using electrical stimulation to restore function to paralyzed muscles william durfee department of...
Post on 21-Dec-2015
213 views
TRANSCRIPT
![Page 1: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/1.jpg)
USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES
William DurfeeDepartment of Mechanical Engineering
University of Minnesota
Kurt Korkowski, Ben Dunn,
Karl Oberjohn, Brent Harrold
Department of Mechanical Engineering
University of Minnesota
Michael Goldfarb, Heather Beck
Karen Palmer, Jeff Chiou
Department of Mechanical Engineering
Massachusetts Institute of Technology
Gary Goldish, Rich ScarlottoPhysical Med. & Rehab. ServiceVA Medical Center, Minneapolis, MN
Allen Wiegner, Nancy WalshSpinal Cord Injury ServiceVA Medical Center, West Roxbury, MA
![Page 2: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/2.jpg)
• Stimulated Muscles = Power • Brace = Trajectory guidance • Brake = Control, stability
HUMAN/MACHINE DESIGN LABDepartment of Mechanical EngineeringUniversity of Minnesota(www.me.umn.edu/divisions/design/hmd/)
Fu
x,vT
X
PE Force-Velocity
CE Force-Velocity
Fscale
IRC
CE Force-Length
Activation Dynamics (2nd order)
PE Force-Length
u
V
X
V
X
Force
Passive Element
Active Element
Muscle mechanicsSmart orthotics +
electrical stimulation for gait restoration
Haptic interfaces for virtual product prototyping
![Page 3: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/3.jpg)
OUTLINE
What is Functional Electrical Stimulation (FES)?
How FES can be used to restore motion State-of-the-art
– Why it’s hard– Commercial products
FES research at the U:– FES + "smart" orthosis– Modeling and control
![Page 4: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/4.jpg)
![Page 5: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/5.jpg)
![Page 6: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/6.jpg)
FES APPLICATIONS
Bladder stimulation (incontinence) Cerebellar stimulation (movement disorders) Sensory substitution Visual prostheses (blindness) Auditory prostheses (deafness) Pain suppression (TNS) Pacemakers Limb control (paralysis)
![Page 7: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/7.jpg)
HOW FES WORKS
Brain
Spinal Cord
Limb
Stimulator
![Page 8: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/8.jpg)
SPINAL CORD INJURY
NUMBERS 150,000 in U.S. 8,000 new cases each
year 1/2 quadriplegic, 1/2
paraplegic LEADING CAUSES
– Automobiles– Guns– Sports (diving)– Falls
AGE GROUPRange: 15-29Mean: 23
![Page 9: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/9.jpg)
SPINAL CORD INJURY
NUMBERS 150,000 in U.S. 8,000 new cases each
year 1/2 quadriplegic, 1/2
paraplegic LEADING CAUSES
– Automobiles– Guns– Sports (diving)– Falls
AGE GROUPRange: 15-29Mean: 23
![Page 10: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/10.jpg)
FES BICYCLE ERGOMETER
![Page 11: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/11.jpg)
FreeHand
![Page 12: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/12.jpg)
FreeHand
![Page 13: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/13.jpg)
FreeHand (NeuroControl)
![Page 14: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/14.jpg)
Handmaster (NESS)
![Page 15: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/15.jpg)
Parastep (Sigmedics)
![Page 16: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/16.jpg)
WalkAide (Neuromotion)
![Page 17: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/17.jpg)
New Mobility, June 1997
Wired ForWalking: BY Sam Maddox
Fifteen years ago, Bassam "Sam" Khawam, a 22-year-old Lebanese American living in the Cleveland suburbs, was paralyzed at T8-9 by a bullet. Khawam was big and physical, a black belt in karate. He was given the usual spinal cord injury prognosis: Get used to the chair, son, you're not going far without it. Khawam took the news the way many young, strong guys who join the gimp world do. He didn't buy it.
"You are 22 and it happens to you," says Khawam, now a rehab engineer and father of two in Spokane, Wash. "You would want to walk again too."
This is the story of Khawam and of a handful of other paralyzed research subjects, of the multimillion-dollar functional electrical stimulation (FES) project that got them walking, and of the scientist who lined up the money and ran the lab. It's a story of good intentions and good press on the side of science, and bad luck and bad faith as seen by the project's participants. It's about an ambitious but flawed technology and of questionable medical ethics. And it's about a tightknit research community so convinced of its promise that it would tolerate less-than-acceptable standards of care for its human subjects.
