1. Week 5 Stimulating and recording of nerves and neurons
Neuroprosthetics
Week 5
Stimulating and recording of nerves and neurons

2. Realising an Action Potential
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

3. Electrical activation of ion channels
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.

4. Intracellular Electrodes
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!

5. Functional Magnetic Resonance Imaging (fMRI)
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.

6. Charge balanced stimulation
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).

7. Biphasic response

8. Example – Cochlear implants
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

9. Stimulating Interface
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.

10. Extracellular recording
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.

11. Closed-Loop prostheses
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!!

12. Cellular level nervous system
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.

13. Pipette Electrodes
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.

14. Microelectrodes
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.

15. Multichannel recordings
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.

16. Multichannel methods Photoengraved microelectrodes
Microelectrode arrays
SOI wafer fabrication
Polymide electrodes
Michigan probe
Three-dimensional array
Acute probes

17. Example 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.

19. Design Considerations
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.

20. Future directions
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.