Lecture 5: Bioelectricity, Functional Electrical Stimulation and Neuromuscular Prosthetics

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Lecture 5 Bioelectricity, Functional Electrical
Stimulation and Neuromuscular Prosthetics
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Lecture Overview

• "Animal Electricity" Brief History
• Biopotentials
• Muscles revisited A Bioelectric Perspective
• Equivalent Circuits A Model for bioelectronic
  Implant
• Functional Electrical Stimulation
• Device Design and Challenges
• Bioelectrodes The Weakest Link
• Smart Materials for Neuroprosthetic Devices
• Case Study Biohybrid device for Peripheral Nerve
  Injuries
• Next week Biosensors, Biomonitoring devices

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Discovery of Bioelectricity

• " The idea grew that in the animal itself there
  was an indwelling electricity. We were
  strengthened in such an assumption of a very fine
  nervous fluid that during the phenomenon flowed
  into the muscle from the nerve, similar to
  electric current   
• Luigi Galvani, 1791

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Bioelectricity Opposition and Evolution

• Galvani attributed bioelectricity to a life
  force vitalism vs. mechanism
• Alessandro Volta(1794) suggested that the effect
  was only due to dissimilar metals thus
  discrediting Galvanis hypothesis
• Carlo Metteuci broke the deadl-lock and confirmed
  Galvanis experiment
• Metteuci showed that action potential precedes
  contraction of skeletal muscles
• Led to emergence of new discipline
  Electrophysiology
• Basic Notions of Electrophysiology necessary to
  design certain medical devices

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Ringers experiment Placed two hearts One in
tap water, one in distilled waterOnly one of
the hearts ceased to contract. Which one ? Why?
How is electricity "produced" in the
body?What are the basic components of an
electric circuit ?What would their biological
equivalent be?
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Origin of Bioelectricity Membrane Potentials

• Electric currents  in the body occur through
  ion flow
• Ions Na, K, Cl-, Ca2
• Ion permeability is regulated through
• Osmosis
• Ion pumps
• Ion channels
• Ion exchangers
• Concepts
• Resting Potential
• Action Potential
• Depolarization
• Hyperpolarization

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Circuit Elements in Bioelectricity

• Capacitor
• Inductor
• Resistor
• Impedance Total Opposition to currents in the
  circuit

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Resting Potentials
ion cytoplasm outside Veq
Na 12 140 64 mV
K 135 4 -92 mV
Cl- 5 150 -89 mV
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Measuring Ion Concentrations
Nernst Equation
Goldman Equation
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Muscles Revisited
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• Draw equivalent circuit models for muscle
  contraction combining the electrical and
  mechanical compenents

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What is a Bionic Device ?

• Intelligent, adaptive device capable of
  interacting with the human body in a
  bidirectional manner
• Direct contact with bodys command and control
  systems
• Seamless, intertwining of electronics, mechanics
  and materials
• Biomimicry Closely replicates physiological
  function

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Functional Electric Stimulation

• Rehabilitation technique using low-level
  electrical current to enhance that patients
  ability to function and live independently
• Device
• Controller
• Stimulator
• Leads/ Electrodes

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Spinal Cord Injury A snapshot
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Functional Electrical Stimulation to restore
movement

• Bionic Gloves for reaching, grasping and
  releasing
• Peripheral Prosthetics for Elbow extensions
• Restoring shoulder movement
• Locomotion Peripheral, Spinal and Central

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Biopotentials for Control and Feedback
ENG
EEG
EMG
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Functional Electric Stimulation and
Rehabilitation The carry-over effect

• Improves the fitness and strength of remaining
  units
• Reduces amount of spasticity
• Improves connectivity tissue stretch
• Reasons unclear, but possibly related to cortical
  reorganization

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Functional Electric Stimulation Limitations of
first generation of implants

• Reverse recruitment fast twitch before slow
  twitch leading to muscle fatigue
• Virtual lack of closed-loop control via afferent
  pathways
• Steep recruitment of muscle fibers results in
  robot-like movements
• Inflexibility of stimulation patterns ,
  non-adaptibility
• Adverse reaction at electrode interface
• Encapsulation and surface fouling of electrodes
• Spill-over Neighbouring muscles inadvertently
  recruited

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BIONIC DEVICES TO RESTORE LOCOMOTION
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Epidural Spinal-Cord Stimulation

• Stimulation of the dorsal structures
  (locomotion-like ENGs)
• Combined with weight-bearing threadmill therapy
• Facilitates gait
• Effective only in incomplete injuries
• Lack of selectivity, muscle synergy

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An Improvement Intraspinal Microstimulation

• Tapping into patterned movements of the legs
• Ventral horn is the best place to activate
  weight-generating extension
• Ventral Horn Interneurons, motor neurons
• Able to produce all the muscle synergies required
  for stepping
• Microstimulation in lamina IX of the lumbosacral
  spinal cord
• Stable recordings over time
• Near-normal recruitment order
• ISMS can generate weight-bearing stepping after
  SCI
• Superior to epidural or peripheral nerve
  stimulation

