Neuroprosthetics

1. 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.