Transcript Introduction to Machine Intelligence

Neuroprosthetics
Motor Prostheses
Background
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Damage to the Central Nervous System
(CNS) can result in sensory loss, muscle
contraction, cognitive problems, loss of
motor control & biological function loss
Treatments include – drugs, physical
therapy, surgery & rehab (+ future –
neural regeneration)
Also possible – Neural Motor Prosthesis
Definition
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Motor Prosthesis (MP) is a device that
electrically stimulates nerves innervating a
series of muscles for restoring functional
movement or biological function.
Here we look at how electrical stimulation
can be used to overcome motor and
functional loss
Clinical Applications
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Spinal cord injury
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Brain injury
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Diseases that effect neural function
Spinal Cord Injury
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Greatest level of success in this area
Usually caused by car or diving accident (UK)
Severance or compression of cord by a
fractured bone & tissue swelling
Mobile areas of vertebral column are most
susceptible
Paraplegia – paralysis of the lower extremeties
Tetraplegia – paralysis of upper & lower ext.
Spinal Cord Injury
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Muscle Atrophy – degeneration of muscle
tissue due to loss of neuronal input
Muscles to be stimulated need
strengthening regime prior to introduction
Second level + higher injuries – loss of
ability of brain stem to control breathing
Cervical + lumbar level lesions – loss of
bladder, bowel & sexual functions
Brain Injury
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Many different disorders result – cognitive
and sensory not issue here
Use of MP can be hindered by cognitive
problems
But MP can be used for motor relearning
Stroke – blood vessel in brain is blocked or
ruptured – loss of blood flow to an area
Result is neuronal death
Most common is stroke in one motor area,
resulting in paralysis in one side (opposite)
Brain Injury
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Stroke also causes – hyperreflexia, muscle
spasticity, muscle atrophy
Cerebral Palsy (CP) – occurs shortly after birth
Caused by accident, infection (meningitis or
enchepalitis) or brain asphyxsia
All these lead to neuronal death
Result is difficult to perform motor tasks – spastic
CP – muscles permanently contracted
Diseases
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Some lead to neuronal death, some to loss of the
myelin sheath around neurons thus preventing
action potential conduction and some affect the
generation or release of neurotransmitters
Only first type can benefit from prosthetics – do
not affect nerves going to muscles
MP generates action potentials in nerves to cause
muscle contractions
So use in MS & MND very limited at present
Motor Prosthesis Design
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Stimulus delivery system – electrode +
wires for stimulation
Control unit – interpret user commands,
convert info into muscle stimulations
Command interface – records signals
generated by user & converts into
commands for the MP
Stimulus Delivery System
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All MPs operate by electrical stimulation of
nerves to elicit a muscle contraction – three
methods exist for this
Surface electrodes – on surface of skin over
point where nerve & muscle join
Require conductive adhesives or pad for contact
Low cost & noninvasive
Not all muscles can be activated (those nearest
skin) & Large power due to large voltages (80V)
to drive current across skin impedance
Stimulus Delivery System
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Percutaneous Electrodes – Inserted through skin
with needle near motor point of muscle
Single or multiple wire strands
Barbed at the end to ensure anchoring
Power requirements much reduced
Skin irritation + infection can occur
Stress on wires at interface – poss breakage
Stimulus Delivery System
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Implanted Electrodes – implanted in body
Epimysial – platinum-iridium disk in a silicon pad,
sutured near muscle motor point
Intramuscular – inserted with needle, stainless steel
with umbrella anchor
Nerve Cuff – Platinum-iridium bands that encircle
the nerve to the muscle
Advantages – highly selective (not nerve cuff), less
power
Disadvantage – Invasive, surgery for placement and
replacement
Control Unit
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Consists of command processor & control processor
Command Processor – interprets user generated
signals to operate various motor prosthesis
functions
E.g. System state (on/off), activation pattern
selection
Control Processor – converts signals from command
processor into actual function
Decides specific muscle stimulations required
Command Interface
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Records user generated signals to operate
the MP
Examples EMG, Voice, switches, respiration
Use least number of input channels
Reduce amount of noise (SNR)
Transition rate is important
Performance reliability
Interface must be cheap, simple and
invisible
Cardiac Pacemaker
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Implanted device that delivers electrical
impulses to cardiac tissue to control heart
contractions
Stimulus via lead wires and electrodes on heart
surface or in heart muscles
Electrodes have to withstand movement
Platinum alloy for biological compatibility –
avoiding corrosion
IPG – Implanted Pulse Generator – Titanium
package
Foot Stimulators
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First used for correction of footdrop –
inability to lift foot during swing in walking
(toes do not clear the ground)
Liberson (1961) stimulated peroneal nerve
using surface electrodes – resulted in
dorsiflexion of foot (toes avoided ground)
Switch on sole of shoe – closed when lifted
Hand Stimulators
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For those with cervical level injury at 4th and 5th
levels
Electrical stimulation combined with hand &
wrist splint
Surface electrode for finger extension
Flexion by using spring between thumb and
middle/index fingers
Increase in stimulation causes extension, as
current decreases so spring returns finger
Limitations of MPs
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Denervation – Charge needed to drive
muscles directly is large, causing tissue
heating. MPs not good if nerves damaged
Muscle Spasticity – Action potentials
spontaneously active, overriding any
possible MP action
Limited Feedback available – usually
limited to visual + auditory. Cognitive
stress on user
Commercially Available
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Estimated there will be over 100,000 MPs
in use by 2010
Upper Extremity
Lower Extremity
Organ Systems
Upper Extremity MPs
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Bionic Glove – augments grasp using surface
electrodes for finger movements
Handmaster System – Electrodes mounted in a
brace, so easier to use – fingers again
AutoMove – surface electrodes but control
signals augment muscle movements
Freehand System – Implanted system:
Stimulator in chest – works with palmar grasp
(glass) and lateral grasp (pencil)
Lower Extremity MPs
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Mostly for footdrop or standing for
paraplegics (limited success with walking)
Odstock, MicroFES & Footlifter – use
surface electrodes for foot & knee flexion
Parastep System – restores standing &
walking (about 1,000 recipients) – six
electrodes follow pattern of stimulation
Organ System Prostheses
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Stimulation of sacral nerves to control bladder &
urethral contraction
VOCARE – two (implanted) electrode pairs, user
controlled – problem in loss of erection – not so
acceptable with males
Quik-Coff – surface electrodes on abdominal wall
– help coughing – user activated
Atrostim & T154 – implanted devices stimulate
phrenic nerve to restore respiration
New Clinical Applications
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Current technology requires intact motor nerve
to muscle for muscle contraction
Device needed where myelin sheath has
degenerated – no action potentials – nerve
regeneration so MP can work
Neural regeneration in spinal cord – MPs guide
growth for regenerated nerves and maintain
muscle strength
Use MP to block unwanted signals for CP
Technology Development
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Drive towards implanted technology
Multichannel stimulation via RF link
Modular implanted systems that
communicate – limited need for wires
Power/battery requirements – biofuel
cells, make use of body chemistry
Think & respond system to replace
externally generated control signals
Final Words
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Present MPs mainly limited to use where
nerve fibres to muscle link is OK – e.g.
spinal cord injury
Exciting area of development for implant
technology
Have only considered therapy here – not
nervous system extension or supergrip!!
Next Week
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Emerging Technologies