hand openlonger than 2 s prior to grasping, thesubject has to press the push buttonevery second after he/she has al-ready issued the hand-closing com-mand. The subject has to press thebutton repetitively for as long as thehand is to remain open. The push

button control is less attractive thanthe discrete EMG control strategy,since the subject needs more time topress the push button than to gener-ate an EMG signal.

� Sliding potentiometer control issimilar to the proportional EMG con-trol strategy. By sliding the potenti-ometer in one direction, the subjectgenerates finger extension (handopening), and by sliding it in the op-posite direction, finger and thumbflexion are generated (hand closing).The resistance of the potentiometer(i.e., excursion of the slider) is usedto control the grasping force gener-ated by the prosthesis (Fig. 6).

88 IEEE ENGINEERING IN MEDICINE AND BIOLOGY January/February 2001

Table 3. Experimental Results with the Grasping Neuroprosthesis

Subject Sex Born Disability Arm After Injury Control Strategy Outcome

M.T. M 1962 C5 complete Right 8 months Proport. EMG Accepted

A.M. M 1979 C4 incomplete Right 3 months Slding resistor Accepted

C.K. F 1959 C5 complete Right 5 years Push button Accepted

S.O. M 1966 C4-C5 complete Right 2 months Push button Accepted

Z.S. F 1928 C6 incomplete Right 2 months Push button Rejected

K.D. M 1977 C6 incomplete Left 7 months Dscrete EMG Rejected

H.R. M 1983 C5-C6 complete Right 4 months Push button Rejected

B.R. M 1935 C3 incomplete Right 2 months Push button Rejected

The walking

neuroprosthesis can

enable a subject with

the “drop foot”

problem to walk or to

improve walking by

generating a gait

sequence in the

impaired leg.

Results and Patient AcceptanceThe ETHZ-ParaCare grasping

neuroprosthesis was tested with eight SCIsubjects (see Table 3), four of whom ac-cepted the prosthesis. The term accepted isused to indicate that the neuroprosthesiswas able to generate the desired functionand that the subject adopted the prosthesisand used it to perform daily living func-tions. When it is stated that the system wasrejected, this means that the FES systemcould not generate the desired function dueto physiological reasons, or the subject re-fused to use the prosthesis despite the factthat it performed successfully, or the sub-ject recovered to the point that he/she couldgenerate the desired function without usingthe neuroprosthesis. The graspingneuroprosthesis was rejected by four sub-jects for the following reasons. SubjectH.R. was emotionally unstable and refusedto collaborate with our group. Subject B.R.already had a good tenodesis grasp and didnot benefit much from the neuroprosthesis.As for subjects K.D. and Z.S., they im-proved their grasp during FES training tothe point that they did not need theneuroprosthesis any longer. Our tests haveshown that the best candidates for the pro-posed grasping neuroprosthesis are sub-jects with C4-C5 or C5 complete SCIlesions, or equivalent. Subjects with lowerlesion levels, such as C6 complete SCI, orincomplete SCI, could also benefit fromthe grasping neuroprosthesis, but the suc-cess rate is much lower since these subjectsoften have a partially functional grasp.

Some of the tasks the subjects (Table3) were able to perform with the graspingneuroprosthesis were: (1) to grasp, to lift,and to place a variety of objects (up to 3

kg); (2) to lift a telephone receiver, to diala number, to maintain a conversation, andto hang up; (3) to pour a liquid from a bot-tle into a glass and to drink it from theglass; (4) to grasp a fork or a spoon and eatwith it; (5) to grasp an apple and eat it; (6)to grasp a pencil and write with it; (7) tobrush the teeth; and (8) to shave using anelectrical or a manual razor. Currently,subjects A.M., S.O. and C.K. use thegrasping neuroprosthesis in daily livingactivities (A.M. - two years, S.O. - sevenmonths, and C.K. - six months). SubjectM.T. was released from our hospital inearly 1997, before we were able to pro-vide him with the portable FES system.

