Neuroprosthetics Motor Prostheses

Background Damage to the Central Nervous System (CNS) can result in sensory loss, muscle contraction, cognitive problems, loss of motor control & biological function loss 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) Treatments include – drugs, physical therapy, surgery & rehab (+ future – neural regeneration) Also possible – Neural Motor Prosthesis Also possible – Neural Motor Prosthesis

Definition Motor Prosthesis (MP) is a device that electrically stimulates nerves innervating a series of muscles for restoring functional movement or biological function. 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 Here we look at how electrical stimulation can be used to overcome motor and functional loss

Clinical Applications Spinal cord injury Spinal cord injury Brain injury Brain injury Diseases that effect neural function Diseases that effect neural function

Spinal Cord Injury Greatest level of success in this area Greatest level of success in this area Usually caused by car or diving accident (UK) Usually caused by car or diving accident (UK) Severance or compression of cord by a fractured bone & tissue swelling Severance or compression of cord by a fractured bone & tissue swelling Mobile areas of vertebral column are most susceptible Mobile areas of vertebral column are most susceptible Paraplegia – paralysis of the lower extremeties Paraplegia – paralysis of the lower extremeties Tetraplegia – paralysis of upper & lower ext. Tetraplegia – paralysis of upper & lower ext.

Spinal Cord Injury Muscle Atrophy – degeneration of muscle tissue due to loss of neuronal input Muscle Atrophy – degeneration of muscle tissue due to loss of neuronal input Muscles to be stimulated need strengthening regime prior to introduction Muscles to be stimulated need strengthening regime prior to introduction Second level + higher injuries – loss of ability of brain stem to control breathing Second level + higher injuries – loss of ability of brain stem to control breathing Cervical + lumbar level lesions – loss of bladder, bowel & sexual functions Cervical + lumbar level lesions – loss of bladder, bowel & sexual functions

Brain Injury Many different disorders result – cognitive and sensory not issue here Many different disorders result – cognitive and sensory not issue here Use of MP can be hindered by cognitive problems Use of MP can be hindered by cognitive problems But MP can be used for motor relearning But MP can be used for motor relearning Stroke – blood vessel in brain is blocked or ruptured – loss of blood flow to an area Stroke – blood vessel in brain is blocked or ruptured – loss of blood flow to an area Result is neuronal death Result is neuronal death Most common is stroke in one motor area, resulting in paralysis in one side (opposite) Most common is stroke in one motor area, resulting in paralysis in one side (opposite)

Brain Injury Stroke also causes – hyperreflexia, muscle spasticity, muscle atrophy Stroke also causes – hyperreflexia, muscle spasticity, muscle atrophy Cerebral Palsy (CP) – occurs shortly after birth Cerebral Palsy (CP) – occurs shortly after birth Caused by accident, infection (meningitis or enchepalitis) or brain asphyxsia Caused by accident, infection (meningitis or enchepalitis) or brain asphyxsia All these lead to neuronal death All these lead to neuronal death Result is difficult to perform motor tasks – spastic CP – muscles permanently contracted Result is difficult to perform motor tasks – spastic CP – muscles permanently contracted

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 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 Only first type can benefit from prosthetics – do not affect nerves going to muscles MP generates action potentials in nerves to cause muscle contractions MP generates action potentials in nerves to cause muscle contractions So use in MS & MND very limited at present So use in MS & MND very limited at present

Motor Prosthesis Design Stimulus delivery system – electrode + wires for stimulation Stimulus delivery system – electrode + wires for stimulation Control unit – interpret user commands, convert info into muscle stimulations Control unit – interpret user commands, convert info into muscle stimulations Command interface – records signals generated by user & converts into commands for the MP Command interface – records signals generated by user & converts into commands for the MP

Stimulus Delivery System All MPs operate by electrical stimulation of nerves to elicit a muscle contraction – three methods exist for this 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 Surface electrodes – on surface of skin over point where nerve & muscle join Require conductive adhesives or pad for contact Require conductive adhesives or pad for contact Low cost & noninvasive 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 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 Percutaneous Electrodes – Inserted through skin with needle near motor point of muscle Percutaneous Electrodes – Inserted through skin with needle near motor point of muscle Single or multiple wire strands Single or multiple wire strands Barbed at the end to ensure anchoring Barbed at the end to ensure anchoring Power requirements much reduced Power requirements much reduced Skin irritation + infection can occur Skin irritation + infection can occur Stress on wires at interface – poss breakage Stress on wires at interface – poss breakage

Stimulus Delivery System Implanted Electrodes – implanted in body Implanted Electrodes – implanted in body Epimysial – platinum-iridium disk in a silicon pad, sutured near muscle motor point Epimysial – platinum-iridium disk in a silicon pad, sutured near muscle motor point Intramuscular – inserted with needle, stainless steel with umbrella anchor Intramuscular – inserted with needle, stainless steel with umbrella anchor Nerve Cuff – Platinum-iridium bands that encircle the nerve to the muscle Nerve Cuff – Platinum-iridium bands that encircle the nerve to the muscle Advantages – highly selective (not nerve cuff), less power Advantages – highly selective (not nerve cuff), less power Disadvantage – Invasive, surgery for placement and replacement Disadvantage – Invasive, surgery for placement and replacement

