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WAn innovative, five-fingered hand prosthesis lets amputeestype and even play musical instruments.

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Technologylends a hand

Gary Legg, Contributing Editor

But graduate student RickiAbboudi and Craelius, director of theOrthotics and Prosthetics Laboratoryin Rutgers’ Department of BiomedicalEngineering, thought finger tap-ping—even though a big step forwardin hand prostheses—wasn’t enough.Instead, they set as their goal an arti-ficial hand that could perform lightoffice work, including typing andusing a mouse. Now, the latest versionof their hand prosthesis, withimprovements from other graduatestudents, is doing that. One amputeehas even used it to play the piano.

The Rutgers group named their artificial handDextra, for the dexterity it provides. All five ofDextra’s fingers function independently, and theycan both tap and grasp objects. What’s more, thefingers are controlled just as anyone’s fingersare—by muscles and tendons in the forearm (in

hen Rutgers professor William Craeliusand one of his biomedical engineeringgraduate students decided to develop animproved hand prosthesis, they first tooknote of what hand amputees said theyneeded most—the ability to tap a finger ona computer keyboard. Existing artificialhands didn’t allow that. Their one or twofingers and a thumb could only grasp andhold things.

The Dextra artificial hand, made with pneumatic sensors todetect forearm muscle and tendon movements, uses ahand amputee’s own natural impulse to move five fingersindependently. In this prototype, an external controller(normally strapped to the upper arm) converts the muscleand tendon movements to finger movements.

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this case, the residual forearm, or what’sleft after hand amputation). An amputeesimply decides to move his or her “phan-tom” fingers—the fingers that are nolonger there—and Dextra’s artificial fin-gers move in their place.

An array of innovative and cleverlyapplied technology makes all of this hap-pen. Special handmade sensors detectsmall movements in the residual arm’smuscles and tendons. A microcontrollerrunning a filter algorithm maps thosemovements into commands for appropri-ate finger movements. The hand itself—concocted from a variety of availablecomponents, including motors fromradio-controlled model airplanes—car-ries out the microcontroller’s commandsand moves its fingers.

A two-part socket that slips over anamputee’s residual arm provides mechan-ical support for the hand and also allowsmeasurement of arm-muscle movementsthat control the hand. Against the resid-ual arm itself is a flexible silicone sleevewith an array of embedded sensors. Overthis sleeve is a hard plastic socket to

which the artificial hand is attached.The amputee’s attempt to move phan-tom fingers activates the sensors in thesilicone sleeve that then press against theouter socket. Ultimately, the forcesdetected by these sensors result in move-ment of the artificial hand’s fingers.

Handmade sensors. Coming up withsuitable sensors wasn’t easy, however. Foran amputee’s comfort, the sensors needto be soft, and most sensors are hard. TheRutgers team’s search for suitable sensorswas fruitless, and attempts to get sensormanufacturers to make custom sensorsled nowhere. The solution, a Craeliusbrainchild, was handmade pneumaticdevices—small, soft, air-filled bladdersglued to small silicone tubes.

Applying the pneumatic sensorsrequired even more ingenuity. Graduatestudent Mike Kogan’s idea was to embedthe soft, air-filled bladders directly intothe silicone sleeve, and he devised a wayto do that when the sleeve gets custommolded to fit an amputee’s residual arm.The tubes from the bladders lead to com-mercially available sensors that actually

measure the air pressure that correlateswith muscle movement. Not only are thepneumatic devices comfortable, Craeliussays, but “the air pressures in the bladdershave similar compliance as arm tissue, sothe signal transfer is optimal.”

One problem still remaining, howev-er, was that of translating the measure-ments of muscle and tendon movementsinto desired movements of as many asfive fingers. Some early methods worked,but not very reliably. Then graduate stu-dents David Curcie, Jim Flint, and SamPhillips came up with a workable con-cept called residual kinetic imaging.Basically, says Craelius, the residual fore-arm is a three-dimensional object whosedynamic shape relates to what theamputee wants to do—move certain fin-gers in certain ways. A particular pressurepattern on the forearm corresponds to aparticular set of muscles that would resultin specific finger movements if theamputee still had fingers.

To determine just what pressure pat-terns correspond to certain motions inindividual fingers, Jim Flint devised a

WHAT DEXTRA CAN DOThe capabilities of the Dextra artificial hand go well beyond those of traditional hand prostheses developed much less

recently and with much less use of technology. It can move five fingers independently, whereas a conventional prosthesis isbasically a three-jaw chuck—two fingers and a thumb that close on each other. Also, Dextra makes natural use of a residualarm’s muscles and tendons for finger control. Until recently, most upper-limb prostheses were controlled by an amputee’sback, neck, or shoulder muscles via a steel cable.

