by Deborah Conn

A device that would “grant an amputee the fine motor control necessary to thread a needle, use a computer keyboard, play a piano or perform fretwork on a guitar.” That’s the ideal prosthetic arm. But design challenges, and funding, have kept the field from advancing—until now.

It’s unfortunate, but true, that the greatest technological advances in prosthetics are driven by soldiers’ loss of limbs. The Iraq war is no exception, and it is stimulating stunning developments in upper-limb prosthetics.

One of the main forces behind this effort is the Defense Advanced Research Projects Agency, or DARPA, which announced the Revolutionizing Prosthetics Initiative in 2005. The initiative encompasses two separate programs: Revolutionizing Prosthetics (RP) 2007 and RP 2009.

The goal of the four-year effort, RP 2009, is to produce a prosthetic arm that will mimic the properties and sensory perception of the real thing—in short, the ideal prosthetic arm described above.

But because returning soldiers need better prostheses now, DARPA also launched the shorter-term program, RP 2007, charged with using current state-of-the-art technology to create a vastly improved prosthetic arm.

“Their spec is much more reduced in terms of speed and torque capabilities than the four-year team,” says Dr. Richard Weir, director of the Biomechatronics Development Laboratory at the Rehabilitation Institute of Chicago. “The 2009 team is trying to create something more comparable to a human arm.”

Each program brings together dozens of teams and researchers. Thirty-five different institutions, for instance, are working on RP 2009 alone. And they’ve made some amazing advances.

RP 2007
DARPA’s RP 2007 program is headed by DEKA Research and Development, of Manchester, N.H., under the direction of Program Manager Tom Faska.

The program is focusing on three separate tasks, says Faska, the first of which is the socket. “Normally, an arm prosthesis uses a vacuum socket that can fall off, or tight straps,” Faska explains. “We’ve taken a deviation from current socket technology and are using pneumatically operated McKibben muscles, which are comfortable for moving around, but at the user’s discretion, can be tightened either by pushing a button or through a signal from the arm.”

For the arm itself, the DEKA team has produced Generation (or Gen) 1 and 2 arms. (The DARPA RP 2009 program uses the terms Prototype or Proto 1 and 2 for its arms, which are being developed separately.)

“We’re working with many different groups,” says Faska. “RIC [the Rehabilitation Institute of Chicago] is doing TMR [targeted muscle reinnervation], the University of New Brunswick, Canada, is working on pattern recognition, and Kinea Design LLP [in Evanston, Ill.] is working on a tactor [a small transducer designed to optimize skin response to vibration] that can provide sensory feedback.”

According to Faska, the biggest challenges with the arm are its weight and degrees of freedom, as well as battery life. At nine pounds, the Gen 1 prosthetic arm weighs about the same as a human arm.

Weight is a serious challenge for most patients because the heavier the arm, the more unwieldy it is. Although a human arm weighs about nine pounds, that is heavy for a prosthesis because it must be attached using a socket or straps rather than bones and muscles.

The hand, with four powered and eight unpowered degrees of freedom, is differentially geared so it can grasp irregularly shaped objects. In addition to the hand, the arm has another six degrees of freedom, all of which can be operated simultaneously.  

“The huge challenge is in the control system,” he says. “Making a highly functional arm is one challenge. Actually getting the patient to control it in a natural manner is another.”

Targeted Muscle Reinnervation
To address that challenge, RP 2007 is exploring the use of different types of controls, including intuitive, noninvasive controls, such as joysticks, footpads, and inertial measurement units that sense the position of the head, meaning that the arm will move when the head moves.

A new technique is targeted muscle reinnervation (TMR), developed by Dr. Todd Kuiken, director of the Neural Engineering Center for Artificial Limbs (NECAL) at the Rehabilitation Institute of Chicago. TMR involves the transfer of residual nerves from an amputated limb to unused muscle regions near to the injury.

“Somewhere between the neck and shoulder, the amputee might still have nerves that used to run down to the arm and hand, but now have nothing to do,” explains Robert Lipschutz, CP, who is on the NECAL team. “What we have done is to surgically move the nerves and suture them to the muscles that remain.

“In the first candidate, we tied the nerves back into the pectoralis muscles. We put a surface myoelectrode over that muscle, and when the amputee thinks about bending his elbow, it tries to fire that muscle. The electrode picks up the signal, amplifies it in the electronic arm, and the elbow flexes.”

Lipschutz is also director of education for the Prosthetics and Orthotics Clinical Center at RIC, as well as clinical instructor at Northwestern University’s Prosthetic-Orthotic Center.

He notes that TMR yielded an unplanned benefit in the first candidate.

“We found that the skin surface itself is being reinnervated by the nerve, so the [area of the] chest that is moving the elbow or the hand is also getting a sensory pattern of the arm. If we touch this person on the chest, he feels it in the missing arm or hand.

“If we can map out where the different sensations are in the hand and then put something that would push on the right area of the chest, then this person could experience the true feeling of grabbing an object again.”

This would be a benefit because amputees would know when they were actually touching an object and when they let it go.

Another area under investigation is pattern recognition. Researchers are trying to pick up electrical signals on the surface of the skin that indicate which muscles are being stimulated when the subject wishes to perform different functions, such as pinching, pointing or making a fist.

One of the challenges, according to Lipschutz, is consistency. “We can have a great session, but when the individual comes back we have to make sure we have the electrodes in exactly the same sites, so we can control it the same way.”

Another challenge is that the locations on the skin that indicate which muscles are being stimulated are different on each individual. Lipschutz speculates that an amputee fitted with these electrodes might need a 15-minute training session each morning to educate the arm’s control program by pinpointing his or her particular locations.

