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Neuroprosthetics
Week 5
Stimulating and recording of
nerves and neurons
Realising an Action Potential
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Depolarization (synaptic transmission or external
stimuli) causes ion channels to open
Response of Na channels causes increase in
outward current – depolarizes membrane further
Once opened, ion channels stay open for a while
Action potential achieved when depolarization
exceeds a threshold – then outward current
exceeds inward current
Depolarization is reversed by closing of Na
channels and opening of K channels
Membrane potential is restored to resting value
Electrical activation of ion channels
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Applying a negative current via a
stimulating electrode depolarizes the
membrane.
Natural response is then evoked.
Electrical stimulus can lead to action
potential.
Stimulating electrode serves as a cathode.
Return electrode required to complete the
current loop.
Intracellular Electrodes
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Technique used to inject current inside the cell is
the patch clamp.
Glass pipette electrodes are used to penetrate
the cell membrane.
A high impedance seal is thus formed between
the membrane and pipette.
Very efficient method but difficult/impossible to
form an implantable device!
Functional Magnetic Resonance
Imaging (fMRI)
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Used to detect brain activity in response to
a specific stimulus.
No direct electrical interface - noninvasive.
Scans can be recorded.
Resolution typically more than 1mm.
Difficult to perform – expensive.
Useful in assisting positioning of
electrodes.
Charge balanced stimulation
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For a single stimulation waveform the total
net charge must be zero.
Either supply equal cathodic and anodic
currents or better to use a blocking
capacitor which slowly discharges.
But capacitor size effects pulse duration.
If charge is not balanced, bubbling can
occur (oxygen + hydrogen produced).
Biphasic response
Example – Cochlear implants
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Use charge balanced, biphasic stimulation
pulses to activate auditory nerve.
When a 200Hz signal is present, short
pulses of 0.1msec at 200 Hz used to
stimulate.
We will consider this more in lecture 7/8
Stimulating Interface
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Electrodes form the interface between stimulus
circuit and biological cells.
Material must be high conductivity - must
deliver current at high charge densities without
corroding or dissolving.
Charge injection capacity, electrochemical
stability and mechanical strength are important.
Need to be chemically inert.
Small area electrodes for better resolution.
Platinum, Iridium, Gold are all good.
Extracellular recording
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Stimulation is the basis for restoration of sensory
and motor function.
Recording/monitoring is also essential for a
prosthesis.
Usually extensive physiological recording
research must be done to map an area into
which a stimulation device will be implanted.
Recordings are also often used to validate
efficacy and optimize design of implants.
Closed-Loop prostheses
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Monitoring is an essential part of any
closed-loop prosthesis.
Example – for spinal cord injuries –
biological control signals can be recorded
to drive implanted stimulating electrodes.
John Chapin’s work (rats) involved signals
from the motor cortex driving a robot arm
– reward/punishment in a maze!!
Cellular level nervous system
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When a neuron receives stimuli from other cells
its membrane depolarizes and causes ionic
currents to flow.
The action potential (voltage drop) associated
with this current can be measured if a suitable
electrode is located near enough.
An action potential is typically 50 to 500
microvolts with a frequency content from 100Hz
to 10 KHz.
Pipette Electrodes
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Electrode must approach the active neuron
without damaging it or other cells acting with it.
Electrode needs to be as small and noninvasive
as possible.
Glass micropipette – heating and pulling 1mm
diameter glass capilary.
Tip tapered and bevelled to 1 microm.
Filled with electrolyte solution for conduction.
Forms a low pass filter – good for up to 1 KHz
only.
Microelectrodes
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Preferred method for detecting action
potentials is with a metal microelectrode.
Use small diameter metal wire sharpened
to a (less than) 1 microm tip
Examples: Tungsten, stainless steel,
platinum
But single electrodes do not give
information on cell networks.
Multichannel recordings
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Important to observe the activity and
interaction of many neurons
simultaneously.
Determine relationships between cells.
Neurons can be separated by spatial
distribution as well as waveform/time.
Dealt with in Lecture 2 by Adam.
Multichannel methods
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Photoengraved microelectrodes
Microelectrode arrays
SOI wafer fabrication
Polymide electrodes
Michigan probe
Three-dimensional array
Acute probes
Example
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Microelectrode array
High density of penetrating shafts
Each shaft is 0.5 to 1.5 mm long
These project down from a glass/silicon
composite base
Silicon shafts are isolated from each other with a
glass frit.
The 50 microm tip of each shaft is coated with
platinum to form the electrode site.
The shafts are spaced on 400 microm centres.
Design Considerations
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Shank length – depends on depth of target and
strength/stiffness.
Shank width – minimize for noninvasiveness but
maximize for strength
Substrate thickness – insertion in tough tissue. Buckling
force is important. Poss high velocity.
Site spacing – could show correlated and uncorrelated
activity.
Site area – smaller site/higher impedance, causing
attenuation and noise. But larger site is less selective.
Future directions
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Solid-state devices – batch fabrication,
reproducible characteristics, small size + on-chip
processing.
Bioactive coatings – improved interface +
enhanced recording stability. Seed coating with
neurotrophins to assist behaviour.
Reduced output leads – reduces motion,
migration and adverse tissue response. Wireless
operation + on-chip design.