Walking NeuroprosthesesThe ETHZ-ParaCare walking

neuroprosthesis was also developed usingthe above rapid prototyping and portableFES systems (see above). This neuro-prosthesis was designed to improve or re-store the walking function in incompleteSCI and stroke subjects who had good con-trol of one leg but have poor or no controlover the other leg [33, 35] (see Fig. 7).These subjects are typically wheelchair us-ers or slow walkers who cannot flex or ex-tend the ankle joint (the “drop foot”problem) and have poor control over thehip and knee joints. However, to benefitfrom the walking neuroprosthesis, they

January/February 2001 IEEE ENGINEERING IN MEDICINE AND BIOLOGY 89

Push Button

Stimulator Electrodes

RectusFemoris M.

Peroneus N.

TibialisAnterior M.

(b)(a)

7. (a) Walking neuroprosthesis for hemiplegic subjects and subjects with unilateralparaplegia. (b) Placement of the stimulation electrodes for the subject in (a).

Table 4. Experimental Results with the Walking Neuroprosthesis

Subject Sex Born Disability Leg After Injury Control Strategy Outcome

D.B. M 1972 C6-C7 incomplete Right 2 years Push button Accepted

M.A. M 1940 C4 incompleteequivalent

Left Push button Accepted

R.C. M 1921 T4 incompleteequivalent

Left 10 years Push button Rejected

D.G. M 1975 T8 incomplete Right 9 months Push button Rejected

A.H. M 1965 Lock in syndrome Right 9 months Push button Rejected

F.M. M 1955 C7 incompleteequivalent

Right 4 months Foot switch Rejected

M.B. M 1956 C5-C6 incompleteequivalent

Right 1 month Foot switch Rejected

N.S. F 1970 Tumor on the spine Left 1 month Foot switch Rejected

must have a good sense of balance andmust be able to stand safely using a supportstructure such as walker or crutches.

The walking neuroprosthesis can en-able a subject with the “drop foot” prob-lem to walk or to improve walking bygenerating a gait sequence in the impairedleg. In most cases, a movement that is sim-ilar to the swing phase of the natural walk-ing cycle can be evoked by stimulating theperoneal nerve, which elicits a flexion re-flex. This reflex activates a simultaneouscontraction of the hip, knee, and ankleflexor muscles that lift the leg off theground. The subject’s forward movementof the upper body, combined with theflexion reflex, generates the desired step-ping motion. Besides peroneal nerve stim-ulation, the following muscles and musclegroups can be stimulated to provide addi-tional support or smoother movement ofthe impaired leg during walking: tibialisanterior, gastrocnemius, rectus femoris,

biceps femoris, vastus lateralis, vastusmedialis, and semitendinosus.

The walking neuroprosthesis consistsof the stimulator unit (see above), up tofour pairs of Compex self-adhesive sur-face-stimulation electrodes [6], and eithera push button, a foot switch, or a gaitphase recognition sensor [34] used to trig-ger the stimulation sequences.

The proposed walking neuroprosthesisoffers three different triggering strategiesto control the prosthesis:

� Push button control strategy wasused by the majority of the subjectslisted in Table 4. By pressing a pushbutton, which is attached to a walker[Fig. 7(a)], the subject triggers theonset of the stimulation sequencesthat cause the impaired leg to move.After a few hours of walking with theprosthesis, all subjects learned howto use this control strategy withoutconsciously thinking about it.

� Foot switch control strategy is verysimilar to the push button controlstrategy. Instead of the subject press-ing the push button, the foot switch,which is located in the shoe sole un-der the heel, automatically triggersthe stimulation sequence each timethe subject lifts the heel. This controlstrategy is also intuitive and easy toimplement, similar to the push buttoncontrol strategy. The disadvantage ofthis strategy is that the foot switch of-ten generates erroneous triggeringsignals, due to weight shifting or footsliding during standing [26, 34].