Not all amputees can control all five of Dextra’s fingers, however. In some cases, that’s because injuries that resulted inamputation of a hand also damaged arm tissue, making it difficult to sense muscle movement in what’s left of the arm. Also,some congenital amputees—those born without an arm—are unable to use Dextra at all, because they can’t visualize move-ments of fingers they never had and thus can’t appropriately move the muscles that normally control those fingers.

Most amputees who have tried Dextra have achieved very good results, however. Typically, they can tap their artificial fin-gers at a rate of three taps per second, compared to 4.5 taps per second for the standard human. That’s fast enough for typingand even for playing musical instruments to a degree. Latency, the time between a decision to tap a finger and the actual tap,is also low. The Dextra development team hasn’t yet quantified the delay, but Craelius watched one subject playing “Mary Hada Little Lamb” on the piano and observed that the subject’s real hand and the artificial hand coordinated perfectly.

The hand does have less gripping strength than a human hand. A real hand’s pinch force between thumb and forefingeris 100 newtons, Craelius says, whereas Dextra, using the smallest servo motors available for radio-controlled model airplanes,exerts about 2 newtons per finger or 10 newtons total. Also, the range of flexion in a human metacarpophalangeal joint—where a finger joins the body of the hand—is about 100 degrees, compared to 60 degrees in a finger of the artificial hand.

Nor can the Rutgers hand—or any artificial hand currently envisioned—match the human hand for complex motion. “Thehuman hand has 24 degrees of freedom, not even including the wrist,” Craelius says. “We’re providing one degree of freedomfor each finger, and maybe two for the thumb.”

A wrist and a better thumb, in fact, are what amputees most often include on their wish lists for improved prostheses. TheRutgers thumb is a step in the right direction, because it can both flex and extend, although it can’t yaw, or move from sideto side. A simple motorized wrist that rotates would be immeasurably less functional than a real wrist, Craelius says, althoughit would still be helpful. “With rotation,” he says, “you can at least open a door with a key.”

Some commercially available hand prostheses now have rotating wrists, and some also are myoelectric, meaning that they,like Dextra, use a residual arm’s muscle movement for control. Five independently functioning fingers are not yet available ina commercial prosthesis, however. For that, amputees will have to wait for Dextra or something similar.

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trainable digital filter. Creation of the fil-ter occurs in a training session that stepsan amputee through various finger move-ments while recording a series of pressureimages from the sensor array in the formof mathematical matrices. To convert nchannels of sensory information to mmotors, says Craelius, you need to invertthose matrices with a filter. In the train-ing session, a laboratory personal com-puter performs the matrix inversion tocreate the filter. During use of the hand,an 11-MHz microcontroller from DallasSemiconductor applies the filter to gen-erate finger-movement commands.

Finger-movement commands from themicrocontroller exist as pulse-width-modulated (PWM) control signals. Thehand, an aluminum frame with attachedfingers, is roughly the size and weight of ahuman hand. Inside the frame are rotaryservo motors that pull on nylon fila-ments, or “tendons,” to move the fingers.

Finger mechanics. Determining justhow to move the artificial fingers was aproblem in itself, however. Originally, toenable tapping motions suitable for typ-ing, the Rutgers engineers designed fin-gers to move at just the major jointwhere a finger joins the body of thehand. For grasping, however, a finger hasto curl, which means that it must flex atone or both of its other joints. TheDextra team has experimented withattaching the nylon tendons to the fin-gers at either of these two joints and in

between. Each design provides a slightlydifferent kind of grasp, Craelius says.

The controller for Dextra doesn’tincorporate feedback, at least not in therobotic sense. Humans using the hand,however, soon learn to incorporate theirown feedback, says Craelius. “The personcan hear the motors going,” he says, “andadjust the amount of force by both visu-alization and tactile feedback.”

As of now, Dextra isn’t commerciallyavailable. It remains a prototype, stillneeding miniaturization, packaging, andperhaps ruggedizing before it becomes aproduct. Preparing it for commercial usewill probably also require a differentpower source. Currently, the hand workson eight AA batteries that provide eighthours or less of use.

High cost and a limited market are alsoa deterrent to commercialization, just asthey are to any hand prosthesis. There areno big regulatory barriers to market a newprosthesis, Craelius says, just marketforces and insurance companies’ willing-ness to pay. Currently, he notes, the hard-ware for the most advanced prosthesestypically costs $15,000 to $20,000.

As for when Dextra might actuallycome to market, Craelius says, “Noguess.” In the meantime, he and his grad-uate students continue to improve thehand, with current efforts concentratingon making it easier to put on and takeoff. Many amputees, no doubt, eagerlyawait its availability.

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Dr. William Craelius, principal inventor of the Dextra artificial hand, is Director of the Orthoticsand Prosthetics Laboratory in the Department of Biomedical Engineering at Rutgers University(Piscataway, NJ).

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