Inside Khawam's legs is a virtual birdnest of corroding electrodes that cannot be removed without destructive surgery. He has had infections requiring antibiotics, plastic surgery and hospitalization. He and his doctors see a clear link between the infections and the electrodes, and one doctor suggested removing all of Khawam's thigh and calf muscle as the only way to get the hardware out. Another offers this bleak statement: "Mr. Khawam's prognosis is decidedly poor, as the future course of medical treatment may include either above-or below-the-knee amputation to rid him of a constant source of infection."
This is the story…...of the multimillion-dollar functional electrical stimulation (FES) project that got them walking….. It's about an ambitious but flawed technology and of questionable medical ethics. And it's about a tightknit research community so convinced of its promise that it would tolerate less-than-acceptable standards of care for its human subjects.
Inside Khawam's legs is a virtual birdnest of corroding electrodes ….. He and his doctors see a clear link between the infections and the electrodes….."Mr. Khawam's prognosis is decidedly poor…..either above-or below-the-knee amputation….."
![Page 18: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/18.jpg)
LOWER LIMB FES
FEXTERNAL
CONTROL STIMULATORInputs
Measurements
![Page 19: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/19.jpg)
(Th
e U
NH
Ro
bo
t L
ab
, w
ww
.ece
.un
h.e
du
/ro
bo
ts/r
bt_
ho
me
.htm
)IS IT LIKE A BIPED ROBOT?
![Page 20: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/20.jpg)
ROBOT CONTROL
SEGMENTDYNAMICS/KINEMATICS
MOTORS
SENSORS
CONTROLLER
DisturbancesDesired Task
Commands
![Page 21: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/21.jpg)
FES CONTROLCNS PROCESSOR
BRAIN
SPINAL
VISUALVESTIBULAR
NATURALSENSORS UPPER LIMB SEGMENT
DYNAMICS/KINEMATICS
MUSCLES
NATURALSENSORS
LOWER LIMB SEGMENTDYNAMICS/KINEMATICS
MUSCLES
ARTIFICIALSENSORS
SPINAL CIRCUITS
CONTROLLER
DisturbancesDesired Task
?
?
Disturbances
Spinal Lesion
Commands
Cognitive feedback
![Page 22: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/22.jpg)
SERVOMOTORS AS ACTUATORS
Linear, time-invariant
Torque
Current
Torque
Speed
![Page 23: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/23.jpg)
MUSCLES AS ACTUATORS
High power/weight, but nonlinear, time-varying and uni-directional
Time
FFF
Activation Velocity
Force = f(neural input, length, velocity, time, ...)
![Page 24: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/24.jpg)
WHAT MAKES FES DIFFERENT?
Not enough muscles Not enough sensors Muscle force too low Muscle fatigue Spasticity
Weight constraints Size constraints Cosmetic constraints Ease-of-use constraints Reliability Implanted systems No sensory feedback User control?
![Page 25: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/25.jpg)
WHY MODEL?
