Electronics get muscles moving

June 08, 1998

"Give me a hug, Daddy." Who could deny their child such a simple request? For Sam Sampson, injured in a car accident that left him paralyzed from the waist down, six years passed before he was able to stand and give his three sons and wife a loving embrace. Thanks to an implanted functional electronic stimulation system, Sam not only stands and hugs, he's also stepping and moving around his wheelchair. "Just being able to stand up and give my wife and kids a hug again--if that's all I could do, I'd be happy. But I'm doing so much more than I ever imagined," Sam says with a shy, yet proud, smile.

To give a hug, one has to be able to stand, reach, and grasp. Many physically challenged people cannot perform such simple tasks. Implanted functional electronic stimulation (FES) systems under development at the Cleveland FES Center give them the ability to execute these actions.

The U.S. Food and Drug Administration (FDA) approved the first implantable FES neuroprosthesis last fall, the Freehand from NeuroControl (see news). This device offers individuals with quadriplegia the ability to grip and hold. In another application, a prototype bladder control system allows individuals with paralysis to control their bodies' waste schedule (see sidebar).

Two thousand Americans each year incur debilitating spinal cord injuries. "When a person becomes paralyzed, signals no longer flow from the brain to the muscles," says Primoz Strojnik, D. Sc., a principal researcher at the Cleveland FES Center. Nerve fibers are severed and cannot carry the body's electric signal. Functional electronic stimulation takes over where the nerves stopped. Researchers at the FES Center artificially create electronic pulses that stimulate muscles into action. Through a series of electrodes and wires attached to isolated muscles, users rely upon pulses generated by the implant devices and controlled by an external unit to activate their paralyzed muscles.

The technology originally conceived in the early 1980s continues to go through clinical studies. "But on this level, it's only been around since yesterday," says Strojnik, referring to the Center's on-going lower-extremity technology advancements.

Researchers at the FES Center believe Sam is the first person in the world to have two implanted FES devices for standing and stepping. There are external stimulation systems for mobility. However, according to Strojnik, surface electrodes must continually be repositioned, deep muscles are harder to stimulate, and consistency and repeatability of muscle response is not as good as with implanted electrodes.

Sam's device, developed at the Cleveland FES Center, was the direct result of work done to stimulate and control hand muscles. The Cleveland FES Center is a research consortium of the Cleveland VA Medical Center, Case Western Reserve University, and the Metrohealth Medical Center.

In the original Freehand system, a position sensor attached on the user's chest and shoulder translates small movements into a control signal. An external controller usually mounted on the wheelchair processes this signal into radio frequencies that are transmitted to a customized circuit implanted under the skin on the chest wall. When the eight electrodes wired into the paralyzed hand and forearm muscles receive stimuli from the microprocessor, the muscles contract for either a finger pinch or palmar grip.

FES Center researchers adapted this exact technology for leg movement. Strojnik says, "The technology is basically the same for upper or lower extremities. The implant doesn't know where it is being implanted. The only real difference is the length of wires that connect the electrodes to the muscles."

Many individuals with spinal cord injuries also have sensory damage, which leads to another problem. The Freehand system is based on an open-loop function. Bioengineers pre-program the external control box with specific functions, such as reach and grasp. But the system has no way of knowing if the user's hand is functioning properly and in the proper mode of operation. Except for visual clues, neither does the user. To compensate for this, one channel of stimulation can be used to provide sensory feedback to the user. Seven stimulation channels attached to the surface of the muscles control movement, while the one sensory channel relates "feeling" information. The sensory channel provides stimulation in an area with sensation to create a feeling similar to what you feel when your leg goes to sleep.

In Sam's system, the sensory feedback isn't necessary, as he can see the status of the system by observing the LEDs on the external controller he wears on his belt. He uses visual feedback to coordinate the use of his legs.

The FES implant technology originated with the cardiac pacemaker almost 30 years ago. Cleveland FES Center engineers relied on the lessons learned from that technology as to what materials were compatible with the body's hostile environment. Stainless steel wires coated with Teflon and silicone carry the electrical signal from the silicone- and epoxy-encapsulated titanium implant to the electrodes made from noble metals. Once placed, the electrode will not move. The body encapsulates the electrodes with natural fiber tissue growth.

Wires running through the fatty tissue under the skin connect electrodes to eight leg muscles--four of the major anti-gravity muscles per side--to produce standing. Ron Triolo, a professor of orthopedics and biomedical engineering at Case Western Reserve University and the principal investigator on the implanted walking device, says, "All of these muscles are used to lift the body. Others control side to side motion. Right now, there aren't enough channels on the circuit to control all normal movement." There are several research volunteers standing with a single 8-channel implant using these muscles. Sam went on to receive a second implant to activate not only these eight standing muscles, but another eight required for stepping. The perfect group of muscles is still under investigation. "We don't know the best set yet," Triolo continues. But this set seems to work for Sam.

The beauty of the FES Center system is that it should last for the lifetime of the user, says Strojnik. Pacemakers have to be replaced periodically because they have an internal energy source. Through an external antenna taped to Sam's stomach, where the skin meets the implant, radio frequency waves supply the power needed to generate the electric current. Batteries are not required by the implanted devices.

Hostile environment demands unique solutions. "A great portion of our work is dedicated to reliability," says Strojnik. "All components have to be reliable. This isn't like a TV set. We can't open up a person to make a repair." A wide range of circumstances is taken into consideration. Components are tested in various temperatures and saline solutions, shook, bent, and pulled.

