Neuroprosthetics

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 • Treatments include – drugs, physical therapy, surgery & rehab (+ future – neural regeneration) • 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. • Here we look at how electrical stimulation can be used to overcome motor and functional loss

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

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.

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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!!

Next Week • Emerging Technologies

Neuroprosthetics