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Bypassing the spinal cord .
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Brain-controlled Implants Cortical Neural
Prosthetics

• IMPLANT
• Chronic Multielectrode arrays
• Extraction Algorithms (EA)
• Effectors (robot, intrinsic muscles)
• Primary Motor Cortex organised in a body map
• Movement-related information encoded as firing
  rate
• Firing rate is directionally tuned
• EA convert firing rate into movement displacement
• Successful control of a robotic arm using a
  monkeys brain

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Current Technological Limitations

• Adaptibility DOES NOT extend to activating
  afferent pathways
• Ineffective in activating multiple muscle
  systems, muscle synergies obtained only in rare
  cases.
• Does not adapt to neuronal reorganisation and
  hyperexcitabality in SCI
• Electrodes lead to scarring, encapsulation and
  signal attenuation
• Inefficient power sources
• Inadequate signal processing algorithms
• Lack of concise biomechanical models to correlate
  neuronal discharge rate to movement dynamics and
  kinematics.

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The Future (or Part of it)

• Idea of shared control suggested by Dr. Nicolesis
• Stimulating Central Pattern Generators for
  rhythmic movement (bidirectional interfacing)
• Novel Bioelectrodes
• Building Implantable Neuromuscular Implant based
  on work on artificial muscles by Rajagopalan et
  al.
• Body-powered implants

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Distributed or Shared Control

• Device-to-device communication
• Implant in motor cortex communicates with device
  in limb
• Possibility of more natural movements, sensorial
  feedback
• Not feasible for movement of lower extremities ?

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Novel Bioelectrodes A Biomimetic Neural
Interface

• Biomimicry Design of structures and mechanics
  closely resembling living tissue and system
• Conducting Polymer Electronic conduction, ion
  transport and cellular adhesion and growth
• Polyelectrolyte gel mechanical and chemical
  properties similar to living tissue. Biomimetic
  ion exchange and transport phenomena
• Designed as Stand-Alone Electrode

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Motivation Challenge

• Nerve-electrode interface remains the weakest
  link in neuroprosthetic devices.
• Drawbacks of current bioelectrodes include
• Poor anchoring at nerve interface
• Mechanical mismatch between metal electrodes and
  soft tissue
• Inefficient charge transfer, signal attenuation
• Inflammatory response Build-up of fibroblasts at
  stimulation site
• Current Solutions Surface modification of
  electrodes with conducting polymers, bioadhesive
  molecules
• Results Decreased impedance (?), decrease in
  inflammatory response. Incremental improvements,
  but fundamental problems persist.
• Novel Solution A paradigm shift biomimetic
  approach through field responsive polymers

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Solution A "Smart" Cuff Electrode

• Problem Poor anchoring at interface
• Solution Soft thermo-morphing Armature
• Problem Mechanical mismatch, attenuated charge
  transfer
• Solution Neuro-mimetic electrode
• Problem Inflammatory Response - Build up of
  Fibroblasts
• Solution Drug-eluting Electrode

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Soft Thermomorphing Armature

• Poly(isopropylacrylamide)
• Biocompatible
• Reverse solubility at body temperature
• Widely used in medicine for cell adhesion, drug
  delivery
• Can be fabricated as solid hydrogels

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Soft Thermomorphing Armature
Thermosensitive PNIIPAM layer Passive
polyacrylamide layer
DESIGN FRONTAL VIEW PRINCIPLE LATERAL
VIEW
Tgt 35.5
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Neuro-mimetic Electrode
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The Nerve Sodium Currents
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Neuro-mimetic Electrode

• Electrochemical Polymerisation
• Both pyrolle and poly(sodium acrylate)
  electrochemically deposited on metal electrode
• Chemical Polymerisation
• Polypyrrole electrostatically deposited on
  poly(sodium acrylate) gel

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Conductivity of Composites

• 8515 Ratio of Polypyrrole, PSA produces optimal
  miscibility, texture and conductivity
• Conductivity 10-1 S/cm
• Inferior to results obtained in literature.
  Resistance greater than that of platinum
  electrode
• Results by Martin et. al show decrease in
  impedance with coating of polypyrrole
• Possible explanations microporosity, thickness,
  gel layer
• To be optimised following in-vivo tests
• Na-ion release to be confirmed through atomic
  abosrption spectroscopy

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Drug-eluting Electrodes
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Drug-Electroding Stents
www.fda.gov
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Drug-eluting Polypyrrole The Mechanism
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Anti-inflammatory Drug Fosfosal

• Negatively-charged Molecule.Exists as a salt
• Can serve as counterion to polypyrrole
• Electrically-released on activation
• Benzoic Acid used as stand-in for current
  experiments