"It took a long time to develop electrodes and leads that don't break," says Strojnik. The circuit capsule, receiver coil and lead wires are conformally coated in epoxy and silicone elastomer to provide physical support of the feedthroughs and radio frequency coil, and stress relief to the leads. This makes the device suitable for long-term implantations.

The leads are bifilar wound helices enclosed in a silicone elastomer tube. In-line connectors are used between proximal leads permanently attached to the implant and electrodes or sensors to provide points of connection for components and to facilitate surgical installation and maintenance. Center engineers make remote sensors recording and stimulating electrodes using similar materials and methods.

The first implants had leads permanently attached to the implants. There was no way of replacing one if it broke. The FES Center developed an in-line connector where electrodes can be detached from the implant if something breaks. "Connectors have to be able to be connected, disconnected, and reconnected," says Strojnik. So, researchers made a machine to bend the coupler 100,000 times to see if the design would hold up under such movements. Because connectors must be watertight, they were stretched and flexed under water to simulate body conditions. To prevent corrosion, researchers used stainless steel for both wire and connector.

One of the biggest engineering challenges was the electrical circuitry, says Strojnik. Researchers at the FES Center developed their own silicon ASIC, or application-specific integrated circuit, to incorporate as many functions as possible.

The goal of the group is not just technology advancement, but to prove its feasibility and usefulness. Strojnik sees the technology as a tool, but not a goal. However, he has a strong desire to see it advanced. "We want to add more and more function to the same implant volume," says Strojnik. This is difficult. Hybrid technology helps with this, but the scientists are still limited in what they can do.

"I would love to make the device smaller, but we are mechanically limited," he adds. "We still have so many wires coming out of the package that require a certain amount of space. The number of stimulation outputs and sensory input lines limits the size. Eventually we may have smaller devices that work in concert with each other or individually."

Another aim is complete implanted control methods--have everything implanted except for antenna. Both transducer and sensors would be inside the body. "Nothing limits us but time and money," says Strojnik. "The technology and know-how is there."

Presently, Center engineers are working on a device that combines the implantable joint angle transducer and the EMG with 16 stimulus channels. This comes with 31 wires in the device. The major problem with so many channels will be managing wires during the implanting procedure.

Walking is work. Sam, in gray T-shirt, jean-shorts, and high tops, sits comfortably in his wheelchair. He initially heard of the FES Center through the Learning Channel. "I saw them one night and decided to call and volunteer." After extensive muscle, nerve, and psychological exams, Sam was ready. Surgery took place in November 1996. Doctors installed two 8-channel implants, one for each leg.

"You've heard that it takes a village to raise a child, well, it takes a city to get a person to walk," Triolo says.

One of the biggest challenges was working out the bugs in Sam's external control unit, the "brain" of the system. He spent 11 months total at the FES Center adjusting and fine-tuning the programming in his little white box. Engineers established proper electric thresholds and adjusted stimulation to obtain the strongest muscle response. They also preprogrammed the box with specific functions such as "flex left quadriceps," "short stand," and "sit."

To achieve these functions, engineers program the pulse durations, stimulus amplitudes, and frequencies into Sam's controller for each of the 16 muscles. Using a laptop and a Windows-based program written by the FES research group, engineers change the stimuli with a click of the mouse. Sam's doctors then transfer this information from the computer to the portable controller that he wears on a belt.

Whenever Sam wants to stand or walk, he tapes two antennas directly outside each implant, which are buried beneath his abdomen's skin. Sam selects a movement from a menu with a button control mounted on a small ring. He presses a blue button to activate the radio frequency signal that sends the stimulation timing information to the implant. His legs snap to attention. When finished, Sam pushes the red button, deactivating the signal.

"There can be no delay between control signal and action," says Strojnik. Information processing time is critical. Advances in microprocessing make possible the almost instantaneous muscle reaction. Delays are due to the sluggish way all our muscles respond to signals from our nerves--not from the electronics.

Sam started standing in March 1997 and walking in May of that year. Now Sam exercises 11/2 to 2 hours and walks with the aid of a walker for short periods totaling about 45 minutes a day. Triolo is quick to point out that the goal of the technology is not transportation. At least not yet. It is only to provide additional function within the vicinity of the wheelchair. And perhaps an emotional boost as well.

To date, two individuals have an implanted standing system. Three more are scheduled for surgery. The focus over the next two years will be to prove the effectiveness of the device and receive FDA approval.

Although Sam has had good results, there is still a lot of room for improvement. A stronger signal is needed, says Triolo. A strong response from a muscle must be guaranteed every time. "This is not a cure," Triolo is quick to point out. "Sometimes people see us and think 'miracle.' Although we welcome inquiries, it's not readily available, easy, or for everyone." Candidates must meet certain physical and mental criteria. Specific muscles must react to stimulation.

Thanks to Sam, both he and the FES implantable walking technology have taken their first steps.

Engineering Challenges

- Reliability of materials and parts; components must last the lifetime of the user.

- Materials for electrodes and leads in a hostile environment.

- Easily accessible connectors.

- Electrical circuitry--incorporating more channels on a fixed ASIC.

- User feedback.

- Standardized data collection system.

- Move from prototype to mass production.

- Development of easy-to-use software.

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