� Gait phase recognition sensor con-sists of three force-sensitive resistors(FSRs), a gyroscope, and a rule-basedobserver [34] (Fig. 8). The gait phaserecognition sensor identifies four gaitphases during walking: heel-off,swing phase, heel-strike, and midstance, with a reliability greater than99% [27,28,34]. The sensor is inte-grated into the shoe sole of the dis-abled leg and initiates stimulationeach time the disabled leg is in theheel-off gait phase. In order to bringthe disabled leg into the heel-offphase, the subject has to take a stepforward with the healthy leg. Once thehealthy leg is in mid-stance gait phaseand the weight is shifted to it, the dis-abled leg goes into the heel-off gaitphase. Unlike the foot switch [26] andother available gait phase sensors[24], our sensor is capable of distin-guishing true walking sequences fromweight shifting during standing, and itdoes not give false gait annunciation

when the instrumented foot is slidingduring standing. This control strategyis still being tested, and the test resultswill be made available at the end ofthis year.

Results and Patient AcceptanceThe ETHZ-ParaCare walking

neuroprosthesis was tested with the eightsubjects listed in Table 4, two of whomsuccessfully used the prosthesis (i.e., ac-cepted the system). The prosthesis was re-jected by six subjects for the followingreasons: For subjects R.C. and D.G., wewere unable to successfully stimulate theperoneal nerve. Subject A.H. had bothlegs disabled due to the lock-in syndrome,and our walking neuroprosthesis, which isdesigned to compensate for the drop footproblem, could not help restore walking.Subjects F.M., M.B., and N.S., after ex-tensive physiotherapy and training withthe walking neuroprosthesis, were able torestore normal walking function and didnot need the neuroprosthesis any longer.The tests performed by our group haveshown that subjects who can benefit long-term from the walking neuroprosthesisare those for whom gait cannot be restoredby other means and who have a good re-sponse to peroneal nerve stimulation.

Some of the tasks the subjects (Table4) were able to perform with the walkingneuroprosthesis are: (1) to walk safely us-ing a walker; (2) to walk up to 500 m with-out halting, at an average speed of 1.5km/h; and (3) to walk uphill and downhill.Currently, subject M.A. has used thewalking neuroprosthesis for more that 12months in daily living activities. SubjectD.B. was released from our hospital inearly 1997, before we were able to pro-vide him with the portable FES system.

ConclusionsThe walking and grasping neuro-

prostheses developed by the Swiss Fed-eral Institute of Technology Zurich andthe University Hospital Balgrist repre-sent advanced surface stimulation FESsystems that can be used by SCI andstroke subjects to improve or restore thewalking and grasping functions. Thegrasping neuroprosthesis is used to re-store palmar or lateral grasp in highlesioned SCI subjects. The graspingneuroprosthesis can be tailored to fit thesubjects’ needs, and it can be controlledwith one of the following strategies: pro-portional EMG, discrete EMG, push but-ton, and sliding resistor. Based on our

90 IEEE ENGINEERING IN MEDICINE AND BIOLOGY January/February 2001

Observer

GyroscopeFSRs

8. Gait phase identification sensor.

Rapid prototyping

FES is an ideal

tool to design a

neuroprosthesis and

to immediately assess

its effectiveness.

experience, the best candidates for thegrasping neuroprosthesis are subjectswith C4-C5 or C5 complete spinal cordlesion, or equivalent. Subjects withlower level lesions, such as complete C6or lower level SCI, or incomplete SCI,could also benefit from the graspingneuroprosthesis, but the success rate ismuch less, since these subjects oftenhave a partially functional grasp.

The walking neuroprosthesis is usedmost frequently to compensate for the“drop foot” problem in incomplete SCIand stroke subjects. According to the sub-jects’ needs and preferences, theneuroprosthesis can be controlled eitherby push button, foot switch, or the phasedetection sensor. Our experiments haveshown that only subjects who are moti-vated and whose gait can be significantlyimproved with electrical stimulation canbenefit from the neuroprosthesis.