Control Unit Consists of command processor & control processor Consists of command processor & control processor Command Processor – interprets user generated signals to operate various motor prosthesis functions Command Processor – interprets user generated signals to operate various motor prosthesis functions E.g. System state (on/off), activation pattern selection E.g. System state (on/off), activation pattern selection Control Processor – converts signals from command processor into actual function Control Processor – converts signals from command processor into actual function Decides specific muscle stimulations required Decides specific muscle stimulations required

Command Interface Records user generated signals to operate the MP Records user generated signals to operate the MP Examples EMG, Voice, switches, respiration Examples EMG, Voice, switches, respiration Use least number of input channels Use least number of input channels Reduce amount of noise (SNR) Reduce amount of noise (SNR) Transition rate is important Transition rate is important Performance reliability Performance reliability Interface must be cheap, simple and invisible Interface must be cheap, simple and invisible

Cardiac Pacemaker Implanted device that delivers electrical impulses to cardiac tissue to control heart contractions 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 Stimulus via lead wires and electrodes on heart surface or in heart muscles Electrodes have to withstand movement Electrodes have to withstand movement Platinum alloy for biological compatibility – avoiding corrosion Platinum alloy for biological compatibility – avoiding corrosion IPG – Implanted Pulse Generator – Titanium package IPG – Implanted Pulse Generator – Titanium package

Foot Stimulators First used for correction of footdrop – inability to lift foot during swing in walking (toes do not clear the ground) 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) Liberson (1961) stimulated peroneal nerve using surface electrodes – resulted in dorsiflexion of foot (toes avoided ground) Switch on sole of shoe – closed when lifted Switch on sole of shoe – closed when lifted

Hand Stimulators For those with cervical level injury at 4 th and 5 th levels For those with cervical level injury at 4 th and 5 th levels Electrical stimulation combined with hand & wrist splint Electrical stimulation combined with hand & wrist splint Surface electrode for finger extension Surface electrode for finger extension Flexion by using spring between thumb and middle/index fingers Flexion by using spring between thumb and middle/index fingers Increase in stimulation causes extension, as current decreases so spring returns finger Increase in stimulation causes extension, as current decreases so spring returns finger

Limitations of MPs Denervation – Charge needed to drive muscles directly is large, causing tissue heating. MPs not good if nerves damaged 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 Muscle Spasticity – Action potentials spontaneously active, overriding any possible MP action Limited Feedback available – usually limited to visual + auditory. Cognitive stress on user Limited Feedback available – usually limited to visual + auditory. Cognitive stress on user

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

Upper Extremity MPs Bionic Glove – augments grasp using surface electrodes for finger movements Bionic Glove – augments grasp using surface electrodes for finger movements Handmaster System – Electrodes mounted in a brace, so easier to use – fingers again Handmaster System – Electrodes mounted in a brace, so easier to use – fingers again AutoMove – surface electrodes but control signals augment muscle movements 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) Freehand System – Implanted system: Stimulator in chest – works with palmar grasp (glass) and lateral grasp (pencil)

Lower Extremity MPs Mostly for footdrop or standing for paraplegics (limited success with walking) Mostly for footdrop or standing for paraplegics (limited success with walking) Odstock, MicroFES & Footlifter – use surface electrodes for foot & knee flexion 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 Parastep System – restores standing & walking (about 1,000 recipients) – six electrodes follow pattern of stimulation

Organ System Prostheses Stimulation of sacral nerves to control bladder & urethral contraction 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 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 Quik-Coff – surface electrodes on abdominal wall – help coughing – user activated Atrostim & T154 – implanted devices stimulate phrenic nerve to restore respiration Atrostim & T154 – implanted devices stimulate phrenic nerve to restore respiration

New Clinical Applications Current technology requires intact motor nerve to muscle for muscle contraction 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 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 Neural regeneration in spinal cord – MPs guide growth for regenerated nerves and maintain muscle strength Use MP to block unwanted signals for CP Use MP to block unwanted signals for CP

Technology Development Drive towards implanted technology Drive towards implanted technology Multichannel stimulation via RF link Multichannel stimulation via RF link Modular implanted systems that communicate – limited need for wires Modular implanted systems that communicate – limited need for wires Power/battery requirements – biofuel cells, make use of body chemistry Power/battery requirements – biofuel cells, make use of body chemistry Think & respond system to replace externally generated control signals Think & respond system to replace externally generated control signals

Final Words Present MPs mainly limited to use where nerve fibres to muscle link is OK – e.g. spinal cord injury 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 Exciting area of development for implant technology Have only considered therapy here – not nervous system extension or supergrip!! Have only considered therapy here – not nervous system extension or supergrip!!

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