RP 2009
Taking the lead in RP 2009 is the Applied Physics Laboratory (APL) of The Johns Hopkins University in Baltimore, under the direction of Stuart Harshbarger. DARPA awarded APL a $30.4 million contract for the initial phase of the program.

During the first half of the program, research has focused on control systems as well as the design of the arm itself. In April, APL delivered the first DARPA limb prototype, called Proto 1, which uses TMR as a control system, provides sensory feedback, and allows for eight degrees of freedom.

Richard Weir and his group at RIC worked closely with Otto Bock Vienna to design Proto 1’s components, while APL concentrated on electronics.

DARPA’s 2009 spec calls for a transhumeral prosthesis, which Weir considers somewhat limiting. “Trans-radial amputations are far more common in the United States,” he says. “In our opinion, it’s not a good thing to design just to the specification. If any of our arm systems are to be useful, we have to be designing them so they can fit transradial amputees as well.”

The Vanderbilt Arm
One of the prostheses developed as part of RP 2009 is the Vanderbilt arm. The impetus was a desire by Dr. Michael Goldfarb, professor of mechanical engineering at Vanderbilt University in Nashville, to explore alternate sources of power.

“Batteries and motors are heavy and weak compared to human muscles,” he says. “One of the best robots around, a humanoid built by Honda, weighs 285 pounds, carries 66 pounds of batteries, and the best it can do is a medium slow walk for about 20 minutes.”

Goldfarb’s team uses liquid propellant to power its transhumeral prosthesis. “Our arm, when fully fueled, weighs 4 pounds and has 21 degrees of freedom. It can move much faster, with greater forces, than state-of-the-art myoelectrics.”

The Vanderbilt arm is not quite ready for market, however. “One problem is that it uses an unconventional power source. You can’t buy this fuel off the shelf anywhere. Commercializing the arm would require building a whole new power supply infrastructure.

“It also requires a lot of input from the user. None of the neural interface groups can provide the number of control channels we’d need to control this arm.”

Because of these challenges, Goldfarb does not expect his arm will be part of the final DARPA prototype. But he’s pleased with what he and his team have accomplished. “Our role was to push to the limits of technology. And that’s what we did.”

What’s next?
RP 2007 is right on time and scheduled for completion in January 2008. “Once we’ve completed the Gen 2 arm, we’ll do the final trials, and then we will go with the components that give us the best functionality,” says Faska. When the final prototype is identified, the system will move into human clinical trials.

According to RIC’s Richard Weir, RP 2009 is on schedule. The teams are now working on Proto 2. “We’re testing to see if there are conventional electric motors we can use that meet DARPA specs. We’re also exploring different hand architectures. Because we want a prosthesis that can work for transradial amputees, our lab and Otto Bock are building a hand with 15 motors and microcontrollers in the hand itself, rather than in the forearm.”

In the second phase of RP 2009, during 2008 and 2009, DARPA will decide which architecture to pursue, according to Weir, who notes that the 2009 teams are working on alternate control schemes as well.

His team has developed injectable or surgically implantable sensors called IMES devices (Injectable MyoElectric Sensors), which provide greater signal isolation than the surface electrodes on the skin used on Proto 1.

Other researchers are looking into splicing nerves with computer chips and implanting chips in the brain.
“All of these control strategies are more invasive, pushing these arm systems into the realm of FDA Class III medical devices,” says Weir.

“I do expect we’ll meet the timeline. Two years ago I thought the spec was outrageous, but I’ve been really surprised at what we have been able to achieve.”

Other upper-limb advances
The DARPA initiative, of course, is not the only work being done in upper-limb prosthetics.

In the November 2007 issue of the O&P Almanac, for example, Stuart Mead of Scotland-based Touch Bionics talked about the company’s i-LIMB Hand, an advanced myoelectric hand that has individually powered digits and a thumb that can rotate to 90 degrees. Mead is also working on a wrist rotator and an electric elbow, shoulder, and mid-humeral rotator, which moves the arm from side to side.

Among the companies working on upper-limb prosthetics is Liberating Technologies, in Holliston, Mass., a commercial partner on the RP 2007 team. “We collaborated with RIC clinicians to build systems for targeted muscle reinnervation patients that could accept multiple inputs (up to 10) and run multiple devices,” says William Hanson, president.

The company introduced the Boston Digital Arm System in 2001, providing the ability to simultaneously control multiple devices (up to five), and is continually improving it. “Now that TMR is here, we may want to run more than five devices,” Hanson says. “So we’re working on a more robust system that allows the arm to accept even more signals from the patient and run more powered devices.”

“[A] challenge is that as products in the upper-limb field become more complex, we have to find ways for them all to work together. If the industry doesn’t come up with a standard, we’ll find that certain components won’t work with other manufacturers’ systems.”

Another well-known prosthesis is the Utah Arm, developed by Motion Control, in Salt Lake City. Company president Harold Sears says the Utah Arm 3+ should be released by the end of the year.

“We were one of the first companies to make it possible for two functions to operate at the same time. [But] this can be tricky for the patient, [because] the control site is usually a back motion—it’s like patting your head and rubbing your tummy at the same time. The 3+ hand is activated as soon as the elbow stops, and this makes it more practical to use the elbow and hand together.”

Another innovation allows the elbow to swing freely when not in use without using power. The 3+ also incorporates existing technology that frees the wrist. “When you push and pull on something, it doesn’t feel stiff,” explains Sears.

Unlike the DARPA-funded efforts, says Sears, “We focus on incremental improvements that the hand and elbow can provide. We have to be much more practical. We can’t waste our time on things that won’t actually become a product.”

Whether incremental or revolutionary, the O&P industry seems poised for significant advances in upper-limb prosthetics technology. Inspired by soldiers, that work will mean new capabilities and freedom for amputees in all walks of life.

Deborah Conn is a freelance writer based in Falls Church, Va.