Neuroprosthetics

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Neuroprosthetics

• Motor Prostheses

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Background

• 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

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Definition

• 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

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Clinical Applications

• Spinal cord injury
• Brain injury
• Diseases that effect neural function

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Spinal Cord Injury

• 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.

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Spinal Cord Injury

• 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

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Brain Injury

• 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)

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Brain Injury

• 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

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Diseases

• 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

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Motor Prosthesis Design

• 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

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Stimulus Delivery System

• 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

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Stimulus Delivery System

• 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

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Stimulus Delivery System

• 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

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Control Unit

• 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

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Command Interface

• 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

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Cardiac Pacemaker

• 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

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Foot Stimulators

• 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

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Hand Stimulators

• 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

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Limitations of MPs

• 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

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Commercially Available

• Estimated there will be over 100,000 MPs in use
  by 2010
• Upper Extremity
• Lower Extremity
• Organ Systems

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Upper Extremity MPs

• 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)

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Lower Extremity MPs

• 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

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Organ System Prostheses

• 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

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New Clinical Applications

• 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

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Technology Development

• 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

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Final Words

• 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!!

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Next Week

• Emerging Technologies