Complex system– Multi-link inverted pendulum– Nonlinear, time-varying actuators
(muscles)
Better models Better control Use model for:
– Designing "generic" controllers– Prescribing/tuning custom
systems
CONTROLInputs
Measurements
STIMULATOR
![Page 26: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/26.jpg)
MODEL VERIFICATION
Direct comparison with experimental data Predictive capability Parameterization to the subject
![Page 27: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/27.jpg)
MODELING FOR CONTROL OF GAIT
•Rigid body links (10)•Ideal joints•Passive torques•Active torques (muscles)
![Page 28: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/28.jpg)
MODELING MUSCLE
T
CEKSE
BP
KP
Active (AE)
Passive (PE)
u
Fu
x,v
3 inputs (u,x,v), 1 output, modified Hill-type model
“Muscle” = activity from single stim channelJoint-space model --> no knowledge of anatomy needed
![Page 29: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/29.jpg)
ISOMETRIC MUSCLE
Staticnonlinearity
Lineardynamic system
Hammerstein model
stim force
Identify LDS with impluse response
Identify SL by deconvolution
![Page 30: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/30.jpg)
MODELING MUSCLE
Force
Fu
x,vX
PE Force-Velocity
CE Force-Velocity
Fscale
IRC
CE Force-Length
Activation Dynamics (2nd order)
PE Force-Length
u
V
X
V
X
Passive Element
Active Element
![Page 31: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/31.jpg)
WHAT'S WRONG WITH THE MUSCLE MODEL
Invariant F-A, F-L, F-V (no change with activation) Invariant twitch dynamics (uniform fiber types) Time-invariant (no fatigue) Zero neural time-delay Rigid SEC
CEKSE
XCEXSE
XMT
![Page 32: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/32.jpg)
ISOLATED, ANIMAL MODEL MUSCLE
0
5
10
15
20
25
30
35
0 4 8 12 16
Fo
rce
(N
)
Time (s)
(Durfee and Palmer, IEEE Trans. Biomed. Eng., 41(3):205-216, 1994)
Experiment
Model
![Page 33: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/33.jpg)
INTACT, HUMAN MUSCLE
(Abushanab: Ph.D. Thesis, MIT, 1995)
![Page 34: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/34.jpg)
EXPT VS. MODEL
ExperimentSimulation
Hip
flexio
n (
deg)
Knee fl
exio
n (
deg)
Time (sec) Time (sec)
0 2 4 6 8 0 2 4 6 820
30
40
0
20
40
![Page 35: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/35.jpg)
WHERE WE ARE WITH MODELING & IDENTIFICATION
Goal of modeling: simulation matches experiment
Subject-to-subject variation is large ==> calibration is required
How good is "good enough" will be determined by control strategy
Must extend to subjects with SCI Better experimental ID methods evolving More diverse verification tests evolving
![Page 36: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/36.jpg)
PROBLEMS WITH FES-AIDED GAIT
PROPOSED SOLUTION:
Stimulation plus "smart" orthotics
Requires precise, stable control for repeatable steps
Muscles are nonlinear, time-varying
(1)
Need to walk for reasonable distances
Muscles fatigue rapidly
(2)
![Page 37: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/37.jpg)
BRACE (CBO) + FES
• Stimulated Muscles = Power • Brace = Trajectory guidance • Brake = Control, stability
![Page 38: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/38.jpg)
CBO OVERVIEW, SPECIFICATIONS
Designed for RESEARCH use
JOINTS– 2-dof hip, 1-dof knee, fixed ankle– hip adduction stop– magnetic particle brakes– Evoloid gear, 16:1 transmission
STRUCTURE– aluminum, chromoly
WEIGHT, INERTIA– 12.5 lbs, 10% of limb inertia
STIMULATION AND CONTROL– 4-channel stimulation– on/off stimulation control– closed-loop brake control
(Goldfarb and Durfee, IEEE Tran Rehab Eng, 4(1):13-24, 1996)
![Page 39: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/39.jpg)
CBO EVALUATION PROTOCOL
4-channel stimulation (quad + peroneal) Parallel bars, walker 5 - 10 m lengths Compare gait with and without CBO Speed/distance, quadriceps use,
repeatability Four subjects with paraplegia
![Page 40: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/40.jpg)
FES
FES + CBO
![Page 41: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/41.jpg)
INCREASED SPEED, DISTANCE
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
Without CBO With CBO
Gait Speed
0.09
0.12
Sp
eed
(m
/s)
0
10
20
30
40
50
60
Without CBO With CBO
Gait Distance
25
50
Dis
tan
ce (
m)
![Page 42: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/42.jpg)
BETTER REPEATABILITY
0
20
40
60
80
100
120
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
20
40
60
80
100
120
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Time (sec)
With CBO
Time (sec)
Without CBO
Kn
ee a
ng
le (
deg
)
Kn
ee a
ng
le (
deg
)
![Page 43: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/43.jpg)
OPEN ISSUES
Substantial improvement in FES-aided gait.....but preliminary, laboratory results only
Consumer-driven design (size, weight, ease of use)
Technical issues
Handling upper-limb inputs ???Startle, stumble response ???Fault tolerant equipment ???
Commercialization issues
Market size ???User acceptance ???Who pays !!??
![Page 44: USING ELECTRICAL STIMULATION TO RESTORE FUNCTION TO PARALYZED MUSCLES William Durfee Department of Mechanical Engineering University of Minnesota Kurt](https://reader030.vdocument.in/reader030/viewer/2022032522/56649d6a5503460f94a49242/html5/thumbnails/44.jpg)