After five years of experimenting withthe walking and grasping neuroprostheses,we conclude that these systems can signifi-cantly improve the quality of life for strokeand SCI subjects. These devices can beused for either permanent prosthetic or re-habilitation purposes. For both devices, wehave observed that the sooner the subjectstarts training, the faster and the better re-covery tends to be. Also, the subject whostarts training with the FES system earlyafter the injury can potentially benefit fromthe positive effect that FES training has onthe plasticity of the central nervous system.Some subjects can recover enough due toFES training that they do not need theneuroprosthesis any longer. Since the sur-face FES systems can be applied to sub-jects almost immediately after the injury,they are ideal for such rehabilitation appli-cations. If the subject cannot recover func-tion through FES training, which happensmore frequently, then the system is used asa neuroprosthesis. For these subjects, itwas also observed that the sooner use of theneuroprosthesis starts (and the more oftenit is used), the higher the acceptance rate is.An early start with FES training also pre-vents loss of muscle volume, shortening oftendons, and joint contractures. The sub-jects are generally more motivated to learn“FES skills” in the early stage after the in-jury, rather than later, when they have be-come accustomed to daily living routines.We believe that a subject prior to decidingto have an implanted FES system shouldfirst learn to use a surface FES system indaily living activities. Subjects who aremotivated and in good psychological con-

dition are more likely to accept and bene-fit from the neuroprosthesis.

Most of the existing FES systems weredeveloped with a specific application inmind. As a result, their hardware-softwarearchitectures have limited flexibility, and asubject often has to adjust to the system in-stead of the system being adjusted to thesubject. We believe that FES systems mustbe flexible and suitable for a wide range ofapplications. The FES system should beable to adjust to different subjects withtheir specific needs as well as be controlledby various man-machine interfaces.

The man-machine interfaces and thestimulators must be robust, reliable, easyto implement, and appealing to the sub-jects. The rapid prototyping and the porta-ble FES systems discussed in this articlewere designed to provide greater hard-ware and software flexibility, and to pro-vide the subjects with the option to usedifferent man-machine interfaces to con-trol the neuroprosthesis. Although thesystems developed could be further sim-plified and improved, they clearly showedthat rapid prototyping FES is an ideal toolto design a neuroprosthesis and to imme-diately assess its effectiveness. In addi-tion, the flexibility of the systems allowsthe use of any sensor the subject finds con-venient to control the neuroprosthesis.Capability to download the neuroprosthe-sis developed with the rapid prototypingsystem into the portable FES system fur-ther minimizes prosthesis developmenttime and ensures that every subject has asystem specifically designed to meethis/her needs.

Our current efforts are aimed at de-signing a new generation of the portableelectrical stimulator with surface-stimu-lation electrodes. This project is beingcarried out in collaboration with Swisscompany Compex SA from Ecublens.This stimulator will be suitable for vari-ous neuroprosthetic, rehabilitation andscientific applications.

AcknowledgmentsThe project presented in this article is

supported by the Swiss National ScienceFoundation (project No. 5002-057811.1),the Kommission für Technologie und In-novat ion des Bundesamtes fürKonjunkturfragen (project No. 3960.1),and company Compex SA, all located inSwitzerland. The authors also wish tothank Professor Dejan B. Popovic fromthe Faculty of Technical Sciences at theUniversity of Novi Sad, Yugoslavia, and

SMI at Aalborg University, Denmark, forvaluable advice that helped us improvethe quality of the manuscript. Additionalinformation regarding this and our otherprojects can be found at ht tp: / /www.aut.ee.ethz.ch/~fes/.

Milos R. Popovic received the Ph.D. de-gree in mechanical engineering from theUniversity of Toronto, Canada, in 1996,and the Dipl. Electrical Engineer degreefrom the University of Belgrade, Yugo-slavia in 1990. Since 1997 he has beenleading the Functional Electrical Stimula-tion Group at the Swiss Federal Instituteof Technology (ETH) and the ParaplegicCenter of the University Hospital Balgrist(ParaCare), both in Zurich, Switzerland.From 1996 until 1997, he worked forHoneywell Aerospace in Toronto. Hisfields of expertise are functional electricalstimulation, modeling, and control of lin-ear and nonlinear dynamic systems, ro-botics, powers systems, signal processingand safety analysis. In 1997, togetherwith Mr. Thierry Keller, he received theSwiss National Science Foundation Tech-nology Transfer Award - 1st place.

