8, 1998 Design News
SPECIAL MEDICAL ISSUE
FUNCTIONAL ELECTRICAL STIMULATION
Electronics get muscles moving
Functional electrical stimulation
motivates dormant muscles, granting independence to
people incurring spinal cord injuries
Laurie Peach, Associate Editor
"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. 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 below).
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
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
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
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,
"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
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
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. Thi