Thierry Keller received his Dipl. Ing. de-gree in electrical engineering (M.Sc.E.E.)from the Swiss Federal Institute of Tech-nology Zurich (ETHZ), Switzerland in1995. He is currently working as a re-search engineer at the Paraplegic Centerof University Hospital Balgrist, Zurich,and at ETHZ. There he developed walk-ing and grasping neuroprostheses for spi-nal cord injured subjects. His research

January/February 2001 IEEE ENGINEERING IN MEDICINE AND BIOLOGY 91

A subject prior to

deciding to have an

implanted FES system

should first learn to

use a surface FES

system in daily living

activities.

interests are in the development of controlstrategies for neuroprostheses using EMGrecordings. In 1997, together with Dr.Milos R. Popovic, he has received theSwiss National Science Foundation Tech-nology Transfer Award - 1st place.

Ion Pappas grew up in Athens, Greece. Hereceived his Dipl. Ing. degree from the de-partment of “Microtechnique” at the SwissFederal Institute of Technology inLausanne (EPFL) in 1994. He completedhis doctoral thesis on the field of robotic vi-sual control and micromanipulation at theSwiss Federal Institute of Technology inZurich (ETHZ) in 1998. Since then, he hasbecome a member of the Automatic Con-trol Laboratory at the ETHZ. His researchinterests include functional electrical stim-ulation, robotics, modeling, and control.

Volker Dietz was appointed head of the Para-plegic Center and the chair of Paraplegiologyat the Universtiy of Zurich, Switzerland, in1992. Before that, he was assistant professorat the Department of Neurology andNeurophysiology at the University ofFreiburg, Germany. He obtained the MD de-gree from the University of Tubingen in1971 and has specialized Neurology since1977. Prof. Dietz’s main research activitiesare human neuronal control of functionalmovements; pathophysiological basis ofmovement disorders; development aspectsof stance and gait; electrophysiological as-sessment of motor defects; spinal cord (dys-)function; prognostic clinical andelectrophysiological outcome parameters ofspinal cord injury; and establishment of newtraining programs in para- and tetraplegicsubjects. He is a consultant to the GermanResearch Council, VW Foundation, SwissNational Science Foundation, Danish Na-tional Research Foundation, British MRCCouncil, Research Council of theNASA/ESA, European Commission, and amember of the Scientific Committee “Inter-national Spinal Research Trust.” In recogni-tion of his research, in 1998 he received the“Preis der Hoffnung” of the German Re-search Council.

Manfred Morari was appointed head of theAutomatic Control Laboratory at the SwissFederal Institute of Technology (ETH) inZurich in 1994. Before that, he was theMcCollum-Corcoran Professor and Exec-utive Officer for Control and DynamicalSystems at the California Institute of Tech-nology. He obtained the diploma fromETH Zurich and the Ph.D. from the Uni-versity of Minnesota. His interests are inhybrid systems and the control of biomedi-cal systems. In recognition of his research,

he has received numerous awards, amongthem the Eckman Award of the AACC, theColburn Award, and the ProfessionalProgress Award of the AIChE, and waselected to the National Academy of Engi-neering (U.S.). Professor Morari has heldappointments with Exxon R&E and ICIand has consulted internationally for anumber of major corporations.

Address for Correspondence: M.R.Popovic, Automatic Control Laboratory,Swiss Federal Institute of Technology,Zurich - ETHZ, ETH Zentrum/ETL K28,CH-8092 Zurich, Switzerland. E-mail:popovic@aut .ee .e thz.ch. Web:http://www.aut.ee.ethz.ch/~fes/

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